The Mammalian Heart (HL) (HL IB Biology)

Adaptations of the mammalian heart to deliver blood to the arteries

• The heart muscle is known as cardiac muscle; a specialised form of muscle tissue that is able to contract repeatedly and continuously without becoming fatigued
• The heart is divided into four chambers; the two top chambers are atria and the bottom two chambers are ventricles
  • The atria
    • These chambers receive blood from the veins that return from the lungs and the body
    • They are surrounded by a thin layer of muscle, allowing them pump blood over a short distance into the ventricles below
  • The ventricles
    • These chambers receive blood from the atria
    • They have thick muscle walls which can generate high pressures for pumping blood to the lungs and to the body
    • The left ventricle has thicker muscle than the right ventricle, as it must pump blood all around the body
• The left and right sides of the heart are separated by a wall of muscular tissue, called the septum. 
  • The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart
• The wall of the right atrium contain a small region of tissue known as the pacemaker, or sinoatrial node (SAN)
  • The pacemaker initiates the heart beat by sending a wave of excitation across the atria
    • This causes atrial contraction
  • The electrical impulse travels down to the base of the ventricles
  • From the base, it spreads upwards across the ventricular walls
    • This causes ventricular contraction
• There are two blood vessels bringing blood to the heart; the vena cava and pulmonary vein
• There are two blood vessels taking blood away from the heart; the pulmonary artery and aorta

Valves in the heart

• Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards
• Valves in the heart:
  • Open when the pressure of blood behind them is greater than the pressure in front of them
  • Close when the pressure of blood in front of them is greater than the pressure behind them
• The valves between the atria and the ventricles are the atrioventricular valves; they prevent blood from flowing back from the ventricles into the atria
  • The right atrium and right ventricle are separated by the tricuspid valve
  • The left atrium and left ventricle are separated by the bicuspid valve
• The valves between the ventricles and the arteries are the semilunar valves
  • The right ventricle and the pulmonary artery are separated by the pulmonary valve
  • The left ventricle and aorta are separated by a valve called the aortic valve

Heart structure diagram

Deoxygenated blood flows into the right atrium from the body, from which it flows downwards into the right ventricle, and then upwards to the pulmonary artery which carries it to the lungs 

Oxygenated blood from the lungs enters the left atrium, flows down into the left ventricle, and then upwards to the aorta which carries it to the rest of the body

Coronary arteries

• The heart is a muscle and so requires its own blood supply; blood supplies the cells with oxygen and glucose for aerobic respiration
• The heart receives blood through arteries called coronary arteries
  • These arteries are visible on the front of the heart

Coronary arteries diagram

The coronary arteries supply the heart muscle with oxygenated blood