Digestion and Absorption

Absorption and Assimilation

So once you eat your food, how does the body extract the ingredients that it requires? Would you be surprised to know that this function actually happens in your small intestine? The process is known as Absorption and Assimilation. Let us learn about it here.

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Absorption and Assimilation

As the food gets broken down into smaller and simpler particles, it has to get absorbed into the bloodstream. It is only through the blood and circulatory system that the digested food reaches various cells and tissues. Blood coming from the digestive organs carries simple sugars, glycerol, amino acids, and a few vitamins and salts to the liver. The liver stores and processes these substances. They are also detoxified here. It delivers the nutrients to the rest of the body, as and when needed. This entire process is summed up as Absorption and Assimilation.

The process through which the end products of digestion are absorbed into the blood or lymph from the intestinal mucosa is called as Absorption. This process occurs either by the passive, active or facilitated transport mechanisms in the body. The small intestine is the organ where absorption occurs. It is specially adapted to carry out this function.

Absorption and Assimilation

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What happens to the Digested Food in the Small Intestine?

The small intestine has special cells that help absorb nutrients from the intestinal lining into the bloodstream. It has many physiological features that help in this absorption process. It is a long convoluted tube-like organ, which is around ten feet in length and has a diameter of one inch. There is a thin membrane called the mesentery that surrounds the small intestine and anchors it in place.

Many blood vessels, lymphatic vessels, nerves pass through the mesentery. They provide support to the tissues of the small intestine and also help in the transport of nutrients from intestines to the rest of the body.

The small intestine is divided into three regions:

  • Duodenum- It is the first section which connects to the pyloric sphincter of the stomach. It is the shortest region of the intestine. The chyme gets mixed with bile and pancreatic juice here.
  • Jejunum – It is the middle section and is the primary site of nutrient absorption. This region measures around 3 feet in length.
  • Ileum – It is the final section of the small intestine that empties into the large intestine. At a length of 6 feet, it completes the absorption of the remaining nutrients.

(Source – Wikimedia Common)

Absorptive area of the Small Intestine

The huge absorptive surface area of the small intestine is due to the presence of the mucosal folds, villi, and microvilli. These are the three distinguishing features of the small intestine. The rest of the structure is similar to the other areas of the alimentary canal.

The villi and microvilli are exposed to the intestinal lumen. The vast microvillar surface creates a brush border. This increases the rate of nutrient absorption to a great extent. Through the epithelium of the villi, the digested food molecules pass from the lumen of the intestine to the blood capillary network or lacteals.

Amino acids and monosaccharides that enter the blood capillary network are carried away by the blood. But, larger molecules from the digestion of fat enter the lacteal. These are then emptied into the lymphatic system, which eventually discharges its contents into the blood system.

Through the processes of Osmosis and Diffusion, water and fatty acids are absorbed. The other nutrients such as glucose, amino acids, & minerals are absorbed by active transport.


In the process of absorption and assimilation after digestion and absorption, the nutrients that are present in the blood reach the target cells and tissues which utilize them for their activities. This process of synthesizing the biological compounds (macromolecules) from the absorbed simple molecules is called assimilation.  It helps in the cell growth and development and new cell production.

Solved Questions For You

Q: Why are villi present in the small intestine and not in the stomach?

Ans:  The small intestine is mainly responsible for the absorption process. The villi and microvilli increase the surface area of absorption. The stomach, on the other hand, is an organ that primarily stores food temporarily along with the digesting proteins. Hence the small intestine has villi and not the stomach.

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