4. Small intestine

Small intestine

Duodenum, jejunum, ileum

Physiologically similar between species

Digestion, absorption, endocrine, immune defence

Motility functions

Mixing chyme with digestive enzymes

Maximise contact with mucosa for absorption

Propelling chyme to large intestine

Names of 3 small intestine movements

Mixing contractions

Propulsive contractions

Migrating myoelectric complex

Mixing contractions (segmentation contractions)

During food intake

Triggered by chyme presence + wall stretching

Short and at regular intervals

Propulsive contractions (peristalsis)

During food intake

Slow migration of contents to large intestine

Migrating myoelectric complex (MMC)

Cleans intestine during fasting

Moves undigested food and bacteria to large intestine

Motilin stimulates MMC via enteric + autonomous NS

Phases of migrating myoelectric complex

Phase 1 - period of resting, no contractions

Phase 2 - increased frequency of action potentials + small intestine contractions, irregular mixing contractions

Phase 3 - peak electrical + mechanical activity, strong contractions for propulsion

Regulation of small intestine motility

Stimuli - wall stretching, gastroenteric reflex, parasympathetic (Ach increase motility), sympathetic (decreases motility)

Hormonal - increases (gastrin, CCK, insulin, motilin, serotonin…), decreases (secretin, glucagon…)

What are pancreatic secretions produced by in the small intestine

Acinar and centroacinar (duct) cells

What do pancreatic secretions contain

Water + ions (Na+, K+)

Bicarbonate (HCO3-) - neutralise acid, inhibit pepsin, pH for pancreatic enzymes

Digestive enzymes - protein dig (trypsin, chymotrypsin), carb dig (pancreatic amylase), fats dig (pancreatic lipase, phospholipase), nuclease (deoxyribonuclease, ribonuclease)

Activation of pancreatic secretions

Enterokinase in duodenum activates trypsinogen into trypsin which then activates other enzymes

Regulation of pancreatic secretions

Acetylcholine - vagus nerve to acini cells

Cholecystokinin - duodenal/jejunal mucosa to stimulate acini cells to secrete dig enzymes

Secretin - duodenal/jejunal mucosa to stimulate duct cells to secrete bicarbonate and water

Phases of regulation of pancreatic secretions

Cephalic + gastric phase - stimulated by Ach, enzyme secretion (no fluid)

Intestinal phase - major, by CCK + secretin

Bile secretion

Produced in liver by hepatocytes, stored in gall bladder

Functions - Fat digestion + absorption (bile salt emulsify fat, lipase digestion) and excretion (excretion of bilirubin and excess cholesterol)

Composition of bile secretion

Water, bile salts, bilirubin, cholesterol, fatty acids, lecithin, Na+, K+, Cl+, HCO3-

Enterohepatic circulation

Bile salts are recycled

Primary bile salts → produced in liver

Secondary bile salts → modified by intestinal bacteria

Bile flow

Hepatic duct → common bile duct → gall bladder

Bile salts

Precursor - cholesterol

Emulsificaiton - large fat droplets into smaller ones

Fat absorption - micelles (bile salts surround fat droplets) → micelles deliver to intestinal cells

Bilirubin

Yellow brown colour to urine, feces and bruises

Hemoglobin breakdown (degragation of heme group)

Excess bilirubin leads to jaundice

Bilirubin metabolism

Biliverdin → unconjugated bilirubin (not water soluble)

Unconjugated bilirubin bound to albumin to transport to liver

Unconjugated bilirubin + glucoronic acid → conjugated bilirubin

Now water soluble and secreted in bile

Regulation of bile secretion

CCK (gall bladder contractions)

Secretin (bile secretion)

Bile salt recycling

Vagal stimulation