Function: The Blood Circulation

Blood circulates in two linked circuits: the pulmonary, which carries blood to the lungs to be oxygenated, and the systemic, which supplies oxygenated blood to the body. Arteries carrying blood from the heart divide into smaller vessels called arterioles and then into capillaries, where nutrient and waste exchange occurs. Capillaries join up to form venules, which in turn join to form veins that carry blood back to the heart. The portal vein does not return blood to the heart but carries it to the liver.

Blood supply

In both the pulmonary and systemic circulations, the exchange of oxygen, nutrients, and waste products occurs in the capillaries that join arterioles to venules.

A double circuit

The heart powers the pulmonary and the systemic circulations. In the pulmonary circulation, deoxygenated blood (blue) travels to the lungs, where it absorbs oxygen before returning to the heart. This oxygenated blood (red) is pumped around the body in the systemic circulation. Body tissues absorb oxygen, and deoxygenated blood returns to the heart to be pumped to the lungs again.

Distribution of blood in the circulation

At rest, the veins act as a reservoir for blood, holding most of the body’s blood volume. If an increase in blood supply is needed, the veins constrict and return more blood to the heart.

Venous return

The blood pressure in the veins is about a tenth of that in the arteries. Various physical mechanisms ensure that there is adequate venous return (blood flow back to the heart). Many deep veins lie within muscles. When the muscles contract, they squeeze the veins and force blood back to the heart. The action of inhalation during breathing also draws blood to the heart. In addition, venous return from the upper body is assisted by gravity.

Respiratory pump during inhalation

While inhaling, the chest cavity expands, lowering the pressure in the chest. Higher pressure in the rest of the body pushes blood in the veins towards the heart.

Muscular pump

Muscles contract and relax as we move, squeezing the veins that pass through them and pushing blood back to the heart. One-way valves prevent backflow.

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

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