Summary
Highlights
Ingestion is the act of putting food into the mouth. Propulsion, the movement of food through the digestive tract, begins with swallowing and involves peristalsis—wave-like muscle contractions that move food from the mouth to the anus.
Mechanical breakdown physically reduces food into smaller particles through chewing, mixing, and churning, increasing its surface area for enzyme action. Chemical digestion breaks down large food molecules into their building blocks using enzymes.
The digestive system's main functions are absorption and digestion, involving six essential activities: ingestion, propulsion, mechanical breakdown, chemical digestion, absorption, and defecation. Each step is crucial for processing food and extracting nutrients.
Absorption is the process where digested nutrients and reabsorbed water mostly enter the bloodstream in the small intestine, aided by microvilli. Defecation is the elimination of undigested residues from the gastrointestinal tract as fecal material.
The alimentary canal, or GI tract, is a continuous muscular tube from the mouth to the anus. The mouth is where ingestion begins, supported by structures like lips, cheeks, teeth, tongue, and palate. Food then passes through the pharynx, which has three divisions: nasopharynx, oropharynx, and laryngopharynx.
The esophagus, or gullet, is a muscular tube that propels food from the pharynx to the stomach via peristalsis. The walls of the alimentary canal (from esophagus to large intestine) consist of four tissue layers: mucosa, submucosa, muscularis externa, and serosa.
The stomach is a C-shaped organ in the upper left abdomen that stores food for 4-6 hours. It has regions like the cardial region, fundus, body, and pylorus. The muscularis externa of the stomach has three layers (circular, longitudinal, and oblique) to facilitate mixing and churning food into chyme. It also produces gastric juices containing hydrochloric acid and protein-digesting enzymes like pepsin.
The small intestine is the major digestive organ, divided into three regions: duodenum, jejunum, and ileum. The duodenum, the first C-shaped segment, receives chyme from the stomach and secretions from accessory organs like the pancreas, playing a crucial role in chemical digestion. It measures 15-25 cm in length.