The kidneys maintain water volume and balance as well as regulate blood composition.

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Introduction to How Your Kidneys Work

­Did you­ know that kidney stone plagued even the ancient Egyptians [source: NKUDIC]? Or that 26 million Americans have chronic kidney disease [source: National Kidney Foundation]? Hundreds of thousan­ds of people suffer from renal failure each year and undergo dialysis or await a kidney transplant.

But what do your kidneys do? Why are they so important? Don't they just produce urine? In this article, we'll take a close look at our kidneys and find out exactly what they do.

Your kidneys are two bean-shaped organs, each about the size of your fist. They are located in the middle of your back, just below your rib cage, on either side of your spine. Your kidneys weigh about 0.5 percent of your total body weight. Although the kidneys are small organs by weight, they receive a huge amount -- 20 percent -- of the blood pumped by the heart. The large blood supply to your kidneys enables them to do the following tasks:

  • Regulate the composition of your blood: Keep the concentrations of various ions and other important substances constant; Keep the volume of water in your body constant; Remove wastes from your body (urea, ammonia, drugs, toxic substances); Keep the acid/base concentration of your blood constant
  • Help regulate your blood pressure
  • Stimulate the making of red blood cells
  • Maintain your body's calcium levels

­­Your kidneys receive the blood from the renal artery, process it, return the processed blood to the body through the renal vein and­ remove the wastes and other unwanted substances in the urine. Urine flows from the kidneys through the ureters to the bladder. In the bladder, the urine is stored until it is excreted from the body through the urethra.

Now let's take a look inside your kidneys.

Diagram showing the parts of the kidney and the nephron

Inside Your Kidneys

If you were to cut a kidney in half, you would see the following parts:

  • Renal capsule - a thin, outer membrane that helps protect the kidney
  • Cortex - a lightly colored outer region
  • Medulla - a darker, reddish-brown, inner region
  • Renal pelvis - a flat, funnel-shaped cavity that collects the urine into the ureters

If you look closely at the cortex and medulla, you can see many tiny, tubular structures that stretch across both regions perpendicular to the surface of the kidney. In each kidney, there are one million of these structures, called nephrons. The nephron is the basic unit of the kidney. It's a long, thin tube that is closed at one end, has two twisted regions interspaced with a long hairpin loop, ends in a long straight portion and is surrounded by capillaries.

The parts of the nephron are as follows:

  • Bowman's capsule - This closed end at the beginning of the nephron is located in the cortex.
  • Proximal convoluted tubule or proximal tubule - The first twisted region after the Bowman's capsule; it's in the cortex.
  • Loop of Henle - A long, hairpin loop after the proximal tubule, it extends from the cortex down into the medulla and back.
  • Distal convoluted tubule or distal tubule - This second twisted portion of the nephron after the loop of Henle is located in the cortex.
  • Collecting duct - This long straight portion after the distal tubule that is the open end of the nephron extends from the cortex down through the medulla.

Each part of the nephron has different types of cells with different properties -- this is important in understanding how the kidney regulates the composition of the blood.

The nephron has a unique blood supply compared to other organs:

  • Afferent arteriole - connects the renal artery with the glomerular capillaries
  • Glomerular capillaries - coiled capillaries that are inside the Bowman's capsule
  • Efferent arteriole - connects the glomerular capillaries with the peritubular capillaries
  • Peritubular capillaries - located after the glomerular capillaries and surrounding the proximal tubule, loop of Henle, and distal tubule
  • Interlobular veins - drain the peritubular capillaries into the renal vein

The kidney is the only organ of the body in which two capillary beds, in series, connect arteries with veins. This arrangement is important for maintaining a constant blood flow through and around the nephron despite fluctuations in systemic blood pressure.

