The Heart (OCR GCSE Biology A (Gateway))

Circulation & Gas Exchange

• The circulatory system consists of a network of blood vessels connected to the heart and lungs

• The heart pumps deoxygenated blood to the lungs where gas exchange surfaces take up oxygen

  • Oxygen diffuses into the blood from the alveoli

    • Alveoli are specialised structures that have a large surface area to volume ratio and thin walls to maximise gas exchange 

  • Carbon dioxide (waste gas) diffuses from the blood into the lungs (to be exhaled) via the alveoli

• Oxygenated blood then returns to the heart which pumps it out in arteries around the body towards organs

• Arteries narrow to capillaries as they pass through the organ

  • A network of small capillaries allows for gas exchange with organ tissues 

• Capillaries widen to veins as they move away from the organ to carry deoxygenated blood back towards the heart

The alveolus is the gas exchange surface in humans

A network of small capillaries allows for efficient gas exchange in organ tissues

The Double Circulatory System

• The human heart is part of a double circulatory system

  • This means there are two circuits joined together

• The circulatory system is a system of blood vessels with a pump (the heart) and valves that maintain a one-way flow of blood around the body

• 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)

• Mammals (and birds) have a double circulatory system

  • Not all animals do though - fish don't

• There are 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

  • 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

  • This is important for mammals as they require a lot of oxygen to maintain their body temperature

The double circulatory system in humans

The Structure of the Heart

• The heart is labelled as if it was in the chest so what is your left on a diagram is actually the right-hand side (and vice versa)

• The heart is a pumping organ that ensures blood continuously flows around the body

• The heart has two distinct halves and acts as a double pump 

  • The right side of the heart receives deoxygenated blood from the body and pumps it to the lungs 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 chambers at the top of the heart are the atria, the chambers at the bottom the ventricles

• Valves are present to prevent blood flowing backwards

  • The heart is made of a special type of muscle called cardiac muscle which never gets tired

    • Cardiac muscle cells contain far more mitochondria than skeletal muscle to provide lots of ATP for energy

  • The heart beats continuously so needs a constant supply of oxygen (and glucose) for aerobic respiration

    • Cardiac muscle has its own blood supply via the coronary arteries which branch off from the aorta

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 actually the right-hand side of the heart

Pathway of blood through the heart

• Deoxygenated blood (from the body) enters the heart via the vena cava, emptying into the right atrium

• Oxygenated blood (from the lungs) returns to the heart via the pulmonary vein, emptying into the left atrium

• The atria contract pushing the blood into the ventricles 

• The right ventricle contracts forcing blood through the pulmonary artery to the nearby lungs where gas exchange occurs (and the blood becomes oxygenated)

• At the same time the left ventricle contracts forcing the oxygenated blood through the aorta, out of the heart

• The oxygenated blood flows through arteries around the body delivering oxygen to organs and tissues

• Once the oxygen has been used up, deoxygenated blood returns to the heart through veins