Gastro Intestibal Tract

8/9/2019 Gastro Intestibal Tract

1/15

The Human Gastrointestinal (GI) Tract


2/15

Ingestion

Food placed in the mouth is

ground into finer particles by the teeth,

moistened and lubricated by saliva (secreted by three

pairs ofsalivary glands) small amounts of starch are digested by the amylase

present in saliva

the resulting bolus of food is swallowed into the

esophagus and

carried by peristalsis to the stomach.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.html


3/15

The stomach

a J-shaped area of thegastrointestinal (GI) tract thatsits in the upper left side ofthe abdomen.

It is the most dilated area ofthe tract and has severalregions - the fundus (theexpanded part of thestomach), the cardia, thebody, and the funnel-shapedantrum.

Food enters the stomach

through the cardiac orgastroesophageal sphincterand leaves it via the pyloricsphincter.

approximately 25cm long

and can expand to hold up to

4L of food and drink,

although its empty volume is

only 50ml collapses in on itself when it

is empty and forms folds or

rugae.


4/15

Absorption in the stomach

As the contents of the stomach become

thoroughly liquefied, they pass into the

duodenum, the first segment

Two ducts enter the duodenum:

one draining the gall bladder and hence the

liver

the other draining the exocrine portion of

the pancreas.


5/15

FUNCTIONS OF THE STOMACH

Food reservoir

Absorption

Mucus secretion.

Gastric juice secretion

The wall of the stomach is lined with millions ofgastric glands, which together secrete

400800 ml of gastric juice at each meal. Several kinds of cells are found in the gastricglands

parietal cells

chief cells

-It stops the proliferation of bacteria in the stomach;

- It inactivates salivary amylase, mixed with the food in the mouth;

- It curdles milk to prepare it for digestion;

- It tenderises proteins (by denaturing them);- It converts the pepsinogen produced by chief cells into pepsin, which starts to digest

protein;


6/15

FUNCTIONS OF THE STOMACH

Churning food

- Food that enters the stomach is mixed with and diluted by the

gastric secretions into a thick soup-like substance called chyme. The

chyme is churned by waves of peristalsis. Each wave lasts about half a

minute and 'flows' from the top of the stomach to the bottom;

Production of intrinsic factor. The parietal cells in the stomach also produce intrinsic factor, which is

essential for the absorption of vitamin B12 from the ileum in the small

intestine. Vitamin B12 is necessary for the healthy functioning of nerve

fibres in the body, for the formation of myelin sheaths on the nerves in the

spinal cord and for the formation of red blood cells.


7/15

The liver

The liver secretes bile. Between meals it accumulates in

the gall bladder. When food, especially when it contains

fat, enters the duodenum, the release of the hormone

cholecystokinin (CCK) stimulates the gall bladder to

contract and discharge its bile into the duodenum.

Bile contains ;

bile acids

bile pigments
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.html


8/15


9/15

The pancreasThe pancreas consists of clusters if endocrine cells (

the islets ofLangerhans) and exocrine cells whosesecretions drain into the duodenum.

Pancreatic fluid contains:

sodium bicarbonate (NaHCO3). This neutralizesthe acidity of the fluid arriving from the stomach

raising its pH to about 8.

pancreatic amylase. This enzyme hydrolyzes

starch into a mixture ofmaltose and glucose.

pancreatic lipase. The enzyme hydrolyzes

ingested fats into a mixture offatty acids and

monoglycerides. Its action is enhanced by the

detergent effect of bile.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pancreas.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pancreas.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fats.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/M.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/M.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fats.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pancreas.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pancreas.html


10/15

The secretion of pancreatic fluid is

controlled by two hormones:

secretin, which mainly affects the release of

sodium bicarbonate, and

cholecystokinin (CCK), which stimulates

the release of the digestive enzymes.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/G/GutHormones.html


11/15

The Small Intestine

Digestion within the small

intestine produces a

mixture of disaccharides,

peptides, fatty acids, and

monoglycerides. The finaldigestion and absorption

of these substances occurs

in the villi, which line the

inner surface of the smallintestine.


12/15

Incorporated in the plasma membrane of the

microvilli are a number of enzymes that complete

digestion:aminopeptidases attack the amino terminal (N-

terminal) of peptides producing amino acids.

disaccharidasesThese enzymes convertdisaccharides into their monosaccharide subunits.

omaltase hydrolyzes maltose into glucose.

osucrase hydrolyzes sucrose (common table sugar)into glucose and fructose.

olactase hydrolyzes lactose (milk sugar) into glucose

and galactose.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/Carbohydrates.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.html


13/15

Fructose simply diffuses into the villi, but both

glucose and galactose are absorbed by

active transport.

fatty acids and monoglycerides. These become

resynthesized into fats as they enter the cells of the

villus. The resulting small droplets of fat are thendischarged by exocytosis into the lymph vessels,

called lacteals, draining the villi.

Humans with a rare genetic inability to form

microvilli die of starvation.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fats.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/M.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/E.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/E.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/M.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/Fats.htmlhttp://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/A.html


14/15

The large intestine (colon)

The large intestine receives the liquid residue after digestion and absorption arecomplete. This residue consists mostly of water as well as materials (e.g. cellulose)that were not digested. While the contents of the small intestine are normally sterile,the colon contains an enormous population of microorganisms.

Reabsorption of water is the chief function of the large intestine. Thelarge amounts of water secreted into the stomach and small intestine

by the various digestive glands must be reclaimed to avoiddehydration. If the large intestine becomes irritated, it may dischargeits contents before water reabsorption is complete causing diarrhea.On the other hand, if the colon retains its contents too long, the fecalmatter becomes dried out and compressed into hard masses causing

constipation.


15/15

Documents

Gastro Intestibal Tract