Regulating the composition of the blood involves the following:

  • Keeping the concentrations of various ions and other important substances constant
  • Keeping the volume of water in your body constant
  • Removing wastes from your body
  • Keeping the acid/base concentration of your blood constant

The kidney does this by a combination of three processes:

  • It filters 20 percent of the plasma and non-cell elements from the blood into the inside of the nephron (the lumen).
  • It reabsorbs the components that the body needs from the lumen back into the blood.
  • It secretes some unwanted components from the blood into the lumen of the nephron.

Anything (fluid, ions, small molecules) that has not been reabsorbed from the lumen gets swept away to form the urine, which ultimately leaves the body. Through these processes, the blood is maintained with the proper composition, and excess or unwanted substances are removed from the blood into the urine.

Next, we'll look at how the kidneys regulate blood composition by three main processes:

  • Filtration
  • Reabsorption
  • Secretion
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The glomerular capillaries, peritubular capillaries and the Bowman's capsule are all integral to the filtration process.

Kidney Filtration

In the nephron, approximately 20 percent of the blood gets filtered under pressure through the walls of the glomerular capillaries and Bowman's capsule. The filtrate is composed of water, ions (sodium, potassium, chloride), glucose and small proteins (less than 30,000 daltons -- a dalton is a unit of molecular weight). The rate of filtration is approximately 125 ml/min or 45 gallons (180 liters) each day. Considering that you have 7 to 8 liters of blood in your body, this means that your entire blood volume gets filtered approximately 20 to 25 times each day! Also, the amount of any substance that gets filtered is the product of the concentration of that substance in the blood and the rate of filtration. So the higher the concentration, the greater the amount filtered or the greater the filtration rate, the more substance gets filtered.

This filtration process is much like the making of espresso or cappuccino. In a cappuccino machine, water is forced under pressure through a fine sieve containing ground coffee; the filtrate is the brewed coffee. The arrangement of the glomerular capillaries in series with the peritubular capillaries is important to maintain a constant pressure in the glomerular capillaries, and thus a constant rate of filtration, despite momentary fluctuations in blood pressure. Once the filtrate has entered the Bowman's capsule, it flows through the lumen of the nephron into the proximal tubule.

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Kidney Reabsorption

Once inside the lumen of the nephron, small molecules, such as ions, glucose and amino acids, get reabsorbed from the filtrate:

  • Specialized proteins called transporters are located on the membranes of the various cells of the nephron.
  • These transporters grab the small molecules from the filtrate as it flows by them.
  • Each transporter grabs only one or two types of molecules. For example, glucose is reabsorbed by a transporter that also grabs sodium.
  • Transporters are concentrated in different parts of the nephron. For example, most of the Na transporters are located in the proximal tubule, while fewer ones are spread out through other segments.
  • Some transporters require energy, usually in the form of adenosine triphosphate (active transport), while others don't (passive transport).
  • Water gets reabsorbed passively by osmosis in response to the buildup of reabsorbed Na in spaces between the cells that form the walls of the nephron.
  • Other molecules get reabsorbed passively when they are caught up in the flow of water (solvent drag).
  • Reabsorption of most substances is related to the reabsorption of Na, either directly, via sharing a transporter, or indirectly via solvent drag, which is set up by the reabsorption of Na.

The reabsorption process is similar to the "fish pond" game that you see in some amusement parks or state fairs. In these games, there is a stream that contains different colored plastic fish with magnets. The children playing the game each have a fishing pole with an attached magnet to catch the fish as they move by. Different colored fish have different prize values associated with them, so some children will be selective and try to grab the colored fish with the highest prize value. Now suppose our nephron is the stream, the filtered molecules are the various colored fish, and our children are the transporters. Furthermore, each child is fishing for a specific colored fish. Most children start at the beginning of the stream and some spread out further downstream. By the end of the stream, most of the fish have been caught. This is what happens as the filtrate travels through the nephron.

