Small intestine and associated structures

Small intestine

tube that carries chyme away from stomach and deposits it in the large intestine

•  3 distinct parts: duodenum jejunum ileum

Duodenum

• Receives chyme through pyloric sphincter

Jejunum

• Longest part

• Most chemical digestion and absorption occur here

Ileum

• Prominent pyers patches

• Empties into large intestine at cecum (horse), at colon (dog and cat), or at cecum and colon (ruminants and pig)

Small Intestine Surface Area

• Adaptations that increase surface area include the long organ with many loops and coils.

• Plications are folds in the mucosal lining.

• Villi are found in the mucosa and contain microvilli. (The brush border)

• Intestinal crypts (crypts of Langerhans.)

Lacteals and Blood Capillaries

• Lacteals are lymphatic capillaries.

• They carry absorbed lipids and fat-soluble substances to the thoracic duct and into the vena cava.

• Blood capillaries collect some absorbed nutrients and transport them to the liver.

Duodenal Mucosa Secretions

Cholecystokinin, (CCK)

functions to inhibit gastric emptying.

• causes increased secretion of bicarbonate and pancreatic digestive enzymes.

• CCK stimulates secretion of enteropeptidase.

Stimuli for secretion

•  high amino acid or fatty acid concentrations, or low pH of chyme entering the duodenum.

Duodenal Mucosa Secretions

Secretin

Functions to decrease HCl production in the stomach.

• Secretin increases pancreatic and biliary bicarbonate secretions.

Duodenal Mucosa Secretions

Pancreas

Exocrine function: 

• Lipase, amylase, nuclease, protease

Endocrine function:

• Glucagon,  insulin

The liver

• The liver is the largest digestive gland in the body.

• It has a multitude of functions, many not related to digestion.

Functions:

• The liver secretes substances essential for digestion and absorption of nutrients.

• synthesizes nutrients and regulates their release into the bloodstream.

• excretes toxic substances (those originating within the body and from outside the body)

• produces plasma proteins, cholesterol, and blood coagulation factors.

where liver works:

• The mammalian liver consists of lobes.

• The liver is strategically placed to process blood leaving the GI tract.

• It prevents toxic substances from entering general circulation.

• Known as the triad and includes the hepatic portal vein, hepatic artery, and bile ducts.

• Kupffer cells engulf foreign objects.

• Site for protein production- proteins made by hepatocytes enter the bloodstream through pores in adjacent sinusoids.

• Nearly all plasma proteins , including albumin and blood clotting proteins.

Gallbladder and Bile

• concentrates and stores bile until it is needed.

• Bile enters the duodenum to digest high fat and peptide concentrations.

• provides a means for the liver to excrete waste products even when the animal is not eating.

Composition:

• bile salts

• Phospholipids

• Cholesterol,

• bile pigments

• Conjugated (water soluble) bilirubin

Functions of bile

• is released into the intestine to emulsify fat.

• salts are reabsorbed when they reach the ileum.

• salts enter the hepatic portal vein and return to the liver.

• The liver reabsorbs bile salts and recycles them back to bile.

• The amount of bile salts that recirculate from intestine to liver influences how much bile is synthesized by hepatocytes.

Bilirubin Excretion

is a waste product formed when old blood cells are broken down

• Phagocytized by macrophages

•  Recycled into some usable parts by the liver but in excess it can be dangerous

• Stored with bile and released slowly into small intestine 

•  some further breakdown occurs 

• some excreted via feces (brown colour) 

• Some excreted via urine (contributes to yellow color)