Blood Vessels

• The circulatory system is composed of the heart, which acts as the system's pump, and blood vessels such as arteries, capillaries, and veins.
• Arteries carry blood away from the heart, capillaries exchange nutrients and oxygen with the tissues, and veins carry the blood back to the heart.
• The structure of each type of vessel is related to its function, with arteries carrying blood directly from the heart, the blood inside them being at high pressures due to being recently pumped out of the ventricles.
• Arteries need to be both strong and elastic to bear these high pressures, with a thick layer of muscle tissue to keep them strong and a thick layer of elastic tissue to stretch and recoil.
• The wall of the artery is quite thick when compared to the lumen, which is the hole or space in the middle of the vessel.
• Once the arteries have transported the blood to the different parts of the body, it enters the capillaries, which are small vessels that come into close contact with all of the cells in our body, exchanging substances with them.
• Capillaries are adapted to this function, with walls that are only a single cell thick and are permeable, allowing substances to diffuse through them easily.
• The lumen of an individual capillary is small, but the total cross-sectional area of all the capillaries added together is large, far higher than for arteries, meaning that the pressure of the blood is much lower and it flows more slowly, giving it time to exchange things with the tissues.
• Capillaries join up into larger vessels called veins which transport the blood back to the heart.
• Veins are relatively large with thin walls, and have valves to prevent blood from flowing backwards, ensuring that it always flows towards the heart.
• To calculate the rate of blood flow, divide the amount of blood that has flowed by the time it took, for example, 2500 ml of blood passing through the aorta in eight minutes would result in a rate of blood flow of 320 milliliters per minute.