It's easy not to think too much about your horse's kidneys. As long as there's some indication that they're working -- and the wet bedding in the stall seems evidence enough -- you probably don't invest much time pondering just how well they function. That's partly due to hearing few complaints about them these days.
Extremely efficient at processing every drop of your horse's 10 gallons of blood twice each hour, the kidneys are among the most reliable of all of his body's regulatory systems. Studies show that a fraction of one percent of horses are diagnosed with kidney disease. Even fewer die from it. One case study conducted at Purdue University from 1964 to 1996 reported that only 515 of 442,535 horses admitted to participating veterinary teaching hospitals had kidney disease -- that's about 0.12 percent.
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.
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.
Signs of system kidney failure
In those relatively rare instances when kidney function is disrupted, waste products accumulate in the blood and eventually poison the system. Yet a horse with the usual subacute or chronic kidney dysfunction usually seems more "off" than ill. He may be a little lethargic or depressed. He may have a little less appetite and his performance level may not be up to par. His coat may lose some of its usual luster. Unfortunately, these signs appear only in the later stages of most kidney disease, and can easily be attributed to another illnesses. Taken together, however, poor performance, lethargy or depression and a rough coat earn a careful investigation for kidney disease.
