Digestion in Small Intestine — Definition

Definition

The small intestine is a vital organ in the digestive system, a long, coiled tube extending from the stomach to the large intestine. It's where the bulk of chemical digestion and nutrient absorption takes place. Imagine it as the 'main processing plant' for the food you eat after it leaves the stomach. It's divided into three main sections: the duodenum, the jejunum, and the ileum.

When partially digested food, called chyme, leaves the stomach, it's highly acidic and still contains large macromolecules. It first enters the duodenum, which is the shortest segment. Here, it mixes with three crucial digestive juices: bile from the liver (stored in the gallbladder), pancreatic juice from the pancreas, and intestinal juice (succus entericus) secreted by the small intestine itself.

Bile helps in the emulsification of fats, breaking down large fat globules into smaller ones, making them easier for enzymes to act upon. Pancreatic juice is a powerhouse, containing enzymes like pancreatic amylase for carbohydrates, trypsin and chymotrypsin for proteins, and pancreatic lipase for fats.

These enzymes work in a slightly alkaline environment, which is provided by bicarbonate ions in the pancreatic juice, neutralizing the acidic chyme from the stomach.

As the chyme moves into the jejunum and then the ileum, the process of digestion continues with the help of enzymes embedded in the brush border of the intestinal lining. These 'brush border enzymes' are the finalizers, breaking down disaccharides into monosaccharides, small peptides into amino acids, and emulsified fats into fatty acids and glycerol.

Once these nutrients are broken down into their simplest forms, they are ready for absorption. The inner lining of the small intestine is not smooth; it has millions of tiny, finger-like projections called villi, and each villus has even tinier projections called microvilli.

This incredible folding dramatically increases the surface area, allowing for maximum absorption of nutrients into the bloodstream (for carbohydrates and proteins) and lymphatic system (for fats). So, in essence, the small intestine is where the food you eat is finally broken down into usable building blocks and then taken into your body.