Blood Vessels (HL IB Biology)

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

Blood vessels diagram

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

Adaptations of capillaries for exchange of materials

• 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
    • 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 of some capillaries have gaps between them called fenestrations which allow blood plasma to leak out and form tissue fluid
    • Tissue fluid surrounds the cells, enabling exchange of substances such as oxygen, glucose, and carbon dioxide
    • Tissue fluid contains oxygen, glucose and other small molecules from the blood plasma
    • Large molecules such as proteins usually can't fit through the fenestrations into the tissue fluid
    • The permeability of capillaries can vary depending on the requirements of a tissue
  • 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 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
    • It also reduces the diffusion distance as red blood cells are brought in close contact with the capillary wall

Capillary structure diagram

Capillaries have a narrow lumen and walls that are one cell thick to increase the rate of diffusion between the blood and cells

Adaptations 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 innermost layer is an endothelial layer, consisting of squamous epithelium
    • 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 middle layer contains smooth muscle cells and a thick layer of elastic tissue
    • This layer is very thick in the walls of arteries
    • The layer of muscle:
      • Strengthen the arteries so they can withstand high pressure
      • Can 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 when the heart beats
    • Further from the heart there is more smooth muscle and less elastic tissue due to smaller fluctuations in blood pressure
  • The outer layer covers the exterior of the artery and is mostly made up of collagen and elastic fibres
    • Collagen is a strong protein and protects blood vessels from damage by over-stretching
    • Along with elastic fibres, it prevents the arterial wall from rupturing as blood surges from the ventricles
• Arteries have a narrow lumen which helps to maintain high blood pressure

Artery structure diagram

Arteries have thick muscular walls and a narrow lumen

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

Adaptations 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 capillaries converge to form venules, which deliver blood to veins
• The structure of veins differs from arteries:
  • The middle layer 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 walls of veins are flexible, allowing surrounding muscles and tissues to compress them
    • This facilitates the movement of blood back to the heart
  • 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
  • Veins have a wide lumen
    • This maximises the volume of blood that can flow at any one time

Vein structure diagram

Veins have thin walls and a wide lumen