Describe the process of digestion in the small intestine in detail.

(N/A) The semi-digested food,known as chyme,enters the small intestine from the stomach through the pyloric sphincter. In the small intestine,chyme comes into contact with three major secretions: pancreatic juice,bile juice,and intestinal juice.
$1$. Mechanical Digestion: The muscularis layer of the small intestine generates various movements (peristalsis and segmentation) that facilitate the thorough mixing of food with these secretions.
$2$. Pancreatic Juice Action: Pancreatic juice contains inactive enzymes: trypsinogen,chymotrypsinogen,and procarboxypeptidases,along with amylases,lipases,and nucleases. Trypsinogen is activated by enterokinase (secreted by the intestinal mucosa) into active trypsin,which then activates other pancreatic enzymes.
- Proteins,peptones,and proteoses are acted upon by trypsin,chymotrypsin,and carboxypeptidase to form dipeptides.
- Carbohydrates (starch) are hydrolyzed by pancreatic amylase into disaccharides.
- Fats are broken down by lipases into diglycerides and monoglycerides.
- Nucleic acids are acted upon by nucleases to form nucleotides and nucleosides.
$3$. Bile Action: Bile contains bile pigments (bilirubin and biliverdin),bile salts,cholesterol,and phospholipids,but no enzymes. It aids in the emulsification of fats,breaking them down into small micelles,which increases the surface area for lipase action.
$4$. Intestinal Juice (Succus Entericus) Action: The final steps of digestion occur in the jejunum and ileum,where enzymes in the intestinal juice convert complex molecules into absorbable simple forms:
- Dipeptides $\xrightarrow{\text{Dipeptidases}}$ Amino acids
- Maltose $\xrightarrow{\text{Maltase}}$ Glucose + Glucose
- Lactose $\xrightarrow{\text{Lactase}}$ Glucose + Galactose
- Sucrose $\xrightarrow{\text{Sucrase}}$ Glucose + Fructose
- Nucleotides $\xrightarrow{\text{Nucleotidases}}$ Nucleosides $\xrightarrow{\text{Nucleosidases}}$ Sugars + Nitrogenous bases
- Di and Monoglycerides $\xrightarrow{\text{Lipases}}$ Fatty acids + Glycerol
The resulting simple components are absorbed in the jejunum and ileum,while undigested and unabsorbed substances pass into the large intestine.