The Circulatory System (Edexcel GCSE Biology: Combined Science)

• The heart organ is a double pump
  • Oxygenated blood from the lungs enters the left side of the heart and is pumped to the rest of the body (the systemic circuit)
    • The left ventricle has a thicker muscle wall than the right ventricle as it has to pump blood at high pressure around the entire body,

  • Deoxygenated blood from the body enters the right side of the heart and is pumped to the lungs (the pulmonary circuit)
    • The right ventricle is pumping blood at lower pressure to the lungs

  • A muscle wall called the septum separates the two sides of the heart

• Blood is pumped towards the heart in veins and away from the heart in arteries
• The coronary arteries supply the cardiac muscle tissue of the heart with oxygenated blood
  • As the heart is a muscle it needs a constant supply of oxygen (and glucose) for aerobic respiration to release energy to allow continued muscle contraction

• Valves are present to prevent blood flowing backwards

Structure of the Heart

The pathway of blood through the heart

• Deoxygenated blood coming from the body flows through the vena cava and into the right atrium
• The atrium contracts and the blood is forced through the tricuspid (atrioventricular) valve into the right ventricle
• The ventricle contracts and the blood is pushed through the semilunar valve into the pulmonary artery
• The blood travels to the lungs and moves through the capillaries past the alveoli where gas exchange takes place
  • Low pressure blood flow on this side of the heart prevents damage to the capillaries in the lungs

• Oxygenated blood returns via the pulmonary vein to the left atrium
• The atrium contracts and forces the blood through the bicuspid (atrioventricular) valve into the left ventricle
• The ventricle contracts and the blood is forced through the semilunar valve and out through the aorta
  • Thicker muscle walls of the left ventricle produce a high enough pressure for the blood to travel around the whole body

• The circulatory system consists of a closed network of blood vessels connected to the heart
  • Oxygenated blood is carried away from the heart and towards organs in arteries
  • These narrow to arterioles and then capillaries as they pass through the organ
  • In the organs, respiring cells use up the oxygen from the blood
  • The capillaries widen to venules and finally veins as they move away from the organs
  • Veins carry deoxygenated blood back towards the heart

• A different network of lymphatic vessels collect all the excess tissue fluid that leaks out of the capillaries and delivers it back to the circulatory system

The circulatory system

Main Blood Vessels of the Circulatory System Table