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The Digestive System
Chapter 24
The Digestive System The digestive system
– Takes in food– Breaks it down into nutrient molecules– Absorbs the nutrient molecules into the
bloodstream– Rids the body of indigestible remains
The Digestive System
The organs of the digestive system can be separated into two main groups; those of the alimentary canal and the accessory organs
The Digestive System
The alimentary canal or gastrointestinal (GI) tract is the continuous muscular digestive tube that winds through the body
The Digestive System The organs of the alimentary canal are
– Mouth, pharynx, esophagus, stomach, small intestine and large intestine
– Food in this canal is technically out of the body
The accessory digestive organs are– Teeth, tongue, gallbladder, salivary glands,
liver and pancreas– The accessory organs produce saliva, bile and
digestive enzymes that contribute to the breakdown of foodstuffs
Digestive Processes The digestive tract
can be viewed as a process by which food becomes less complex at each step of processing and nutrients become available to the body
Ingestion Ingestion
– Simply the process of taking food into the digestive tract, usually via the mouth
Propulsion Propulsion is the process that moves food
through the alimentary canal It includes swallowing (voluntary
process) and peristalsis (involuntary process)
Propulsion Peristalsis involves
alternate waves of contraction and relaxation of muscles in the organ walls
Its main effect is to squeeze food from one organ to the next
Some mixing occurs as well
Mechanical Digestion Mechanical digestion physically prepares
food for chemical digestion by enzymes
Mechanical Digestion Mechanical processes
include chewing, mixing of food with saliva by the tongue, churning of food by the stomach, and segmentation
Segmentation mixes food with digestive juices and increases the rate of absorption by moving food over the intestinal wall
Chemical Digestion Chemical digestion is a series of catabolic
steps in which complex food molecules are broken down to their chemical building blocks
Chemical digestion is accomplished by enzymes secreted by various glands into the lumen of the alimentary canal
The enzymatic breakdown of foodstuffs begins in the mouth and is essentially complete in the small intestine
Absorption Absorption is the passage of digested end
products (plus vitamins, mineral and water) from the lumen of the GI tract into the blood or lymph
For absorption to occur these substances must first enter the mucosal cells by active or passive transport processes
The small intestine is the main absorption site
Defecation Defecation is the elimination of
indigestible substances from the body via the anus
Basic Functional Concepts Most organ systems respond to changes in
the internal environment either by attempting to restore some plasma variable or by changing their own function
The digestive system creates an optimal environment for its functioning in the lumen of the GI tract
Essentially all digestive tract regulatory mechanisms act to control luminal conditions so that digestion and absorption can occur there as effectively as possible
Basic Functional Concepts Digestive activity is provoked by a range
of mechanical and chemical stimuli– Receptors are located in the walls of the tract
organs– These receptors respond to several stimuli– The most important being the stretching of
the organ by food in the lumen, osmolarity (solute concentration) and pH of the contents and the presence of substrates and end products of digestion
Basic Functional Concepts When appropriately
stimulated, these receptors initiate reflexes that– Activate or inhibit
glands that secrete digestive juices into the lumen or hormones into the blood
– Mix lumen contents along the length of the tract by stimulating the smooth muscle of the GI tract walls
Basic Functional Concepts Controls of digestive activity are both
extrinsic and intrinsic– Another novel trait of the digestive tract is
that many of its controlling systems are intrinsic - a product of in-house nerve plexuses or local hormone-producing cells
– The walls of the alimentary canal contain nerve plexuses
– These plexuses extend essentially the entire length of the GI tract and influence each other both in the same and in different organs
Digestive Processes Two kinds of reflex
activity occur Short reflexes are
mediated entirely by the local enteric plexuses in response to GI tract stimuli
Long reflexes are initiated by stimuli arising from within or outside of the GI tract and involve CNS centers and ANS
Digestive Processes The stomach and small intestine also
contain hormone-producing cells that, when stimulated by chemicals, nerve fibers, or local stretch, release their products to the extracellular space
These hormones circulate in the blood and are distributed to their target cells within the same or different tract organs, which they prod into secretory or contractile activity
Digestive System Organs Most of the digestive
organs reside in the abdominal-pelvic cavity
All ventral body cavities contain serous membranes
The peritoneum of the abdominal cavity is the most extensive serous membrane of the body
Digestive System Organs The visceral peritoneum
covers the external surface of most digestive organs and is continuous with the parietal peritoneum that lines the walls of the abdomino-pelvic cavity
Between the two layers is the peritoneal cavity, a slitlike potential space containing fluid secreted by the serous membranes
Digestive System Organs The serous fluid lubricates
the mobile digestive organs, allowing them to glide easily across one another as they carry out their digestive activities
Digestive System Organs
A mesentery is a double layer of peritoneum - a sheet of two serous membranes fused back to back - that extends to the digestive organ from the body wall
Digestive System Organs
Mesenteries provide routes for blood vessels, lymphatics and nerves to reach the digestive viscera
Digestive System Organs
Mesenteries also suspend the visceral organs in place as well as serving as a site for fat storage
Digestive Processes
Not all alimentary canal organs are suspended with the peritoneal cavity by a mesentery
Some parts of the small intestine originate the cavity but then adhere to the dorsal abdominal wall (Figure 24.5b) above
Digestive Processes
Organs that adhere to the dorsal abdominal wall lose their mesentery and lie posterior to the peritoneum
These organs, which also include most of the pancreas and parts of the large intestine are called retro-peritoneal organs
Digestive Processes
Digestive organs like the stomach that keep their mesentery and remain in the peritoneal cavity are called interperitoneal or peritoneal organs
It is not known why some digestive organs end up in the retroperitoneal position
Blood Supply The splanchnic circulation includes those
arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation
The hepatic, splenic and left gastric branches of the celiac trunk serve the spleen, liver, and stomach
The mesenteric arteries (superior and inferior) serve the small and large intestine
Blood Supply The arterial supply to the abdominal organs
is approximately one quarter of the cardiac output
The hepatic portal circulation collects nutrient-rich venous blood draining from the digestive viscera and delivers it to the liver
The liver collects the absorbed nutrients for metabolic processing or for storage before releasing them back to the bloodstream for general cellular use
Histology of the Alimentary Canal From the esophagus to the anal canal, the
walls of every organ of the alimentary canal are made up of the same four basic layers or tunics– Mucosa– Submucosa– Muscularis externa– Serosa
Each tunic contains a predominant tissue type that plays a specific role in food breakdown
Histology of the Alimentary Canal From internal to
external the four layers of the alimentary canal are– Mucosa
– Submucosa
– Muscularis Externa
– Serosa
Histology: Mucosa The mucosa is the
moist epithelial membrane that lines the length of the lumen of the alimentary canal
Major functions are– Secretion of mucus,
digestive enzymes and hormones
– Absorption
– Protection
Histology: Mucosa The mucosa is the
moist epithelial membrane that lines the length of the lumen of the alimentary canal
Major functions are– Secretion of mucus,
digestive enzymes and hormones
– Absorption
– Protection
Histology: Mucosa More complex than most other mucosae
the typical digestive mucosa consists of three sublayers– A surface epithelium– A lamina propria– A deep muscularis mucosae
Histology: Mucosa The epithelium of
the mucosa is a simple columnar epithelium that is rich in mucus secreting goblet cells
Histology: Mucosa The slippery mucus it produces protects
certain digestive organs from digesting themselves by enzymes working within their cavities and eases food passage
In the stomach and small intestine the mucosa contain both enzyme-secreting and hormone-secreting cells
Thus, in such sites, the mucosa is a diffuse kind of endocrine organ as well as part of the digestive organ
Histology: Mucosa The lamina propria
which underlies the epithelium is loose areolar connective
Note lymph nodule
Histology: Mucosa Its capillaries nourish the epithelium and
absorb digested nutrients Its isolated lymph nodules are part of the
