What are the steps of chemical digestion in the small intestine?

Chemical digestion in the small intestine involves the breakdown of nutrients by enzymes into absorbable forms.

The small intestine is the primary site for chemical digestion and absorption of nutrients in the human body. This process begins in the duodenum, the first part of the small intestine, where the partially digested food from the stomach, known as chyme, is mixed with bile from the liver and gallbladder, and pancreatic juice from the pancreas.

Bile, which is alkaline in nature, neutralises the acidic chyme, creating a suitable pH for the action of digestive enzymes. It also emulsifies fats, breaking them down into smaller droplets, which increases their surface area for enzymatic action. The pancreatic juice contains a variety of enzymes including amylase, which continues the breakdown of carbohydrates into simpler sugars; lipase, which digests fats into fatty acids and glycerol; and proteases like trypsin and chymotrypsin, which break down proteins into peptides and amino acids.

The next stage of chemical digestion occurs in the jejunum and ileum, the remaining parts of the small intestine. Here, the lining of the intestine, or the mucosa, secretes enzymes known as brush border enzymes. These include disaccharidases, which break down disaccharides into monosaccharides; peptidases, which break down peptides into amino acids; and nucleases, which break down nucleic acids into nucleotides.

The final products of this chemical digestion - monosaccharides, amino acids, fatty acids, glycerol, and nucleotides - are then absorbed through the intestinal wall into the bloodstream, where they are transported to the cells of the body for use. This complex process of chemical digestion in the small intestine ensures that the nutrients in the food we eat are broken down into their simplest forms, ready for absorption and use by the body.