A human digestive system is a group of organs. The primary goal of the digestive system is to break down large food molecules into smaller ones. The human body absorbs these small molecules easily. It is from the food we consume that we derive energy to perform our daily chores. Also, the food that we eat helps to carry out the growth, repair, and maintenance of the body. Cells in the human body utilise biomolecules and enzymes, and hence, carry out the respiratory chain. All these biomolecules and enzymes are derived from the food that we consume. How is the food that we eat made available to each and every cell of the body? This is made possible only with the help of our digestive system, with the help of which food is digested and absorbed in our body.
Let’s discuss the important functions of the human digestive system.
Parts of the Digestive System
We voluntarily put the food in our mouth, chew it, and the chewed food mixed with saliva (also known as the bolus) enters the food pipe. We might stop eating after some time, but the process of digestion goes on for hours until the waste products are eliminated. 
The human digestive tract, also known as the Gastrointestinal Tract, consists of the alimentary canal and the associated glands. The alimentary canal begins with the mouth. The mouth is an anterior opening and leads to the buccal cavity. Teeth and tongue are located in the buccal cavity. Next to the buccal cavity is the pharynx. The pharynx has the opening for the oesophagus and the trachea (windpipe). The gastro-oesophageal sphincter is present at the opening of the oesophagus in the stomach. The stomach is divided into three segments; upper cardiac (in which the oesophagus opens), middle fundus, and the lower pyloric region. It is the pyloric segment of the stomach which leads to the duodenum; a pyloric sphincter regulates the opening of the stomach into the duodenum. The small intestine is also divided into three parts. The duodenum represents the first part of the small intestine. The middle coiled segment of the small intestine is the jejunum, and the highly coiled part is the ileum. The ileum has its opening in the large intestine. The large intestine is also divided into three parts, namely, caecum, colon, and rectum. The first part of the large intestine, caecum, leads into the colon; which is further segmented into the ascending, transverse and descending parts. The descending part of the colon extends into the rectum. The rectum leads into the anus.
Functions of the Digestive System
Now that we have an overview of the parts of the human digestive system, we will now talk about the functions of the digestive system.
The human digestive system not only makes digestion of the food possible but also its assimilation and elimination.
Digestion is the process whereby the complex substances are converted into simpler ones, which are easily absorbed. Various chemical and mechanical processes aid the very process of digestion. The mechanical processes begin right at the moment you put food in your mouth. Since chewing is a physical process, it is referred to as a mechanical process. Chemical digestion also initiates when the food is in your mouth. In mouth, the food gets mixed with saliva; which contains salivary enzymes. These salivary enzymes start breaking down food molecules. When the food reaches your stomach, chemical processes start taking place. The food, now, starts mixing with various digestive juices.
1. Saliva helps in proper mixing and mastication of the food particles. The food mixed with saliva is called bolus. The saliva contains the electrolytes, Na+, K+, Cl-, and HCO3-, and salivary enzymes, salivary lipase, and lysozyme.
- The salivary amylase breaks down the starch molecules. The salivary amylase, which is present in saliva, is also called ptyalin. Starch is broken down into maltose and dextrin. Further digestion takes place in the small intestine. Nearly 30 per cent of the starch is digested in the oral cavity itself.
Starch + Salivary amylase → Maltose
- Salivary lipase aids in the digestion of the fat. The salivary lipase plays a significant role in infants; because the pancreatic lipase is still not developed in infants.
- The lysozyme of the saliva plays the role of an antibacterial agent; hence, it prevents infections.
- The electrolytes/ions maintain the acidity of the oral cavity between the pH range of 6.2–7.4; as the ions present in the saliva act as an effective buffer.
2. The bolus passes through the oesophagus into the stomach. The bolus is pushed through the intestine by the alternating waves of muscular contractions called peristalsis.