mucosa associated lymphatic tissue (MALT) which collectively act as a defense against bacteria and other pathogens
Large collections of lymph nodules occur at strategic locations such as within the pharynx (tonsils) and appendix
Histology: Mucosa The muscularis
mucosae is a scant layer of smooth muscle cells that produces local movements of the mucosa
Histology: Mucosa The twitching of this muscle layer dislodges
food particles that have adhered to the mucosa
In the small intestine, it throws the mucosa into a series of small folds that immensely increase its surface area
Histology: Submucosa The submocosa is a
moderately dense connective tissue containing blood and lymphatic vessels, lymph nodules, and nerve fibers
Its rich supply of elastic fibers enables the stomach to regain its normal shape after storing a large meal
Histology: Submucosa The submocosa is a
moderately dense connective tissue containing blood and lymphatic vessels, lymph nodules, and nerve fibers
Its rich supply of elastic fibers enables the stomach to regain its normal shape after storing a large meal
Histology: Muscularis Externa The muscularis
externa is responsible for segmentation and peristalsis
It mixes and propels foodstuffs along the digestive tract
This thick muscular layer has an inner circular and an outer longitudinal layer
Histology: Muscularis Externa In several places along the GI tract, the
circular layer thickens to form sphincters Sphincters act as valves to prevent backflow
and control food passage from one organ to the next
Histology: Serosa The serosa is the
protective outermost layer of inter- peritoneal organ
This visceral peritoneum is formed of areolar connective tissue covered with meso- thelium, a single layer of squamous epithelial cells
Histology: Serosa In the esophagus, which is located in the
thoracic instead of the abdominopelvic cavity, the serosa is replaced by an adventitia
The adventitia is an ordinary fibrous connective tissue that binds the esophagus to surrounding structures
Retroperitoneal organs have both a serosa (on the side facing the peritoneal cavity) and an adventitia (on the side abutting the dorsal body wall)
Enteric Nervous System The alimentary
canal has its own in-house nerve supply
Enteric neurons communicate widely with each other to regulate digestive system activity
IntrinsicNervePlexes
Enteric Nervous System These enteric
neurons constitute the bulk of the two major intrinsic nerve plexuses found within the walls of the alimentary canal– Submucosal nerve
plexus
– Myenteric nerve plexus
Myentericplexus
Submucosalplexus
Enteric Nervous System A smaller third
plexus is found within the serosa layer– Subsersora nerve
plexus
Subserosa nerveplexus
Enteric Nervous System The submucosal
nerve plexus chiefly regulates the activity of glands and smooth muscle in the mucosa tunic
The myenteric nerve plexus lies between the circular and longitudinal layers of smooth muscle of the muscularis externa
Myentericplexus
Submucosalplexus
Enteric Nervous System Via their communication with each other,
with smooth muscle layers, and with submucosal plexus, the enteric neurons of the myenteric plexus provide the major nerve supply to the GI tract
This plexus controls GI tract mobility by controlling the patterns of segmentation and peristalsis
Control comes from local reflex arcs between enteric neurons in the same or different plexus or organs
Enteric Nervous System The enteric nervous system is also linked
to the CNS by afferent visceral fibers and sympathetic and parasympathetic branches of the ANS
Digestive activity is subject to extrinsic control exerted by ANS which can speed up or slow secretory activity and mobility
Digestive System
Mouth, Pharynx, and Esophagus The mouth is the only part of the
digestive system that is involved in the ingestion of food
Most digestive function of the mouth reflect the activity of accessory organs chewing the food and mixing it with salvia to begin the process of chemical digestion
The mouth also begin the propulsive process by which food is carried through the pharynx and esophagus to the stomach
The Mouth The oral cavity is a
lined with mucosa It bounded by the
lips anteriorly, and the tongue inferiorly and the cheeks laterally
Its anterior opening is the oral orifice
Posteriorly the oral cavity is continuous with the oropharynx
The Mouth The walls of the
mouth are lined with stratified squamous epithelium
The epithelium is highly ketatinized for extra protection against abrasion during eating
The mucosa also produces defensins to fight microbes in the mouth
The Lips and Cheeks The labia and the
cheeks have a core of skeletal muscle covered by skin
The orbicularis oris muscle forms the bulk of the lips
The cheeks are formed largely by the buccinators
The area between the teeth and gums is the vestibule
The Lips and Cheeks The lips extend
from the inferior margin of the nose to the superior boundary of the chin
The reddened area is called red margin
The labial frenulum is a median fold that joins the internal aspect of each lip to the gum
The Palate The palate which
forms the roof of the mouth has two distinct parts– Hard palate
– Soft palate
The Palate The hard palate is
underlain by bone and is a rigid surface against which the tongue forces food during chewing
There exists a centerline ridge called a raphe
The mucosa is corrugated for friction
The Palate The soft palate is a
mobile fold formed by skeletal muscle
Projecting down from its free edge is the uvula
The soft palate rises reflexively to close off the nasopharynx when swallowing
The Palate The soft palate is
anchored to the tongue by the palantoglossal arches and to the wall of oropharynx by the palantopharyngeal arches
These arches form the boundary of the facuces
The Tongue The tongue occupies the floor of the
mouth and fills most of the oral cavity when closed
The tongue is composed of interlacing masses of skeletal muscle fibers
The tongue grips the food and constantly repositions it between the teeth
The tongue also mixes the food with salvia and form it into a mass called a bolus and then initiates swallowing by moving the mass into the pharynx
The Tongue The tongue has both
intrinsic and extrinsic skeletal muscles
The intrinsic muscles are confined within the tongue and are not attached bone
The fibers allow the tongue to change its shape for speech and swallowing but not its position
The Tongue The extrinsic muscles
extend the tongue from their points of origin
The extrinsic muscles allow the tongue to be protruded, retracted and moved side to side
The tongue is divided by a median septum of connective tissue
The Tongue A fold of mucosa
called the lingual frenulum secures the tongue to the floor of the mouth
This frenulum limits the posterior move- ment of the tongue
You cannot swallow your tongue
The Tongue The conical filaform
papillae give the tongue surface a roughness that aids in manipulating foods in the mouth
They align in parallel rows on the dorsum
They contain keratin which stiffens them
House taste buds
The Tongue The mushroom
shaped fungiform palillae are scattered over the surface
Each has a vascular core that gives it a reddish hue
Houses taste buds
The Tongue The circumvallate
are located in a V-shaped row at the back of the tongue
Appear similar to the fungiform papillae but with an additional surrounding furrow
The Salivary Glands A number of glands both inside and
outside the oral cavity produce and secrete saliva
Saliva functions to – Cleanses the mouth– Dissolves food chemical so that they can be
tasted– Moistens food and aids in compacting it into
a bolus– Contains enzymes that begin the chemical
breakdown of starches
The Salivary Glands Most saliva is
produced by three pairs of extrinsic salivary glands– Parotid
– Submandibular
– Sublingual These glands lie
outside the oral cavity and empty their secretions into it
The Salivary Glands The intrinsic
salivary glands are small and are scattered throughout the oral cavity
The Salivary Glands The salivary glands are composed of two
types of secretory cells; mucus and serous The serous cells produce a watery
secretion containing enzymes and the ions of saliva
The mucus cells produce mucus a stringy viscous solution
The Teeth The teeth lie in sockets in the gum
covered margins of the mandible and maxilla
Teeth function to tear and grind food and begin the mechanical process of digestion
Dentition Ordinarily we have two sets of teeth the
primary and permanent dentitions The primary dentition consists of
deciduous teeth The first teeth appear at six months and
additional teeth continue to erupt until about 24 months when all 20 teeth have emerged
Dentition As the deeper permanent teeth enlarge
and develop, the root of the milk teeth are resorbed from below causing them to loosen and fall out between the ages of 6 and 12 years
Generally, all the teeth of the permanent dentition have erupted by adolescence
The Teeth Teeth are classified
according to their shape and function– Incisors / cutting
– Canines / tear
– Premolars / grind
– Molars / crush There are 20 milk
teeth and 32 permanent teeth
Tooth Structure Each tooth has two
major regions; the crown and the root
The crown represents the visible portion of the tooth exposed above the gum
The root is the portion of the tooth that is imbedded in the jawbone
