We all recognize the familiar ‘kidney shape’ of the human kidneys, seen here along with their blood supply and the ureters, joining them to the bladder. The main arteries to the kidneys are the renal arteries, which join directly to the aorta. They carry blood rich in waste products to be filtered by the kidneys. The renal veins return the filtered blood to the major veins of the body. Each kidney is separated into two regions, the cortex and the medulla. We’ll learn more about these regions later in this activity.

If we section a kidney and look in detail at its structure, we can see that it is made up of a large number of filtering units called nephrons. Each nephron is equivalent to one of the nephridia that we saw in the earthworm. A similar filtering principle is involved here; however, the funnel leading to the coiled tube now encloses a cluster of capillaries from the renal artery. This structure is called a Bowman’s capsule. Notice that the Bowman’s capsule and the outer part of each nephron lies in the cortex of the kidney. The inner part lies in the kidney’s medulla. The Bowman’s capsule is joined to a tubule that forms a long U-shaped bend called the loop of the nephron before emptying into a collecting duct that leads to the ureter.

The region leading from the Bowman’s capsule to the loop is called the proximal tubule. The region leading from the loop to the collecting duct is called the distal tubule. Now that we know the parts of a nephron, let’s look at how it functions in filtering the blood.

If you look at the blood vessels of the nephron, you’ll see that the arteriole leading away from the Bowman’s capsule doesn’t go straight to the renal vein. Instead, it loops and twists around the tubule. Blood entering the capsule is under high pressure. The capillary cluster acts as a filter, forcing water and salts out of the blood and into the tubule. In addition to water, the filtrate that enters the tubule contains many useful small ions, like sodium, and small molecules, such as glucose and vitamins.

If this nonselective filtering process ended here, we’d risk losing these important nutrients. However, the blood vessels that twist around the tubule reabsorb the nutrients and water, leaving the residue as urine in the collecting duct. How are these blood vessels able to reabsorb nutrients? The answer lies in the transport epithelium that lines the tubule, providing a layer of cells across which ions and molecules can be shuttled. As the filtrate passes through the proximal tubule, bicarbonate ions, sodium ions, and nutrients diffuse into the epithelium.

Hydrogen ions, potassium ions, and ammonia are passed back.

The loop of the nephron, connecting the proximal and distal tubules, is where most of the water in the filtrate is reabsorbed.

We’ve already seen that the loop of the nephron lies in the medulla of the kidney.

Here, the salt concentration of the interstitial fluid is much higher than that in the cortex.

As the filtrate reaches the medulla, water diffuses out from the tubule, diluting the interstitial fluid to maintain osmotic balance.

Inside the distal tubule, more sodium ions are reclaimed by active transport and still more water diffuses out. Potassium and hydrogen ions move into the tubule in the opposite direction.

The collecting duct itself is permeable to water, but not to sodium ions. As the filtrate passes through the collecting duct it becomes more concentrated as more water is reabsorbed.

The vertebrate kidney is a complex structure, consisting of more than a million filtering units called nephrons.

See if you can you identify the various parts of the kidney shown in this diagram.

That’s right!

Not quite right!

All: The cluster of capillaries is in the Bowman’s capsule.

The proximal tubule is the part of the tubule leading out of the capsule. The loop of the nephron is where much of the water in the urine is reabsorbed. The distal tubule leads to a collecting duct.

If you drink a modest amount of water each day, your urine will be more concentrated than your body fluids. But you don’t always drink the same amount. What would happen if your urine was always the same concentration, and you drank a large volume of water? The water would not be excreted, and you’d become very bloated. Conversely, if you drank very little water, your body would lose valuable fluids.

Just how does the body overcome these problems and maintain homeostasis? When there is less water in your body fluids and the salt concentration of the blood increases, you become dehydrated and begin to feel thirsty. Specialized neurons in the hypothalamus of the brain trigger the pituitary gland to secrete a substance called ADH, or antidiuretic hormone. ADH travels in the bloodstream to the kidneys. Its presence increases the permeability of the distal tubules and collecting ducts to water, so more water is reabsorbed. Once the salt concentration of the blood has decreased to an acceptable level, the sensation of thirst and the production of ADH diminish. The negative feedback process is complete.

Sometimes the kidneys don’t function properly. The commonest causes of kidney failure are diabetes and high blood pressure. There are several different forms of diabetes. In the most common form, diabetes mellitus, blood glucose levels get too high, and some of it filters through the kidneys and into the urine. The person becomes very thirsty, and produces large amounts of urine. Valuable ions, especially those necessary for maintaining nerve and muscle function are lost along with the excess glucose. The strain put on the kidneys by the disease may eventually cause kidney failure. Kidney failure due to diabetes mellitus occurs in more than 20,000 people each year in the United States.

High blood pressure makes the heart work harder and, over time, can damage blood vessels throughout the body. If the blood vessels in the kidneys are damaged, they may stop removing wastes and extra fluid from the blood. The extra fluid may then raise blood pressure even more.

After diabetes, high blood pressure is the leading cause of kidney failure with about 15,000 new cases in the US every year.

People with kidney failure must either go on dialysis or receive a new kidney through a transplant. Dialysis consists of diverting blood from an artery through a machine that brings the blood and dialysis fluid into close contact through a membrane. Pores in the membrane allow most dissolved substances in the blood to pass out of the tubing, while retaining the proteins and cells. The external solution in which the tubing is immersed is a salt solution with ionic concentrations slightly lower than the desired concentrations in the blood. To maintain the blood's concentration of a specific chemical, the external solution is adjusted to the same concentration as that of the blood. In such a case, the two solutions are in equilibrium, so the blood's concentration of that chemical does not change.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education