3.2.3 Blood Vessels

The blood vessels form a continuous network; the structure of each allows it to carry out its function.

Structure of arteries

• Artery walls consist of three layers: tunica adventitia/externa, tunica media and tunica intima
  • The tunica intima is made up of an endothelial layer, a layer of connective tissue and a layer of elastic fibres
    • The endothelium is one cell thick and lines the lumen of all blood vessels. 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
• Arteries have a 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

• The tunica adventitia covers the exterior of the artery and is mostly made up of collagen
  • Collagen is a strong protein protects blood vessels from damage by over-stretching

• Arteries have a narrow lumen which helps to maintain a high blood pressure
• A pulse is present in arteries

Structure of arterioles

• Arterioles possess a muscular layer that means they can contract and partially cut off blood flow to specific organs
  • Eg. During exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles
  • Unlike arteries, arterioles have a lower proportion of elastic fibres and a large number of muscle cells
  • The presence of muscle cells allows them to contract and close their lumen to stop and regulate blood flow

Structure of veins

• Veins return blood to the heart
• They receive blood that has passed through capillary networks (blood pressure is very low and it must be returned to the heart)
• The tunica media is much thinner in veins
  • There is no need for a thick muscular layer as veins don't have to withstand high pressure

• The lumen of the vein is much larger than that of an artery
  • A larger lumen helps to ensure 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 a larger lumen means the volume of blood delivered per unit of time is equal

• Veins contain valves
  • These prevent the backflow of blood, helping return blood to the heart

• A pulse is absent in veins

Structure of venules

• Venules connect the capillaries to the 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

The structures of arteries and veins. The distinctive shape of the blood vessels can be seen in the photomicrographs.

The correct answer is B.

This is because elastic arteries do not contain many smooth muscle fibres and are not able to perform vasoconstriction or vasodilation. In contrast, muscular arteries contain much more smooth muscle. Veins have large amounts of collagen for strength and keeping their shape, while capillaries are only one-cell thick and so do not contain any elastic, smooth muscle or collagen.

• Capillaries are a type of blood vessel present in the circulatory system
• They have thin walls which are “leaky”, allowing substances to leave the blood to reach the body’s tissues
• They can form networks called capillary beds which are very important exchange surfaces within the circulatory system

Structure and function of capillaries

• Capillaries have a very small diameter (lumen)
  • This forces the blood to travel slowly which provides more opportunity for diffusion to occur

• A large number of capillaries branch between cells
  • Substances can diffuse between the blood and cells quickly as there is a short diffusion distance

• The wall of the capillary is made solely from a single layer of endothelial cells (this layer also lines the lumen in arteries and veins)
  • The wall is only one cell thick – this reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
  • The cells of the wall have gaps called pores which 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

The structure of a capillary