The blood vessels
- Arteries are adapted for carrying blood under high pressure away from the heart towards individual organs.
Thick wall, an outer layer of fibrous tissue for protection, a thick middle layer containing elastic tissue which allows stretching when blood surges (during systole) and recoils to continue pushing (during diastole) and muscle tissue which provides support but can also constrict or dilate, providing more or less blood to an organ. The endothelium provides a smooth inner surface which reduces the friction caused by blood flow through the lumen.The lumen is small, narrow which maintains high blood pressure and may be constricted or dilated. NOTE: the muscle in the arterial walls is not used to push the blood along, its contraction constricts (narrows) the artery.
- Veins are adapted for carrying blood under low pressure and returning it to the heart.
Thin wall, an outer layer of fibrous tissue for protection, a thin middle layer containing some smoth muscle and few elastic fibres since the blood is under low pressure. The endothelium of squamous endothelium provides a smooth inner surface, reducing friction as blood flows through. The large lumen makes it easier for blood to enter from the capillaries while friction is reduced as blood flows back to the heart. Veins have semilunar valves to prevent backflow of blood. Blood is squeezed along when skeletal muscles contract.
- Capillaries are adapted for the exchange of material between the blood and the tissue cells. The vast extensive network of capillaries slows the blood, providing a large surface area and giving time for the diffusion to occur.