Digestion PhysiologyChapter 26

Test (Chapters 25 & 26) – Tuesday, 3/30 (Tentative Date

)

Primary Mechanisms of Digestive System (Table 26-1)

• Ingestion – take food in• Digestion – break down complex nutrients

into simpler nutrients• Motility – movement of GI tract; aids in

digestion• Secretion – enzymes are required for

digestion to take place• Absorption – movement across GI mucosa

into the internal environment• Elimination – process of eliminating

unabsorbed material

Mechanical Digestion

• Includes all motility of the digestive tract that cause the following changes:– Change in physical state of the food

from large particles to smaller particles– Churning of the GI lumen to mix

particles with digestive juices– Propelling food forward ending with

elimination

Mastication

• Mastication = chewing• Requires tongue, cheek and lips• Reduces particle size• Mix food with saliva

Deglutition• Deglutition = swallowing1. Oral stage

• Bolus is formed voluntarily• Tongue pushes bolus against the palate and into

the oropharynx• Soft palate acts as a valve to prevent food from

entering the nasopharynx

2. Pharyngeal stage• Involuntary reflexes push bolus toward

esophagus• Epiglottis prevents food from entering trachea

3. Esophageal stage• Involuntary reflexes move bolus towards stomach

Motility

• Smooth muscle contractions take over in the lower portion of the esophagus

• Peristalsis – progressive wavelike ripple of the muscle layer of a hollow organ– Bolus stretches the GI tract wall

triggers contraction of circular smooth muscle bolus moves forward

Motility

• Segmentation – mixing movement; back and forward movement within a single region– Mechanically breakdown food particles– Mix food and digestive juices together– Facilitate absorption

• Peristalsis and segmentation can occur in alternating sequence to churn/mix and progress food

Peristalsis vs. Segmentation

Regulation of Motility

• Gastric Motility– Emptying the stomach takes approx 2-6

hours after a meal– Food is churned with digestive juices to

form chyme • Ejection every 20 seconds into the

duodenum• Controlled by hormonal and nervous

mechanisms

Regulation of Motility

• Hormonal Control– Fats and nutrients in duodenum cause

secretion of gastric inhibitory peptide (GIP) from the intestinal mucosa into the bloodstream• Slows peristalsis in stomach; decreasing

passage of food into the duodenum

Regulation of Motility

• Nervous Control– Nerve receptors in duodenum are

sensitive to acid and distention • Vagus nerve (CN X) causes inhibition (stops)

gastric peristalsis

Regulation of Motility• Intestinal Motility• Takes approx 5 hours for food to pass through

the small intestine– Segmentation

• Mixes chyme and digestive juices from liver, pancreas and intestinal mucosa

• causes contact with intestinal mucosa to increase absorption

– Peristalsis• Continues in the jejunum to move food into the large intestine • Stimulated by the hormone cholecystokinin-pancreozymin

(CCK)– Secreted by intestinal endocrine cells in the presence of chyme

Mechanical Digestion

• Summarized in Table 26-2; page 775

Chemical Digestion

• Consists of all the changes in the chemical composition of food

• Result of hydrolysis– Compounds combine with water then

split into simpler compounds– Enzymes catalyze the hydrolysis of

foods

Chemical Digestion

• Six main types of chemical substances:– Carbohydrates, proteins, fats, vitamins,

mineral salts, water– Only carbohydrates, proteins and fats

must undergo chemical digestion to be absorbed

Properties of Digestive Enzymes

• Extracellular enzymes• Classified as hydrolases• Function optimally at a specific pH

– Ex: amylase vs pepsin

• Continually destroyed or eliminated

Carbohydrate Digestion

• Carbohydrates are saccharide compounds– Contain one or more saccharide groups

• Polysaccharides – starches & glycogen• Disaccharides – sucrose, lactose and

maltose• Monosaccharides – glucose, fructose &

galactose

Carbohydrate Digestion

• Polysaccharides are hydrolyzed by amylases– Present in saliva and pancreatic juice

