Explain why an extensive network of capillaries and ventilation of the lungs helps to maximise the effectiveness of gas exchange.

The capillaries deliver blood that has high carbon dioxide levels and low oxygen levels and take away blood that has high oxygen levels and low carbon dioxide levels. This means there is always a concentration gradient between the blood and the air in the lungs. Ventilation of the lungs replaces air with higher concentrations of carbon dioxide and lower concentration of oxygen with air that has a higher concentration of oxygen and lower concentration of carbon dioxide. These factors maintain a high concentration gradient between the body and air for the two gases.