Final answer:
The long-term increase in hematocrit at altitude occurs because plasma volume is decreasing, which is true. The decrease in oxygen concentration at high altitudes stimulates the production of red blood cells, leading to an increase in hematocrit. This adaptation allows individuals to perform physical activities without experiencing altitude sickness.
Step-by-step explanation:
The statement 'The long-term increase in hematocrit at altitude occurs because plasma volume is decreasing.' is true. When a person is at a high altitude, the body experiences a decrease in oxygen concentration. As a result, the kidneys produce the hormone erythropoietin (EPO) to stimulate the production of red blood cells, which leads to an increase in hematocrit. The increase in hematocrit helps the body transport more oxygen with the available red blood cells. This adaptation allows individuals to engage in physical exertion without developing altitude sickness.