Endurance training increases blood volume. There are two mechanisms responsible for this. Firstly, exercise increases the release of two hormones (ADH and aldosterone), which result in the kidneys retaining water. Secondly, exercise increases the amount of protein in blood, and this increases the osmotic pressure in the blood, which means more fluid is retained in the blood.
The increase in blood volume means more blood is returning to the heart, which causes an increase in stroke volume. This ultimately leads to an increase in the athletes VO2max because at maximal work loads, the heart rate is maximal, and the stroke volume has increased, causing an increase in cardiac output. This means more oxygen is available for the active muscles, which results in an increase in the VO2max.
The increase in blood volume means more blood is returning to the heart, which causes an increase in stroke volume. This ultimately leads to an increase in the athletes VO2max because at maximal work loads, the heart rate is maximal, and the stroke volume has increased, causing an increase in cardiac output. This means more oxygen is available for the active muscles, which results in an increase in the VO2max.