This month, we will start a series on the function of the major organs of the feline body. Everyone knows that organs can potentially fail as pets age, but what do these organs actually do? What are their daily functions in maintaining health? This month we will start with the most common organ to fail in cats: the kidneys.
For those who don’t know, the kidneys produce urine and filter toxins out of the body, but they also do much more! They prevent the buildup of wastes and extra fluid in the body; keep levels of electrolytes stable (such as sodium, potassium, and phosphate); and make substances which help regulate blood pressure, stimulate red blood cell production, and regulate calcium in the body.
The kidneys are relatively tiny organs (only 0.4% of the total body weight), but they receive 25% of the cardiac (heart) output of the body. For example, the average human adult contains 4.7-5.5 liters of blood, yet the kidneys filter about 180 liters of blood a day. This means your entire blood volume is filtered by the kidneys dozens of times every day! The kidneys receive more blood than any other organ in the body.
Most cats are born with two kidneys. They are the same shape (bean shaped) and located in the same area (just below the rib cage, one on each side of the spine) as human kidneys, except cat kidneys are much smaller. Each kidney contains hundreds of thousands of tiny filtration units called nephrons. Each nephron itself is composed of multiple structures, with the glomerulus being most important for filtration of blood. The glomerulus keeps normal blood proteins and cells in the bloodstream, while allowing extra fluid and wastes to pass into the urine. Approximately 99% of fluid is retained in the blood due to the nephron: (For reference, if fluid wasn’t retained, the average human would have to drink 12 ounces of liquid every 3 minutes to make up the volume of fluid filtered through the kidneys!)
The kidneys are vital for regulation of the electrolytes sodium, potassium, and phosphate. They maintain electrolyte concentrations through filtering electrolytes from blood. Needed electrolytes are returned to the blood, while excess electrolytes are excreted into the urine. Normal blood electrolyte levels have a narrow range for health and the kidneys help maintain a balance between daily consumption and excretion.
Healthy nephrons also produce several hormones and enzymes, which are important for normal body function: erythropoietin, renin, and calcitriol.
Erythropoietin is a hormone and is important for the production of new red blood cells. It is released from kidney cells in response to hypoxia (low oxygen in the blood) and stimulates the bone marrow to produce more red blood cells. Decreasing amounts of erythropoietin is one reason that cats can become anemic when they have kidney disease.
Renin is an enzyme and acts on the hormone angiotensin to regulate blood pressure. Renin is released from kidney cells in response to low blood pressure and therefore acts to increase blood pressure back to normal range. It is still unknown exactly why some cats with kidney disease will develop high blood pressure but seems to be multifactorial.
Calcitriol is a hormone and is important for calcium regulation in the body, both directly and indirectly. It is the active form of Vitamin D and is produced in the kidneys from Vitamin D. Directly, calcitriol acts on the intestinal tract and bone to regulate blood levels of calcium. Indirectly, it acts to decrease levels of parathyroid hormone (PTH) when it becomes elevated. The effects of unregulated elevations of PTH on the body due to kidney disease, and lowered levels of calcitriol, are still being evaluated.
As you can see, the kidneys do so much more than just produce urine! Their role in the body is so integral to its proper functioning, which is why kidney disease can be so devastating. That being said, the kidneys are remarkably resilient, and typical signs of chronic kidney disease will not be noticed until the equivalent of half a kidney (or less) is all that is functioning. For more information on chronic kidney disease, you can read our December 2015 newsletter!
Until next month, we hope you and your feline friends enjoy the cool weather and crisp colors of Autumn! See you next time! =^_^=