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Exchange surfaces: 5. The function of villi in the exchange surface of the human small intestine

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5. The function of villi in the exchange surface of the human small intestine

See also Digestion and enzymes - section on human digestive system, metabolism and synthesis

The small intestine is about 7 m long, and is where dissolved digested food particles are absorbed from the digestive system into the bloodstream to supply the cells with the necessary nutrients.

The long length and large surface gives plenty of time for the soluble food molecules to absorbed into the bloodstream as the food moves slowly along. It takes at least 6-8 hours to travel through the small intestine.

The transfer through the partially permeable membrane might be by 'natural' diffusion down a diffusion gradient or by active transport against a diffusion gradient.

The partially permeable membrane regulates the transfer of substances.

The efficiency of the process is considerably increased by the structure of the small intestine  - adaptations:

(i) a single layer of surface cells - short diffusion time and distance - fast diffusion through the permeable membrane,

(ii) long length - increase contact time for breakdown and absorption of food molecules.

(iii) a large surface area for absorption - result of many small projections called villi which have microvilli to increase the surface area even more,

(iv) and a good blood supply from numerous capillaries that transport the nutrients away efficiently and maintain the concentration gradient in the direction of absorption.

all of which speed up the process, so read on for the transport details below in text and on diagram below.

Know and understand that the villi in the small intestine provide a large surface area with an extensive network of thin blood capillaries to absorb the products of digestion by diffusion and active transport.

The tissue lining in the small intestine is covered with millions of protuberances called villi, which poke up from the intestine surface into the partially or wholly digested food /mush'.

The villi consist of a single very thin layer of epithelial cells on the very large surface area of the intestine.

Both factors considerably speeds up the food molecule absorption process.

Each villus (of the millions of villi) has single layer of surface cells and each villus contains a multitude of fine blood capillaries into which the small digested food molecules can rapidly diffuse into and be absorbed into the body.

These molecules include amino acids, sugars like glucose, glycerol, fatty acids and important ions of sodium, iron and calcium.

A good blood supply is needed to efficiently carry the digested food away to where they are needed.

A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine.

The food molecules can diffuse into the bloodstream down a normal concentration gradient, but sometimes active transport is required.

For example ...

After a meal has been digested, the concentration of food molecules in the blood can be higher than in the intestine. In this situation, molecules are conveyed into the blood by active transport e.g.

When there is a higher concentration of glucose in the intestine than in the bloodstream, glucose molecules will naturally diffuse into the blood stream down the diffusion gradient (concentration gradient from higher to lower concentration).

However, if there is a lower concentration of glucose in the intestine, your body still needs glucose for respiration, therefore active transport must be deployed. This uses energy in such a way as to transfer glucose molecules from the intestine against the natural concentration (diffusion) gradient.

When more food is required simple sugars, amino acids, vitamins and minerals from digestion are actively transported into the villi from an area of lower concentration to an area of higher concentration, against the concentration gradient, and into the bloodstream of the many micro capillary blood vessels surrounding the villi.

As well as the uptake of glucose by epithelial cells of villi, this also applies to kidney tubules.

For more on active transport see Diffusion, osmosis and active transport

See also Enzymes - structure, functions, optimum conditions, investigation experiments

Summary of learning objectives and key words or phrases

Be able to describe and explain the function of villi in the exchange of substances on the surface of the human small intestine.

Describe how the large surface area to volume ratio makes transfer very efficient, but appreciate active transport is sometimes needed.



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