4 Digestion and Nutrient Absorption

The digestive tract and its organs

Digestion transforms food into small molecules that can cross the intestinal lining. Absorption moves those molecules, as well as water, vitamins, and minerals, into the blood or lymph so they can be carried throughout the body.

The gastrointestinal (GI) tract is a continuous passage from mouth to anus. Food passes through the mouth, esophagus, stomach, small intestine, and large intestine; waste leaves through the rectum and anus. The liver, gallbladder, and pancreas are accessory organs: food does not pass through them, but their secretions aid digestion.

How food moves through the tract

Chewing breaks food into smaller pieces, while saliva moistens it and begins starch digestion. Swallowed food forms a soft mass called a bolus. Waves of muscular contraction called peristalsis move the bolus through the esophagus toward the stomach.

In the stomach, muscular mixing combines food with acid and enzymes to produce a semifluid mixture called chyme. The stomach begins substantial protein digestion and releases chyme gradually into the small intestine.

The small intestine consists of the duodenum, jejunum, and ileum. These regions receive digestive secretions and are the main sites of digestion and nutrient absorption. Folds, villi, and microscopic microvilli greatly increase the surface area available for absorption.

The colon, which is part of the large intestine, absorbs much of the remaining water and electrolytes. Gut microbes ferment some undigested carbohydrates, and the remaining material becomes stool.

Mechanical and chemical digestion

Mechanical digestion physically breaks food apart and mixes it with digestive fluids. Chewing, stomach churning, and intestinal mixing expose more food to enzymes. Peristalsis propels contents along the tract, while rhythmic mixing movements in the intestine bring contents into contact with the intestinal lining.

Chemical digestion uses enzymes to split large food molecules into absorbable units. Different enzymes act on particular nutrients:

• Carbohydrates: Salivary amylase begins breaking down starch in the mouth. Pancreatic amylase and enzymes on the small-intestinal brush border continue the process. Digestible carbohydrates are ultimately reduced to simple sugars, mainly glucose, galactose, and fructose.

• Proteins: Stomach acid unfolds proteins, and the enzyme pepsin begins cutting them into smaller peptides. Pancreatic and intestinal enzymes break them down further, chiefly into amino acids and small peptides.

• Fats: Bile disperses fat droplets in the small intestine, increasing the area available to enzymes. Pancreatic lipase breaks triglycerides mainly into fatty acids and monoglycerides.

Vitamins, minerals, and water generally do not need to be digested into smaller molecules, but they must still cross the intestinal lining to be absorbed. Some fiber resists human digestive enzymes and passes into the large intestine, where some types are fermented by gut microbes.

Digestive secretions and accessory organs

The pancreas supplies digestive enzymes and bicarbonate, which helps neutralize acidic chyme. The liver makes bile, and the gallbladder stores and releases it into the small intestine. Bile helps disperse dietary fat into small droplets; it is not an enzyme.

Absorption and transport

Most nutrient absorption takes place in the small intestine. Nutrients cross the lining through specialized cells called enterocytes, using processes such as transport proteins or diffusion. The route taken after absorption depends partly on the nutrient's solubility.

• Sugars and amino acids: These enter small blood vessels in the intestinal villi. Blood from the intestines travels first to the liver through the hepatic portal vein. The liver processes, stores, or releases absorbed substances before they circulate to the rest of the body.

• Most dietary fats: These are packaged inside intestinal cells into particles called chylomicrons. Chylomicrons enter lymph vessels in the villi, travel through the lymphatic system, and then drain into the bloodstream. Some smaller fat-related molecules can enter portal blood directly.

• Vitamins: Water-soluble vitamins generally enter blood. Fat-soluble vitamins (A, D, E, and K) are absorbed along with dietary fat and commonly travel in chylomicrons.

• Minerals: Minerals use different absorption pathways; for example, iron and calcium absorption is regulated according to the body's needs.

• Water and electrolytes: These are absorbed in both the small and large intestines. The colon reclaims much of the water remaining in intestinal contents.

An example of digestion and absorption

After a meal containing bread, eggs, and oil, starch is broken down into simple sugars, egg proteins into amino acids and small peptides, and much of the oil into fatty-acid-containing products. Sugars and amino acids enter portal blood and pass to the liver; most absorbed fat travels first through lymph in chylomicrons.

In general, mechanical digestion breaks food into smaller pieces and mixes it, while chemical digestion uses enzymes to make nutrients small enough to absorb. The small intestine is the main site of nutrient absorption. Sugars and amino acids generally travel in portal blood to the liver, while most dietary fats enter lymph before reaching the bloodstream. The large intestine absorbs remaining water and electrolytes and helps form stool.