Digestion in Small Intestine

Human Physiology

Lesson 20 : Digestion In Intestines

Digestion In Small Intestine

Small intestine is divided into three segments namely duodenum, jejunum and ileum. Duodenum receives acidic chyme from stomach through pyloric sphincter. Wall of duodenum contains numerous mucus secreting glands called Brunner's glands. Enormous mucus secreted by these glands protects the duodenal wall from erosive action of acidic chyme. Duodenum receives digestive secretions from two organs i.e. liver contributing bile juice and pancreas contributing pancreatic juice. Two main functions of small intestine are

  1. digestion by enzymes secreted from pancreas and intestines and.
  2. absorption of digested food products.

Liver synthesizes bile juice which is a yellowish in colour, alkaline in pH and contains bile salts, bile pigments several kinds of lipids and electrolytes. Bile is collected from liver by numerous bile ducts in diluted form and is poured into gall bladder for storage. In gall bladder, apart from storage, bile gets concentrated with absorption of water and mucus is added. Bile from gall bladder is discharged into duodenum through cystic duct. The discharge of bile coincides with arrival of chyme into duodenum and the secretion is stimulated by cholecystokinin hormone which is released from duodenum in the presence of chyme.

Digestive function of bile juice resides in its two components. The bicarbonate ions helps to neutralize the acid present in chyme and provides favourable near neutral pH required for action of pancreatic and intestinal enzymes. Second component of bile is bile salts (glycol-cholates and tauro-cholates of sodium) which help in emulsification of dietary lipids. Lipids, due to their poor miscibility with water (aqueous medium), assumes shape of lipid globules, rendering them unexposed to various pancreatic and intestinal enzymes. Bile salt, by their detergent action, reduces the surface tension and breaks fat globules into finer droplets, thus making an emulsion of fat in water (Fig 1). In this form, the lipids easily interact with enzymes and digested. There is no specific digestive enzyme present in bile of animals.

Pancreatic juice is a mixture of several digestive enzymes helping in digestion of carbohydrates, proteins, lipids and nucleic acid. Apart from enzymes, the pancreatic juice is rich in sodium bicarbonate which helps in neutralizing acidity of chyme. Pancreatic enzymes are also secreted after the entry of chyme in duodenum under the influence of hormone cholecystokinin or pancreozymin. Secretion of bicarbonates is influenced by secretion hormone. Both these hormones are secreted by the duodenal epithelial cells following entry of chyme. Most favourable pH for the activity of pancreatic enzymes is around 7.0-7.5. End products of digestion such as glucose (Fig 2), fructose and other simple sugars (from polysaccharides and starch digestion), amino acids (from protein digestion, Fig 3 & 4), fatty acids & glycerol (from lipid digestion, Fig 5) and nucleotides (from DNA and RNA) undergo absorption in the ileum.

Some of the pictorial representations are as follows:

  1. Emulsification and Digestion of Fats
  2. Digestion of Starch
  3. Digestion of Proteins
  4. Action of proteases
  5. Action of Lipases
Last modified: Wednesday, 11 April 2012, 6:53 AM