Digestive System

The digestive system performs four functions viz. ingestion, digestion, absorption and elimination.  Ingestion is intake of food. Digestion is of two types viz. mechanical (food is broken down into smaller pieces, this begins as soon as we put food in our mouth) and chemical (use of enzymes and acids to break down consumed food). Absorption is the assimilation of digested food in cells while elimination is passing out of what we cannot digest. The entire digestive system is made of alimentary canal and accessory digestive organs. Alimentary canal is made of salivary glands, Pharynx, Oesophagus, stomach, small intestine and large intestine.  Accessory digestive organs are liver, gallbladder and pancreas.

Digestion in Mouth Cavity

Salivary glands release saliva in mouth cavity which contains Salivary Amylaze enzyme that digests starch into sugars. Further, another enzyme called lingual lipase also begins digestion of the lipids / fats in mouth cavity only. Thus, while digestion of carbohydrates and lipids begins in mouth cavity, digestion of proteins begins only in stomach in highly acidic environment. Mouth cavity leads to pharynx that is common passage for food and air. When we swallow the food, the windpipe is closed by a flap of cartilage behind the root of the tongue. This flap is called epiglottis. Beyond epiglottis is Oesophagus, a food pipe which ends in stomach. At the junction of the oesophagus and stomach, there is a ring like muscle, called the oesophageal sphincter that relaxes and allows the food to pass through to the stomach. No digestion takes place in oesophagus.

Digestion in stomach

The Stomach has three mechanical tasks. To store the swallowed food, to mix up the food, liquid, and digestive juice produced by the stomach and to empty its contents slowly into the small intestine. Digestion of carbohydrates, proteins as well as fats takes place in stomach. We note here that least time is needed to digest carbohydrates, more for protein and maximum for fats.

Key Enzymes in Stomach

The main gastric enzyme is Pepsin which is secreted in inactive form called Pepsinogen. It is activated by stomach acid (HCl). It breaks protein into peptide fragments and amino acids. Here, Hydrochloric acid plays role to denture the proteins and kill any bacteria or viruses in the food. Another stomach enzyme is Gastric lipase. It works in acidic environment {other lipases such as pancreatic lipase work in alkaline environment} digests fats and lipids.

How stomach saves itself from acids secreted?

Stomach has highly acidic environment. To protect its own lining from digestion by digestive juices, it secrets Mucin and carbonate from its mucous cells. This is one way to save its own cells. Another way is a high turnover of stomach cells.

Function of Gastrin Hormone

Gastrin is an important hormone of G-cells of stomach. It stimulates stomach cells to produce hydrochloric acid (HCl) and another chemical called Intrinsic factor (IF).

Digestion in Small Intestine

Small intestine is largest part of digestive system (around 6 meters) and divided into three parts viz. the duodenum, jejunum and ileum.

By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme. A walnut-sized muscular valve at the outlet of the stomach called the pylorus keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Once entered into duodenum, the chyme comes into contact with pancreatic juice with a pH of approximately 8.5. Thus, the hitherto acidic environment of stomach (pH near 2) is turned into alkaline environment. Here we note that the neutralization of the acidity of the chyme is necessary for the functioning of the digestive enzymes that act in the duodenum. Further, without neutralization of the acidity of the chyme, mucous membrane of the intestine would be damaged.

What happens here is that the acidity of chyme stimulates production of a hormone called secretin in the duodenum. Secretin stimulates the pancreas to release pancreatic juice and also signals the gallbladder to expel bile in the duodenum. The pancreatic secretion, rich in bicarbonate ions, is released in the duodenum and neutralizes the chyme acidity; this acidity is also neutralized by the secretion of bile in the duodenal lumen.

Further, most of the chemical digestion of fats begins only in duodenum via so called emulsification process.

Role of Liver: Bile Juice and Emulsification

Bile, an emulsifier liquid, is made by the liver and later stored in the gallbladder and released in the duodenum. Bile is composed of bile salts, cholesterol and bile pigments. Bile salts are detergents, amphiphilic molecules, or rather, molecules with a polar water-soluble portion and a non-polar fat-soluble portion. This feature allows bile salts to enclose fats inside water-soluble micelles in a process called emulsification. Through this process, fats come into contact with intestinal lipases, enzymes that break them down into simpler fatty acids and glycerol.

Why patients with gall stones are not allowed to take fatty foods?

Bile is concentrated and stored in the gallbladder. When foods high in fat are ingested, the gallbladder contracts to release bile into the duodenum. This is the reason why patients with gallstones are advised to not to eat fatty foods, because the reactive contraction of the gallbladder may move some of the stones to the point of blocking the duct that drains bile into the duodenum, causing pain and other complications.