The Pharynx From the mouth, the
food passes posteriorly into the oropharnyx
The mucosa consists of stratified squamous epithelium
The epithelium is supplied with mucus producing glands for lubrication
The Pharynx The external muscle
layer consists of two skeletal muscle layers
The cells of the inner layer run longitudinally
The outer layer of muscles pharyngeal constrictor muscles, encircle the wall
Sequential contractions propel food into esophagus
The Esophagus
The esophagus takes a fairly straight course through the mediastinum of the thorax, pierces the diaphragm at the esophageal hiatus to enter the abdomen
The Esophagus The esophagus joins
the stomach at the cardiac orifice
The cardica orifice is surrounded by the cardiac esophogeal sphincter
The Pharynx The esophageal mucosa contains a non-
ketatinized stratified squamous epithelium which changes abruptly simple columnar epithelium upon reaching the stomach
When empty the esophagus is empty with its mucosa drawn into folds which flatten out when food is in passage
The mucosa contains mucus secreting esophageal glands which are compressed by a passing bolus of food resulting in the glands secreting a lubricant
The Pharynx The muscularis externa changes from
skeletal muscle to a mix of skeletal and smooth to finally all smooth as it approaches the stomach
Instead of a serosa, the esophagus has a fibrous adventitia composed entirely of connective tissue, which blends with surrounding structures along its route
Digestive Processes The mouth and its accessory digestive
organs are involved in most digestive processes– The mouth ingests food– Begins mechanical digestion by chewing– Initiates propulsion by swallowing– Starts the process of chemical digestion
– The pharynx and the esophagus serve as conduits to pass food from the mouth to the stomach
Digestive Processes: Mastication Mastication is the mechanical process of
breaking down food The cheeks and closed lips hold the food
between the teeth The tongue mixes the food with saliva to
soften it The teeth cut and grind food into smaller
pieces
Digestive Processes: Deglutition In deglutition, food is
first compacted by the tongue into a bolus and swallowed
Swallowing is a process that requires the coordination of tongue soft palate, pharynx, esophagus and 22 separate muscles
Digestive Processes: Deglutition In deglutition, food is
first compacted by the tongue into a bolus and swallowed
Swallowing is a process that requires the coordination of tongue soft palate, pharynx, esophagus and 22 separate muscles
Digestive Processes: Deglutition Food passage into
respiratory passageways by rising of the uvula and larynx
Relaxation of the upper esophageal sphincter allows food entry into the esophagus
Digestive Processes: Deglutition The constrictor muscles
of the pharynx contract, forcing food into the esophagus inferiorly
The upper esophageal sphincter contracts after entry
Digestive Processes: Deglutition Food is conducted along
the length of the esophagus to the stomach by peristaltic waves
Digestive Processes The gastroesophageal
sphincter enters opens and food enters the stomach
The Stomach The stomach functions as a temporary
storage tank where the chemical breakdown of protein begins and food is converted to a creamy paste called chyme
The stomach lies in the upper left quadrant of the abdominal cavity
Though relatively fixed at both ends, it is free to move in between
The Stomach: Gross Anatomy The stomach varies
from 6 to 10 inches in length, but its diameter and volume depend on how much food it contains
Empty it may contain on 50 ml but can expand to hold about 4 liters of food
The Stomach: Gross Anatomy
When empty, the stomach collapses inward, throwing its mucosa into large, longitudinal folds called rugae
The Stomach: Gross Anatomy
The major region of the stomach are the cardia region, the fundus, body, pyloric region, and the greater and lesser curvatures
The Stomach: Gross Anatomy
The lesser omentum runs from the liver to the lesser curvature where it becomes continuous with the visceral peritoneum of the stomach
The Stomach: Gross Anatomy
The greater omentum drapes inferior from the greater curvature of the stomach to cover the coils of the small intestine
Stomach: Microscopic Anatomy The stomach wall exhibits the four tunics
of most of the alimentary canal but its muscularis and mucosa are modified for the special roles of stomach
The muscularis externa has an extra oblique layer of muscle that enables it to mix, churn and pummel food
The epithelium lining the stomach mucosa is simple columnar epithelium composed entirely of goblet cells, which produce a protective coating of mucus
Microscopic Anatomy The four tunics
typical of the alimentary canal– Mucosa
– Submucosa
– Muscularis Externia
– Serosa
Microscopic Anatomy The otherwise smooth
lining is dotted with millions of gastric pits which lead to gastric glands that produce gastric juice
The glands of the stomach body are substantially larger and produce the majority of the stomach secretions
Microscopic Anatomy Mucus neck cells
produce a different type of mucus from that secreted by the mucus secreting cells of the surface epithelium
The special function of this unique mucus is not yet understood
Microscopic Anatomy Parietal cells scattered
among the chief cells secrete hydrochloric acid (HCl) and intrinsic factor
The parietal cells have a large surface area adapted for secreting HCl in the stomach
Intrinsic factor is required for absorption of B12 in the small intestine
Microscopic Anatomy Chief cells produce
pepsinogen, the inactive form of the protein- digesting enzyme pepsin
The cells occur mainly in the basal regions of the gastric glands
Pepsinogen is activated by HCl
Microscopic Anatomy Parietal cells scattered
among the chief cells secrete hydrochloric acid (HCL) and intrinsic factor
The parietal cells have a large surface area adapted for secreting HCL in the stomach
Intrinsic factor is required for absorption of B12 in the small intestine
Microscopic Anatomy Enteroendocrine
release a variety of hormones directly into the lamina propria
These products diffuse into capillaries and ultimately influence several digestive system target organs which regulate stomach secretion and mobility
Mucosal Barrier Gastric juice is 100,000 more concentrated
than that found in the blood Under such harsh conditions the stomach
must protect itself from self digestion with a mucosal barrier– Bicarbonate rich mucus is on the stomach wall– Epithelial cells are joined by tight junctions– Glandular cells are impermeable to HCl– Surface epithelium is replace every 3 to 6 days
Digestive Processes: Stomach The stomach is involved in the whole
range of digestive activities– It serves as a holding area for ingested food– Breaks down food further chemically and
mechanically– It delivers chyme to the small intestine at a
controlled rate
Digestive Processes: Stomach Protein digestion is initiated in the
stomach and is essentially the only type of enyzmatic digestion that occurs there
The most important protein digesting enzyme produced by the gastric mucosa is pepsin
In children, the stomach glands also secrete rennin, an enzyme that acts on milk protein converting it to a curdy substance appearing like sour milk
Digestive Processes: Stomach Despite its many functions in the
digestive system the only one that is essential for life is secretion of intrinsic factor
Intrinsic factor is required for intestinal absorption of vitamin B12, needed to produce mature erythrocytes
Without B12 the individual will develop prenicious anemia unless administered by injection
Regulation of Gastric Secretion Gastric secretion is controlled by both
neural and hormonal mechanisms Under normal conditions the gastric
mucosa creates as much as 3 liters of gastric juice every day
Gastric juice is an acid solution that has the potential to dissolve nails
Regulation of Gastric Secretion Nervous control is regulated by long
(vagus nerve mediated) and short (local enteric) nerve reflexes
When the vagus nerves actively stimulate the stomach, secretory activity of virtually all of its glands increase
The sympathetic nerves depress secretory activity
Regulation of Gastric Secretion Hormonal control of gastric secretion is
largely from the presence of gastrin Gastrin stimulates the secretion of both
enzymes and HCL in the stomach Hormones produced by the small
intestine are largely gastrin antagonists
Regulation of Gastric Secretion Stimuli acting at three distinct sites, the
head, stomach, and small intestine, provoke or inhibit gastric secretory activity
Accordingly the three phases are called cephalic, gastric, and intestinal phases
However, the effector site is the stomach in all cases and once initiated, one or all threephases may be occurring at the same time
Phase 1: Cephalic reflex The cephalic reflex phase of gastric
secretion occurs before food enters the stomach
It is triggered by the aroma, taste, sight, or though of food
During this phase the brain gets the stomach ready for food
Phase 1: Cephalic reflex Inputs from activated olfactory receptors
and taste buds are relayed to the hypothalamus which in turn stimulates the vagal nuclei of the medulla oblongata, causing motor impulses to be transmitted via the vagus nerves to the parasympathetic nerve ganglia
Eneteric ganglionic neurons in turn stimulate the stomach glands
