The Heart (AQA GCSE Biology): Revision Note

The heart

• The heart is an organ that pumps blood around the body

• It is part of the circulatory system, which contains:

  • blood vessels

  • a pump (the heart)

  • valves that maintain a one-way flow of blood around the body

Double circulatory system

• The human heart is part of a double-circulatory system, meaning that:

  • it consists of two circuits; one to the body and one to the lungs

  • blood passes through the heart twice for every complete journey around the body

• The heart has four chambers; two on the right and two on the left:

  • 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 to the body; this is the systemic circuit

• The benefits of a double circulatory system include:

  • Blood is sent to the lungs at a lower pressure, protecting delicate lung capillaries from damage while allowing sufficient time for gas exchange

  • Returning blood to the heart to be pumped a second time increases its pressure before it travels to the rest of the body; cells can therefore be supplied with oxygen more quickly

  • The heart structure prevents oxygenated blood from mixing with deoxygenated blood, ensuring that cells receive fully oxygenated blood

Heart structure

• The heart has four chambers; two upper atria and two lower ventricles

  • Atria: receive blood from the body or the lungs and pass it to the ventricles

  • Ventricles: receive blood from the atria and pump it to the lungs / body

• The right side of the heart contains deoxygenated blood

• The left side of the heart contains oxygenated blood

• Blood travels towards the heart in veins

  • Vena cava: blood travels from the body back towards the heart

  • Pulmonary vein: blood travels from the lungs to the heart

• The blood is pumped away from the heart in arteries

  • Aorta: blood is pumped out of the heart towards the body

  • Pulmonary artery: blood is pumped out of the heart towards the lungs

• The heart muscle itself is supplied with blood by the coronary arteries

  • Blood in the coronary arteries supplies oxygen and nutrients and removes waste carbon dioxide

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Pathway of blood through the heart

• Deoxygenated blood enters the heart via the vena cava, emptying into the right atrium

• Blood flows through a set of valves into the right ventricle

• When the right ventricle contracts, blood travels up through the pulmonary artery to the lungs, where gas exchange occurs

• Oxygenated blood returns to the heart via the pulmonary vein, emptying into the left atrium

• Blood flows through another set of valves into the left ventricle

• When the left ventricle contracts, blood travels up through the aorta, and to the rest of the body

Adaptations of the heart

• The structure of the heart ensures that blood is pumped efficiently:

  • Differences in muscle thickness

    • The walls of the ventricles are much thicker than those of the atria as they need to pump blood out of the heart at 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

  • Maintenance of correct blood flow

    • Two sets of valves inside the heart which prevent the backflow of blood

  • Separation of left and right

    • A muscle wall between left and right chambers 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

Heart rate

• The 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

• 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