The Heart (AQA GCSE Biology: Combined Science)

The double circulatory system

• The human heart is part of a double-circulatory system
• The circulatory system is a system of:
  • blood vessels
  • a pump (the heart)
  • valves that maintain a one-way flow of blood around the body
• The heart has four chambers separated into two halves:
  • The right side of the heart pumps blood to the lungs for gas exchange (this is the pulmonary circuit)
  • The left side of the heart pumps blood under high pressure to the body (this is systemic circulation)
• The benefits of a double circulatory system:
  • Blood travelling through the small capillaries in the lungs loses a lot of pressure which reduces the speed at which it can flow meaning more time for diffusion of gases at the alveoli
  • By returning oxygenated blood to the heart from the lungs, the pressure can be raised before sending it to the body, meaning cells can be supplied with oxygenated blood more quickly

The heart structure

• The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs
  • This is where oxygen diffuses in from the alveoli and carbon dioxide diffuses out
• The left side of the heart receives oxygenated blood from the lungs and pumps it to the body
• Blood is pumped towards the heart in veins and away from the heart in arteries
• The four chambers of the heart are divided into top and bottom:
  • Chambers at the top are the atria
  • Chambers at the bottom are the ventricles

Heart diagram: the inside

In a heart diagram, the heart is labelled as if you are looking at it directly in someone’s body – so the left-hand side of an image of the heart is the right-hand side of the heart

Heart diagram: the outside

Coronary arteries are found on the outside of the heart

Pathway of blood through the heart

• Deoxygenated blood enters the heart via the vena cava, emptying into the right atrium
• Blood flows down through a set of valves into the right ventricle
• When the ventricles contract, blood travels up through the pulmonary artery to the nearby lungs where gas exchange occurs (and the blood becomes oxygenated)
• Oxygenated blood returns to the heart via the pulmonary vein, emptying into the left atrium
• Blood flows down through a set of valves into the left ventricle
• When the ventricles contract, blood travels up through the aorta, and to the rest of the body

Adaptations of the heart

• The walls of the ventricles are much thicker than those of the atria as they are responsible for pumping blood out of the heart and so need to generate a higher pressure
  • The wall of the left ventricle is much thicker than that of the right ventricle as it has to pump blood at high pressure around the entire body,
  • The right ventricle pumps blood at lower pressure to the lungs which are close to the heart so do not require high-pressure
• There are two sets of valves inside the heart which function to prevent the backflow of blood in the heart
• The two sides of the heart are separated by the septum 
  • The septum prevents the mixing of deoxygenated and oxygenated blood inside the heart
  • If blood were to mix, then less oxygenated blood would be pumped around the body
• The heart is made of a special type of cardiac muscle tissue which does not fatigue like skeletal muscle
• The coronary arteries supply the tissue of the heart with oxygenated blood
  • The heart needs a constant supply of oxygen (and glucose) for aerobic respiration to release energy to allow continued muscle contraction

Resting heart rate

• The natural resting heart rate is controlled by a group of cells located in the right atrium
• These cells form a structure called the pacemaker
• The role of the pacemaker is to coordinate the contraction of the heart muscle, therefore it regulates the heart rate
• Up to a point, the faster the heart contracts, the more quickly oxygenated blood can be delivered around the body
  • When a person is at rest, the oxygen demand of their cells is relatively low and so a lower heart rate is maintained
  • When a person is exercising, the oxygen demand of their muscle cells increases so a higher heart rate is necessary
• The pacemaker sends out an electrical impulse which spreads to the surrounding muscle cells of the heart, causing them to contract
  • The pacemaker does this every time the heart needs to “beat”, so if a person has a resting heart rate of 60 beats per minute (bpm), then the pacemaker will be sending out electrical impulses on average once every second

Artificial pacemakers

• Sometimes, the pacemaker of the heart stops functioning properly (this can cause an irregular heartbeat)
• Artificial pacemakers are electrical devices used to correct irregularities in the heart rate
• The device is implanted just under the skin, with a wire that delivers an electrical current to the heart to help it contract regularly