PancreasWeighs about 100g, and produces about 1L of juice/day. Secretions are both endocrine and exocrine.

Islets of LangerhansSecret insulin and glucagon

Acini: secret pancreatic juice

Metabolicfunction

Digestive function

Pacreatic Juice• Pancreatic juice contains:

– Water and electrolytes (Na+, K+, Cl+, Ca++)– Bicarbonate (HCO3-)– Digestive enzymes:

• Amylase• Trypsin• Lipase• Ribonuclease• Deoxyribonuclease• Etc.

Sodium BicarbonateFor neutralization of acid

Enzymes for digestionof food

Regulation of the Digestive System• The activities of the G.I. Tract is regulated by:

– Neural mechanisms:• Automomic nervous system

– Sympathetic– Parasympathetic

• Enteric nervous system– myenteric– Submucosal

– Action of hormones (endocrine):• Hormones secreted from the glands located in the G.I. Tract act on effector

cells of the G.I. Itself. Examples of such hormones are: secretin, gastrin, CCK (cholecystokinin)

From stomachFrom small intestine

Regulation of Gastric Secretion• Contractions and secretions of the stomach to some extent are automatic. For

example: the presence of food causes distension in the stomach wall and this will lead to contractions. Presence of some food materials in the stomach stimulate its glands to secret gastric juice.

• However, when ANS or hormones are active their effects dominate the function of the stomach.

• There are 3 phases in the control of gastric secretion:

1- Cephalic phase: sight, smell, taste, even thoughts of food can initiate this phase.

2- Gastric phase: when food enters the stomach this phase becomes active.

3- Intestinal phase: when food enters samll intestine this phase becomes active.

Cephalic Phase

Vagal nuclei

SightSmellSound Thoughts

TasteTouchTemp.Etc.

Mouth

Parietal cells HClChief cells EnzymeMucus cells MucusG cells Gastrin

Gastric Gland

- Cephalic phase is abolished by cutting vagus (vagotomy).

ACh

Vagus

Exoc

rine

Endocrine

Gastric Phase

Food entering stomach

Parietal cells HClChief cells EnzymeMucus cells MucusG cells Gastrin

Gastric Gland

AChDistension, protein digestion products

Vagal nuclei

Enteric Plexus

ACh

- Gastric secretion is abolished after administration of atropine

Vagus

LOCAL

CENTRAL

Atropine is blocker of ACh

Intestinal Phase

HClG cell

Gastrin

Blood

Food entering small intestine

1- Stimulation (weak):

2- Inhibition (strong):

HCl

SecretinBlood

Food entering small intestine

CCK- CCK is released in response to fat and - secretin is released in response to low pH.

QUESTION

Regulation of Intestinal Function

The GI has its own nervous system (enteric nervous system), it can sense andform 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.

QUESTION

Regulation of Pancreatic Secretion

1- 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

1- 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

BloodCCKQUESTION

Gallbladder Emptying

Gallbladder

Pancreas

Liver

Sphincter of Oddi

Vagus

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

QUESTION

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.

Hydro

lysis

Carbohydrates in FoodMost 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.

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”

K+

Na+

Glucose

Glucose

Na+ Intestinal Lumen

Blood

Fructose

Fructose

FacilitatedDiffusion

Final products ofCarbohydrates Digestion

QUESTION

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

Enterokinase

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

QUESTION

- 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

Passive diffusion

- 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.

QUESTION

Digestion of Lipids

QUESTION

Absorption of Lipids

Chylomicrons