Capillaries veins and arteries

• Blood circulates from the heart through the arteries to

  the capillaries, then to the veins, before returning back

  to the heart.
• As blood moves away from the heart, there is a

  decrease in blood pressure.
• The central cavity of a blood vessel is called the lumen
• The lumen is lined with a layer of cells called the

  endothelium.
• The endothelium is surrounded by layers of tissue that

  differ in the different blood vessels.
• Arteries have:-

  • an outer layer of connective tissue containing elastic

  fibres

  • a middle layer containing smooth muscle with more

  elastic fibres
• The elastic walls of the arteries stretch and recoil to

  accommodate the surge of blood after each

  contraction of the heart.
• To control blood flow, the smooth muscle in the walls of

  the arteries can:-

  • contract causing vasoconstriction

  • relax causing vasodilation
• vasoconstriction

  the diameter of the

  central lumen decreases and this decreases the flow of

  blood
• vasodilation

  the diameter of the central

  lumen increases and this increases the flow of blood
• Veins have:-

  • an outer layer of connective tissue containing elastic

  fibres

  • a much thinner muscular wall than arterie
• Veins contain valves to prevent the backflow of

  blood
• Capillaries have very thin walls
• Capillaries allow exchange of substances carried in the

  blood with the body tissues/cells through their thin

  walls.
• Pressure filtration causes plasma to pass through the

  capillary walls into the tissue fluid surrounding the cells.
• Tissue fluid and blood plasma are similar in

  composition.
• However, tissue fluid does not contain plasma

  proteins as they are too large to be filtered through

  the capillary walls.
• Tissue fluid contains and supplies the cells with glucose,

  oxygen and other substance
• Carbon dioxide and other metabolic wastes diffuse out

  of the cells and into the tissue fluid to be excreted
• Much of the tissue fluid returns to the blood.

  Lymphatic vessels absorb excess tissue fluid and return

  it as lymph to the circulatory system.