Physiology of Digestion: Have you ever wondered what happens to the food which we ingest? Food that we eat breaks down into its simpler form, which is called digestion and here we will know about the Physiology of Digestion. The digestive system includes the digestive tract and its accessory organs such as salivary glands, liver, gallbladder, and pancreas, which process food into molecules that can be absorbed and utilized by the cells of the body. The simpler form of food gives us nutrients and energy when absorbed. The entire alimentary canal is involved in the digestion of food and its absorption.

Physiology of Digestion- Definition

The process by which complex food is broken down into simpler substances or molecules that is suitable for absorption is called digestion. Like carbohydrates are broken down into simple sugars, e.g., glucose, protein is broken down to amino acids, fats are broken down to fatty acids and glycerol.

Physiology of Digestion- Process

Digestion of food is a very complex process. For proper utilization of food in our body, it needs to be properly digested. Most of our food is complicated chemical structures. Digestion of the following nutrients present in the food:-

Digestion of Carbohydrates

Carbohydrates are of three kinds- Monosaccharides, disaccharides and polysaccharides. Polysaccharides and disaccharides are broken down into monosaccharides. Starch and glycogen are polysaccharides, while sucrose, lactose, maltose are disaccharides. Glucose is a monosaccharide. Digestion of carbohydrates in different parts of the digestive tract-
In Oral Cavity– In the oral cavity, the salivary gland secretes saliva, containing a carbohydrate digesting enzyme salivary amylase or ptyalin. This converts starch into maltose, isomaltose and \(\alpha \)-dextrins.

About \(30\% \) of carbohydrates are digested in the oral cavity.

In Stomach– The digestion of carbohydrates does not take place in the stomach. Although gastric juice contains gastric amylase, its activity gets inhibited in the highly acidic medium of the stomach created by \({\text{HCl.}}\)

In the Small Intestine– In the first part of the intestine, i.e., duodenum, due to the stimulus received from the secretin hormone, Brunner’s gland secretes mucoid fluid. This fluid does not contain enzymes, but it neutralizes the effect of acids present in acidic chyme entering the duodenum until bile, and pancreatic juice is secreted. Mucus is secreted by goblet or mucous cells, which protects the wall of the intestine from being digested. Water and electrolytes are also secreted from the enterocytes present in the intestinal crypts.
(i) Pancreatic juice secreted from the pancreas contains pancreatic \(\alpha \)-amylase. This enzyme breaks down starch to maltose, isomaltose and \(\alpha \)-dextrin.

Pancreatic amylase is several times more powerful than salivary amylase. In the duodenum, almost all starches are digested to maltose and a few into polymers of glucose. Bicarbonate ions are also present in the pancreatic juice, which neutralizes the effect of acidic chyme.
(ii) Intestinal juice secreted from the walls of the intestine contains maltase, isomaltase, sucrase, lactase, and \(\alpha \)-dextrinase. These break disaccharides into monosaccharides.

Whatever carbohydrates are present in our food, almost \(80\% \) of it is digested to glucose as its final product.
In mammals, only human beings can digest the lactose of the milk in adults. However, few adults are unable to digest it due to the incapability of the intestinal wall to secrete lactase. If such adults take milk, then due to the absence of its digesting enzyme, it gets fermented in the intestine, producing gas and acids. This is called Lactose Intolerance. So, these adults can take curd instead of milk because lactose is fermented to lactic acid in curd. This acid in curd does not cause any digestive problems.

Fig: Digestion of Carbohydrate

Digestion of Proteins

Proteins are digested into amino acids. Protein digesting enzymes are collectively called proteases. Protein digesting enzymes are secreted in their inactive form and get activated at their site of action.
Saliva does not contain protein-digesting enzymes. So digestion of proteins does not take place in the oral cavity.
In stomach: Gastric glands secretes gastric juice, which contains proteases as proenzymes and \({\text{HCl.}}\) This \({\text{HCl}}\) provides an acidic medium \(\left({{\text{pH}}\,1 – 2} \right)\) in the stomach and kills the germs coming along with the food in the stomach. It also activates the proenzymes, i.e., pepsinogen and prorennin, into their active form, pepsin and rennin, respectively. The semi-digested food with gastric juice is called chyme. Mucus in the stomach protects the lining of the stomach from the actions of \({\text{HCl}}\) and proteases.

Pepsin breaks proteins into smaller molecules called proteases and peptones. Its action in the duodenum is ceased by the alkaline medium. Pepsin hydrolyses soluble casein to paracasein, which leads to milk coagulation. Rennin (rennet or chymosin) is also milk coagulating protease, but it is only present during infancy and is absent in adults. In calves also, it is present. Rennin also hydrolyses soluble casein to paracasein, which leads to milk coagulation.

We have summarized various reactions below:

In Small Intestine– Most of the protein digestion occurs in the duodenum. Chyme contains semi-digested proteins in the form of proteoses, peptones and large polypeptide chains. These are broken down into amino acids by different enzymes of the pancreatic juice and intestinal juice.

