Blood and Circulation

• blood 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, which need it for respiration
  • do not contain a nucleus, so there is more room for haemoglobin
  • are very small, so they can fit through the tiny capillaries
  • are shaped like biconcave discs, giving them a large surface area that oxygen can quickly diffuse across
• white blood cells:

  • help to protect the body against infection
  • can change shape, so they can squeeze out of the blood vessels into the tissues or surround and engulf microorganisms
• platelets are fragments of cells, which collect at wounds and trigger blood clotting
• arteries

  • take blood from your heart to your organs
  • thick walls made from muscle and elastic fibres
• veins

  • take blood from your organs to your heart
  • thinner walls and 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
  • narrow, thin-walled blood vessels
• the heart pumps blood around the body in a double circulatory system
• the blood passes through the heart twice on each circuit
• there are four chambers in the heart:

  • the left and right atria, which receive blood from the veins
  • the left and ventricles, which pump the blood out into arteries
• heart

  A) pulmonary artery

  B) aorta

  C) pulmonary vein

  D) left atrium

  E) heart valve

  F) left ventricle

  G) right ventricle

  H) right atrium

  I) vena cava

  
• blood enters the heart through the atria
• the atria contract and force blood into the ventricles
• the ventricles then contract and force blood out of the heart
• valves make sure the blood flows in the correct direction
• the natural resting heart rate is controlled by a group of cells located in the right atrium, which act as a pacemaker
• artificial pacemakers are electrical devices used to correct irregularities in the heart rate
• the heart sends blood to the lungs via the pulmonary artery
• air obtained by breathing reaches the lungs through the trachea
• the trachea divides into two tubes - the bronchi
• the bronchi divide to form bronchioles
• the bronchioles divide until they end in tiny air sacs called alveoli
• there are millions of alveoli and they are adapted to very efficient at exchanging oxygen and carbon dioxide:

  • they have a large, moist surface area
  • they have a very rich blood supply
  • they are very close to the blood capillaries, so the distance for gases to diffuse is small
• the blood is taken back to the heart through the pulmonary vein