(ii) State two ways in which the structure of blood vessel A differs from the structure of blood vessel J.

4. (a) (ii)
Here are two ways in which the structure of blood vessel A (pulmonary vein) differs from the structure of blood vessel J (pulmonary artery):

1. Vessel J (pulmonary artery) has a thicker, more muscular and elastic wall compared to vessel A (pulmonary vein). This allows it to withstand the higher pressure of blood pumped from the heart.
2. Vessel J (pulmonary artery) has a narrower lumen (internal diameter) compared to vessel A (pulmonary vein).

4. (b)
The change in the mean number of capillaries improves an athlete's performance in the following ways:

• The increase in the mean number of capillaries per mm² after training means there is a greater surface area for exchange between the blood and the muscle cells.
• This allows for more efficient delivery of oxygen and nutrients (like glucose) to the working muscles, which are essential for aerobic respiration and energy production.
• It also facilitates more efficient removal of metabolic waste products such as carbon dioxide and lactic acid from the muscles.
• Improved oxygen supply and waste removal mean muscles can perform aerobic respiration for longer, producing more ATP (energy) and delaying the onset of fatigue, thereby enhancing the athlete's endurance and overall performance.