1. Small Intestine
  2. Large Intestine
  3. The Liver

In this section, you will learn and understand the digestive system of humans. First of all, you will know the parts of the small intestine and their functions. After discussing the small intestine we will discuss about the parts and functions of the large intestine and at the end the functions of the liver.

So let us first start understanding the small intestine.

Small Intestine

In the small intestine, there is complete digestion of meals takes place. The small intestine is a tubular part of the digestive system. It consists of the following three main portions,

  • Duodenum (25 cm)
  • Jejunum        (2.4 m)
  • Ileum (3.5 m)

Amongst the three portions of the small intestine, we first discuss the duodenum.


Duodenum is composed of the first 10 inches (25 cm) of the small intestine. It is that portion of the small intestine where maximum digestion takes place. In the duodenum, the meal is mixed with three different secretions:

  1. Bile from the liver helps in the digestion of fats through the process of emulsification. Emulsification is a process in which the droplets of lipids separate from each other.
  2. Pancreatic juice which is consists of three enzymes (1) lipase (2) amylase (3) trypsin.

Functions of three enzymes present in pancreatic juice

  • Lipase: helps in the digestion of lipids
  • Amylase: helps in the digestion of carbohydrates
  • Trypsin: helps in the digestion of protein
  • Intestinal juice present in intestinal walls contains many enzymes for the complete digestion of all kinds of food.


After duodenum jejunum is present. This portion of the small intestine consists of 2.4 meters in length. It is related to the rest of the digestion of proteins, carbohydrates, and lipids in our food.


This is the 3.5 meters long portion of the small intestine and is present after the jejunum. Its primary role is the absorption of digested food. Circular infoldings are present in the inner walls of the ileum. These infoldings have a very large number of finger-like projections called villi (singular villus).

Villi increase the surface area in the walls of the ileum and they help in the absorption of digested food. Each villus is richly supplied with blood capillaries and a vessel of the lymphatic system which is called a lacteal. The thickness of the walls of villi is only single-celled.

The digested food e.g. sugars, and amino acids, are absorbed from the intestine into the blood present in the villi. Blood carries these substances from the small intestine through the hepatic portal vein and goes to the liver. The liver filters these substances remove toxins and stores extra water.

From the liver, the required food material goes toward the heart through the hepatic vein. Lacteal carries them to the major lymphatic duct from where they enter the bloodstream.

Large Intestine

After the absorption of digested products of our meal in the blood, the rest mass of undigested food enters the large intestine. The large intestine is composed of the 3 main parts namely (a) the Caecum (b) the colon (c) the rectum. The caecum is a pouch and forms the T-junctions with the small intestine. From the colon, extra water is absorbed into the blood.  

When extra water is absorbed the solid remains of food are called feces.  Feces mainly consist of undigested material, a very large number of bacteria, sloughed cells of digested system, bile pigments, and water are also part of feces.

 Bile also contains pigments that are waste products of red blood cell destruction in the liver; these bile pigments are excreted out of the body. Feces are temporarily stored in the rectum, which opens out through the anus.

Under favorable conditions when the rectum is full of feces, it gives rise to a reflex and the anus is open for defecation. This reflex is voluntarily inhibited in adults but in children, it is controlled involuntarily. During growth, an infant learns to bring this reflex action under voluntary control.

An important  function of bacteria present in the colon:

Many bacteria live in the colon. They produce vitamin K, which is necessary for the coagulation of blood.

What is an appendix?

A non-functional finger-like process called appendix originates from the caecum. Inflammation of the appendix is created due to infection which causes pain. The infected appendix should be removed surgically, otherwise, it may burst and inflammation may spread in the abdomen.

Besides the elimination of feces or wastes from the body large intestine also absorbs water and salts.

Role of Liver

The largest gland of the body is the liver. It is reddish in appearance. In an adult human, its weight is about 1.5 kg. It is present below the diaphragm on the right side in front of the stomach in the abdomen. The gallbladder is present along the right ventral side of the liver. The gallbladder stores a fluid called bile juice, which is secreted by the liver.

Bile is secreted into the duodenum by the contraction of the gallbladder because it is a muscular organ. Bile does not contain any enzyme.  It just contains salts that help in the emulsification of lipids. Bile helps lipid-digesting enzymes attack fats.

Besides digestion, the liver performs a number of different functions, amongst them, there are 8 functions of the liver described in the following.

  • It produces heat which helps to maintain the temperature of the body.
  • Stores fat-soluble vitamins (A, E, D, and K) and mineral ions such as iron.
  • A blood clotting protein is produced by the liver.
  • The liver destroys the old red blood cells.
  • The amino group is removed from amino acids, termed (de-amination).
  • Converts harmful ammonia into less or non-toxic form e.g. Urea.
  • Converts carbohydrates and proteins into lipids and produces cholesterol.
  • Converts carbohydrates to glycogen, when there is a need to convert glycogen to carbohydrates.

A question arises as for as the modern age is concerned:

What are the harmful effects of soft drinks on the body?

Harmful effects of soft drinks,

There is a growing concern about the harmful effects of carbonated soft drinks. They are acidic in nature and make our bodies poor in oxygen. They contain phosphoric acid which dissolves the calcium from the bones, hence results weakening the bones. Caffeine is also present in colas which increases heart rate and causes hypertension i.e. high blood pressure.

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