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Thursday 22 April 2010

Arteries, Veins and Capillaries

The vascular system is like a road system. As the heart beats, blood is propelled into large arteries leaving the heart and then moves into smaller and smaller arteries until it reaches the arterioles which feed the capillary beds in the tissues. Capillary beds are drained by venules which empty into veins, that eventually empty into great veins that enter into the heart.

Arteries carry oxygenated blood away from the heart to the vital organs and tissues, this means that the arteries walls need to be thicker than veins as they need to be able to expand as the  blood is pumped through them. The walls need to be strong and stretchy as they are continuously under changes in pressure.
Arteries do not have valves (except for the semi-lunar valves of the pulmonary artery and the aorta).

Veins tend to have thinner walls than arteries as the blood flowing through them is deoxygenated and under low pressure. Veins are far from the heart in the circulatory pathway. They tend to have less muscle than arteries and contain valves. The valves throughout the main veins prevent blood flowing in the wrong direction. Veins drain blood from the tissues and return the deoxygenated blood back to the heart.

Capillaries are extremely tiny and narrow. They are found in most organs and tissues of the body. They are supplied with blood from the arterioles and drained by venules. Capillary walls are one cell thick and this enables exchanges of oxygen, carbon dioxide, water and salts etc between blood and the surrounding body tissues.-www.ivy-rose.co.uk



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