Biology 1.4

• Main components of blood

  • Red blood cells
  • White blood cells
  • Platelets
  • Plasma
• Red blood cells


  • Transport blood around the body from the lungs through blood vessels for aerobic respiration
  • Contain haemoglobin which is a red protein that combines with oxygen
  • Have no nucleus, to carry more oxygen
  • Have a biconcave shape (flattened disc shape) to maximise their surface area for oxygen absorption
• White blood cells


  • Defend the body against diseases
  • Most white blood cells are called phagocytes
  • Once engulfed by the phagocytes, bacteria and pathogens break down because of the enzymes inside the phagocyte
• Plasma


  • A straw coloured liquid that makes up 55% of blood volume
  • Transports carbon dioxide to the lungs, digested foods to respiring cells, area from the liver to the kidneys for excretion, and hormones from the glands where they are made to the target organs
• Platelets


  • Remain against cells called Megakaryo-cyte and contain no nucleus
  • They are involved in the clotting of blood and scab forming
• Arteries


  • Carry blood to the heart (always deoxygenated apart from the pulmonary artery)
  • Have thick elastic and muscular walls to withstand high blood pressure
  • The largest artery is the aorta
• Veins


  • Carry blood to the heart (always deoxygenated apart from the pulmonary vein)
  • Have thin and less muscular walls
  • Have larger passageways for blood (large lumen)
  • Contains blood under low pressure and valves to prevent blood from flowing backwards
• Capillaries


  • Allow the diffusion of gases and nutrients from the blood into the body cells
  • Have walls that are one cell thick and small lumen that allow blood to pass are one at a time
  • Have very low blood pressure
• Heart


  • A muscular organ that pumps blood around the body
  • The heart is divided into 2 halves - the right hand side and the left hand side
  • The walls of the heart are thick and made from special heart muscle tissue
• Travel of the blood

  1. Deoxygenated blood from the body enters the right atrium through the vena cava

  2. Blood moves into the right ventricle through the tricuspid valve

  3. Blood is pushed through the pulmonary valve into the pulmonary artery to the lungs

  4. Oxygenated blood returns to the left atrium through the pulmonary vein

  5. Blood moves through the bicuspid valve into the left ventricle

  6. Blood is pushed through the semi-lunar valve into the aorta to the rest of the body
• The left hand side of the heart has thicker walls than the right hand side as the blood has to travel further
• The circulatory system is called a double circulatory system because blood passes through the heart twice per circuit, in pulmonary circulation and systemic circulation
• Arteries mainly carry oxygenated blood except the pulmonary vein which carries oxygenated blood but is still carrying blood to the heart