Two major factors affect the reabsorption process:

  • Concentration of small molecules in the filtrate - the higher the concentration, the more molecules can be reabsorbed. Like our children in the fish pond game, if you increase the number of fish in the stream, the children will have an easier time catching them. In the kidney, this is true only to a certain extent because: There is only a fixed number of transporters for a given molecule present in the nephron. There is a limit to how many molecules the transporters can grab in a given period of time.
  • Rate of flow of the filtrate - flow rate affects the time available for the transporters to reabsorb molecules. As with our fish pond, if the stream moves by slowly, the children will have more time to catch fish than if the stream were moving faster.

To give you an idea of the quantity of reabsorption across the nephron, let's look at the sodium ion (Na) as an example:

  • Proximal tubule - reabsorbs 65 percent of filtered Na. In addition, the proximal tubule passively reabsorbs about 2/3 of water and most other substances.
  • Loop of Henle - reabsorbs 25 percent of filtered Na.
  • Distal tubule - reabsorbs 8 percent of filtered Na.
  • Collecting duct - reabsorbs the remaining 2 percent only if the hormone aldosterone is present.
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Your blood needs your kidneys to deal with those potato chips.

©iStockphoto.com/henrik5000

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Kidney Processes Working Together

Some substances are secreted from the plasma into the lumen by the cells of the nephron. Examples of such substances are ammonia (NH3). As in reabsorption,­ there are transporters on the cells that can move these specific subs­tances into the lumen.

­Now let's put all of these processes -- filtration, reabsorption and secretion -- together to understand how the kidneys maintain a constant composition of the blood. Let's say that you decide to eat several bags of salty (NaCl) potato chips at one sitting. The Na will be absorbed into your blood by your intestines, increasing the concentration of Na in your blood. The increased Na in the blood will be filtered into the nephron. While the Na transporters will attempt to reabsorb all of the filtered Na, it's likely that the amount will exceed their ability. Therefore, excess Na will remain in the lumen; water will also remain, due to osmosis. The excess Na will be excreted into the urine and eliminated from the body. So whether a substance remains in the blood depends on the amount filtered into the nephron and the amount reabsorbed or secreted by various transporters.

Let's look at an another example: Why do you have to keep taking repeated doses of any given medicine? Well, once you take the medicine, it gets absorbed by the intestine into the blood. The medicine in the blood acts on its target cell and also gets filtered into the nephron. Most medicines don't have transporters in the nephron to reabsorb them from the filtrate. In fact, some transporters actively secrete medicines into the nephron. Therefore, the medicine gets eliminated in the urine and you must take another dosage later.

We've seen how the kidney can regulate ions and small molecules and eliminate unwanted substances. In the next section, we'll see how the kidney maintains water balance.

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Maintaining Water Volume

Your kidneys have the ability to conserve or waste water. For example, if you drink a large glass of water, you'll find that you will have the urge to urinate within an hour or so. In contrast, if you don't drink for a while, such as overnight, you will not produce much urine and it will usually be very concentrated (i.e. darker). How does your kidney know the difference? The answer to this question involves two mechanisms:

  • The structure and transport properties of the loop of Henle in the nephron.
  • The anti-diuretic hormone (ADH), also called vasopressin, secreted by the pituitary gland.

The loop of Henle has a descending limb and an ascending limb. As filtrate moves down the loop of Henle, water is reabsorbed, but ions (Na,Cl) aren't. The removal of water serves to concentrate the Na and Cl in the lumen. Now, as the filtrate moves up the other side (ascending limb), Na and Cl are reabsorbed, but water isn't. What these two transport properties do is set up a concentration difference in NaCl along the length of the loop, with the highest concentration at the bottom and lowest concentration at the top. The loop of Henle can then concentrate NaCl in the medulla. The longer the loop, the bigger the concentration gradient. This also means that the medulla tissue tends to be saltier than the cortex tissue.

Now, as the filtrate flows through the collecting ducts, which go back down through the medulla, water can be reabsorbed from the filtrate by osmosis. Water moves from an area of low Na concentration (high water concentration) in the collecting ducts to an area of high Na concentration (low water concentration) in the medullary tissue. If you remove water from the filtrate at this final stage, you can concentrate the urine.

