Regulation of Intestinal Function

The GI has its own nervous system (enteric nervous system), it can sense and form reflex actions due to the presence of food.

In the intestine the enteric plexus is very extensive and this is involved in several reflexes:

1- Gastroilial reflex =increase gastric activity causes increased motility of ileum and movement of chyme (food mixed with digestive juice)through the ileocecal valve.

2- Ileogastric reflex =when ileum is distended this decreases the motility and emptyingof the stomach.

3- Intestino-intestinal reflex = over-distension of one part of the intestine causes reflex relaxation of the rest of the intestine.

Regulation of Pancreatic Secretion

HORMONES :

a) Secretin = released from intestinal mucosa in response to presence of acidic chyme in small intestine.

AcidicChyme

Disfunction of the above may cause duodenal ulcer.

IntestinalMucosalCell

Glandular Cells of Pancreas

SodiumBicarbonate

Neutralization

BloodSecretin

Regulation of Pancreatic Secretion

HORMONES :

b) CCK = released mainly in response to presence of fat, proteins and carbohydrates in small intestine.

FattyChyme

IntestinalMucosalCell

Glandular Cells of Pancreas

Digestiveenzymes

Digestion

BloodCCK

Gallbladder Emptying

Gallbladder

Pancreas

LiverVagus

ACh

Duodenum

Blood

. . .FAT

CCK

CCK

+

+

The bile is secreted by the liver and stored in the gallbladder. Both secretin and CCK stimulate secretion of bile by the liver. CCK is major stimulator of gallbladder contraction.Neural reflexes also help with gallbladder contraction and bile release into duodenum.

CCK

Digestion and Absorption

Also in small amounts food contains:

- Nucleic acids, DNA and RNA- Salts and minerals, Na, Cl, Ca, Fe, PO4, Cu, etc.- Vitamins, C, D, Thiamin (B1), Riboflavin (B2), B12 etc.

Food mainly contains:

Carbohydrates – mainly polysaccharides (starches) and disaccharides.

Lipids (fat) – mainly triglycerids

Proteins – polymers of amino acids

DigestionDigestion = breaking down of large compounds in food into smaller and simpler substances that can be absorbed and used by the cells of the body.

Carbohydrates in Food

Most common carbohydrates ingested are:

- Starch – long chain of glucose (polysaccharides) with occasional branching.

- Sucrose (table sugar) – a disaccharide consist of glucose and fructose.

- Lactose (milk sugar) – a disaccharide consist of glucose and galactose.

= glucose= fructose= galactose

Lactose Intolerance

Deficiency in enzyme lactase

Inadequate lactose digestion (Lactose in milk)

High level of lactose in intestine (Colon)

Osmotic effect (water moves from blood to lumen)

Intraluminal volume and gas

DIARRHEOA & GAS

Bacterial action onundigested lactose.

Digestion of Carbohydrates- The digestion of carbohydrates occur in mouth and it continues in the duodenum.

Digestion & Absorption of Carbohydrates- Short oligosaccharids (-limit dextrins), maltotrios and maltose are broken down to single sugars (monosaccharides) by the brush border enzymes.

- The resulted monosaccharides are : Glucose (80%)Fructose (10%)Galactose (10%)

-The monosaccharides (glucose & galactose) are transported into epithelial cells by secondary active transport and fructose is transported by facilitated diffusion.

K+

Na+

Glucose

Glucose

Na+ Intestinal Lumen

Blood

Fructose

Fructose

Digestion of Proteins- Proteins are polymeres of amino acids, they can be formed from less than 100 amino acids up to thousands.

-There is no digestion of proteins in mouth, the digestion starts in the stomach by the action of pepsin.

Protein Pepsin in the stomachPolypeptides + amino acids

- Most protein digestion occurs in duodenum and jejunum.

Food fromthe stomach

Duodenum (small intestine)

Pancreatic enzymes

pepsinogen Chief cells

Enterokinase

- Pancreatic enzymes are activated in the duodenum by the action of enterokinase.

- Final Products of protein digestion are:

Amino acids, Dipeptides and Tripeptides.

- There are five different transport systems for protein digestion products:

Absorption of Proteins

Neutral amino acidsAcidic amino acidsIMINO acids

Basic amino acids

Di- and Tri-peptides

Secondary active transport

Facilitated diffusion

Tertiary active transport

- For their digestion, lipids must be first emulsified.

Digestion of Lipids

- The bile secreted into duodenum combines with lipids and reduces their surface tension breaking the lipids into small droplets.

Digestion of Lipids

Micelles Formation and Absorption

Absorption of Lipids

Absorption of Water - Approximate daily intake = 2 L

- Approximate GIT secretion each day = 7 L

- Only 50 to 100 ml/day is excreted in the feces.