Final answer:
The amount of plasma volume in the blood increases due to endurance training or repeated heat exposure, assisting in the body's cooling process through sweating and blood vessel dilation. Additionally, erythropoietin levels rise, which can be risky due to increased blood viscosity.
Step-by-step explanation:
In response to endurance training or repeated and prolonged heat exposure, the amount of plasma volume in the blood increases. This physiological adaptation helps to dissipate excess heat through mechanisms such as sweating and dilation of dermal blood vessels. Elevated body temperature, or hyperthermia, promotes an increase in heart rate and strength of contraction to aid in these thermoregulatory processes. Conversely, endurance training also leads to an increase in erythropoietin, which stimulates the production of red blood cells, enhancing the oxygen supply to muscles and improving endurance. However, an increase in erythropoietin can be dangerous, as it can lead to increased blood viscosity and subsequently make it difficult for blood to circulate, posing health risks to athletes.