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1. The body's transport system

Blood travels around the body in over 75,000 miles of arteries, veins and capillaries. Stretched end-to-end, that's three times around the world! They transport oxygen from the lungs, remove carbon dioxide from the cells and carry nutrients, hormones and water to all parts of the body.

Picture 1.2 The structure of arteries, veins and capillaries

	Arteries	Capillaries	Veins
Carry blood:	away from the heart	from arteries to veins	towards the heart
Pressure:	high	low	very low
Walls are:	thick and muscular with elastic fibres	very thin and leaky - one cell thick	thin
Blood is:	oxygenated (except pulmonary artery)	exchanging nutrients and gases with cells	de-oxygenated (except pulmonary vein)
Pulse:	Strong	none	none

Humans are endothermic and keep a constant body temperature of 37 °C. The circulatory system plays a big part in regulating body temperature and it is self-repairing. A blood clot quickly forms to seal up a cut before new tissue grows to replace the damaged area.

Interactive graphic of circulatory system

Picture 1.3a Circulation system

Layout of the circulation

The circulation in humans, and other mammals, is a closed loop of tubes that run in a continuous figure of "8" centred on the heart.

The heart is the pump that keeps the blood flowing. This is called a "double circulation" system: one loop carries blood around the lungs . The blood is carried to the lungs in the pulmonary arteries and returns in the pulmonary veins.

The other loop supplies blood at high pressure to the head and body. The main artery from the heart is the aorta. It branches to carry blood to the head and body. The blood returns to the heart in the vena cava.

Picture 1.3b. Figure of 8.

Question 1

Look at each of the descriptions below and decide whether they relate to veins, arteries or capillaries. Make your choice using the buttons then press the 'Show answers' button to see how you got on.

		vein	artery	capillary
a)	Oxygen and glucose move out into the tissues
b)	The walls are under a lot of pressure
c)	Carbon dioxide is picked up and dissolves in the plasma
d)	contains valves to stop the backflow of blood
e)	The blood flows very quickly
f)	There is one specific vessel that carries oxygenated blood at a low pressure. Type your answer in the box (make sure you spell it correctly).