What happens in the small inestine?

The small intestine is a vital organ in the human digestive system responsible for digesting and absorbing nutrients from the food we eat. The various processes performed by the small intestine include:

Mechanical Digestion: Peristaltic contractions, which are involuntary muscle movements, occur throughout the small intestine, propelling the partially digested food forward and creating a churning action. This mechanical action further breaks down the food into smaller particles, increasing the surface area for enzymes to work on.

Chemical Digestion: The small intestine is the primary site for chemical digestion, where various enzymes break down carbohydrates, proteins, and fats into smaller molecules that can be absorbed by the body.

1. Carbohydrates: Amylase, an enzyme produced by the pancreas and the cells lining the small intestine (enterocytes), breaks down complex carbohydrates like starches into simple sugars (e.g., glucose) that can be absorbed.

2. Proteins: Proteases, including trypsin and chymotrypsin from the pancreas, act on proteins, breaking them down into smaller peptide fragments. Aminopeptidases and dipeptidases, produced by the enterocytes, further hydrolyze these peptides into individual amino acids.

3. Fats: Pancreatic lipase works in conjunction with bile salts produced by the liver to break down triglycerides into free fatty acids and glycerol. The fatty acids can then be absorbed through the intestinal lining.

Absorption of Nutrients: The epithelial cells lining the small intestine have specialized structures that aid in the absorption of digested nutrients.

Microvilli: The epithelial cells are covered with millions of tiny finger-like projections called microvilli, increasing the surface area available for nutrient absorption.

Brush Border Enzymes: The microvilli contain digestive enzymes called brush border enzymes, including lactase, sucrase, and maltase, which break down complex sugars into simple sugars that can be absorbed by the enterocytes.

Active and Passive Transport: Some nutrients, such as amino acids and monosaccharides, are actively transported against a concentration gradient into the bloodstream with the help of carrier proteins. Passive transport mechanisms, such as diffusion and osmosis, facilitate the movement of other substances like water, electrolytes, and lipids across the intestinal barrier.

Specialized Transport Mechanisms: Various specialized transport mechanisms exist for certain nutrients. For instance, vitamin B12 is bound to intrinsic factor produced by the stomach, facilitating its absorption in the small intestine. Bile salts also aid in the digestion and absorption of lipids by forming micelles that help solubilize and transport fats.

After being absorbed by the enterocytes, the nutrients are further transported through the bloodstream to different cells and organs in the body for utilization or storage. Any undigested material along with water and mucus moves into the large intestine for further processing and eventual elimination.