The Human Heart (WJEC GCSE Biology: Combined Science)

• The heart is an organ made of muscle called cardiac muscle which contracts to pump blood around the body
• 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

Structure of the heart diagram

Structure of the heart showing the four main chambers, the four main blood vessels and the valves

The outside of the heart

• The external surface of the heart is covered in small blood vessels called coronary arteries
• Coronary arteries supply blood to the heart muscle itself
  • The blood supply to the heart muscle provides oxygen and glucose for the respiring cardiac muscle cells and removes the waste products of metabolism

The external view of the heart diagram

The external view of the heart showing the coronary arteries surrounding the outside of 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 valve into the right ventricle
• The ventricle contracts and the blood is pushed through the semi-lunar 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 valve into the left ventricle
• The ventricle contracts and the blood is forced through the semi-lunar 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 events within the right and left sides of the heart occur simultaneously 

Valves of the heart

• The heart contains four valves, two on each side, that control the unidirectional flow of blood through the heart
• They open and close to prevent blood flowing backwards as the heart beats and pushes the blood around
• The bicuspid and tricuspid valves are positioned between the atria and the ventricles and for this reason are sometimes called the atrioventricular valves
  • The tricuspid is on the right side of the heart and is named 'tri' because it has three "flaps"
  • The bicuspid is on the left side of the heart and is named 'bi' because it has two "flaps"
• The semi-lunar valves are both located in the arteries
  • There is one in the base of the aorta and one in base of the pulmonary artery
  • These both prevent the backflow of blood into the ventricles when the heart muscle is relaxed

A cross section of a heart viewed from above diagram

The four valves of the heart with the number of 'flaps' visible when viewed as a cross section from above