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Even in those rare instances when a horse develops chronic kidney disease, the ailment may go undetected for several years. The kidneys' ability to function satisfactorily even when their capacity is drastically diminished means that clinical signs don't become apparent until the condition is in an advanced and often fatal stage. Additionally, the signs of equine kidney disease are identical to other more common health problems. Of course, the extremely low incidence of equine kidney disease makes it a pretty safe bet that your horse won't be stricken. But you can keep your mind at ease and deepen your appreciation for that wet bedding you remove each day from his stall by gaining some vital insights on those truly remarkable kidneys. A sophisticated waste-processing system Situated on either side of the spine, just beneath the last few ribs and slightly behind the area that your saddle covers, your horse's kidneys function very much like a waste-water treatment plant. Protected from impact and injury by a cushioning layer of fat, a layer of bone and several inches of muscle, the two purple organs, each weighing about a pound and a half, sort through the various substances carried in the blood, conserving those that your horse needs to survive and disposing of those that could do him harm. As the kidneys carry out their basic filtration duties they also monitor and regulate fluid volume and composition. They promote red blood cell production, modulate blood pressure and control the blood's pH (acidity). Blood arrives in the kidneys via the renal artery--a branch of the aorta--and is processed in two layers of kidney tissue that lie beneath the organs' skinlike covering. Within the tissue, more than a million microscopic units called nephrons handle the purification duties. Only about 25 to 30 percent of the nephrons actively process fluid full-time. The remainder stand by in case of an increase in blood flow, which may be triggered by illness, excitement or cold temperatures. Additional nephrons also may be called into service when a horse consumes more water than usual.
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Each nephron begins at a tuft of capillaries on one of the tiny branches of the renal artery. Known as the glomerulus, this tuft is surrounded by a cup-shaped funnel, called Bowman's capsule, that receives the watery part of the blood for processing. Molecules of protein, fats and blood cells that are too large to fit through the filters of Bowman's capsule are rejected and remain in the circulation for other destinations. The fluid that passes through the capsule then moves into in a long winding tube called the loop of Henle. Here, appropriate amounts of salt, potassium, sulfate, phosphate, glucose and amino acids are extracted and returned to the blood to replenish deficits. Water that is needed by the circulatory system is absorbed at the end of the nephron, and waste substances, such as creatinine, urea, uric acid, pigments and excess salts and water, are sent to the bladder to be discharged. Almost 99 percent of the filtered blood is reabsorbed, while the remaining blood constituents are unwanted and excreted in the urine. Any potential waste that proves too large to fit through the kidneys' tubes is rerouted to the liver or the gut wall, where it is processed for excretion with solid feces. The entire blood volume (on average, about 9 gallons) passes through the kidneys more than 60 times in 24 hours
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