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Pulmonary circulationand systemic circulationis shown on thisdiagram.

Remember pulmonary

circulation is thecirculation between theheart and lungs.

Systemic circulation isaround the body.

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The circulation in humans, and othermammals, is a closed loop of tubes that runin a continuous figure of "8" centred on theheart.The heart is the pump that keeps the bloodflowing. This is called a "double circulation"

system: one loop carries blood around thelungs . The blood is carried to the lungs inthe pulmonary arteries and returns in thepulmonary veins.The other loop supplies blood at highpressure to the head and body . The mainartery from the heart is the aorta . Itbranches to carry blood to the head andbody. The blood returns to the heart in the

vena cava .

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Arteries carryblood awayfrom the heartand deal withblood that has ahigh pressure.

Veins carryblood flowingback toward the

heart, they dealwith lowpressure blood.

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High pressure blood inarteries means theyhave to be much moreelastic. They havethick muscular walls to

deal with the pressureincrease every time theheart beats.

Veins carry blood that has

come from lots of capillariesin the working muscles. Thisis low pressure so the wallsare not as thick and they arenot as stretchy.

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Veins have one way valves so that the blood can travel up yourlegs and not pool in your feet.

Capillaries walls are so small that they are only one cell thick and the diameter of a red blood cell.

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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 elasticfibres

very thin and leaky - one cell thick thin

Blood is: oxygenated (exceptpulmonary artery)

exchanging nutrientsand gases with cells

de-oxygenated(except pulmonary vein)

Pulse: Strong none none

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Question 1

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

Oxygen and glucose move out into the tissues

The walls are under a lot of pressure

Carbon dioxide is picked up and dissolves in the plasma

Contains valves to stop the backflow of blood

The blood flows very quickly

For each statement decide if the answer is VEIN, ATERY or CAPILLARY.

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A cross section of blood vessels showing the thicknessof their walls.

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What are capillaries? Capillaries are extremely small vessels located within the tissues of the bodythat transport blood from the arteries to the veins .

Capillary walls are thin and are composed of endothelium (a single layer of overlapping flat cells). Oxygen, carbon dioxide, nutrients and wastes areexchanged through the thin walls of the capillaries.

Capillary Size Capillaries are so small that red blood cells can only travel through them insingle file.

5-10 microns in diameter.

http://biology.about.com/library/organs/heart/blarteries.htmhttp://biology.about.com/library/organs/heart/blveins.htmhttp://www.med.virginia.edu/medicine/basic-sci/biomed/ley/endothelium.htmhttp://www.med.virginia.edu/medicine/basic-sci/biomed/ley/endothelium.htmhttp://biology.about.com/library/organs/heart/blveins.htmhttp://biology.about.com/library/organs/heart/blarteries.htm

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ArteriesArteries carry oxygen-rich blood from your heart to the tissuesand organs in your body, like your brain, kidneys, and liver.Because they carry blood with oxygen, arteries appear red.Blood flows through arteries with great force. So the walls ofarteries are thick and flexible. The thicker walls help protect the

arteries against damage from the high pressure.Arteries get smaller and smaller as they get farther from yourheart. At their smallest point, arteries become capillaries.

CapillariesCapillaries connect arteries to veins. As the tiniest blood

vessels, capillaries carry blood to and from every cell inyour body. In an adult body, that means trillions of cells.Capillary walls are so thin that oxygen and nutrients canpass right through them into your body's cells. Wasteproducts and carbon dioxide from the cells can also passthrough the walls of capillaries back into your bloodstream .

Veins

Capillaries get larger and larger as they leave each celland quickly become veins. Veins carry the oxygen-poorblood back to your heart. Because they carry bloodwithout oxygen, veins appear blue. The walls of veins aremuch thinner than artery walls they don't have to be asthick because blood flows through veins at a lowerpressure.

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