The Heart, Double Circulation and the Pacemaker

Describe the structure and function of the human heart and lungs.
Explain the double circulatory system and the role of the right and left ventricles.
Identify key blood vessels: aorta, vena cava, pulmonary artery, pulmonary vein, and coronary arteries.
Explain the role of the natural pacemaker and artificial pacemakers.
Relate the structure of arteries, veins, and capillaries to their functions.
Calculate the rate of blood flow.

Why the heart and lungs work together

Body cells need oxygen for aerobic respiration and produce carbon dioxide that must be removed. The lungs exchange these gases with the air, while blood carries them between the lungs and body cells. The heart is the muscular pump that keeps the blood moving. Together, these organs supply cells much faster than diffusion across the whole body could.

The heart’s chambers and the two journeys

The heart has four chambers. The two atria at the top receive blood; the two ventricles below pump it out. Valves keep blood moving forwards rather than flowing backwards. The right and left sides handle different journeys, so blood passes through the heart twice in one complete circuit: this is a double circulatory system.

Blood returning from the body has given up much of its oxygen. It enters the right atrium through the vena cava, moves into the right ventricle, and is pumped through the pulmonary artery to the lungs. There, it gains oxygen and loses carbon dioxide. The oxygenated blood returns through the pulmonary vein to the left atrium, moves into the left ventricle, and is pumped through the aorta to the rest of the body. The left ventricle has a thicker muscular wall than the right because it must pump blood around the whole body, rather than just to the nearby lungs.

Blood flows from body tissues through the vena cava, right atrium and right ventricle, then through the pulmonary artery to the lungs. A gap at the vessel crossing shows that the vena cava and pulmonary artery do not connect directly. Higher-oxygen blood returns through the pulmonary vein to the left atrium and left ventricle, then leaves through the aorta for the body. The left ventricular wall is thicker than the right. Coronary arteries branch from the aorta to supply heart muscle, not the ventricular cavity. Schematic, not to scale. — In a double circulatory system, blood returns to the heart after passing through the lungs before being pumped around the rest of the body.

The heart muscle also needs its own supply of oxygen for respiration. Coronary arteries branch from the aorta and carry oxygenated blood to the heart muscle itself; they are not a shortcut for blood travelling through the heart’s chambers.

Where blood meets air in the lungs

Air passes down the trachea, which divides into two bronchi, one leading into each lung. The airways branch further and ultimately lead to many tiny air sacs called alveoli. A network of very small blood vessels, or capillaries, surrounds the alveoli. This puts blood close to the air inside each sac.

Oxygen diffuses from the air in an alveolus into the surrounding blood because its concentration is higher in the alveolus. Carbon dioxide diffuses the other way, from the blood into the alveolus, and is breathed out. Diffusion is movement from a higher to a lower concentration. The lungs are well adapted for this exchange: their many alveoli provide a large surface area, and the thin alveolar and capillary walls make the distance for diffusion short. Breathing brings in fresh air, while blood flow carries oxygen away and brings more carbon dioxide to the lungs; both help maintain the concentration differences that drive diffusion.

Many thin-walled alveoli provide a large surface for rapid gas exchange with blood in the surrounding capillaries.

Keeping the heart beating regularly

At rest, the heart’s natural rate is controlled by a small group of cells in the right atrium. This natural pacemaker produces electrical signals that coordinate the heartbeat. If the heart rate becomes irregular, an artificial pacemaker can be used. It is an electrical device that sends signals to the heart to help correct the irregularity.

Blood’s route

Body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body. The right ventricle pumps to the lungs; the thicker-walled left ventricle pumps around the body. Coronary arteries supply the heart muscle.

Lungs and gas exchange

Air travels through the trachea and bronchi to the alveoli. Oxygen diffuses from alveoli into capillary blood; carbon dioxide diffuses from blood into alveoli. Many alveoli give a large surface area, thin walls give a short diffusion distance, and ventilation plus blood flow maintain concentration differences.

Heart rhythm

Cells in the right atrium act as the natural pacemaker, controlling the resting heart rate with electrical signals. An artificial pacemaker is an electrical device used to correct irregularities in that rate.

Trace blood in the direction of flow: body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body.

Name a blood vessel by the direction blood travels relative to the heart, not by whether the blood contains much oxygen. The pulmonary artery carries deoxygenated blood; the pulmonary vein carries oxygenated blood.

When explaining gas exchange, give the direction of diffusion for both gases and link the alveoli’s large surface area and thin walls to rapid exchange.

You need to understand that heart valves prevent backflow, but AQA does not require their individual names.

An organ made of muscle that pumps blood through the lungs and around the rest of the body.

A system in which blood passes through the heart twice during one complete journey around the body: once on its way to the lungs and once on its way to the rest of the body.

An upper chamber of the heart that receives blood returning to the heart.

A lower chamber of the heart that pumps blood out of the heart.

A tiny air sac in the lungs where oxygen enters the blood and carbon dioxide leaves it.

The movement of particles from an area of higher concentration to an area of lower concentration.

A group of cells in the right atrium that produces electrical signals controlling the natural resting heart rate.

An electrical device that sends signals to the heart to correct an irregular heart rate.