­­ADH, which is secreted by the pituitary gland, controls the ability of water to pass through the cells in the walls of the collecting ducts. If no ADH is present, then no water can pass through the walls of the ducts. The more ADH present, the more water can pass through.

Specialized nerve cells, called osmoreceptors, in the hypothalamus of the brain sense the Na concentration of the blood. The nerve endings of these osmoreceptors are located in the posterior pituitary gland and secrete ADH. If the Na concentration of the blood is high, the osmoreceptors secrete ADH. If the Na concentration of the blood is low, they don't secrete ADH. In reality, there is always some very low level of ADH secreted from the osmoreceptors.

Now let's look at how your kidneys maintain water volume.

©iStockphoto.com/lovleah He's going to need a bathroom break pretty soon.

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Maintaining Water Balance

When you drink a large glass of water, the water gets absorbed into the blood and the following happens:

  • The absorbed water increases the amount of water filtered in the glomerulus.
  • The absorbed water in the blood reduces the Na concentration a little.
  • The reduced Na concentration lowers the amount of Na filtered in the glomerulus.
  • The nephron reabsorbs all of the reduced Na load and some of the accompanying water, leaving excess water in the filtrate.
  • The reduced Na concentration is sensed by the osmoreceptors.
  • The osmoreceptors do not secrete as much ADH.
  • Because the collecting ducts don't see as much ADH, they don't allow much water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
  • The excess water gets excreted in the urine.
  • When the excess water is excreted, the Na concentration of the blood returns to normal.

Typically, we don't drink water overnight when we sleep. So, our intestines aren't absorbing water:

  • Decreased water absorption by the intestine reduces the amount of water in the blood.
  • Decreased water in the blood reduces the amount of water filtered in the glomerulus.
  • Decreased water in the blood increases the Na concentration in the blood.
  • Increased Na concentration in the blood increases the amount of Na filtered in the glomerulus.
  • The nephron doesn't reabsorb all of the filtered Na, and some water remains with it in the filtrate.
  • The increased Na concentration in the blood is sensed by the osmoreceptors.
  • The osmoreceptors secrete ADH.
  • The collecting ducts see more ADH and allow water to be reabsorbed in response to the Na concentration gradient set up by the loop of Henle.
  • More water gets reabsorbed from the collecting ducts, producing a concentrated urine. A little water is lost in the urine because of the Na; we can't excrete solid urine.
  • The removal of Na and increased reabsorption of water help return the blood concentration of Na to normal.

So, the loop of Henle sets up the Na concentration gradient across the medulla, allowing for water to be reabsorbed from the collecting ducts, and ADH allows the water to pass through those collecting ducts.

Your blood maintains a constant concentration of hydrogen ion (pH) by a chemical mixture of hydrogen ions and sodium bicarbonate. The sodium bicarbonate is produced by the carbon dioxide (CO2) formed in the cells as a byproduct of many chemical reactions. The CO2 enters the blood in the capillaries, where red blood cells contain an enzyme called carbonic anhydrase that helps combine CO 2 and water (H 2O) to form carbonic acid (H 2 CO3 ) quickly. The carbonic acid formed then rapidly separates into hydrogen ions (H+ ) and bicarbonate ions (HCO3-). This reaction can also proceed in the reverse direction, whereby sodium bicarbonate plus hydrogen ion yields carbon dioxide and water.

Carbonic Anhydrase

CO 2 + H 2 O <---------> H 2 CO3 <---------> H+ + HCO 3-

The correct pH is maintained by keeping the ratio of hydrogen ion to bicarbonate in the blood constant. If you add acid (hydrogen ion) to the blood, then you will reduce the bicarbonate concentration and alter the pH of the blood. Similarly, if you reduce the hydrogen ion by adding alkali, you will increase the bicarbonate concentration and alter the pH of the blood.

