Human Digestive System - contd…

Pharynx

It is the portion of the alimentary canal after the mouth. It is a cavity common to air and food.

The Gut Wall

From the oesphagus to rectum, the gut wall is mainly composed of two regions:

Mucous coat

It is a the mucous membrane lined with epithelium showing numerous folds.

Muscle coat

It has an inner circular and the outer longitudinal muscles. Both these layers facilitate the movement of food along the alimentary canal. The mucous coat keeps the passage smooth and mixes the food with mucus. The muscle layer by periodic contraction and expansion moves the food along the canal. This movement is called peristalsis.

Peristalsis

Oesophagus

Pharynx along the alimentary canal leads to a narrow, tubular, muscular organ called the oesophagus.

Stomach

It is a sac-like organ which is connected to the oesophagus at the anterior by a cardiac sphincter and to the small intestine at the posterior end by a pyloric sphincter.

Small Intestine

It is a long structure which is narrower than the stomach. It has three main portions:

• Duodenum

• Jejunum

• Ileum

Duodenum

It is a short portion of the small intestine which is looped and measures about 25 cm.

Jejunum

It is a region of about 2.5m length where no digestion occurs.

Ileum

It is a highly coiled and narrow region measuring about 3.6m in length.

Villi

Villi are finger-like projections in the small intestine. Each villus is made up of connective tissue and smooth muscles. The villi are lined by a single layer of thin walled epithelial cells through which the absorption takes place. The fats are absorbed into the lymphatic system. In the centre of each villus, is a lymphatic duct. It is also called sinus or lacteal. The space between the lacteal and the epithelium is criss-crossed by several small blood capillaries.

Villi

Large Intestine

Small intestine enlarges slightly to form a tubular large intestine which is 5cm wide and 2m long. It has three portions:

• Caecum

• Colon

• Rectum

Caecum

It is a portion of the large intestine that is not very well developed in man. It is more developed in cattle and other ruminants. In man, it shows a small projection called the appendix. The infection of appendix is called appendicitis. It is a vestigeal organ and can be removed if it is infected.

Colon

Colon has four parts:

• ascending colon

• transverse colon

• descending colon

• pelvic colon

Rectum

It is a bag-like structure which ends in an opening called the anus. The rectum stores the undigested food as faecal matter till it can be passed out. The anus is guarded by external and internal sphincter muscles.