• Sucrose, lactose and maltose are hydrolyzed by sucrase, lactase, and maltase– Located on epithelial cells lining villi in

small intestine– End product (usually glucose) is absorbed

in the small intestine

Protein Digestion

• Proteins are large molecules composed of twisted chains of amino acids

• Proteases catalyze the hydrolysis of proteins into smaller compounds– Proteins proteoses or peptides

amino acids• The peptides bonds holding amino

acids together can vary = increased need for varying proteases

Protein Digestion - Proteases

• Pepsin – gastric juice (stomach)• Trypsin and chymotrypsin –

pancreatic juice (pancreas)• Peptidases – intestinal brush border

(small intestine)

Proteoses

Fat Digestion

• Fats are insoluble in water so they must be emulsified prior to digestion– Emulsify = dispersed as small droplets– Lecithin and bile salts emulsify oils and

fats in the small intestine by forming micelles (fig 26-8, page 778)

• Micelles are further broken down by lipase

Residuals of Digestion• Certain compounds cannot be

digested in humans b/c we lack the enzyme required for hydrolysis

• These compounds are excreted in the feces

• Cellulose (dietary fiber), connective tissue from meat (collagen), undigested fats combined with calcium and magnesium, bacteria, pigments, water, mucous

Chemical Digestion

• Summarized in Table 26-3, page 779

Secretion

• Release of substances from exocrine glands in the GI tract– Examples: Saliva, gastric juice, bile,

pancreatic juice, intestinal juice

Saliva• Secreted from salivary glands• Water component helps liquefies food

chyme– Allows enzymes to mix with food particles

• Mucus lubricates food to protect mucosa lining• Amylase – chemically digest (breakdown)

complex carbohydrates• Lipase (small amounts) – digest lipids

– Decreased function when fat are not emulsified

• Sodium bicarbonate (NaHCO3)– Dissociated in water– Bicarbonate ions bind with H+ to increase pH

Control of Salivary Secretion

• Controlled by reflex mechanisms:– Olfactory & visual stimuli send afferent

impulses to centers in the brainstem efferent impulses to salivary glands

– Chemical and mechanical stimuli come from the presence of food in the mouth

Gastric Juice

• Gastric juice = water, mucous, enzymes & acid

• Secreted by gastric glands in the stomach

• Chief cells – secrete enzymes of digestive juices– Pepsin

• Parietal cells – Secrete HCl

• Decreases stomach pH

Control of Gastric Secretion• Gastric secretion is controlled by 3

phases:1. Cephalic phase (“psychic phase”)

– Sight, smell, taste, thought of food activate control centers in medulla oblongata

– Parasympathetic fibers of the vagus nerve conduct impulses to gastric glands

– Vagal impulses stimulate production of gastrin• Gastrin stimulates gastric secretion

Control of Gastric Secretion

2. Gastric phase: – Gastrin secretion is further stimulated

by the presence of products of protein digestion & distention

– Gastrin continues to stimulate the secretion of gastric juices (pepsin and HCl)

Control of Gastric Secretion3. Intestinal phase:

– Gastric inhibitory peptide (GIP) in secreted in the small intestine in the presence of fats and carbohydrates

• Decrease gastric motility and secretion– Secretin secreted in the small intestine in the

presence of acid, digested proteins and fats• Inhibit gastric secretion• Simulate secretion of pancreatic enzymes• Stimulate ejection of bile into small intestine

– CCK• Secreted in the small intestine in the presence of chyme• Stimulates ejection of bile from gallbladder• Stimulates secretion of pancreatic juices• Opposes action of gastrin; raises pH of gastric juice

Pancreatic Juice

• Secreted by exocrine portion of the pancreas• Mostly water• Enzymes:

