The Digestive SystemThe Digestive SystemChapter 25Chapter 25

Function of the Digestive System

To break down food into a “usable” (absorbable) form

To supply our cells with the nutrients they need for energy, growth & repair

Organs of the Digestive System

Gastrointestinal tract (GIT) – continuous passageway which contains the food from the time it enters the body, until it leaves; organs include:

mouth (oral cavity), pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus

Accessory organs - participate in digestive processes; organs include:

teeth, tongue, salivary glands, liver, gall bladder, pancreas

Processes of Digestion

1. Ingestion

2. Movement along GIT

Voluntary – e.g. swallowing

Involuntary – e.g. peristalsis

3. Secretion – release of water, enzymes, acids, buffers, mucous, etc. into GIT for physical (mechanical) & chemical digestive processes

Processes of Digestion4. Digestion

Mechanical processing – physical breakdown of food; e.g. mastication, emulsification, mixing waves, segmentationChemical digestion – chemical breakdown of food; disassembling of organic molecules into their component parts; requires enzymes

carbohydrates disaccharides monosaccharides

proteins amino acids

lipids fatty acids & monoglycerides

Processes of Digestion

5. Absorption – movement of nutrients from GIT into blood capillaries (monosaccharides, amino acids, H2O, vitamins, minerals) or lymphatic capillaries (fatty acids)

6. Excretion (Defecation) – removal of waste products from GIT

Histology of the GIT

epithelium – stratified squamous or simple columnar

lamina propria – loose CT

muscuaris mucosa – smooth muscle

Mucosa

Submucosa CT with BV’s, nerves & lymphatics

Muscularis externaSkeletal muscle at beginning & end of GIT, smooth muscle (inner circular; outer longitudinal layer) from lower esophagus to rectum

Serosa (a.k.a. viseral peritoneum)

4 layers of tissue surround the lumen of the GIT

Peritoneum & Mesenteries

The abdominal cavity is lined with parietal peritoneum & many of the organs within are covered with visceral peritoneum

Folds of peritoneum called “mesenteries” attach some organs to others

greater omentum

lesser omentum

mesentery proper

mesocolon

Mouth (oral cavity) Regions include the vestibule & oral cavity proper

Roof comprised of hard & soft palate; floor primarily comprised of tongue

Mucosa of stratified squamous epithelium

(non-keratinized)

Joins to the oropharynx at the fauces

Tongue – stratified sqamous epith. over skeletal muscle

intrinsic & extrinsic muscles

papillae

filiform

fungiform

circumvallate

taste buds

Parotid gland

Submandibular gland

Sublingual gland

Salivary glands – secrete saliva – made of H2O, salts & “salivary amylase”

Parotid duct

Submandibular duct

Teeth – involved in “mastication” 2 sets of teeth – deciduous & permanent 4 types of teeth – incisors, cuspids (canines), bicuspids (premolars), molars

Parts of a tooth –

crown – dentin surrounded by enamel, has hollowed pulp cavity filled with CT pulp

neck – at gingival border

root – within mandible & maxilla, has hollowed root canal with BVs & nerves, root canal opens at apical foramen

Pharynx

nasopharynx

uvula

oropharynx

epiglottis

laryngopharynx

Common passageway for air & food

oropharynx & laryngopharynx lined with stratified squamous epithelium (nasopharynx lined with PSCC)

uvula & epiglottis protect airway when swallowing (“deglutition”)

Esophagus muscular tube running from pharynx, posterior to trachea, down thoracic cavity, through “esophageal hiatus” of diaphragm, to lower esophageal (a.k.a. cardiac) sphincter at junction of stomach

functions in “deglutition” through action of peristalsis

mucosa is stratified squamous epithelium

variations in muscularis externa – begins as skeletal muscle at upper 1/3, mixed skeletal & smooth muscle in middle, smooth muscle by lower 1/3

Stomach - Gross Anatomy

Lower esophageal (cardiac) sphincter

Pyloric sphincter

Stomach - Histology

Rugae – folds of mucosa & submucosa to allow for expansion of stomach

Mucosa of simple columnar epithelium with mucous cells

Gastric pit leading to gastric glands

Stomach – Histology (cont)

- Secrete mucus to protect epithelial cells from enzymes & acid

- Secrete HCl (for protein digestion) & intrinsic factor (for B12 absorption)

- Secrete pepsinogen which gets converted to “pepsin” when mixed with HCl; for protein digestion

- Secrete gastrin to regulate stomach emptying

Entero-

(peptic cells)

(G-cells)

Stomach

Modifications in stomach include 3 layers of smooth muscle in muscularis externa – outer longitudinal, middle circular, innermost oblique layer

polypeptidespepsin

HClproteins

Functions of stomach include temporary storage of food mechanical breakdown of food to “chyme” through powerful mixing waves intrinsic factor for vitamin B12 absorption start of chemical digestion of proteins –

Small Intestine - Anatomy- connects stomach to large intestine; 15-20’ long; 1” diameter; held together in abdominal cavity by “mesentery proper”

- site for completion of chemical digestion & absorption of nutrients

- comprised of three regions:

Duodenum – 10” in length; receives chyme from stomach, secretions from liver, gallbladder & pancreas