3. When the food reaches the stomach, it stores it for nearly 4 to 5 hours. The gastric glands, which are present in the stomach, secrete mucus, proenzyme pepsinogen, HCl, and intrinsic factor. The acidic gastric juices, which are released by the gastric glands, mix completely with the food. The mixing occurs with the help of churning movement of the stomach, and the food is now, called chyme.
- HCl creates an acidic environment for the digestion of the food. The acidic environment is required for the efficient functioning of the pepsin enzymes. The proenzyme, pepsinogen, is converted into the active form, pepsin, only after the action of the Hydrochloric acid (HCl).
- Once pepsin is activated, it starts digesting proteins; and converts the proteins into peptones and proteases.
- The mucus and the bicarbonates secreted by the gastric glands protect the lining of the stomach from the highly acidic HCl.
- The intrinsic factor is critical for the absorption of the vitamin B12.
- Another proenzyme, prorenin, is secreted in infants. The proenzyme is converted into an active form, renin, also by the action of the HCl. Renin is responsible for the conversion of casein into peptides.
- Besides the above gastric enzymes, gastric lipases are also secreted by the gastric glands in small quantities.
4. Now, the chyme reaches the small intestine. The chyme passes through the small intestine via the process of peristalsis only. The partially digested food, chyme, is further mixed and churned in the small intestine. The enzymes/juices released by the pancreas and the liver are also released in the small intestine itself. Therefore, the intestinal juice also contains pancreatic enzymes, bile juice, and mucus. The intestinal juice contains disaccharidases (e.g., maltase), dipeptidases, lipases, nucleosidases, etc. The pancreatic juice includes the proenzymes- trypsinogen, chymotrypsinogen, procarboxypeptidases, amylases, lipases, and nucleases. The bile carries bile pigments (bilirubin and biliverdin), bile salts, cholesterol, and phospholipids. However, the bile does not have any enzymes present in it.
- The intestinal juice is also known as the succus entericus.
- The enzyme enterokinase, which is released by the small intestine, activates trypsinogen into trypsin. The active form, trypsin, further activates other proenzymes present in the pancreatic juice.
- Bile aids in the break down of fat into smaller molecules called micelles. This process is known as emulsification of the fats. The lipases, which are present in an inactive form in the intestinal juice, are also activated by the bile.
- Since all these enzymes are optimum in an alkaline medium, hence, the bicarbonates, as well as the mucus, helps in creating an alkaline environment.
- Partially digested proteins are further broken down into dipeptides by the action of pancreatic enzymes.
- Carbohydrates and nucleic acids are hydrolysed into smaller compounds with the help of pancreatic enzymes; whereas, bile juices help in the breakdown of fats.
5. Once the biomacromolecules are broken down in the small intestine and absorbed by the blood, the undigested/unabsorbed compounds/food particles reach the large intestine. There is no digestion of food in the large intestine. However, it carries out some important functions;
- The leftover/undigested food is acted upon by the bacteria in the large intestine.
- The main function of the large intestine is the absorption of the water, electrolytes, minerals, and some drugs.
- Also, the mucus, which is secreted by the large intestine, adheres the undigested/unabsorbed/waste particle together. Besides this, it also helps in lubrication; for easy passage of the waste.
6. The undigested food is, now, known as faecal matter. It passes through the caecum to the rectum; where it is stored temporarily till egestion.
Absorption
When the process of digestion is completed, the next process which occurs is absorption. It is only through the process of the absorption that the broken down compounds go into the bloodstream. The simple nutrient molecules travel from the gastrointestinal tract into the blood or lymph vessels. The following mechanisms carry out the process of absorption:
- Active Transport,
- Passive Transport, and
- Facilitated Transport
Let’s discuss how absorption of different biomolecules takes place into the bloodstream
1. Glucose, amino acids and some electrolytes- via simple diffusion
2. Glucose and amino acids- via facilitated transport
3. Amino acids, monosaccharides (like glucose), and electrolytes (Na+)- via active transport
4. Fatty acids and glycerol- in the form of chylomicrons