Phase 1: Cephalic reflex The enhanced secretory activity that
results when we see or think of food is a conditioned reflex and occurs only when we like or want the food
If we are depressed or have no appetite, this part of the cephalic reflex is suppressed
Phase 2: Gastric reflex Once food reaches the stomach, local
neural and hormonal mechanisms initiate the gastric phase
This phase provides about two-thirds of the gastric juice released
The most important stimuli are distension, peptids, and low acidity
Phase 2: Gastric reflex Stomach distension activates stretch
receptors and initiates both local (myentertic) reflexes and the long vagovagal reflexes
In vagovagal reflex, impulses travel to the medulla and then back to the stomach via vagal fibers
Both types of reflexes lead to acetylcholine (ACH) release, which in turn stimulates the output of more gastric juice by cells
Phase 2: Gastric reflex Though neural influences initiated by
stomach distension are important, the hormone gastrin probably plays a greater role in stimulating stomach gland secretion during the gastric phase
Chemical stimuli provided by partially digested proteins (peptids)caffine (colas, coffee) and rising pH directly active gastrin secreting entoendocrine cells called G cells
Phase 2: Gastric reflex Although gastrin also stimulates the
release of enzymes, its main target is the HCL secreting parietal cells, which it prods to spew out even more HCL
Highly acidic (pH below 2) gastric contents inhibit gastrin secretion
Phase 2: Gastric reflex When protein foods are in the stomach,
the pH of the gastric contents generally rises because proteins act as buffers to tie up H+
The rise in pH stimulates gastrin and subsequently HCL release, which in turn provides the acidic conditions needed for protein digestion
Phase 2: Gastric reflex The more protein in the meal, the greater
the amount of gastrin and HCL released As proteins are digested, the gastric
contents gradually become more acidic, which again inhibits the gastrin secreting cells
This negative feedback mechanism helps maintain optimal pH and working conditions for the gastric enzymes
Phase 2: Gastric reflex G cells are also activated by the neural
reflexes already described Emotional upsets, fear, anxiety, or
anything that triggers the fight-or-flight response inhibits gastric secretion because (during such times) the sympathetic division overrides parasympathetic controls of digestion
Phase 2: Gastric reflex The control of the HCL secreting parietal
cells is unique and multifaceted Basically, HCL secretion is stimulated by
three chemicals, all of which work through second-messenger systems Ach released by parasympathetic nerve fibers and gastrin secreted by G cells
Phase 2: Gastric reflex Ach released by
para-sympathetic nerve fibers and gastrin secreted by G cells bring about their effects by increasing intercellular Ca+
+ levels
Phase 2: Gastric reflex Histamine
released by mucosal cells called histaminocytes acts through cyclic AMP (cAMP)
Phase 2: Gastric reflex When only one of the three chemicals
binds to the parietal cells, HCL secretions are minimul
When all three of the chemicals bind to the parietal cells volumes of HCL pour forth as if pushed out under pressure
Phase 2: Gastric reflex The process of HCL formation within the
parietal cells is complicated and poorly understood
The consensus is that H+ is actively pumped into the stomach lumen against a tremendous concentration gradient
Phase 2: Gastric reflex As hydrogen ions are secreted, chloride
ions (Cl-) are also pumped into the lumen to maintain an electrical balance in the stomach
The Cl- is obtained from blood plasma, while the H+ appears to come from a breakdown of carbonic acid formed by the combination of carbon dioxide and water and within the parietal cells
Phase 2: Gastric reflex CO2 + H2O
H2CO3 H+ + HCO3
-
As H+ is pumped from the cell and HCO3
- is ejected through the basal cell membrane into the capillary blood
Phase 2: Gastric reflex The result of ejection of the bicarbonate
ion into the capillary blood is that blood draining from the stomach is more alkaline than the blood serving it
The phenomenon is called the alkaline tide
Phase 3: Intestinal The intestinal phase of gastric secretion
has two components– One excitatory– One inhibitory
Phase 3: Intestinal The excitatory aspect is set into motion as
partially digested food begins to fill the initial part (duodenum) of the small intestine
This stimulates intestinal mucosal cells to release a hormone that encourages the gastric glands to continue their secretory activity
Phase 3: Intestinal The effects of this hormone imitate those
of gastrin, so it has been named intestinal (enteric) gastrin
However, intestinal mechanisms stimulate gastrin secretion only briefly
As the intestine distends with chyme containing large amounts of H+, fats, partially digested proteins, and irritating substances, the inhibitatory component is triggered in the form of the enterogastric reflex
Phase 3: Intestinal The enterogastric reflex is actually a trio
of reflexes that– Inhibit the vagal nuclei in the medulla– Inhibit local reflexes– Activate sympathetic fibers that cause the
pyloric sphincter to tighten and prevent further food entry into the small intestine
As a result, gastric secretory activity declines
Phase 3: Intestinal These inhibitions on gastric activity
product the small intestine to harm due to excessive acidity and match the small intestine’s processing abilities to the amount of chyme entering it at a given time
Phase 3: Intestinal In addition, the factors just named trigger
the release of several intestinal hormones collectively called enterogastrones which include– Secretin– Cholecystokinin (CCK)– Vasoactive intestinal peptide (VIP)– Gastric inhibitory peptide (GIP)
All of these hormones inhibit gastric secretion when the stomach is very active
Gastric Motility and Emptying Stomach contractions, accomplished by the
tri-layered muscularis, not only cause its emptying but also compress, knead, twist, and continually mix the food with gastric juice to produce chyme
Because the mixing movements are accomplished by a unique type of peristalis (bidirectional) the process of mechanical digestion and propulsion are inseparable in the stomach
Gastric Motility: Stomach Filling Although the stomach stretches to
accommodate incoming food, internal stomach pressure remains constant until about 1 liter of food has been ingested
The relatively unchanging pressure in the filling stomach is due to 1) reflex mediated relaxation of the stomach muscle and 2) plasticity of visceral smooth muscle
Gastric Motility: Stomach Filling Reflexive relaxation of stomach muscle in
the fundus and body occurs both in anticipation of and in response to food entry into the stomach
As food travels through the esophagus, the stomach muscles relax
This receptive relaxation is coordinated by the swallowing center in the brain stem and mediated by the vagus nerves
Gastric Motility:Stomach Filling The stomach also actively dilates in
response to gastric filling, which activates stretch receptors in the wall
The phenomenon called adaptive relaxation appears to depend on local reflexes involving nitric oxide (NO) releasing hormones
Gastric Motility: Stomach Filling Plasticity is the intrinsic ability of
visceral smooth muscle to exhibit the stress- relaxation response, that is, to be stretched without greatly increasing its tension and contractile strength
Gastric Motility and Emptying
After a meal peristalsis begins near the cardiac sphincter, where it produces only gentle rippling movements of the stomach wall
Gastric Motility and Emptying
As contractions approach the pylorus, where the stomach musculature is thicker, the contractions become more powerful
Gastric Motility and Emptying
Consequently, the contents of the fundus remain relatively undisturbed, while the foodstuffs close to the pylorus receive a very active mixing
Gastric Motility and Emptying
The pyloric region of the stomach, which holds about 30 ml of chyme, acts as a “dynamic filter” that allows only liquids and small particles of food to pass
Gastric Motility and Emptying
Normally, each peristaltic wave reaching the pyloric muscle squirts 3 ml or less of chyme into the small intestine
Gastric Motility and Emptying
While the stomach delivers small amounts of chyme into the doudenum it also simultaneously forces most of the contained material backward into the stomach for further mixing
Gastric Motility and Emptying Although the intensity of the stomach’s
peristaltic waves can be modified, the rate is always constant at around 3 per minute
The contractile rhythm is set by the spontaneous activity of pacemaker cells located at the margins of the longitudinal smooth muscle layer
Gastric Motility and Emptying The pacemaker cells, are believed to be
muscle-like noncontractile cells called interstitial cells of Cajal which depolarize the repolarize spontaneously three times each minute
This depolarization and repolarization establish the so-called cyclic slow waves of the stomach or its basic electrical rhythm (BER)
Gastric Motility and Emptying Since the pacemakers are electrically
coupled to the rest of the smooth muscle sheet by gap junctions, their “beat” is transmitted efficiently and quickly to the entire muscularis