Now, the acid/base balance of our blood changes in response to many things including:

  • Diet - diets rich in meats provide acids to the bloods when digested. In contrast, diets rich in fruits and vegetables make our blood alkaline because they are rich in bicarbonates.
  • Exercise - exercising muscles produce lactic acid that must be eliminated from the body or metabolized.
  • Breathing - high altitude causes rapid breathing that makes our blood alkaline. In contrast, certain lung diseases that block the diffusion of oxygen can cause the blood to be acidic.

In the next section, we'll take a look at how the kidneys regulate blood composition.

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Regulating Blood Composition

The kidney can correct any imbalances by:

  • Removing excess acid (hydrogen ion) or bases (bicarbonate) in the urine and
  • Restoring the bicarbonate concentration in the blood to normal

The kidney cells produce a constant amount of hydrogen ion and bicarbonate because of their own cellular metabolism (production of carbon dioxide). Through a carbonic anhydrase reaction similar to the red blood cells, hydrogen ions get produced and secreted into the lumen of the nephron. Also, bicarbonate ions get produced and secreted into the blood. In the lumen of the nephron, filtered bicarbonate combines with secreted hydrogen ions to form carbon dioxide and water (carbonic anhydrase is also present on the luminal surface of the kidney cells). Whether the kidney removes hydrogen ions or bicarbonate ions in the urine depends upon the amount of bicarbonate filtered in the glomerulus from the blood relative to the amount of hydrogen ions secreted by the kidney cells. If the amount of filtered bicarbonate is greater than the amount of secreted hydrogen ions, then bicarbonate will be lost in the urine. Likewise, If the amount of secreted hydrogen ion is greater than the amount of filtered bicarbonate, then hydrogen ions will be lost in the urine (i.e. acidic urine).

Let's consider a few examples:

  • Acid Diet Hydrogen ions added to the blood by breaking down a meat-rich diet combine with bicarbonate in the blood and form carbon dioxide and water. This reaction reduces the bicarbonate concentration and the pH in the blood. The decreased bicarbonate concentration in the blood reduces the amount of bicarbonate filtered in the glomerulus. All of the filtered bicarbonate combines with the hydrogen ion secreted by the kidney cells in the lumen to form carbon dioxide and water. Because the filtered load of bicarbonate was less than the amount of hydrogen ion secreted by the kidney cells, there is an excess of hydrogen ion in the urine. The amount of bicarbonate secreted from the kidney cells into the blood was equal to the hydrogen ion secreted into the lumen and greater than the filtered load of bicarbonate from the blood -- therefore, the blood has a net gain of bicarbonate. This process continues to lose hydrogen ions in the urine and gain bicarbonate in the blood until the concentrations of hydrogen (pH) and bicarbonate ions in the blood are restored to normal.
  • Alkaline Diet Bicarbonate added to the blood from the fruit or vegetable-rich diet combines with hydrogen ions to form carbon dioxide and water. This reaction reduces the hydrogen ion concentration and increases the pH. The increased bicarbonate concentration increases the amount of bicarbonate filtered in the glomerulus. The filtered bicarbonate exceeds the amount of hydrogen ion secreted by the kidney cell, and excess bicarbonate is lost in the urine. The amount of bicarbonate secreted from the kidney cells into the blood was equal to the hydrogen ions secreted into the lumen and less than the filtered load of bicarbonate from the blood -- therefore, the blood has a net loss of bicarbonate. This process continues to lose bicarbonate in the urine and reduce the bicarbonate in the blood until the concentrations of hydrogen (pH) and bicarbonate ions in the blood are restored to normal.

Now that we have seen how the kidneys regulate the composition of our blood, let's look at how they help regulate our blood pressure.

