Digestive system in Humans Part 1

Contents of the topic:

Some terms to know e. g. Ingestion, Digestion, Absorption, Assimilation, and Defecation.

  • Human Alimentary Canal
  • Oral cavity-Selection, grinding, partial digestion
  • Oesophagus and Peristalsis
  • Stomach

Our cells need oxygen, water, salts, amino acids sugars, carbohydrates, fatty acids, minerals, dietary fibers, and vitamins. All of these can cross the cell membrane to enter the cell. Proteins sugars and lipids are rare in our environment. These are larger molecules and cannot cross the membranes. There is a requirement to convert such large and non-diffusible into small and diffusible molecules (that can cross the membranes). This can only be done by the process of digestion.

After digestion, the smaller and diffusible molecules from the digestive system reach body cells through the blood. Here they are assimilated (to get energy and to manufacture our own structures). At the same time, the indigestible part of food is excreted out of the body by the process of defecation. In order to understand the process of digestion let us learn the following terms.

  • Ingestion

It is the process of taking food.

  • Digestion

The process of breaking up larger and more complex substances into smaller and simpler ones is called digestion.

  • Absorption

Diffusion of digested food into blood and lymph.

  • Assimilation

The conversion of absorbed simple food into complex substances forms the structures of the body.

  • Defecation

The excretion of undigested food out of the body is termed defecation.


The human digestive system consists of a long tube that originates from the mouth and ends at the anus. This

tube is called the alimentary canal. Its major part are the oral cavity, pharynx, esophagus, stomach, small

Intestine and large intestine. In addition, there are many glands related to the alimentary canal. These

are in the form of three pairs of salivary glands, a pancreas, and a liver.

Here we will go through the structure and functioning of the digestive system by considering how a bite of bread taken with some dish (like mutton) is digested and how smaller and simpler molecules like amino acids, simple sugars, fatty acids, salts, vitamins, and water are provided to the cells.

Oral Cavity Selection and Partial Digestion

The oral cavity is behind the mouth and has many important functions in the whole process of digestion. Food selection is one of them. When food enters the oral cavity, it is, first of all, it is tasted and felt. If the taste of food which is eaten is suggested as old or impure, it is rejected. If teeth or tongue feel some hard object, such as dirt, we also reject that bite. The two senses (smell and vision) also assist the oral cavity in the selection of food.

The second function of the oral cavity is the grinding of food by teeth. It is so-called chewing or mastication. This is useful because the esophagus can pass only small pieces of food. Enzymes also cannot act on large pieces of food. Basically, enzymes need small pieces of food with a large surface area to attack.

The third and fourth functions of the oral cavity are lubrication and chemical digestion of food. Mastication or chewing stimulates the three pairs of salivary glands (under the tongue, behind the jaws, and in front of the ears) to release a fluid called saliva in the oral cavity. Saliva adds water and mucous to food and acts as a lubricant to ease the passage of food through the esophagus. Saliva also contains the enzyme salivary amylase, which helps in the partial digestion of cellulose (starch).

During the process of mastication, lubrication, and partial digestion, the pieces of food are rolled up by the tongue into a small, slippery, round mass called a bolus. We swallow the bolus and push it into the esophagus through the pharynx.

Oesophagus and Peristalsis

During swallowing, the bolus is pushed to the back of the mouth by the tongue. When the tongue pushes the bolus, the soft palate also moves upwards and to the rear. In this way, the opening of the nasal cavity is closed and the process of breathing stops during swallowing. When swallowed, the bolus passes the pharynx to enter the esophagus. The pharynx has the ability to prevent the entry of bolus particles into the trachea (windpipe to lungs). During swallowing, the larynx the top of the trachea moves upward and forces the epiglottis (a flap of cartilage ) into a horizontal position. Thus glottis i.e. the opening of the trachea is closed. The beginning of swallowing action is voluntary, but once food reaches the back of the mouth, swallowing becomes automatic.

After swallowing, food enters the tube called the esophagus, which connects the pharynx to the stomach. Both esophagus and pharynx do not contribute to digestion. They are only the passage of food from the mouth to the stomach. The previous salivary digestion lasts as food passes from the pharynx, esophagus, and then to the stomach.

Peristalsis moves food from the oral cavity to the anus. Peristalsis is defined as the waves of contraction and relaxation in smooth muscles of alimentary canal walls.

The Stomach

The stomach is a broad dilated part of the alimentary canal. It is J-shaped, located on the left of the abdomen, just below the diaphragm. The stomach has two major parts. A cardiac portion of the stomach is situated after the esophagus and the Pyloric portion is present below the cardiac portion. The cardiac sphincter is between the stomach and the esophagus while the pyloric sphincter is between the stomach and the small intestine. Bolus enters the stomach from the esophagus through the cardiac sphincter.

When food enters the stomach, the gastric glands found in the stomach wall are stimulated to secrete gastric juice. Gastric juice is composed mainly of mucous, hydrochloric acid, and a protein digestive enzyme called pepsinogen. Hydrochloric acid converts the inactive enzyme pepsinogen into an active enzyme i.e. pepsin. HCl can also germs present in food. Pepsin partially digests the protein portion of food into polypeptides and shorter peptide chains.

Pepsin is a powerful protein-digesting enzyme. A question arises here why does it not digest the stomach walls which are mainly composed of protein? The answer is that we saw pepsin is not in its active form, but rather it is released in an inactive form i.e. pepsinogen, which requires HCl for activation. The mucous of gastric juice creates a thick coating over the inner walls of the stomach and neutralizes the HCl there. It makes pepsinogen difficult to be activated and attack stomach walls. Hence stomach remains safe.

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