Digestive System: Process and Organs Explained

Digesting Food

The Digestive Tract

The digestive tract is a long tube that extends from the mouth to the anus. Digestion begins in the mouth with chewing, breaking down food into smaller pieces for easier enzyme action.

Salivary glands secrete saliva containing enzymes like salivary amylase (begins carbohydrate digestion) and salivary lipase (begins fat digestion).

The Stomach

This mixture, called a bolus, is swallowed and travels down the esophagus to the stomach, passing through the lower esophageal sphincter. In the stomach, the bolus mixes with hydrochloric acid (HCl), pepsin (digests proteins), and gastric lipase (continues fat digestion). This creates a liquid called chyme, which exits the stomach through the pyloric sphincter to the duodenum (the first part of the small intestine).

The Small Intestine

The duodenum completes the breakdown of food into absorbable units. Two accessory organs contribute:

• The liver secretes bile, stored in the gallbladder, which aids fat digestion.
• The pancreas secretes enzymes like pancreatic amylase (continues carbohydrate digestion), pancreatic lipase (continues fat digestion), trypsin, and chymotrypsin (continue protein digestion).

The jejunum and ileum, the remaining parts of the small intestine, absorb these units.

The Large Intestine

Undigested material passes through the ileocecal valve (near the appendix) to the large intestine. The large intestine consists of the ascending, transverse, and descending portions, leading to the sigmoid colon and finally the anus. It absorbs water and forms feces.

Intestinal Structure

The small intestine's inner lining has fingerlike projections called villi, increasing surface area for nutrient absorption. Microvilli on the villi's surface (forming the brush border) further enhance absorption.

Enterocytes, cells covering the villi, contain microvilli. Inside the villi are projections of the lymphatic system (central lacteals) that receive digested fats. Capillaries within the villi absorb digested carbohydrates (monosaccharides) and proteins (amino acids) into the bloodstream.