​Physical exercise is universally recognized for its profound benefits on cardiovascular health, metabolic regulation, and psychological well-being. However, when physical exertion crosses the threshold from moderate fitness to extreme, high-intensity training—such as ultramarathons, intense bodybuilding, or military-style combat training—the body undergoes severe physiological stress. One of the vital organ systems heavily impacted by this extreme exertion is the renal system. Understanding how strenuous exercise affects kidney function is crucial for athletes aiming to optimize performance while safeguarding their health. ​During high-intensity exercise, the body undergoes a massive redistribution of blood flow. To sustain the high metabolic demands of working skeletal muscles and the heart, blood is shunted away from internal organs. Consequently, renal blood flow can drop by as much as 70% to 80% during maximal exertion. This sharp decline in perfusion leads to a temporary reduction in the Glomerular Filtration Rate (GFR)—the rate at which the kidneys filter waste from the blood. While the kidneys are highly resilient and typically recover their full baseline function within hours of resting, prolonged periods of restricted blood flow can induce cellular stress within renal tissues. ​The diagram below illustrates the basic functional unit of the kidney, the nephron, where this filtration and reabsorption process takes place continuously. Compounding this reduction in blood flow is the challenge of exercise-induced dehydration. Severe sweating during strenuous workouts significantly decreases blood volume and increases blood osmolality. In response, the kidneys must work exponentially harder to conserve water, producing highly concentrated urine. The combination of decreased blood supply (ischemia) and dehydration significantly increases the risk of temporary Acute Kidney Injury (AKI), a condition where the kidneys abruptly struggle to clear metabolic waste from the bloodstream. ​Furthermore, extreme physical overexertion carries the risk of a dangerous medical condition known as Exertional Rhabdomyolysis. This occurs when muscle tissue breaks down rapidly due to mechanical or metabolic stress, releasing cellular contents into the bloodstream—most notably a large oxygen-binding protein called myoglobin. When myoglobin reaches the kidneys, its large molecular size can physically obstruct the delicate renal tubules. Additionally, myoglobin breaks down into toxic byproducts that directly damage kidney cells, potentially leading to severe, life-threatening acute renal failure. A hallmark symptom of this condition is dark, tea-colored urine, accompanied by profound muscle weakness and swelling Prepared by: Dr. Taqa Hazim Abdullah AL_mustaqbal University is the first university in Iraq