6.2.2 The Blood System: Vessels

Introduction to blood vessels

• The circulatory system of the human body contains several different types of blood vessel:
  • Arteries
  • Arterioles
  • Capillaries
  • Venules
  • Veins
• Each type of blood vessel has a specialised structure that relates to the function of that vessel

The circulatory system includes several blood vessels, each specialised to carry out its function

Structure and function of arteries

• Arteries transport blood away from the heart at high pressure
  • Blood travels from the ventricles to the tissues of the body
  • Remember; arteries carry blood away from the heart

• Artery walls consist of three layers:
  • The tunica intima is the innermost layer and 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
      • Blood leaves the heart under high pressure
    • Muscles cells can also contract or relax to control the diameter of the lumen and regulate blood pressure
    • The elastic tissue helps to maintain blood pressure in the arteries. It stretches and recoils to even out fluctuations in pressure
  • The tunica externa covers the exterior of the artery and is mostly made up of collagen
    • Collagen is a strong protein and 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 due to blood leaving the heart under high pressure

Arteries have thick muscular walls made up of three layers of tissue and a narrow lumen

Structure and function of arterioles

• Arterioles branch off from arteries forming narrower blood vessels which transport blood into capillaries
• Arterioles are similar in structure to arteries, but they 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 regulate blood flow to specific organs
  • Eg. during exercise blood flow to the stomach and intestine is reduced while blood flow to the muscles increases

Arterial blood pressure

• Arteries, and to a slightly lesser extent arterioles, must be able to withstand high pressure generated by the contracting heart, and both must maintain this pressure when the heart is relaxed
• Muscle and elastic fibres in the arteries help to maintain the blood pressure as the heart contracts and relaxes
  • Systolic pressure is the peak pressure point reached in the arteries as the blood is forced out of the ventricles at high pressure
    • At this point, the walls of the arteries are forced outwards, enabled by the stretching of elastic fibres

  • Diastolic pressure is the lowest pressure point reached within the artery as the heart relaxes
    • At this point, the stretched elastic fibres recoil and force the blood onward through the lumen of the arteries
  • This maintains high pressure throughout the heart beat cycle

• Vasoconstriction of the circular muscles of the arteries can increase blood pressure by decreasing the diameter of the lumen
• Vasodilation of the circular muscles causes blood pressure to decrease by increasing the diameter of the lumen

Structure and function of capillaries

• Capillaries provide the exchange surface in the tissues of the body through a network of vessels called capillary beds
  • The wall of a capillary is made from a single layer of endothelial cells (this layer is also found lining the lumen in arteries and veins)
    • Being just one cell thick reduces the diffusion distance for oxygen and carbon dioxide between the blood and the tissues of the body
  • The thin endothelium cells also have gaps between them called pores which allow blood plasma to leak out and form tissue fluid
    • Tissue fluid contains oxygen, glucose and other small molecules from the blood plasma
    • Large molecules such as proteins usually can't fit through the pores into the tissue fluid
    • Tissue fluid surrounds the cells, enabling exchange of substances such as oxygen, glucose, and carbon dioxide
    • The permeability of capillaries can vary depending on the requirements of a tissue
  • Capillaries have a lumen with a small diameter
    • Red blood cells squeeze through capillaries in single-file
    • This forces the blood to travel slowly which provides more opportunity for diffusion to occur
  • Capillaries form branches in between the cells; this is the capillary bed
    • These branches increase the surface area for diffusion of substances to and from the cells
    • Being so close to the cells also reduces the diffusion distance

Capillaries have a narrow lumen and walls that are one cell thick

Structure and function of veins

• Veins transport blood to the heart at low pressure
  • Remember; veins carry blood into the heart
• They receive blood that has passed through capillary networks, across which pressure has dropped due to the slow flow of blood
• The structure of veins differs from arteries:
  • The tunica media is much thinner in veins
    • There is no need for a thick muscular and elastic layer as veins don't have to maintain or withstand high pressure
  • The lumen of veins is much wider in diameter than that of arteries
    • 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 back flow of blood that can result under low pressure, helping return blood to the heart
    • Movement of the skeletal muscles pushes the blood through the veins, and any blood that gets pushed backwards gets caught in the valves; this blood can then be moved forwards by the next skeletal muscle movement

• A pulse is absent in veins; the pressure changes taking place due to the beating of the heart are no longer present

Veins have a structure similar to arteries with three layers of tissue and a large lumen

Structure and function 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 to maintain pressure
  • The large lumen enables a large volume of blood to be transported