The pacemakers set the maximum rate of contraction, but they do not initiate the contractions or regulate their force
They generate subthreshold depolarization waves, which are then enhance by neural and hormonal factors
Gastric Motility and Emptying Factors that increase the strength of
stomach contractions are the same factors that enhance gastric secetory activity
Distension of the stomach wall by food activates stretch receptors and gastric secreting cells, which both ultimately gastric smooth muscle and so increase gastric motility
Gastric Motility and Emptying Thus, the more food there is in the
stomach, the more vigorous the stomach mixing and emptying movements will be evident
The stomach usually empties completely within four hours after a meal
However, the larger the meal (greater distension) and the more liquid the meal, the faster the stomach empties
Gastric Motility and Emptying Fluids pass quickly through the stomach Solids linger, remaining until they are
well mixed with gastric juice and converted to a liquid state
Gastric Motility and Emptying The rate of emptying depends as much on the
contents of the duodenum as on whats happening in the stomach
The stomach and duodenum act in tandem As chyme enters the duodenum, receptors in
its wall respond to chemical signals and to stretch, initiating the enterogastric reflex and hormonal mechanisms described earlier
These factors inhibit gastric secretory activity and prevent further duodenal filling by reducing the force of pyloric contractions
Gastric Motility and Emptying A carbohydrate-rich meal moves through
the duodenum rapidly, but fats form an oily layer at the top of the chyme and are digested more slowly by enzymes acting in the intestines
Thus, when chyme entering the duodenum is fatty, food may remain in the stomach six hours or more
The Small Intestine and Associated Structures
In the small intestine, usable food is finally prepared for its journey into the cells of the body
However, this vital function cannot be accomplished without the aid of secretions from the liver (bile) and pancreas (digestive enzymes)
Thus the accessory organ are discussed in this section
Small Intestine The small
intestine is a convoluted tube extending from the pyloric sphincter in the epigastric region to the iliocecal valve where it joins the large intestine
Small Intestine It is the longest part of the alimentary
tube, but its diameter is only about 2.5 cm In the cadaver, the small intestine is 6 - 7
meters long because of loss of muscle tone, while it is only 2 - 4 meters long in the living individual
The small intestine has three subdivisions– Duodenum– Jejunum– Ileum
Gross Anatomy
The relatively immovable duodenum which curves about the head of the pancreas
Small Intestine The duodenum is about 10 inches long Although it is the shortest subdivision,
the duodenum has the most features of interest– The bile duct– Main pancreatic duct– Hepatopancreatic ampulla– Major duodenal papilla
Gross Anatomy
The bile duct, delivering bile from the liver The main pancreatic duct, carries pancreatic juice from
the pancreas
Gross Anatomy
The hepatopancreatic ampulla is where these two ducts unite in the wall of the duodenum
The papilla is where this sphincter enters the duodenum
Small Intestine The jejunum is
about 8 ft long and extends from the duodenum to the ileum
This central section twists back and forth within the abdominal cavity
Small Intestine The ileum is
approximately 12 ft. in length
It joins the large intestine at the ileocecal valve
Small Intestine The jejunum
and ileum hang in coils in the central and lower part of the abdominal cavity
Small Intestine The jejunum
and ileum are suspended from the posterior abdominal wall by the fan shaped mesentery
Small Intestine Nerve fibers serving the small intestine
include the parasympathetics from the vagus nerves and sympathetics from the long splanchic nerves
These are relayed through the superior mesenteric and celiac plexus
Small Intestine The arterial supply is primarily from the
superior and mesenteric artery The veins run parallel to the arteries and
typically drain into the superior mesenteric vein
From the mesenteric vein, the nutrient rich venous blood from the small intestine drains into the hepatic portal vein which carries it to the liver
Microscopic Anatomy The small intestine is highly adapted for
nutrient absorption Its length provides a huge surface area
for absorption There are three structural modifications
which increase the surface area for absorption– Plicae circulares– Villi– Microvilli
Microscopic Anatomy
Digestive System Organs
In this view you can see the plicae circulares and the villi of the small intestine
Microscopic Anatomy Structural modifications increase the
intestinal surface area tremendously It is estimated that the surface area of the
small intestine is equal to 200 square meters or roughly equivalent to the floor space of a two story house
Most absorption occurs in the proximal part of the small intestine, with these structural modifications decreasing toward the distal end
Microscopic Anatomy The circular
folds or plicae circularis are deep permanent folds of the mucosa and submucosa
These folds are nearly 1 cm tall
Microscopic Anatomy The folds force
chyme to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption
Microscopic Anatomy Villi are finger
like projections of the mucosa
Over 1 mm tall they give a velvety texture to the mucosa
Microscopic Anatomy The epithelial
cells of the villi are chiefly absorptive columnar cells called enterocytes
Microscopic Anatomy In each villus is a capillary
bed and a wide lymphatic capillary called a lacteal
Digested food is absorbed through the epithelial cells into both the capillary blood and the lacteal
Villi become gradually narrower and shorter along the length of the sm. intestine
Enterocyte
Microscopic Anatomy Microvilli are tiny
projections of the plasma membrane of the absorptive cells of the mucosa
It gives the mucosal surface a fuzzy appearance sometimes called a brush border
Microscopic Anatomy Beside increasing the absorptive surface,
the plasma membrane of the microvilli bear enzymes referred to as the brush border enzymes
These enzymes complete the final stages of digestion of carbohydrates and proteins in the small intestine
Histology of the Wall The four tunics of
the digestive tract are modified in the small intestine by variations in mucosa and sub- mucosa
Histology of the Wall The epithelium of the mucosa is largely
simple columnar epithelium serving as absorptive cells
The cells are bound by tight junctions and richly endowed with microvilli
Also present are many mucus-secreting goblet cells
Histology of the Wall Scattered among the epithelial cells of the
wall are T cells called intraepithelial lymphocytes
These T cells provide an immunological component
Finally, there scattered enteroendocrine cells which are the source of secretin and cholecystokinin
Histology of the Wall
Between villi the mucosa is studded with pits that lead into tubular intestinal glands called intestinal crypts or crypts of Lieberkuhn
Histology of the Wall The epithelial cells that line these crypts
secrete intestinal juice Intestinal juice is a watery mixture
containing mucus that serves as a carrier fluid for absorption of nutrients from chyme
Histology of the Wall Located deep on the crypts are
specialized secretory cells called Paneth cells
Paneth cells fortify the small intestine by releasing lysozyme an antibacterial enzyme
The number of crypts decreases along the length of the wall of the small intestine, but the number of goblet cells becomes more abundant
Histology of the Wall The various epithelial cells arise from
rapidly dividing stem cells at the base of the crypts
The daughter cells gradually migrate up the villi where they are shed from the villis tips
In this way the villus of the epithelium is renewed every three to six days
Histology of the Wall The rapid replacement of the intestinal
(and gastric) epithelial cells has clinical as well as physiological implications
Treatments for cancer, such as radiation therapy and chemotherapy preferentially target the cells in the body that divide most quickly
This kills cancer cells, but it also nearly obliterates the GI epithelium causing nausea, vomiting, and diarrhea after each treatment
Histology of the Wall The submucosa is typical areolar
connective tissue, and it contains both individual and aggregated lymphoid follicles (Peyer’s patches)
Peyer’s patches increase in abundance toward the end of the small intestine, reflecting the fact that the large intestine contains huge numbers of bacteria that must be prevented from entering the bloodstream
Histology of the Wall
A set of elaborated mucus-secreting duodenal glands (Brunner’s) is found in the submucosa of the duodenum only
Histology of the Wall These glands produce an alkaline
(bicarbonate-rich) mucus that helps neutralize the acidic chyme moving in from the stomach
When this protective mucus barrier is inadequate, the intestinal wall is eroded and duodenal ulcers results
Histology of the Wall The muscularis is typical and bilayered Except for the bulk of the duodenum,
which is retroperitoneal and has an adventitia, the external intestinal surface is covered by visceral peritoneum (serosa)
Intestinal Juice The intestinal glands normally secrete
