The athlete is running a marathon, therefore their muscles are constantly contracting and therefore using up ATP (Energy). ATP is generated through respiration in cells in the human body. Respiration generates heat. It is also a summers day suggesting temperatures would be higher. Therefore, the blood temperature of the athlete would rise, meaning a negative feedback response is needed to cool the body down. One way the body cools itself down, is by sweating. Because the body is secreting sweat, it loses water. This makes the internal solute concentration of the blood too high. Therefore, the body needs to preserve more water to maintain its osmotic balance. The body does this by secreting ADH (anti-diuretic hormone), which makes the plasma membranes of the cells of the collecting duct (in the nephrons in the kidneys) more permeable to water by adding aquaporins (which are water channels) to their membranes. The high solute concentration of the medulla means that most of the water in the collecting duct is reabsorbed by osmosis. Thus, ADH levels in the athlete are high to preserve water in response to a loss in water due to sweat secretion to cool down the body's blood temperature.
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