Answers:

1. The kidneys constitute less than 0.5% of the body mass, yet they receive almost one fourth (22%) of the cardiac output at rest.  During exercise, renal blood flow to the kidney is greatly reduced, as the muscles demand for oxygen and blood flow is increased.  The reduction is related to the intensity of exercise and renal blood flow may fall to 25% of the resting value when strenuous work is performed. The combination of sympathetic nervous activity and the release of catecholamine substances is involved in this process. Hormones that constrict afferent and efferent arterioles, causing reductions in GFR and renal blood flow, include norepinephrine and epinephrine released from the adrenal medulla. In general, blood levels of these hormones parallels the activity of the sympathetic nervous system. The degree of hydration also has an important influence on the glomerular filtration rate. An antidiuretic effect is observed during intense exercise (such as endurance sports). Changes in urine flow are dependent on the plasma antidiuretic hormone levels which are increased by intense exercise. Heavy exercise (such as power/explosive sports) have an inhibitory effect on most electrolytes (Na, Cl, Ca, P). With potassium, however, most studies report that potassium excretion is not consistently affected by moderate to heavy exercise. Increased aldosterone production helps the body to maintain sodium by increasing its reabsorption from the filtered tubular fluid. 

2. Numerous physiological factors may affect renal function during exercise. A state of dehydration such as intense thirst during exercise, alcohol consumption the previous evening, or a low fluid intake on the day of training may greatly affect renal function. An intense arousal state associated with heavy exercise would also result in a large sympathetic discharge prior to and during training. Mass sympathetic discharge can potentially lower RBF to three percent of cardiac output. During short duration, high exertion exercises (such as most power/explosive sports), decreases in GFR of 78% have been shown. Other studies have demonstrated no change in filtered fraction during exercise, suggesting reduction in RBF are also around 78%. The combination of the aforementioned factors could easily lead to a massive reduction in RBF having several impacts in renal function eventually leading to renal failure. In addition, short term anaerobic exercises may also result in acute metabolic acidosis. Thus, all three predisposing factors of acute tubular necrosis (dehydration, renal hypoxia due to decreased blood flow, and acidosis) would contribute to renal failure. 

3. Strenuous exercises in untrained individuals leads to an increased excretion of erythrocytes and leucocytes in urine. Cylindruria has been regularly found in postexercise urine in different sports. Postexercise proteinuria is a common phenomenon in humans. It seems to be directly related to the intensity of exercise, rather than to its duration. This excretion of proteins in urine is a transient state with a half-time of approximately 1 hour. Postexercise proteinuria has a pattern different from normal physiological proteinuria. Immunochemical techniques demonstrate that postexercise proteinuria is of the mixed glomerular-tubular type, the former being predominant. The increased clearance of plasma proteins suggests an increased glomerular permeability and a partial inhibition of tubular reabsorption of macromolecules. Hemoglobinuria and myoglobinuria may also be observed. Hemoglobin is cleared from plasma primarily by the liver; the primary route for elimination of myoglobin is renal. Hemolysis resulting in hemoglobinemia is also known to occur in athletes, but this is seldom manifest as hemoglobinuria due to a high renal threshold of hemoglobin (128 mg%) as compared to myoglobin (less than 15 mg% and maybe as low as 0.3 mg%). The presence of myoglobin in urine after exercise of long duration is more common and sometimes the pigment is present in such concentrations that the urine is red in color, mistakenly prompting a diagnosis of hematuria. Whether or not myoglobinuria occurs following anaerobic exercise in the trained athlete is unclear, although it is common in untrained individuals. The degree of hydration appears to be important to reduce these abnormalities.