What Do Kidneys Do and What Happens When They Fail

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Before we look at what you can do when your kidneys start to fail, it's a good idea to review the basics on how the kidneys work in the body. With this knowledge, you will get a better understanding of why the kidneys are so important in the functioning of your body and the extent of the damage that can occur to your health if things do go wrong.

The two kidneys lie in the abdomen on the muscles of the back, near the waist, and are about 5 inches long. The urine formed from each kidney passes down a long tube called the ureter into the bladder, which can expand to contain and store urine. When urine is passed out of the body, it goes through a tube called the urethra (see Figure 1.1 on the following page).

Each kidney is made up of about 1 million units, called nephrons, which begin with a filter, comprising a tuft of capillaries (the tiniest blood vessels), called the glomerulus. At the glomerulus, liquid is derived from the blood plasma, comprising a solution from which most of the protein

Picture Kidneys Labeled

Figure 1.1: The position of the kidneys.

From the kidneys, the ureters conduct urine to the bladder, which empties through the urethra. Blood is supplied to the kidneys by two renal arteries from the aorta, the main artery of the body. Reprinted by permission from Kidney Failure, the Facts, by Stewart Cameron, Oxford University Press, 1996.

Figure 1.1: The position of the kidneys.

From the kidneys, the ureters conduct urine to the bladder, which empties through the urethra. Blood is supplied to the kidneys by two renal arteries from the aorta, the main artery of the body. Reprinted by permission from Kidney Failure, the Facts, by Stewart Cameron, Oxford University Press, 1996.

has been filtered out by the glomerular membrane. This solution, called the glomerular filtrate, passes down a long, winding tubule (meaning little tube) and finally into a pouch called the kidney pelvis, which in turn drains into the ureter. During its passage down the tubule, most of the filtrate is reabsorbed, but some constituents are more completely reabsorbed than others; tubular secretion adds other constituents to the fluid. The final urine is small in volume compared to the glomerular filtrate, and differs from it considerably in composition.

By these multiple mechanisms the kidneys achieve their remarkable regulatory capacity. It is very important to recognize that the kidneys' main function is not to excrete wastes; instead, they play a very important role in keeping what is called extracellular fluid constant in its makeup. Extracellular fluid is the medium in which the millions of cells that make up our bodies are bathed. Blood plasma is part of the extracellular fluid, and it circulates throughout the body by the pumping of the heart. The kidneys keep constant the composition of the extracellular fluid, namely its content of salts, acid, nutrients, and many other constituents. The lungs play a similar role in that they remove carbon dioxide and add oxygen to the blood, so as to keep these two constituents constant.

The principal function of the kidneys is to keep constant the composition of the extracellular fluid, with respect to all other constituents. The kidneys also keep the volume of the extracellular fluid constant. By extraordinarily complex and efficient mechanisms, the kidneys regulate the excretion of water, salt, potassium, calcium, acid, and many other elements, whatever the intake of these substances may be.

Hormones regulate many of the kidney's functions. Hormones are like chemical messengers that are produced in other organs and sent to the kidney via the blood. For example, antidiuretic hormone, a hormone produced in the brain, is secreted in response to the concentration of dissolved solutes in body fluids. A high concentration of dissolved solutes, such as might occur after water loss in a hot environment, stimulates the production of this hormone. The kidney responds to the hormone by making the urine more concentrated and lower in volume, thus conserving water in the body. At the other extreme, a low concentration of dissolved substances, such as might occur after drinking a lot of water, turns off antidiuretic hormone production. As soon as the hormone disappears from the blood (about 30 minutes), the kidneys stop conserving water and do the opposite: The urine becomes very dilute and increases in flow, thus excreting the water load.

Urine flow can range widely depending on a person's intake of fluids: Minimal urine volume, during severe dehydration, may be as little as 250 ml a day (less than a pint), while maximum urine volume, in the absence of this antidiuretic hormone, is many gallons a day. People who can't make this hormone or who don't respond to it, because they have one form or another of a condition called diabetes insipidus (no relation to sugar diabetes), excrete huge volumes of urine and as a result get thirsty. Their fluid intake usually keeps up with their urine output, and they remain only slightly dehydrated most of the time.

The regulation of salt excretion is closely related to the regulation of body water because salt is the dominant dissolved substance in the extracellular fluid. Salt excretion is regulated by hormones that are produced by the adrenal cortex and by the heart. The regulation of salt balance is discussed in Chapter 8.

Another function of the kidneys is the production of important hormones, including:

• Angiotensin, a hormone that raises blood pressure by constricting blood vessels and also stimulates the adrenal cortex to produce yet another hormone, aldosterone, an important regulator of sodium excretion

• Erythropoietin, a hormone that stimulates the bone marrow to produce more red cells whenever their number is reduced; again, this explains why anemia is such a common feature of kidney failure

• Prostaglandins, which help regulate blood pressure, sodium excretion, and other functions.

In addition, the body's production of vitamin D takes place, in part, in the kidneys, which explains why vitamin D deficiency is a prominent feature of kidney failure.

Clearly, the kidney has many functions besides the excretion of wastes. You cannot live without your kidneys because of the important role they play. Complete loss of kidney function causes death within a few weeks. The good news is that we seem to have much more kidney function than we need, because with adequate care a person can survive with as little as 5 percent of normal kidney function. Thus donation of one kidney does not cause any signs of kidney dysfunction in the donor.

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