Other functions of Liver

Apart from making bile for releasing in small intestine, Liver is also a site for storing, processing and inactivating poisons in food. This work is done by a network of veins in the liver called mesenteric circulation. Liver also polymerizes glucose and stores it as Glycogen. It stores many vitamins and the iron absorbed in the intestine. It detoxifies poisonous substances such as alcohol, nicotine, drugs etc.

Role of Pancreas

The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also makes a substance that neutralizes stomach acid. The pancreatic juice is released into the mixture that contains the following enzymes to help chemically digest fats and carbohydrates:

• Pancreatic Lipase breaks apart fat molecules into fatty acids and glycerol.
• Pancreatic amylase breaks long carbohydrates into disaccharides, which are short chains of two sugars. The disaccharidases then break apart into monosaccharides that can be absorbed by the cells lining the small intestine.
• Trypsin and chymotrypsin are enzymes that break apart peptide fragments. After they break the peptides down into small chains, amino peptidases finish them off by breaking apart the peptides into individual, absorbable amino acids.

Intestinal Villi and Microvilli

After digestion, the next step is absorption by cells of the mucous membrane of the intestine. For this to happen, a large absorption surface is needed. This is done by two ways. Firstly, intestine itself is long and tubular and closely folded and numerous loop. Secondly, a more efficient process is done by intestinal Villi and the microvilli of the mucosal membrane cells. These are like gloved fingers which scale up the process of absorption by increasing absorption area manifold.

The majority of water, vitamins and mineral ions are absorbed by the small intestine.

Digestion in Large Intestine

The large intestine is not responsible for digestion but only for reabsorption of nearly 10% of ingested water, a significant amount that gives consistency to feces. If there is some disease in colon, water will not be absorbed and person will suffer from diarrhoea.

How food is assimilated?

The sugars, amino acids, mineral salts and water are taken from the capillary vessels of small intestine to mesenteric circulation. The blood from the mesenteric circulation distributes nutrients to tissues in body.

Role of vegetable fibres

Plant fibers are not absorbed by the intestine but play an important role in its functioning. They retain water inside the bowels and therefore contribute to the softening of the feces. Softer feces are easier to eliminate during defecation. People who eat less dietary fiber may suffer from hard stool and constipation.

Intestinal Microflora

Bacteria that live inside intestine play an important role in digestion. Some polysaccharides such as cellulose, hemicelluloses and pectin are not digested by digestive enzymes secreted by the body; instead, they are broken down by enzymes released by bacteria in the gastrointestinal tract. Intestinal bacteria also produce substances vital to the functioning of the bowels, facilitating or blocking the absorption of nutrients and stimulating or reducing peristalsis. Further, gut bacteria are the main source of vitamin K for the body and, as a result, they are essential for the blood clotting process.

Special features of Birds digestive system

The digestive system of birds contains special structures called crop, the proventriculus and the gizzard. Crop works as temporary storage of ingested food. Proventriculus is the chemical stomach of birds, in which food is mixed with digestive enzymes. Gizzard is a muscular pouch that serves as a mechanical stomach, in which food is ground up to increase the exposure area of the food particles to digestive enzymes.

Mutualistic Digestion of cellulose in Ruminants

Herbivorous animals eat large amounts of cellulose, which is not digested by their digestive enzymes. In such animals, one region of the digestive tract is colonized by microbes that digest cellulose. This type of digestion is found in horses, cows, and rabbits and also in some insects, such as termites.

Further, food ingested by cows and other ruminant animals first passes through two compartments of the digestive tract called the rumen and the reticulum. Within them, the food is subject to the action of digestive enzymes released by microorganisms that live there in a mutualistic digestion. In the reticulum, the food is broken down. After passing through reticulum, the food (cud) is regurgitated to the mouth to be chewed and swallowed once again in a process called rumination.

Disorders of Digestive System

• Appendicitis, an inflammation of the appendix, most often affects kids and teens between 11 and 20 years old, and requires surgery to correct. The classic symptoms of appendicitis are abdominal pain, fever, loss of appetite, and vomiting.
• Gastrointestinal infections can be caused by viruses, by bacteria (such as Salmonella, Shigella, Campylobacter, or E. coli), or by intestinal parasites (such as amebiasis and giardiasis). Abdominal pain or cramps, diarrheal, and sometimes vomiting are the common symptoms of gastrointestinal infections.
• Inflammatory bowel disease (IBD) is chronic inflammation of the intestines that affects older kids, teens, and adults.
• Hepatitis, a condition with many different causes, is when the liver becomes inflamed and may lose its ability to function.