Why do capillaries need to be thin walled? so only a few nutrients can pass through so only a small amount of blood can flow through so they can go through the small spaces within the body so the exchange of gases and nutrients can happen

Being thin would not help to make only some nutrients: rather, it would help for as many nutrients as possible to pass! And this is the case as the function of the capillaries is to allow the exchange of nutrients between blood and tissue - the correct answer is:
so the exchange of gases and nutrients can happen

Capillaries need to be thin walled to exchange the gases and nutrients in blood and tissue.

Further Explanation:

Capillaries are tiny vessels of the blood vessels. They are very small even a single red blood cell cannot fit between them easily. The capillaries connect the veins and arteries to pass the certain molecule between the tissues and blood. That’s why tissues are very active like liver, kidney and muscle because of the presence of many capillaries in it. Connective tissue is less active because of less number of capillaries. They mainly link the arterial system of the body. Arterial system consist many blood vessels which mainly carry blood away from the heart to the venous.

The exchange of nutrients, waste material and oxygen between the tissue and blood occur in the capillaries. This exchange can occur in the capillaries by two methods:

Passive diffusion: It is considered as the movement of a molecule from the region of higher concentration to the lower concentration. Pinocytosis: It is considered as the process by which the cell of the body actively takes small molecules like proteins and fats.

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Answer Details:

Grade: High school

Subject: Biology

Topic: Capillaries

Keywords:

Capillaries, nutrient, fat, protein, blood, process, connective tissue, venous system, blood vessels, arterial system, liver, kidney, muscle, tissue.

Option B, Concept

Explanation:

Any general idea about anything is termed as concept. Concept depicts an abstract idea about any process, notion, observation, individual or beliefs that originates in the mind or speech or thoughts of any individual. Some believe that concepts are the building blocks of though and they play an essential role in cognitive way of thinking and processing information.

These concepts may or may not require basis for existence.

Hence, option B is correct.

A CONCEPT is the general idea of objects, events, animals, or people based on common features, traits, or characteristics.