Arteries carry oxygenated blood away from the heart.
Arteries consist of 3 walls inner, middle and outer. When the
heart contracts blood surges from the heart into the arteries
under high pressure.
Blood from the arteries passes into smaller arteries called
arterioles then smaller vessels called capillaries. The
diameter of a capillary is so small red blood cells need to travel
through the vessels in single file.
Vasodilation is an increase in the diameter of the blood vessel,
increasing the blood flow and the amount of oxygen to the
tissues. Allowing those tissues to benefit from increased blood
flow. (example: in the case of an injury or infection)
Vasoconstriction is a reduction in the diameter of the blood
vessels decreasing blood flow and the amount of oxygen to the
Capillaries are the site of fluid and gas exchange between the
blood and body cells.
Veins carry deoxygenated blood back to the heart.
Pressure in the smaller capillaries which lead to larger venules
is much lower than in the arteries, therefore a series of one-way
valves and skeletal muscles help venous blood flow.