2.2.2 The Heart

• Circulatory system


  A system of blood vessels with a pump (the heart) and valves that maintain a one-way flow of blood around the body
• Heart


  • Part of a double circulatory system
  • Has four chambers separated into two halves
• Pulmonary circuit


  Right side of heart pumps blood to the lungs for gas exchange
• Systemic circulation


  Left side of heart pumps blood under high pressure to the body
• Benefits of double circulatory system

  • Blood travelling through small capillaries in lungs loses a lot of pressure which reduces the speed it can flow
  • Returning oxygenated blood to heart from lungs allows pressure to be raised before sending to body, meaning cells can be supplied with oxygenated blood more quickly
• The heart is labelled as if it was in the chest so what is your left on a diagram is actually the right-hand side (and vice versa)
• Deoxygenated blood flow


  1. Enters heart via vena cava, empties into right atrium

  2. Flows down through atrioventricular valves into right ventricle

  3. When ventricles contract, blood travels up through pulmonary artery to lungs for gas exchange
• Oxygenated blood flow

  1. Returns to heart via pulmonary vein, empties into left atrium

  2. Flows down through atrioventricular valves into left ventricle

  3. When ventricles contract, blood travels up through aorta to rest of body
• Ventricle walls


  • Walls of ventricles much thicker than atria as they are responsible for pumping blood out of heart and need to generate higher pressure
  • Wall of left ventricle much thicker than right ventricle as it has to pump blood at high pressure around entire body, whereas right ventricle pumps blood at lower pressure to lungs
• Heart valves


  • Atrioventricular valves separate atria from ventricles
  • Semilunar valves found in two blood arteries coming out of top of heart, open when ventricles contract to allow blood out but then shut to avoid backflow
• Septum


  • Separates two sides of heart, prevents mixing of deoxygenated and oxygenated blood
• Cardiac muscle


  • Does not fatigue like skeletal muscle
• Coronary arteries

  • Supply heart tissue with oxygenated blood, heart needs constant supply of oxygen and glucose for aerobic respiration to power muscle contraction
• Pacemaker


  Group of cells in right atrium that coordinate contraction of heart muscle and regulate heart rate
• At rest

  Lower heart rate maintained as oxygen demand of cells is relatively low
• During exercise

  Higher heart rate necessary as oxygen demand of muscle cells increases
• Pacemaker function

  1. Sends out electrical impulse which spreads to surrounding muscle cells, causing them to contract

  2. Does this every time heart needs to "beat"
• If pacemaker stops functioning properly, can cause irregular heartbeat
• Artificial pacemaker

  Electrical device used to correct irregularities in heart rate, implanted under skin with wire delivering electrical current to heart