Transport in mammals
Adrian Whyte and Joanna Johnson, Contributor

Students make their way home after a hard day at school.

ALL CELLS require nutrients and most require oxygen as well. Wastes also need to be removed from the cell.

With a small organism this can easily be done through diffusion over the body surface but for larger or very active organisms, they need a transport system with a pump to ensure that the supply meets the demand of all cells, even those deep within the body.

In mammals, the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels, with substances being released out of, or into the blood as it flows through certain vessels called capillaries.

The heart pumps the blood along a series of tubes that are collectively called blood vessels.

There are types of vessels:
Arteries, Capillaries and Veins.

Arteries carry blood away from the heart to every tissue in the body. They have thick, elastic walls to withstand the high pressure of blood from the heart. Each time the heart beats it fires blood into the arteries at a high pressure, so they need to be tough so that they don't burst. They are also quite thick with only a small space, known as the lumen, down the centre. Fortunately arteries are provided with a tough outer layer and another layer inside this that can cope with the stretching the pulses of blood. This elastic layer is made up of elastic fibres and smooth muscle which contracts and helps to keep the blood moving along.

Section through an artery

Capillaries are the site of the exchange of materials between the blood and tissues. Capillaries are tiny, thin-walled vessels and they are made up of a single layer of endothelial cells around a very small lumen. Molecules can easily move into and out of the capillaries by diffusion. This allows food, gas and waste molecules to be taken to and from every cell in the body.

A section of a capillary with living cells surrounding it.

Veins take blood from every tissue in the body back into the heart. The blood returning from the body is at a much lower pressure than that which is leaving the heart, therefore veins do not need to be as strong as arteries. Veins have a cross-sectional structure that is very similar to arteries. One of the obvious differences is that they have a much wider lumen and thinner walls. The other main difference is that veins have valves inside them, while arteries do not. The valves occur occasionally along their length and ensure that blood can only travel in one direction.

Section through a vein

Section of a vein showing the valves

Q1. Why do large organisms need transport systems?

Q2. How does the structure of an artery relate to its role as a blood vessel?

Q3. Compare the structure of an artery with that of a vein.

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School
masterbio@gmail.com.