The gastrointestinal (GI) tract, extending from the mouth to the anus, plays a pivotal role in the digestion and absorption of nutrients. This process involves both mechanical and chemical actions facilitated by various enzymes.

Digestion begins in the mouth, where food undergoes mechanical breakdown by chewing and combines with saliva. Salivary amylase, an enzyme in saliva, starts the breakdown of starches into maltose. The food then travels down the esophagus to the stomach.

In the stomach, a muscular organ with a capacity of approximately 1500 mL, food is transformed into a semi-fluid mass called chyme. The stomach secretes hydrochloric acid (HCl), which helps break down food into absorbable components and neutralize ingested bacteria. Pepsinogen, secreted by gastric glands, is converted to pepsin in this acidic environment, marking the beginning of protein digestion. Simultaneously, the intrinsic factor combines with dietary vitamin B12, preparing it for absorption in the ileum.

Mechanical digestion in the stomach involves churning movements that break down food into smaller particles. The stomach also acts as a reservoir, gradually releasing its contents into the small intestine.

Once chyme enters the small intestine, further digestion occurs. Chyme stimulates motility and secretion of enzymes from the pancreas and bile from the liver. Starches that are not already broken down are cleaved by pancreatic amylase. Disaccharides are broken down into monosaccharides by brush-border enzymes such as sucrase, lactase, and maltase.

Protein digestion continues with enzymes in pancreatic juice — trypsin, chymotrypsin, carboxypeptidase, and elastase — breaking down proteins into peptides. Two peptidases in the brush border complete this process: aminopeptidase and dipeptidase.

Lipid digestion occurs predominantly in the small intestine, with pancreatic lipase breaking down triglycerides into fatty acids and monoglycerides. Additionally, nucleic acids are digested by ribonuclease and deoxyribonuclease in the pancreatic juice, with further digestion by brush-border enzymes.

The small intestine is also the primary site for absorption of the end products of digestion. The movement of villi allows these products to come in contact with the absorbing membrane. Monosaccharides, amino acids, fatty acids, water, electrolytes, vitamins, and minerals are all absorbed in the small intestine.

In summary, the physiology of the gastrointestinal system involves a complex interplay of mechanical movements and enzymatic actions that enable the digestion and absorption of nutrients, ensuring the body's sustenance and survival.