Blood Vessels in Mammals

• The body contains four types of blood vessel that form a continuous network. The structure of each blood vessel allows it to carry out its function:

  1. Arteries: transport blood away from the heart (usually at high pressure) to tissues
  2. Arterioles: arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
  3. Veins: transport blood to the heart (usually at low pressure)
  4. Venules: these narrower blood vessels transport blood from the capillaries to the veins
• Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel. Arteries have a narrow lumen, and veins have a a wider one.
• The walls of each type of blood vessel have a structure that relates to the function of the vessel. Arteries, arterioles, veins & venules all have varying structural features.
• The walls of arteries and veins consist of three layers:

  1. tunica intima
  2. tunica media
  3. tunica adventitia (tunica externa)
• The tunica intima is made from an endothelial layer, a layer of connective tissue and a layer of elastic fibres. The endothelium is one cell thick and lines the lumen. It is very smooth and reduces friction for free blood flow.
• The tunica media is made up of smooth muscle cells and a thick layer of elastic tissue. The layer of muscle cells strengthen the blood vessel. The elastic tissue helps to maintain blood pressure.
• The tunica adventitia covers the exterior of the blood vessel and is mostly made up of collagen, a strong protein which protects against damage to the blood vessel by over-stretching.
• Arteries take blood away from the heart. They have a narrow lumen to maintain high blood pressure. A pulse is present in arteries. Arteries have a very thick tunica media. The layer of muscle cells strengthen the arteries so they can withstand high pressure. It also enables them to contract and narrow the lumen for reduced blood flow. The elastic tissue helps to maintain blood pressure in the arteries. It stretches and recoils to even out any fluctuations in pressure.
• Veins return blood to the heart. They receive blood that has passed through capillary networks. At this point blood pressure is very low. A pulse is absent in veins. The lumen of veins is much wider than that of an artery. This ensures that blood returns to the heart at an adequate speed. A large lumen reduces friction between the blood and the endothelial layer of the vein. The rate of blood flow is slower in veins but the larger lumen means that the volume of blood delivered per unit of time is equal to that of the arteries.
• Veins contain valves. These prevent the backflow of blood, which helps blood return to the heart.
• Arterioles connect arteries to capillaries. They possess a smooth muscle layer that means they can contract and partially cut off blood flow to specific organs. For example, during exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles. Compared to arteries, arterioles have a lower proportion of elastic fibres.
• Venules connect capillaries to veins. They have few or no elastic fibres and a large lumen. As the blood is at low pressure after passing through the capillaries there is no need for a muscular layer.
• Capillaries link arterioles to venules. They have thin walls which are semi-permeable. This allows substances to leave the blood to reach cells and tissues.
• Capillaries can form networks called capillary beds which are very important exchange surfaces within the circulatory system.
• Capillaries have a very narrow lumen. This forces the blood to travel slowly, providing more opportunities for diffusion to occur.
• A large number of capillaries branch between cells. Substances quickly diffuse between the blood and cells as there is a short diffusion distance.
• The wall of the capillary is made solely from a single layer of endothelial cells. This reduces the diffusion distance for oxygen and carbon dioxide between the blood and tissues. Gaps (pores) between the cells allow blood plasma to leak out and form tissue fluid. White blood cells can combat infection in affected tissues by squeezing through the intercellular junctions in the capillary walls.