Students should know the structure and functioning of the human heart and…

The idea

Atria receive blood and pass it to ventricles. The right ventricle pumps deoxygenated blood through the pulmonary artery to the lungs. Oxygenated blood returns through the pulmonary vein to the left atrium, enters the left ventricle and leaves through the aorta for the body. Blood returns through the vena cava to the right atrium. Valves prevent backflow; the septum separates the sides. Coronary arteries supply heart muscle. Air enters via the trachea and bronchi. Many alveoli give a large exchange surface; alveolar and capillary walls are each one cell thick, shortening the diffusion distance. Oxygen diffuses into blood; carbon dioxide diffuses into alveoli. Ventilation refreshes alveolar air; blood flow brings carbon dioxide and carries oxygen away. Both maintain concentration gradients. Surface area is a separate factor increasing gas exchange rate.

What earns the marks

• Distinguishes the receiving role of the atria from the role of the ventricles in pumping blood out of the heart.
• Traces blood through the vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle and aorta; valves prevent backflow.
• Identifies the trachea and bronchi as airways leading into the lungs and the alveoli as the gas-exchange surfaces surrounded by capillaries.
• Links the many alveoli to a large total surface area and the one-cell-thick alveolar and capillary walls to a short diffusion distance.
• Explains that ventilation and blood flow maintain concentration gradients, with oxygen diffusing into the blood and carbon dioxide diffusing into the alveoli.

Common mistakes and how to put them right

• Saying that atria pump directly to the lungs or body.

  Put it right: Atria pass blood to the ventricles; the ventricles pump it out of the heart.

• Saying that a large surface area maintains the concentration gradient.

  Put it right: Many alveoli increase the available exchange area; ventilation and blood flow maintain the gradients.

• Describing alveoli as having thick walls or plant-like cell walls.

  Put it right: The alveolar lining is one cell thick, helping keep the diffusion distance short.

Exam tips

• For an adaptation question, link each feature to its effect: many alveoli give a large surface area, thin walls give a short distance, and ventilation and blood flow maintain gradients.
• Use arrows to check the order of chambers and vessels, then explain what the arrows represent. Do not confuse the pulmonary artery with the pulmonary vein.