Tuesday, September 9, 2008

Of Veins and Valves

What are veins?
Veins in the human body carry deoxygenated blood towards the heart, with one exception: the pulmonary veins. These veins carry oxygenated blood from the lungs to the heart.

The blood in veins flows slower and smoother than in arteries and the pressure, too, is much lower. Because of this, the walls of the veins contain less elastic fibres and muscles than in the walls of the arteries. The picture below shows a photomicrograph of the cross sections of an artery and a vein.


Picture taken from www.spjc.edu/clw/math_science/Nicotera/pnic/Nicotera/artery%20and%20vein%201.jpg

Skeletal muscles and veins
Due to the low pressure and slow flow of blood in veins, skeletal muscles that surround these veins help in moving the blood along. When skeletal muscles contract, they compress the veins, forcing blood to move along more quickly.

Role of valves in veins
Most veins contain a special feature which other blood vessels do not have. They contain valves. Valves prevent the backflow of blood and this is important in veins because as blood flows to the heart, it may momentarily be pulled backwards due to gravity or muscle contraction. These valves would then immediately close to prevent blood from flowing back. The presence of valves also ensures that blood flows in one direction only, which is, back to the heart.


Picture taken from www.merck.com/mmhe/sec03/ch036/ch036a.html

For an animation on this, click here. Observe the blood cells and see how the valves close when blood moves backwards momentarily.

Characteristic of venous valves
Venous valves are folds on the lining layer of a vein, shaped like half-moons.

Lastly, here's a short video clip taken from youtube, which compares the structures of arteries and veins, and discusses how blood moves along in veins.


References:

Written by Mrs Eunice Chan

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