Blood and circulation

• Blood


  • It is a tissue
  • It is made of a liquid called plasma, which has three different components suspended in it: red blood cells, white blood cells, platelets
• Plasma


  Transports various chemical substances around the body such as the products of digestion, hormones, antibodies, urea and carbon dioxide
• Red blood cells


  • Contain haemoglobin, which binds to oxygen to transport it from the lungs to the tissues and cells, where it is needed for respiration
  • Do not contain a nucleus, so there is the maximum capacity for haemoglobin
  • Are very small, so they can fit through tiny capillaries
  • Are shaped like biconcave discs, providing a large surface area for oxygen to quickly diffuse through
• White blood cells


  • Are essential to protect the body against infection
  • Can change shape, squeezing out of the blood vessels into the tissues or to surround and engulf microorganisms
• Platelets


  • Are fragments of dead cells, which collect at wounds and trigger blood clotting
• Arteries


  • Transport blood from the heart to your organs
  • Have thick walls made from muscle and elastic fibres
• Veins


  • Transport blood from the organs to your heart
  • Have thinner walls with valves to prevent backflow
• Capillaries


  • Allow substances needed by the cells to pass out of the blood
  • Allow substances produced by the cells to pass into the blood
  • Are narrow, thin-walled blood vessels
• Blood circulation


  1. Blood passes through the heart twice on each circuit

  2. Blood enters the heart through the atria

  3. The atria contract and force blood into the ventricles

  4. The ventricles then contract and force blood out of the heart

  5. Valves make sure the blood flows in the correct direction
• Pacemaker


  A group of cells located in the right atrium that control the natural resting heart rate
• Artificial pacemaker


  A device that uses electric current to mimic the natural pacemaker and can correct irregularities in the heart rate
• Gaseous exchange


  1. The heart sends blood to the lungs via the pulmonary artery

  2. Air obtained by breathing reaches the lungs through the trachea, which has rings of cartilage to prevent it collapsing

  3. The trachea divides into two tubes - the bronchi

  4. The bronchi divide to form bronchioles

  5. The bronchioles divide until they end in tiny air sacs called alveoli

  6. The alveoli are adapted to be very efficient at exchanging oxygen and carbon dioxide

  7. The blood is taken back to the heart through the pulmonary vein
• When looking at the circulatory system in a book or on an exam paper, the right-hand side of the body is always on the left-hand side of the image
• The pulmonary artery is unusual because, unlike other arteries, it carries deoxygenated blood. The pulmonary vein carries oxygenated blood
• Key words


  • plasma
  • haemoglobin
  • double circulatory system
  • atria
  • ventricle
  • pacemaker
  • pulmonary artery
  • trachea
  • cartilage
  • bronchi
  • bronchioles
  • alveoli
  • pulmonary vein