between 1 and 2 liters of intestional juice daily
The major stimulus for its production is distension or irritation of the intestinal mucosa by hypertonic or acidic chyme
Intestinal Juice Normally, the pH range of intestinal juice
is slightly alkaline (7.4-7.8), and it is isotonic with blood plasma
Intestinal juice is largely water but it also contains some mucus, which is secreted both by the duodenal glands and by goblet cells of the mucosa
Intestinal juice is relatively enzyme poor because intestinal enzymes are largely limited to the bound enzymes of the brush border
The Liver and Gallbladder The liver and gallbladder are accessory
organs associated with the small intestine The liver has many metabolic and
regulatory roles Its digestive function is to produce bile
for export to the duodenum Bile is a fat emulsifier which breaks up
fat into tiny particles so that they are more accessible to digestive enzymes
The gallbladder is a storage site for bile
The Liver
The ruddy, blood rich liver is the largest gland in the body weighing about 1.4 kg in the average adult
The Liver Shaped like a wedge, it
occupies most of the right hypochondriac and epigastric regions extending farther to the right of the body midline than the left
The Liver Located under the diaphragm, the liver
lies almost entirely within the rib cage The location of the liver within the rib
cage offers this organ some degree of protection
The Liver
The liver has four primary lobes; right, left, caudate and quadrate
The Liver
A mesentery, the falciform ligament, separates the right and left lobes anteriorly and suspends the liver from the diaphragm
The Liver
Running along the free inferior edge of the falciform ligament is the ligamentum teres a remnant of the fetal umbilical vein
The Liver
Except for the superiormost liver area, which is fused to the diaphragm, the entire liver is enclosed by a serosa (visceral peritoneum)
The Liver A dorsal
mesentery, the lesser omentum, anchors the liver to the lesser curvature of the stomach
The Liver
The hepatic artery and hepatic portal vein, enter the liver at the porta hepatis
The Liver
The common bile duct, which runs inferiorly from the liver, travels through the lesser omentum
The Liver
The gallbladder rests in a recess of the inferior surface of the right lobe of the liver
The Liver
Bile leaves the liver through several bile ducts that ultimately fuse to form the large common hepatic duct which travels to the duodenum
The Liver
The common hepatic duct and the cystic duct fuse to form the bile duct
Microscopic Anatomy of Liver The liver is
composed of seed sized structural & functional units called liver lobules
Each lobule is roughly hexagonal
Microscopic Anatomy of Liver Hepatocytes
or live cells are organized to radiate out from a central vein running the length of the longitudinal axis of the lobule
Microscopic Anatomy of Liver To make a rough “model” of a liver
lobule, open a paperback book until the two covers meet
The pages represent the plates of hepatocytes and the hollow cylinder formed by the rolled spine represents the central vein
Microscopic Anatomy of Liver The liver’s main function is to filter and
process the nutrient rich blood delivered to it
At each of the six corners of a lobule is a portal triad so named because three basic structures are always present there:– A branch of the hepatic artery– A branch of the hepatic portal vein– A bile duct
Microscopic Anatomy of Liver
The hepatic artery supplies oxygen rich arterial blood to the liver
Microscopic Anatomy of Liver
The hepatic vein carries blood laden with nutrients from the digestive viscera
Microscopic Anatomy of Liver
A bile duct to carry secreted bile toward the common bile duct and ultimately to the duodenum
Microscopic Anatomy of Liver Between the
hepatocyte plates are enlarged, very leaky capillaries, the liver sinusoids
Microscopic Anatomy of Liver Blood from both
the hepatic portal vein and the hepatic artery percolates from the triad regions through these sinusoids and empties into the central vein
Microscopic Anatomy of Liver From the central
vein blood eventually enters the hepatic veins, which drain the liver, and empty into the inferior vena cava
Microscopic Anatomy of Liver Inside the
sinusoids are star shaped hepatic macrophages, also called Kupffer cells, which remove debris such as bacteria and worn-out blood cells
Microscopic Anatomy of Liver The hepatocytes (liver cells) are virtual
organelle storehouses with large amounts of both rough and smooth endoplasmic reticulum, Golgi apparatuses, peroxisomes, and mitochondria
Thus equipped, the hepatocytes produce not only bile but also– Process blood borne nutrients– Store Fat-soluble vitamins– Detoxify the blood
Microscopic Anatomy of Liver In processing nutrients the hepatocytes
store glycogen and make plasma proteins Fat soluble vitamins are stored until such
time as they are needed for metabolism Detoxification occurs are the hepatocytes
rid the blood of ammonia by converting it to urea
The net result is that the blood leaving the liver contains fewer nutrients and waste materials than the blood that entered
Microscopic Anatomy of Liver Secreted bile
flows through tiny canals, called bile canaliculi that run between adjacent hepato cytes toward the bile branch ducts in the portal triad
Microscopic Anatomy of Liver Note that the bile
and the blood flow in opposite directions in the liver lobule
Bile entering the bile ducts eventually leaves the liver via the common hepatic duct
Microscopic Anatomy of Liver Bile is a yellow-green, alkaline solution
containing– Bile salts– Bile pigments– Cholesterol – Neutral fats– Phospholipids (lecithin and others)– Electrolytes
Only bile salts and phospholipids aid the digestive process
Microscopic Anatomy of Liver Bile salts, primarily cholic acid and
chenodeoxycholic acids are cholesterol derivatives
Their role is to emulsify fats which distributes them throughout the watery intestinal contents
As a result, large fat globules entering the small intestine are physically separated into millions of small fatty droplets
Microscopic Anatomy of Liver Millions of tiny fat droplets vastly
increase the surface area for the fat digesting enzymes to work on
Bile salts also facilitate fat and cholesterol absorption and help solubilize cholesterol, both that contained in bile and that entering the small intestine for food
Microscopic Anatomy of Liver Although many substances secreted in bile
leave the body in feces, bile salts are not among them
Bile salts are conserved by a means of a recycling mechanism called enterohepatic circulation
In this process bile salts are– Reabsorbed into the small intestine– Returned to the liver via the hepatic portal vein– Resecreted in newly formed bile
Microscopic Anatomy of Liver The chief bile pigment is bilirubin, a
waste product of hemoglobin (heme) during the breakdown of worn-out erythrocytes
The globin and iron parts of hemoglobin are saved and recycled, but bilirubin is absorbed from the blood by the liver cells and actively excreted into the bile
Microscopic Anatomy of Liver Most of the bilirubin in bile is metabolized
in the small intestine by resident bacteria A breakdown by-product is urobilirubin
which give feces its brown color In the absence of bile, feces are grey-white
in color and have fatty streaks because essentially no fats are digested or absorbed
Microscopic Anatomy of Liver The liver produces 500 to 1000 ml of bile
daily, and bile production is stepped up when the GI tract contains fatty chyme
Bile salts themselves are a major stimulus for enhance bile secretion
Microscopic Anatomy of Liver The single most
important stimulus of bile secretion is an increased level of bile salts in the enterohepatic circulation
The Gallbladder The gallbladder is
a thin-walled, green muscular sac, rouhgly the size of a kiwi fruit
It snuggles in a shallow fossa on the ventral surface of the liver
The Gallbladder
The gallbladder stores bile that is not immediately needed for digestions
The Gallbladder Bile that is not needed is concentrated by
absorbing some of its water and ions When empty, its mucosa adopts the ridge
like folds or rugae of the stomach Its muscular walls can contract to expell
its contents into the cystic duct which then flows into the bile duct
Like most of the liver it is covered by visceral peritoneum
The Gallbladder
When digestion is not occurring, the hepatopancreatic sphincter is tightly closed
The Gallbladder
Bile then backs up the cystic duct into the gallbladder where it is stored until needed
The Gallbladder Although the liver makes bile continuously
bile does not usually enter the small intestine until the gallbladder contract
The major stimulus for gallbladder contraction is the intestinal hormone cholecystokinin (CCK)
CCK is released to the blood when acidic, fatty chyme enters the duodenum
The Gallbladder Besides causing the gallbladder to
contract, CCk has two other important effects– It stimulates secreation of pancreatic juice– It relaxes the hepatppancreatic sphincter so
that bile and pancreatic juice can enter the duodenum
Parasympathetic impulses delivered by the vagus nerves have a minor impact on stimulating gallbladder contraction
The Pancreas
The pancreas is a soft, tadpole-shaped gland that extends across the abdomen
The Pancreas
Most the pancreas is retroperitoneal and lies deep to the greater curvature of stomach
