The passage of nutrients and other substances into the walls of the gastrointestinal tract and then into the bloodstream is called

It is called Absorption

Explanation:

Absorption as correctly stated in the question is the passage of nutrients and other substances into the walls of the gastrointestinal tract and then into the bloodstream where it is transported to the various cells of the body for utilization. Apart from nutrients, drugs, toxins and water can also be absorbed into the body.

Let us take a look at how typical digestion occurs within the body. First it is important to note that only substances digested by the body are absorbed. Digestion is a process that helps to breakdown substances into smaller sizes before they are absorbed. In the body, for most food substances like carbohydrates, digestion actually starts in the mouth where enzymes (salivary amylase) act on the food. But summarily, digestion is brought about by:  

1.mechanical motions in the gut, including, chewing, and peristaltic motions

2.enzymes and

3.Chemicals such as bile, pancreatic juice etc.

After digestion the molecules are then absorbed, this occurs in the small intestine, and it involves the digested substance passing through the walls of the small intestine into the blood stream where they are transported to the parts of the body where they are needed. To make absorption effective there are some characteristics that the small intestine posses and these include:

1.A small cell layer: the small intestinal wall is one cell thick. This means that it is light enough to allow substances pass through into the blood vessels without having to travel through a thick wall. This reduces the time of absorption.

2.A large surface area: there are substances on the cell surface of the small intestine called microvilli (Singular microvillus) this increases the surface area of the intestine in contact with the substances to be absorbed, so that more substances can be absorbed in less time.

3.Has large number of blood vessels: blood capillaries are closely associated with the small intestine and this helps to increase the absorption gradient, because absorbed molecules are moved away quickly to create room for more substances to be absorbed.  

These special characteristics maximize absorption. If they were absent, the substances will move past the small intestine before they get a chance to the absorbed and they will eventually be egested.

Take a look at the pictures of the digestive system especially the small intestine, you will get a mental picture of how it looks and it will help your understanding.

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