Page 9 - 86 human physiology part-2

Chapter 12

surrounded by a thick circular muscular wall which is normally tonically constricted forming a
functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme.

Control of secretion and motility

The movement and the flow of chemicals into the stomach are controlled by both the nervous
system and by the various digestive system hormones.

The hormone gastrin causes an increase in the secretion of HCL, pepsinogen and intrinsic factor
from parietal cells in the stomach. It also causes increased motility in the stomach. Gastrin is released
by G-cells into the stomach. It is inhibited by pH normally less than 4 (high acid), as well as the
hormone somatostatin.

Cholecystokinin (CCK) has most effect on the gall bladder, but it also decreases gastric emptying.
In a different and rare manner, secretin, produced in the small intestine, has most effects on the
pancreas, but will also diminish acid secretion in the stomach.

Gastric inhibitory peptide (GIP) and enteroglucagon decrease both gastric motility and secretion of
pepsin. Other than gastrin, these hormones act to turn off the stomach action. This is in response to
food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs only
to push food into the small intestine when the intestine is not busy. While the intestine is full and still
digesting food, the stomach acts as a storage for food.

Small Intestine

The small intestine is the site where most of the chemical and mechanical digestion is carried out.
Tiny projections called villi line the small intestine which absorbs digested food into the capillaries.
Most of the food absorption takes place in the jejunum and the ileum.

The functions of a small intestine is, the digestion of proteins into peptides and amino acids
principally occurs in the stomach but some also occurs in the small intestine. Peptides are degraded into
amino acids; lipids (fats) are degraded into fatty acids and glycerol; and carbohydrates are degraded
into simple sugars.

The three main sections of the small intestine is The Duodenum, The Jejunum, The Ileum.

The Duodenum

In anatomy of the digestive system, the duodenum is a hollow jointed tube connecting the
stomach to the jejunum. It is the first and shortest part of the small intestine. It begins with the
duodenal bulb and ends at the ligament of Treitz. The duodenum is almost entirely retro peritoneal. The
duodenum is also where the bile and pancreatic juices enter the intestine.

The Jejunum

The Jejunum is a part of the small bowel, located between the distal end of duodenum and the
proximal part of ileum. The jejunum and the ileum are suspended by an extensive mesentery giving the
bowel great mobility within the abdomen. The inner surface of the jejunum, its mucous membrane, is

224 | Human Physiology