The Pancreas An accessory organ, the pancreas is
important to the digestive process because it produces a broad spectrum of enzymes
These enzymes break down all categories of foodstuffs, which the pancreas then delivers to the duodenum
This exocrine product is called pancreatic juice
The Pancreas
Pancreatic juice drains from the pancreas via the centrally located main pancreatic duct
The Pancreas
The pancreatic duct generally fuses with the bile duct just as it enters the duodenum
The Pancreas
A smaller accessory pancreatic duct empties directly into the main duct
The Pancreas Within the
pancreas are the acini, clusters of secretory cells surrounding ducts
The Pancreas The acini cells
are full of rough endoplasmic reticulum and exhibit deeply staining zymogen granules containing digestive enzymes
The Pancreas The pancreas
also has an endocrine function
Scattered amidst the acini are the more lihgtly staining pancreatic islets
The Pancreas These Islets of
Langerhans release insulin and glucagon, hormones that regulate carbohydrate metabolism
Pancreatic Juice Approximately 1200 to 1500 ml of clear
pancreatic juice is produced daily It consists mainly of water and contains
enzymes and electrolytes The acinar cells produce the enzyme rich
pancreatic juice The epithelial cells lining the smallest
pancreatic ducts release the bicarbonate ions that make it alkaline (pH 8)
The Pancreas The high pH enables pancreatic fluid to
neutralize the acid chyme entering the duodenum
It also provides the optimal environment for activity of intestinal and pancreatic enzymes
Like pepsin of the stomach, pancreatic protein digesting enzymes are produced and released in active forms, which are then activated in the duodenum
The Pancreas Within the duodenum trypsinogen is
activated to trypsin by enterokinase an intestinal brush border enzyme
Trypsin in turn activates two other pancreatic enzymes– Procarboxypeptidase > carboxypeptidase– Chymotrypsinogen > chymotrypsin
Other pancreatic enzymes (amylase, lipase, and nucleases) are secreted in active form but require ions in the bile for activity
Regulation of Pancreatic Secretion Secretion of pancreatic juice is regulated
both by local hormones and by the parasympathetic nervous system
Regulation of Pancreatic Secretion Secretin is
released in response to the presence of HCL in the intestine
Cholecystokinin is released in response to the entry of proteins and fats
Regulation of Pancreas Secretion Both hormones act on the pancreas, but
secretin targets the duct cells, prompting their release of watery bicarbonate-rich pancreatic juice, Whereas CCK stimulates the acini to release enzyme-rich pancreatic juice
Vagal stimulation causes release of pancreatic juice primarily during the cephalic and gastric phases of gastric secretion
Regulation of Pancreatic Secretion Normally, the amount of HCL produced
in the stomach is exactly balanced by the amount of bicarbonate (HCO3) actively secreted by the pancreas
HCO3 is secreted into the pancreatic juice, and H+ enters the blood
Regulation of Pancreatic Secretion
Consequently, the pH of venous blood returning to the heart remains relatively unchanged because alkaline blood draining from the stomach is neutralized by the acidic blood draining the pancreas
Digestion: Small Intestine Although food reaching the small intestine
is unrecognizable, it is far from being digested chemically
Carbohydrates and proteins are partially degraded, but virtually no fat digestion has occurred to this point
The process of food digestion is accelerated during the chyme’s journey of 3 to 6 hours through the small intestine, it is here that virtually all nutrient absorption occurs
Optimal Intestinal Activity Although the primary functions of the small
intestine are digestion and absorption, intestinal juice provides little of what is needed to perform these functions
Most substances required for chemical digestion - bile, digestive enzymes (except for brush border enzymes) and bicarbonate ions (to provide the proper pH for enzymatic catalysis) are imported from the liver and pancreas
Optimal Intestinal Activity Anything that impairs liver or pancreatic
function or delivery of their juices to the small intestine severely hinders the individual’s ability to digest food and absorb nutrients
Optimal Intestinal Activity Optimal digestive activity in the small
intestine also depends on a slow, measured delivery of chyme from the stomach
The small intestine can process only small amounts of chyme at one time
Chyme enter the small intestine is usually hypertonic
Optimal Intestinal Activity If large amounts of chyme were rushed
into the small intestine, the osmotic water loss from the blood into the intestinal lumen would result in dangerously low blood volume
Additionally, the low pH of entering chyme must be adjusted upward and the chyme must be well mixed with bile and pancreatic juice for digestion to continue
These adjustments take time
Optimal Intestinal Activity
Food movement into the small intestine is carefully controlled by the pumping action of the stomach pylorus which prevents the duodenum from being overwhelmed
Motility of the Small Intestine Intestinal smooth muscle mixes chyme
thoroughly with bile and pancreatic and intestinal juices and moves food residues through the ileocecal valve and into the large intestine
In contrast to the peristaltic waves of the stomach, which both mix and propel food, segmentation is the most common motion of the small intestine
Motility of the Small Intestine In segmentation,
chyme is moved backward and forward a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscles
Motility of the Small Intestine These segmenting
movements of the intestine are initiated by intrinsic pacemaker cells (interstitial cells of Cajal) in the longitudinal smooth muscle layer
Motility of the Small Intestine Unlike the somach pacemakers, which
have only one rhythm, the pacemakers in the duodenum depolarizes more frequently (12-14 contractions per minute) than those of the ileum (8-9 contractions per minute)
As a result, segmentation moves intestinal contents slowly and steadily toward the ileocecal valve at a rate that allows ample time to complete digestion and absorption
Motility of the Small Intestine The intensity of the segmentation is
altered by hormones and long and short reflexes– Parasympathetic enhances segmentation– Sympathetic decreases segmentation
The more intense the contractions, the greater the mixing effect, however the basic contractile rhythms of the various intestinal regions remain unchanged
Motility of the Small Intestine True peristalsis
occurs only after most nutrients have been absorbed
Segmentation movements wane, and peristaltic waves begin
Motility of the Small Intestine Peristaltic waves initiated in the duodenum
begin to sweep slowly along the intestine, moving 10 - 70 cm before dying out
Each successive wave is initiated a bit more distally, and this pattern of peristaltic activity is called the migrating mobility complex
A complete migration from the duodenum to the ileum takes about two hours and then repeats itself
Motility of the Small Intestine Peristalsis serves to sweep out the last
remnants of the meal plus bacteria, sloughed-off mucosal cells, and other debris into the large intestine
This “housekeeping” function is critical for preventing the overgrowth of bacteria that migrate from the large intestine to the small intestine
As food enters the stomach with the next meal segmentation replaces peristalsis
Motility of the Small Intestine The local enteric neurons of the GI tract
wall coordinate intestinal mobility patterns
The physiological diversity of the enteric neurons allows a variety of effects to occur depending on which neurons are activated or inhibited
Motility of the Small Intestine A given ACh-releasing (cholinergic)
sensory neuron in the small intestine, once activated, may simultaneously send messages to several different interneurons in the myenteric plexus that regulate peristalsis:– Impulses sent proximally by cholingeric
neurons cause contraction and shortening of the circular muscular layer
Motility of the Small Intestine …interneurons in the myenteric plexus
that regulate peristalsis:– Impulses sent distally to certain interneurons
cause shortening of the longitudinal muscle layers and distension of the intestine, in response to Ach-releasing neurons
– Other impulses sent distally by activated VIP or NO-releasing enteric neurons cause relaxation of the circular muscle
Motility of the Small Intestine As a result, as the proximal area constricts
and forces chyme along the tract, the lumen of the distal part of the intestine enlarges to receive it
Motility of the Small Intestine Most of the time, the ileocecal sphincter
is constricted and closed Two mechanisms, one neural and one
hormonal , cause it to relax when ileal mobility increases and allow food residues to entry the cecum
Enhance activity of the stomach initiates the gastroileal reflex, a long reflex than enhances the force of segmentation in the ileum
Motility of the Small Intestine In addition, gastrin released by the
stomach increases the motility of the ileum and relaxes the ileocecal sphincter
Once the chyme has passes through, it exerts backward pressure that closes the valve’s flaps, preventing regurgitation into the ileum