Jejunum – 8’ long; most digestion & absorption occurs here

Ileum – 12’ long; connects to cecum of large intestine at iliocecal valve (sphincter)

Small IntestineModifications in mucosa & submucosa of intestinal wall designed to increase functional surface area:

Plicae circulares

Plicae circulares (circular folds) – large transverse ridges; most abundant in jejunum

Villi – small finger-like projections of mucosal folds across surface of intestine

Small IntestineVilli

Villi lined with “absorptive cells” - mucosal epithelium of simple columnar epithelium with microvilli “brush border” . These cells also produce enzymes (disaccharidases & peptidases) for final digestion of carbs and proteins

Submucosa of each villus contains a capillary network & a “lacteal” (lymphatic capillary) for absorption of nutrients

Small IntestineVilliIntestinal crypts

containing stem cells and intestinal glands

Between villi are intestinal crypts. Stem cells here can replace old cells found lining villi

Intestinal glands within intestinal crypts secrete “intestinal juice” – provides watery medium to keep enzymes & digestive products in solution for help with absorption.

Pancreas

Stomach

Head

Body

Tail

Retroperitoneal elongated organ lying posterior to stomach, from duodenum to spleen Both endocrine (pancreatic islets of Langerhans – secretes insulin & glucagon) & exocrine gland (pancreatic acini – secretes pancreatic juice (aka pancreatin) through pancreatic duct(s) to duodenum

Pancreaticduct

Duodenum

PancreasPancreatic juice – mixture of enzymes & buffers (sodium bicarbonate) secreted by acinar cells into pancreatic duct & released into duodenum

pancreatic amylase

Starch maltose lipase

Lipids fatty acids + monoglycerol

proteases (trypsin, chymotrypsin, carboxypeptidase)

Proteins & polypeptides small peptidestri & dipeptides

nucleases – digest RNA & DNA

sodium bicarbonate – neutralizes acidic chyme because enzymes in small intestine need an alkaline pH

Liver - Anatomy Largest organ within the body

Comprised of 4 lobes:

Large right & left lobes divided by falciform ligament; small caudate (by IVC) & quadrate (by gall bladder ) lobes

falciform ligament continues at inferior margin as ligamentum teres (round ligament) (remnant of umbilical vein)

Lobes of liver functionally divided into microscopic lobules

Liver - Histology Lobules comprised of rows of Hepatocytes arranged radially around a central vein

Hepatocytes surround blood sinusoids (capillary structures) which are partially lined with phagocytic Kupffer (aka stellate reticuloendothelial) cells

hepatocytes central vein sinusoids

Liver One function of hepatocytes is to produce bile, which gets secreted into bile canaliculi of lobule

Bile canaliculi merge to form bile ducts, which are part of the portal triad seen at each corner of the lobules. Bile ducts merge to eventually create the right & left hepatic ducts

Liver & gall bladder Right & left hepatic ducts unite to form common hepatic duct which merges with cystic duct of gall bladder to form common bile duct which joins with pancreatic duct & enters the duodenum

Gall bladder – hollow muscular sac under right lobe of liver; stores & concentrates bile; releases bile through cystic duct

Bile released into duodenum functions in emulsification of lipids, absorption of fats (due to presence of bile salts), & excretion of bilirubin

Left hepatic ductRight hepatic duct

Pancreatic & bile ducts

Stomach

Body

Tail

Head Pancreatic duct

Accessory pancreatic duct

Common bile duct

Liver - FunctionsThe liver has over 200 functions including (but not limited to):

Bile production & excretion

Metabolic regulation –

storage of glycogen, fatty acids, fat-soluble vitamins & minerals

interconversion of nutrients (“gluconeogenesis”)

detoxification & removal of drugs, toxins & hormones

hematological regulation –

phagocytosis of worn-out RBCs, bacteria & other pathogens

synthesis of plasma proteins

Blood Supply to LiverIn order for the liver to perform all of its functions, it receives blood through 2 vessels:

Hepatic artery - delivers oxygenated blood into sinusoids of liver

Hepatic Portal vein – delivers de-oxygenated, nutrient-rich blood from digestive organs to sinusoids of liver

Liver uses O2 & nutrients within blood of sinusoids & then blood drains into central veins of lobule which merge to form the hepatic veins, which drain into the IVC

Large Intestine Begins at the ilium & ends at the anus; 5’ long; 3” in diameter

Rectum

ileumIleocecal sphincter

Cecum

Vermiform appendix

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

Anal canal

Rectum

Rectum

Internal anal sphincter

External anal sphincter

Anal canal

Anus

3 regions: cecum - blind pouch; has appendix attachedcolon – ascending, (hepatic flexure), transverse, (splenic flexure), descending, sigmoidrectum – last 1” known as “anal canal”

Hepatic (rt. Colic) flexure

Splenic (lt. colic) flexure

Large Intestine

Simple columnar epithelium in mucosa, except at anal canal (strat. Squam.)

No plicae circularis or villi

Modifications in muscularis externa & serosa :

longitudinal muscle layer forms bands called “taeniae coli” which create puckers known as “haustra”

serosa forms “epiploic appendages”

haustra

taeniae coli

epiploic appendages

THE END (literally!)

main functions – H2O re-absorption; absorption of some vitamins & minerals; formation & temporary storage of fecal material

no chemical (enzymatic) digestion but some bacterial