The heart and blood vessels: Students should know the structure and functioning of the human heart and lungs, including how lungs are adapted for gaseous exchange.

The idea

The human heart is a muscular double pump with four chambers. Deoxygenated blood returns from the body through the vena cava into the right atrium, passes through the right ventricle and leaves through the pulmonary artery to the lungs. Oxygenated blood returns through the pulmonary vein to the left atrium, enters the left ventricle and is pumped out through the aorta to the body. Valves prevent backflow, and the left ventricle wall is thicker because it pumps blood further. The lungs contain many alveoli, giving a large surface area; their walls are one cell thick for a short diffusion path; they have a rich blood supply to maintain a steep concentration gradient; and they are ventilated by breathing movements.

What earns the marks

• Names the four chambers of the heart (right atrium, right ventricle, left atrium, left ventricle) and describes the direction of blood flow through them.
• Identifies the major vessels: vena cava, pulmonary artery, pulmonary vein and aorta, and states which side of the heart carries oxygenated or deoxygenated blood.
• Explains that valves in the heart and veins prevent the backflow of blood.
• Relates the thicker muscular wall of the left ventricle to the need to pump blood around the whole body.
• Describes the lungs as containing alveoli that provide a large surface area for gaseous exchange.
• Explains that alveolar walls are one cell thick, giving a short diffusion path, and that a rich capillary network maintains a steep concentration gradient.
• Links breathing movements to ventilation of the lungs, which helps maintain concentration gradients for oxygen and carbon dioxide.

Common mistakes and how to put them right

• Confusing the pulmonary artery with the pulmonary vein.

  Put it right: The pulmonary artery carries deoxygenated blood away from the heart to the lungs, while the pulmonary vein carries oxygenated blood back to the heart.

• Saying the left ventricle is thicker because it is on the left.

  Put it right: It is thicker because it must generate higher pressure to pump blood around the whole body.

• Describing alveoli as having a large surface area but omitting the thin walls and blood supply.

  Put it right: Link large surface area, one-cell-thick walls and a rich blood supply together to explain efficient gaseous exchange.

Exam tips

• Use a labelled diagram or flow sequence to show the route of blood through the heart, naming each chamber and vessel in order.
• When explaining lung adaptations, connect each feature to diffusion: surface area, diffusion distance and concentration gradient.
• Use comparative language such as thicker wall and higher pressure when contrasting the ventricles.