How small intestine and large intestine is main site of digestion of food in Humans


Small intestine:
Human small intestine is about 4 cm in diameter and 7 to 8 meter in length. It is intermediate in length between small intestines of typical carnivores and herbivores of similar size. It reflects the human’s omnivorous eating habits. The length of small intestine directly releases to the total surface area available for absorbing nutrients as determined by many circular folds and minute projections of the inner gut surface on the circular folds, thousands of finger like projections called villi project from each square centimetre of mucosa. 

These minute projections are so dense that the inner wall of human small intestine has total surface area of approximately 300 mm2 – the size of a tennis court. First part of small intestine called duodenum functions primarily in digestion. Next part is jejunum and the last part is ileum. Both function in nutrient absorption. The duodenum contains many digestive enzymes that intestinal glands in the duodenal mucosa secrete. The Pancreas secretes other enzymes. In duodenum digestion of carbohydrates and proteins is completed and most lipids are digested. 

The jejunum and ileum absorb the end products of digestion by active transport (amino acids, simple sugars, fatty acids, glycerol, nucleotides, and water). Sugars and amino acids are absorbed into the capillaries of villi, where as free fatty acids enter the epithelial cells of the villi and recombine with glycerol to form small droplets called chylomicrons which enter the lacteals of the villi. From lacteals chylomicrons move into the lymphatics and eventually into the blood stream for transport throughout the body. Small intestine absorbs water and dissolved mineral ions. Small intestine absorbs about 9 litres of water per day and large intestine absorbs the rest.

Large intestine:
Large intestine has small surface area. Small intestine joins large intestine near blind ended sac, the caecum, Human caecum and its extension, the appendix are storage sites and possibly represent the evolutionary remains of large functional caecum such as is found in herbivores. Appendix contains an abundance of lymphoid tissues and many function as part of immune system.

Working:
Functions of large intestine include re-absorption of water and minerals and the formation and storage of faeces. As peristaltic waves move food residue along, minerals diffuse or are actively transported from the residue across the epithelial surface of large intestine into the blood stream water follows osmotically and returns to the lymphatic system and blood stream. When water re-absorption is insufficient diarrhea results. If two much water is reabsorbed, faecal matter becomes too thick resulting in constipation.

Many bacteria and fungi exist symbiotically in large intestine. They feed on the food residue and further breakdown its organic molecules to waste products, in turn, they secrete amino acids and vitamin K, which the hosts’ gut absorbs. What remains faeces is a mixture of bacteria, fungi, undigested plant fibre, sloughed off intestinal cells and other waste products.

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