– Trypsin and chymotrypsin (proteases)– Lipases– Amylase (starch digesting enzyme)

• Secrete bicarbonate into the GI lumen and H+ into the blood to buffer the effects parietal cell secretion (fig 26-10 and fig 26-11)

Control of Pancreatic Secretion

• Secretin & CCK– Stimulates the secretion of pancreatic

fluid high in bicarbonate to neutralize acidity of chyme in the small intestine

– See notes under “Control of Gastric Section – Intestinal phase”

Bile

• Secreted by liver and stored in gallbladder• Bile contains:

– Lecithin and bile salts• Emulsify fats by creating a hydrophilic “shell” around

tiny fats droplets

– Sodium Bicarbonate – increase pH of chyme in small intestine

– Excretions:• Cholesterol, products of detoxification, bilirubin

(product of hemolysis)

Control of Bile Secretion

• Controlled by CCK and secretin• See Table 26-5, page 782

Intestinal Juice

• Mucus – provides lubrication• Sodium bicarbonate – increases pH

to allow intestinal enzymes to function at optimal level

• Water – carries mucus and NaHCO3

**Study These Tables**

Table 26-4: Digestive SecretionsTable 26-5: Actions of Digestive

Hormones

Both on page 782

Absorption

• Passage of substances (digested foods, vitamins, salts, water) across the mucosa into the blood

• Majority of absorption takes place in small intestine where surface area is increased

Elimination

• Expulsion of digestive residuals from the digestive tract in the form of feces– Defecation– Normally rectum is empty– Massive peristalsis of feces into the

rectum stimulate receptors and relax the external anal sphincter

– Voluntary control

Elimination

• Constipation– Contents move through large intestine

at a slower rate– Increased water absorption occurs

resulting in hardened feces

• Diarrhea– Result of increased motility of the small

intestine– Water absorption does not occur

GI Tract Disorders

• Common S/S– Gastroenteritis

• Gastritis – stomach inflammation• Enteritis – intestinal inflammation

– Anorexia: chronic loss of appetite– Nausea: feeling of needing to vomit; may

progress to vomiting– Emesis: vomiting– Diarrhea: elimination of liquid feces; abdominal

cramps may also be present– Constipation: decreased motility of colon;

difficulty in defecating

GI Tract Disorders

• Ulcers– In stomach or duodenum– Cause pain and may lead to perforation

of the wall of the GI tract– Bleeding anemia– Causes:

• Hyperacidity• H. phylori bacterium

GI Tract Disorders

• Stomach cancer– Linked to excessive alcohol use,

chewing tobacco, eating heavily preserved foods

– Early signs:• Heartburn, belching, nausea

– Later signs:• Chronic indigestion, vomiting, anorexia,

stomach pain, blood in feces

GI Tract Disorders

• Diverticulosis – presence of abnormal sac-like projections on the large intestine (diverticula)– When inflamed causes diverticulitis– S/S: pain, tenderness, fever

GI Tract Disorders

• Colitis – inflammation of the large intestine– s/s: diarrhea, abdominal cramps,

constipation, bleeding, intestinal ulcers– Crohn’s Disease: autoimmune colitis– Treatment: surgical removal of affected

portions of the intestine

GI Tract Disorders

• Irritable bowel syndrome (IBS)– “spastic colon”– Noninflammatory condition usually

caused by stress– Diarrhea or constipation

GI Tract Disorders

• Colorectal cancer– Occurs after 50– Associated with low-fiber, high-fat diet– Early signs: change in bowel habits,

fecal blood, rectal bleeding, abdominal pain, unexplained anemia, weight loss, fatigue

Liver Disorders

• Hepatitis – inflammation of the liver– S/S: jaundice, liver enlargement,

anorexia, abdominal discomfort, gray-white feces, dark urine

– Causes: alcohol or drug abuse; bacterial or viral infection

• Cirrhosis – degenerative liver condition– Tissue can no longer regenerate