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Circulatory system consists of heart and blood vessel.
Circulatory system is needed if the organism is too large (small total surface area/volume ratio) so that cells in the body can get enough oxygen.
Factors affect blood flow: vessel radius,fluid resistance, fluid viscosity
There are three different blood vessels namely vein, capillaries and arteries. Three of these blood vessels have different pressure depending on the size of their lumen. Arteries normally carry oxygenated blood to all parts of the body, (except pulmonary artery) so the pressure is higher because the lumen is smaller and more 'force' is needed to pump the blood to all parts of the body, vein has the lowest pressure because the lumen is larger and it loses its pressure as the pressure has been used to pump blood to all parts of the body. Veins normally carry deoxygenated blood, except pulmonary veins.