Bovine biology series
Part 4 - The Kidney
The kidneys are actually two organs, sitting very close to the aorta artery from the heart. They are located outside of the peritoneal cavity. A ureter from each kidney carries urine produced in the kidney to the bladder. The bladder, of course, stores urine until it is emptied through the urethra and out of the body sinus as directed by the parasympathetic nerves in the spinal column.
In our fourth installment, we examine the wonder of an essential organ that primarily does three things: 1. Filter blood so waste products can be removed, 2. Recover and store useful metabolites the body can still use, and 3. Store those metabolites the body cannot use (bladder) in a liquid volume and discard this volume from the body.
How is all this done? Let's take a look.
There is a term that biologist like to use called homeostasis. This word is derived from the beautiful Greek language in which the prefix homeo means likeness, and the suffix stasis refers to standing. Put together, they mean a balance between two differences, a likeness between two systems that together result in the sustenance of biology.
Homeostasis, then, is the state of equilibrium in biological systems, which is the relative consistency of body fluids such as blood, lymph and tissue fluid.
The kidney largely regulates homeostasis by controlling water balance, pH, osmotic pressure levels, electrolyte levels, buffers, and the concentrations of many blood plasma substances.
There are three kidneys, or renal functions that control homeostasis. They are glomerular filtration, tubular reabsorption and tubular secretion. Simply described, as blood is presented to the kidney, it passes through the glomerular capsule. Under pressure, the blood volume is separated into various components, depending on size of the molecule. The larger ones, such as the red blood cells, are held up and discharged from the capsule. It is the smaller molecules, such as electrolytes, salts, and some very small proteins that are held in the glomerulus.
This collected fluid is the blood plasma. The kidney simply and remarkably collects this volume for a time, and either shunts it to the bladder for urine removal or secretes back into the blood or lymph for use again.
This is most significant, because in this way, the balance between the cellular world and the liquid world is maintained. As substances, such as sodium or glucose or some amino acids, are accumulated in the glomerulus, the body says, let's use these for the regulation of osmotic pressure (sodium) or energy (glucose) or amino acids (muscle work), and they are dumped into the blood by tubular reabsorption.
Certainly the rate at which blood is filtered through the glomerulus is controlled by the nervous system adjusting for blood pressure. Thus when blood pressure is high, the glomerular filtration rate is high. And when the blood pressure is low, filtration is slowed.
The intake of water is most significant here. When we think about the most immediate need for water intake, it is in serving the role as a carrier for blood metabolites. If water intake is reduced for whatever reason, blood volume is lowered because blood volume is constantly being used up and must be recharged, so to speak. So in order for enough blood to go through the glomerulus, the blood volume and therefore the blood pressure must be maintained by water intake, primarily.
Here is another concern. Following a large meal, the action of the liver produces an increase in blood metabolites, especially those that feed cells so chemical work may be done, and then physical work may be done as muscles move. The kidney gets into the act here too, because, as muscles move and milk is secreted, there are by-products formed. These by-products include bits and pieces of protein and electrolytes and liver metabolites such as urobilin.
These by-products are carried in the blood but must be removed from the body. Keep in mind that these bits and pieces are the end result of metabolic processes. They are supposed to be in the blood.
As the blood volume is filtered through the glomerulus, these by-products are filtered out in the blood plasma, and wind up in the bladder as urine. When the bladder is or nearly is full, the nervous system says, OK, time to urinate, and the bladder is emptied.
Why is water intake important? Simply because it is water that carries all these bits and pieces from the cells into the blood into the glomerulus and into the bladder. Dehydration reduces blood volume, and therefore reduces the body’s ability to get rid of metabolites no longer required and in fact may be toxic to the body if not discarded.
Urine is at times very yellow to almost completely clear. Why is this? Urine contains the pigment of the liver called bilirubin, which is converted to urobilin. Generally speaking, the more yellow the urine, the higher the metabolic rate, such as that following a large meal, or it could mean a reduced blood volume, in which large amounts of metabolites are accumulating in the bladder over time.
Urine also contains urea, which is formed by the liver from ammonia, a by-product of amino acid metabolism. The kidney serves a very important function of filtering this urea out of the blood volume, storing it in the bladder as urine so it can be excreted.
One more thing about urine. Remember that there are two routes of excretion of food metabolites once past the rumen. One is the feces, which represents that portion of the feed not absorbed by the intestinal tract. The other route is through the kidney and urine. An example is potassium, which is almost completely absorbed in the intestine. If it is fed in excess of metabolic requirement, then it is absorbed in excess in the blood.
What happens? The body only requires so much. After that point is met, the potassium cation must be excreted. Well, it cannot be discharged any other way except through the kidney and the urine. This means the kidneys are obligated to get rid of excess potassium, otherwise osmotic pressure is altered, and homeostasis is adversely affected.
We have talked about filtering of the blood volume, the recycling of metabolites for use, and the accumulation of waste products in the urine volume.
The kidney, therefore, keeps what is essential, discards the rest, and all the while, regulates blood and cellular osmotic pressure by keeping or removing water from the blood volume. What a remarkable organ!
But we are not finished yet, as still another obligation is placed on this organ. It is the regulation (in part) of blood pH.
Why is this important? Well, an increased metabolic rate, such as that occurs when cows are milking very large amounts, or when we are working very hard ourselves physically, results in an increase of carbon dioxide in the blood. We all learned somewhere in school that we breath in oxygen and breath out carbon dioxide. Thus carbon dioxide is a by-product of normal metabolism, in which chemical work is converted into physical work. Carbon dioxide tends to decrease blood pH, making it more acidic. [Normal blood pH (arterial) is 7.4; normal urine pH is 6.]
Carbon dioxide is discharged from the cellular level, the blood transports this little molecule to the lungs, the diaphragm moves up and down, and out it goes after every breath we take in. That is why breathing rate is matched to work activity. The harder we work, the more we breath, not only for the entry of atmospheric oxygen but for the discharge of carbon dioxide.
The kidney plays a role here in secreting more bicarbonate into the bloodstream, attempting to increase and normalize the blood pH. The kidney, therefore, plays a very important role of helping maintain the blood within normal pH parameters by secreting bicarbonate during times of higher than normal metabolic rates. As soon as the proper carbon dioxide level in the blood is established, which occurs when respiration rate discharge equals metabolic rate output, homeostasis is reached. And bicarbonate secretion by the kidney falls off.
If a cow is clinically suffering from acidosis, the kidneys step up and begin secreting more bicarbonate into the blood in an attempt to increase blood pH. Also, respiration rate is increased to speed along the discharge of carbon dioxide. If a cow is suffering from alkalosis, then the kidney slows or stops the secretion of bicarbonate, increases the secretion of hydrogen ions, and even respiration will be slowed until the buildup of carbon dioxide in the bloodstream brings the blood pH back to normal.
As you might guess, failure of the kidney, or renal failure, is a most severe impairment of the body's ability to contain in normal parameters the ebb and flow of blood metabolites, and the maintenance of homeostasis.
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