Large Intestine
The large intestine frames the small intestine on three sides and extends from the ileocecal valve to the anus
Large Intestine Its diameter is greater than that of the
small intestine, but is less than half as long 1.5 meters
Its major function is to absorb water from indigestible food residues (delivered to it in fluid state) and eliminate them from the body as semisolid feces
Large Intestine
Over most of its length, the large intestine exhibits three features not seen elsewhere; teniae coli, haustra, epiploic appendages
Large Intestine
Teniae coli are three bands of smooth muscle which are the remnants of the smooth muscle layer
Large Intestine
The muscle tone of the teniae coli cause the wall of the large intestine to form pocketlike sacs called haustra
Large Intestine
Epiplocic appendages are small fat-filled pouches of visceral peritoneum that hang from its surface. Significance is not known
Large Intestine
The large intestine has the following subdivisions; cecum, appendix, colon, rectum, and anal canal
Large Intestine
The saclike cecum, or blind pouch, lies below the ileocecal valve is the first part of the large intestine
Large Intestine
Attached to the cecum is the blind, wormlike, vermiform appendix
Large Intestine The appendix contains masses of lymphoid
tissue, and as part of the MALT it plays an important role in body immunity
It has a significant structural problem in that its twisted tissue provides an ideal location for enteric bacteria to accumulate and multiply
Large Intestine
The colon has several distinct regions; ascending, transverse, and descending colon segments connected by flexures
Large Intestine
The ascending colon travels up the right side of the abdominal cavity to the level of the right kidney
Large Intestine
At the level of the kidney the colon makes a right-angle turn, the right colic, or hepatic flexure
Large Intestine
The transverse colon travels across the top of the abdominal cavity
Large Intestine
Directly anterior to the spleen, it bends downward to form the left colic or splenic flexure
Large Intestine
The descending colon descends down the left side of the abdominal cavity
Large Intestine
As the descending colon enters the pelvis it forms the S-shaped sigmoid colon
Large Intestine The transverse
and sigmoid portions of the colon are anchored to the posterior abdominal wall by mesentary sheets called mesocolons
Large Intestine
In the pelvis, at the level of the third sacral vertebra, the sigmoid colon joins the rectum, which is positioned anterior to the sacrum
Large Intestine The natural
orientation of the rectum allows for a number of pelvic organs to be examined digitally during a rectal exam
Large Intestine
The rectum has three lateral curves or bends represented internally are transverse folds called rectal valves
Large Intestine Rectal valves
separate feces from flatus, thus allowing gas to passed
Large Intestine The anal canal
lies entirely external to the abdominopelvic cavity
About 3 cm long the canal begins where the rectum penetrates the muscles of the pelvic floor
Large Intestine The anal canal
has two sphincters– External anal
sphincter– Internal anal
sphincter
Large Intestine The involuntary internal anal sphincter
is composed of smooth muscle The voluntary external anal sphincter is
composed of voluntary muscle These sphincters which act rather like
purse strings to open and close the anus, are ordinarily closed excepts during defecation
Large Intestine: Microscopic The wall of the large intestine differs in
several ways from that of the small intestine
The colon mucosa is simple columnar epithelium except in the anal canal
Because most food is absorbed before reaching the large intestine, there are no circular folds, no villi, and no cells that secrete digestive enzymes
Large Intestine: Microscopic Its mucosa is thicker, its abundant crypts
are deeper, and there are tremendous numbers of goblet cells in the crypts
Lubricating mucus produced by goblet cells eases the passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon
Large Intestine: Microscopic The mucosa of
the anal canal is different from the rest of the colon, reflecting the greater abrasion that this region receives
Large Intestine: Microscopic The mucosa
hangs in long ridges or folds called anal columns and contains stratified squamous epithelium
Large Intestine The anal sinuses
are recesses between the anal columns which exude mucus when compressed by feces
This aids in the emptying of the canal
Large Intestine The horizontal
lines that parallels the inferior margin of the anal sinuses is called the pectinate line
The line separates areas of visceral and somatic sensory innervation
Pectinate line
Large Intestine: Microscopic The mucosa superior to the line is
innervated by visceral sensory fibers and so are relatively insensitive to pain
The are inferior to the pectinate line is innervated by somatic sensory fibers and is very sensitive to pain
Large Intestine: Microscopic Two superficial venous plexuses are
associated with the anal canal, one with the anal columns and the other with the anus itself
Where these veins (hemorhoidal) are inflamed, itchy varicosities called hemorrhoids result
Large Intestine: Microscopic In contrast to the more proximal regions
of the large intestine, teniae coli and haustra are absent in the rectum and anal canal
Consistent with its need to generate strong contractions to perform its expulsive role, the rectum’s muscularis muscle layers are complete and well developed
Bacterial Flora Although most bacteria entering the
cecum from the small intestine are dead having been killed by the action of lysozyme, defensins, HCL, and protein digesting enzymes
The bacteria that survive, together with the bacteria that enter the GI tract via the anus, constitute the bacterial flora of the large intestine
Bacterial Flora The bacterial flora colonize the colon and
ferment some of the indigestible carbo- hydrates (cellulose and others) releasing irritating acids and a mixture of gases– Dimethyl sulfide, H2, N2, CH4, and CO2
About 500 ml of gas is produced each day with much more when certain carbohydrate rich foods are eaten
The bacterial flora also synthesize B complex vitamins and most of vitamin K
Processes: Large Intestine What is finally delivered to the large
intestine contains few nutrients, but still has 12 to 24 hours more digestive system
Except for the small amount of digestion of residue by the enteric bacteria, no further food breakdown takes place in the large intestine
Processes: Large Intestine Although the large intestine harvests
vitamins made by the bacterial flora and reclaims most of the remaining water and some of the electrolytes (particularly sodium and chloride) absorption is not a major function of this organ
The primary concern of the large intestine are propulsive activities that force the fecal material toward the anus and then eliminate it from the body
Processes: Large Intestine While the large intestine is undeniably
essential for our comfort, it is not essential for life
Several different surgical procedures remove a part or all of the large intestine in order to save life
Motility: Large Intestine The large intestine musculature is
inactive much of the time, and when it is mobile, its contractions are sluggish and of short duration
The most frequent movements seen in the colon are haustral contractions, which are slow segmenting movements that occurs every 30 minutes or so
Motility: Large Intestine Haustral contractions reflect local controls
of smooth muscle within the walls of individual haustra
As a haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum
These movements also mix the residue which aids in water absorption
Motility: Large Intestine Mass movements (mass peristalsis) are long,
slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum
Typically these movements occur during or just after eating when the presence of food in the stomach activates the gastroileal reflex in the small intestine and the propulsive gastrocolic reflex in the colon
Motility: Large Intestine Bulk, or fiber, in the diet increases the
strength of colon contractions and softens the stool, allowing the colon to act more efficiently
Defecation The rectum is
usually empty, but when feces are forced into it by mass movements, stretching of the rectal walls initiates the defecation reflex
Defecation This is a spinal
cord mediated reflex that causes the walls of the sigmoid colon and the rectum to contract and the anal sphincters to relax
Defecation Distension or
stretch of the rectal walls triggers a depolarization of sensory (afferent) fibers which synapse with the spinal cord
Defecation Parasympathetic
motor (efferent) fibers, in turn, stimulate contraction of the rectal walls and relaxation of the internal anal sphincter
Defecation If it is convenient
to defecate, voluntary signals stimulate the relaxation of the external anal sphincter
Defecation As feces are forced into the anal canal,
impulses reach the brain allowing us to decide whether the external(voluntary) anal sphincter should remain open or closed
If defection is delayed, the reflex contractions end within a few seconds and the walls relax
With the next mass movement, the reflex is initiated again and again until one chooses to defecate
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