(i) Pancreatic juice contains inactive protein-digesting enzymes called proenzymes like trypsinogen, chymotrypsinogen and procarboxypeptidase, and enzyme elastase. Trypsinogen, chymotrypsinogen and procarboxypeptidase are collectively called pancreatic proteases. Intestinal juice secretes Enterokinase, which activates trypsinogen to trypsin. Now, this trypsin activates chymotrypsinogen and procarboxypeptidase to chymotrypsin and carboxypeptidase. Trypsin works efficiently in alkaline pH that is provided by bile.

All these reactions are summarized below:

Enzyme elastase digests elastin.

Although trypsin can digest protein, it cannot digest keratin protein from hair, nails. Some insects secrete enzymes that can digest fibrous proteins, due to which they are able to ruin the silk sarees or woollen clothes. Trypsin in predatory animals can hydrolyze fibrinogen of blood to fibrin which leads to blood coagulation.

(ii) Intestinal juice- It is also called succus entericus. This juice contains three proteolytic enzymes. They act on the semi-digested proteins and not on the naive proteins.
As discussed above, Enterokinase activates trypsinogen to trypsin, and so it is also called an “activator enzyme”. Other enzymes are aminopeptidases and dipeptidases. Reactions are summarized below:

Fig: Digestion of Proteins

Pepsin and Trypsin

1. Pepsin is a protein-digesting enzyme in the stomach, while trypsin is a protein-digesting enzyme in the intestine.
2. Pepsin functions in an acidic medium, while trypsin functions in an alkaline medium.

Digestion of Fats

Lipase is the enzyme that digests fats.
In Oral cavity
Saliva contains no fat-digesting enzymes.
In Stomach
Gastric juice contains gastric lipase, which converts fats into monoglycerides and fatty acids. It lacks fat-emulsifying agents. So, fats are largely digested in the small intestine.
In Small Intestine
Here bile is secreted from the liver, which helps in the emulsification of fats in which larger fat globules are broken into smaller fat globules, on which fat-digesting enzymes act effectively. Also, bile activates the enzyme lipase. Both pancreatic and intestinal juice contain lipase enzymes. Pancreatic lipase is also called steapsin. This enzyme converts triglycerides into diglycerides and then into monoglycerides, releasing fatty acid at each step. At last, all fats are converted into fatty acid and glycerol.

Fig: Digestion of Fat

Digestion of Nucleic Acid

Digestion of nucleic acid does not take place in the mouth and stomach. Its digesting enzymes are present in the intestine. Pancreatic juice and intestinal juice contain enzymes that can digest nucleic acids. Pancreatic juice contains nucleic digesting enzymes called nucleases, i.e., deoxyribonucleases (DNase) and ribonucleases (RNase).

Intestinal juice contains nucleosidases and nucleotidases.

Fig: Digestion of Macromolecules

Importance of Digestion

1. Digestion of food is important to get nutrients from food which helps the cells of the body to grow and repair.
2. It helps break down the complex food into its simpler form, which can be easily absorbed by the cells.
3. Simple sugars like glucose help in the formation of ATP in the body, which on hydrolysis, produces energy.

Summary

The process by which complex food is broken down into simpler substances or molecules is called digestion. Like carbohydrates are broken down into simple sugars, e.g., glucose, protein is broken down to amino acids, fats are broken down to fatty acids and glycerol. Carbohydrate digesting enzymes are present in the oral cavity, stomach and intestine.

Salivary amylase of saliva digests some of the carbohydrates into maltose, isomaltose and \(\alpha \)-dextrins. In contrast, rest carbohydrates are digested in the small intestine. Protein digestion takes place in the stomach and intestine and is known as proteases collectively. Fats are mainly digested in the small intestine. So, the digestion of food is important so that it can be easily absorbed by the body for its different functions.

FAQs on Physiology of Digestion

Q.1. How does digestion take place in the human body?
Ans: Digestion is the process by which complex food is broken down into simpler substances or molecules. Like carbohydrates are broken down into simple sugars, e.g., glucose, protein is broken down to amino acids, fats are broken down to fatty acids and glycerol.

Q.2. How many hours does food stay in the stomach?
Ans: Food stays \(4-5\) hours in the stomach.

Q.3. What is the physiology of digestion and absorption?
Ans: Digestion refers to the process of breaking complex food materials into simpler forms that are absorbed in the bloodstream called absorption. For example, Carbohydrates are broken down into simple sugars, like glucose which can be easily absorbed in the bloodstream and can be delivered to cells for generating ATP.

Q.4. Why do we digest food?
Ans: Digestion of food is important to get energy, growth and repair of cells. Glucose in the cell during cellular respiration helps in the generation of ATP.

Q.5: Is digestion a physiological process?
Ans: Yes, digestion is a physiological process. Digestion involves the physical and chemical breakdown of food which is needed for its easy absorption in the blood.

Q.6. What is the physiological role of the digestive system?
Ans: Digestive system helps in the digestion of food, i.e. complex food is broken down into its simpler forms so that it can be easily absorbed by the body.

Q.7. What food digests fastest?
Ans: Fruits are digested fast. 

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