a) The circulatory system transports substances around the body.
b) The heart is an organ and pumps blood around the body. Much of the wall of the heart is made from muscle tissue.
c) There are four main chambers (left and right atria and ventricles) of the heart.
d) Blood enters the artia of the heart. The atria contract and force blood into the ventricles. The ventricles contracr and force blood out of the heart. Valves in the heart ensure that blood flows in the correct direction. Blood flows from the heart to the organs through arteries and returns through veins. There are two separate circulation systems, one for the lungs and one for all other organs of the body.
e) Arteries have thick walls containing muscle and elastic fibres. Veins have thinner walls and often have valves to prevent back-flow of blood.
f) If arteries begin to narrow and restrict blood flow stents are used to keep them open.
g) In the organs, blood flows through very narrow, thin-walled blood vessels called capillaries. Substances needed by the cells in body tissues pass out of the blood, and substances produced by the cells pass into the blood, through the walls of the capillaries.
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