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Cardiovascular system
Components
The circulatory system consists of the heart, blood vessels
(arteries, arterioles, veins, venules, capillaries, lymph capillaries
and lymph vessels) and blood (red and white blood cells, plasma).
Functions
Exchanges materials and heat vital to the functioning of all
other systems.
Transports oxygen, carbon dioxide, inorganic ions
(electrolytes), hormones and wastes throughout the body.
Blood clotting helps to maintain barriers to invaders and
prevents fluid loss.
White blood cells use phagocytosis and antibodies to defend
the body against invaders.
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Key mechanisms
Blood circulation
The heart circulates blood around the body through all systems
via a network of arteries, capillaries and veins, assisted by lymph
vessels and capillaries.
Arterial circulation is maintained by the heart.
The heart is mainly cardiac muscle which contracts and
relaxes continuously to pump blood to the body. One-way flow of blood is maintained by
valves. It flows from the right side of the heart to the lungs (pulmonary circulation) and is
pumped from the left side of the heart to all other parts of the body (systemic circulation). Blood
under low pressure from veins is pumped from atria into ventricles, filling and expanding them.
It is put under high pressure when ventricles contract and force it through arteries and
arterioles (small arteries) into organs.
Capillary circulation is maintained by osmosis.
Blood enters the capillaries under high hydrostatic pressure generated by the contraction of
heart muscle. Plasma is filtered through the thin walls of the capillary endothelium (single cell
thick) to bathe cells in tissue fluid. Dissolved molecules (including oxygen), ions, hormones and
nutrients such as glucose are carried to cells. Proteins are retained lowering the osmotic
potential of the plasma. As blood flows through capillaries fluid is lost so that the hydrostatic
pressure falls. Water is reabsorbed back into capillaries when the osmotic pressure exceeds
the hydrostatic pressure, maintaining blood flow through the capillaries and into venules (small
veins).
Venous and lymph circulation are maintained by skeletal muscle and valves.
Blood flows into venules from capillaries and thence into veins. Excess tissue fluid that does
not re-enter capillaries drains into lymph capillaries and then into lymph vessels at very low
pressure. Lymph contains no red blood cells and very little protein. Veins and lymph vessels
are organised to pass through muscles. When muscles contract they compress veins and
lymph vessels. They contain valves to prevent backflow, so that blood and lymph are pumped
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See Defences: the immune system.
Figure 1 The circulatory system.
