Digestion and Absorption

CHAPTER 4

Digestion and Absorption

Overview of the Digestive System

Digestive system is composed of two parts:

(1)    The alimentary canal or the gastrointestinal (GI) tract

(2) The accessory digestive organs.

The alimentary canal or the gastrointestinal (GI) tract

It is a continuous tube that extends from the mouth to the anus. Organs of the alimentary canal or the gastrointestinal (GI) tract are:

• Mouth
• Pharynx
• Esophagus
• Stomach
• Small Intestine
• Large Intestine

The lenpth of the alimentary canal is about 5-7 meters

The accessory Digestive Organs

  The accessory digestive organs include:

(i) Teeth

(ii) Tongue

(iii) Salivary glands

(iv) Liver

(v) Gall bladder

 (Vi)   Pancreas

Functions of Digestive System:

• Ingestion: This process involves taking foods and liquids into the mouth (eating).

• Secretion: Alimentary canal and accessory digestive organs releases a water, acid, buffers and various enzymes space of the canal.

•  Mixing and propulsion: Alternating contractions and relaxations of smooth muscle in the walls of the GI tract mix. Food material and various secretions from alimentary canal and accessory digestive organs are mixed with movement of GI tract.

• Defecation: Indigestible, waste material, which cannot be absorbed is excreted out through the anus. This process is called defecation. The waste material which is thrown out is called as feces

Digestive Enzyme

• Digestive enzymes produced by the salivary glands, tongue, stomach, pancreas, and small intestine. These enzymes are responsible for catalyzing various reactions in alimentary canal.

• The various enzymes secreted by various parts of GI tract and their roles are as follows:

• Salivary amylase: It is secreted by salivary glands. This enzyme is responsible for breakdown of starch.

• Lingual lipase: It is secreted by lingual glands in tongue. This enzyme is responsible for breakdown of triglycerides.

• Pepsin: It is secreted by stomach. This enzyme is responsible for breakdown of proteins.
Pancreatic lipase: It is secreted by pancreas. This enzyme is responsible for breakdown of triglycerides

Digestion in Mouth

• Chewing of food material which we take in mouth, is carried out with the help of the tongue. The food material minced by the teeth and mixed with saliva. This activity leads to softening of the food which is too easy to swallow. This softened mass of food is called a bolus (lump).

• Food material starts dissolving in the water in saliva.

• Salivary amylase, which is secreted by the salivary glands, initiates the breakdown of starch.

• Lingual lipase, which is secreted by lingual glands in the tongue. This enzyme becomes activated in the acidic environment of the stomach and thus starts to work after food is swallowed.

Digestion in the Stomach

• After food enters the stomach, movement of stomach mixes food with secretions of the gastric glands and reduce it to a soupy liquid called chyme.

• Digestion by salivary amylase continues in stomach. Acidic environment in stomach activates lingual lipase, which breaks triglycerides into fatty acids and diglycerides.

• Pepsin in stomach digests the proteins.

• Gastric lipase breaks the short-chain triglycerides in fat molecules. 

Digestion in the Small! intestine

• Carbohydrates like sucrose, lactose, and maltose are broken down in small intestine.

• Sucrase breakdowns sucrose into a molecule of glucose and a molecule of fructose.

• Lactase digests lactose into a molecule of glucose and a molecule of galactose; and

• Maltase breaks maltose and malt triose into two or three molecules of glucose, Digestion of carbohydrates ends with the production of monosaccharides, which the digestive system is able to absorb.

• Protein digestion is completed by two peptidases in the brush border: aminopeptidase and dipeptidase.

• Aminopeptidase cleaves off the amino acid at the amino end of a peptide.

• Dipeptidase splits dipeptides (two amino acids joined by a peptide bond) into single amino acids.

Absorption

1. Route of these digested nutrients from the alimentary canal or gastrointestinal tract into the blood or lymph is called absorption.
2. The materials are absorbed through processes like diffusion, facilitated diffusion, osmosis, and active transport.
3. About 10% of nutrients are absorbed in stomach and large intestine while remaining90% nutrients are absorbed in the small intestine.

Assimilation of Food

1. Assimilation of food starts only after digestion of food. One has to remember that food digested is not food estimated. Carbohydrates and proteins are digested first, in the stomach by the action of strong acid and enzymes, while the fats digested to fatty acids in intestine.
2. Assimilation takes place only in small intestine. Intestine is provided with tiny projections known as microvilli on the preface of the cells lining the intestine which are known as epithelial cells. These epithelial cells take nutrients from intestine and pump into the blood, where from it is distributed to body for growth and repair.