Diuretics

People with chronic high blood pressure (hypertension) often take a class of drugs called diuretics to control their blood pressure. Diuretics reduce Na reabsorption from the lumen of the nephron. Water reabsorption is also reduced. Therefore, Na and water are lost in the urine, which increases urine flow. The decreased reabsorption of Na and water from the nephron reduces blood volume, thereby reducing blood pressure.

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How Kidneys Influence Blood Pressure

The blood pressure in your body depends upon the following conditions:

  • The force of contraction of the heart -- related to how much the heart muscle gets stretched by the incoming blood.
  • The degree to which the arteries and arterioles constrict -- increases the resistance to blood flow, thus requiring a higher blood pressure.
  • The circulating blood volume -- the higher the circulating blood volume, the more the heart muscle gets stretched by the incoming blood.

The kidney influences blood pressure by:

  • Causing the arteries and veins to constrict
  • Increasing the circulating blood volume

Specialized cells are located in a portion of the distal tubule located near and in the wall of the afferent arteriole. The distal tubule cells (macula densa) sense the Na in the filtrate, and the arterial cells (juxtaglomerular cells) sense the blood pressure. When the blood pressure drops, the amount of filtered Na also drops. The juxtaglomerular cells sense the drop in blood pressure and the decrease in Na is relayed to them by the macula densa cells. The juxtaglomerular cells then release an enzyme called renin. Renin converts angiotensinogen (a peptide, or amino acid derivative) into angiotensin I. Angiotensin I is then converted to angiotensin II by an angiotensin-converting enzyme (ACE), which is found mainly in the lungs. Angiotensin II causes blood vessels to contract -- the increased blood vessel constrictions elevate the blood pressure.

­­Now let's take a look at how the kidney increases the circulating blood volume. Angiotensin II also stimulates the adrenal gland to secrete a hormone called aldosterone. Aldosterone stimulates more Na reabsorption in the distal tubule, and water gets reabsorbed along with the Na. The increased Na and water reabsorption from the distal tubule reduces urine output and increases the circulating blood volume. The increased blood volume helps stretch the heart muscle and causes it to generate more pressure with each beat, thereby increasing the blood pressure.

The actions taken by the kidney to regulate blood pressure are especially important during traumatic injury, when they are necessary to maintain blood pressure and conserve the loss of fluids.

Your body stores calcium in the bones, but also maintains a constant level of calcium in the blood. If the blood calcium level falls, then the parathyroid glands in your neck release a hormone called parathyroid hormone. Parathyroid hormone increases calcium reabsorption from the distal tubule of the nephron to restore the blood calcium level. Parathyroid hormone also stimulates calcium release from bone and calcium absorption from the intestine.

In addition to parathyroid hormone, your body also requires vitamin D to stimulate calcium absorption from the kidney and intestine. Vitamin D is found in milk products. A precursor to vitamin D (cholecalciferol) is made in the skin and processed in the liver. However, the final step that converts an inactive form of cholecalciferol into active vitamin D occurs in the proximal tubule of the nephron. Once activated, vitamin D stimulates calcium absorption from the proximal tubule and from the intestine, thereby increasing blood calcium levels.

Kidney stones are often caused by problems in the kidney's ability to handle calcium. In addition, the kidney's role in maintaining blood calcium is important in the bone disease osteoporosis that afflicts many elderly people, especially women.

As you can see, the kidneys perform many functions that are important to your body:

  • Controlling the composition of your blood and eliminate wastes -- filtration/reabsorption/secretion method
  • Influencing blood pressure -- renin secretion
  • Helping to regulate your body's calcium -- vitamin D activation

If the kidneys fail to function, then renal dialysis methods (artificial filtration methods) can be used to help you survive by cleansing the blood. This is especially necessary when both kidneys fail. Although you have two kidneys, it is possible to live with only one. One healthy kidney can be donated and transplanted into a compatible person with total kidney failure. Kidney transplants are a common way to help those people survive and live a normal life.

For more information on the kidney, its functions and its diseases, check out the following page.