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The Inner Workings of the Human Kidney

Like most organs in the human body, the function of a kidney is closely tied to its structure. The outer covering on each kidney, called the capsule, is made of stretchy collagen fibers that help anchor your kidneys. Under the capsule, each kidney has three distinct areas:

  • The renal cortex, which is the outer layer.

  • The renal medulla, which is the middle layer. The renal medulla folds into cone-shaped projections called renal pyramids.

  • The renal pelvis, the inner layer that tapers and becomes a ureter.

Each kidney contains more than 1 million nephrons, microscopic tubules that make urine. Each nephron contributes to a collecting duct that carries the urine into the renal pelvis and then down the ureter.

Each of the million tiny nephrons in one of your kidneys is a mass of tiny, looped tubules that begin and end in the renal cortex. The nephrons are closely associated with capillaries in the kidney. Fluid from the capillaries enters the nephrons at their proximal (near) end.

The kidney filters wastes from the fluid as it passes through the nephron, and then returns the useful water and electrolytes to the blood. Concentrated wastes leave the nephrons at their distal (far) end and then enter a collecting duct that empties into the renal pelvis.

The structure of a nephron is closely tied to its function as a filter:

  • At the proximal end, the nephron swells into a cup-shaped structure called the Bowman’s capsule that wraps around a mass of capillaries called the glomerulus. Filtration begins as pressure in the blood forces fluids from the glomerulus and into the Bowman’s capsule.

  • Fluid in the nephron flows into the proximal convoluted tubule, the twisted part of the nephron closest to the Bowman’s capsule. As fluid passes through this part of the nephron, water and electrolytes are reabsorbed into the blood. Meanwhile, drugs and toxins that are still in the capillaries are secreted from the capillaries into the nephron.

  • Fluid passes into the Loop of Henle, a long, dangling, U-shaped portion of the nephron that passes into the renal medulla. As the fluid moves down in the Loop of Henle, the solutes in the renal medulla draw water out of the nephron by osmosis, reabsorbing the water back into the body.

    From the medulla, the water diffuses back into the capillaries. As fluid moves back up the Loop of Henle, electrolytes move out of the nephron and into the medulla.

  • From the Loop of Henle, the fluid moves into the distal convoluted tubule, the twisted part of the nephron farthest from the Bowman’s capsule. In this part of the nephron, water and electrolytes are again reabsorbed into the blood. Electrolytes that maintain blood pH may be secreted from the blood into the nephron.

  • The concentrated wastes move into the collecting duct and then into the renal pelvis. More water is reabsorbed as the fluid passes through the collecting duct. From the renal pelvis, the wastes move into the ureter and down to the bladder.

Urine is continuously spurted from the ureter into the top of the bladder. Although the bladder can hold up to a pint of urine, you typically begin to feel the need to urinate when your bladder is only one-third full.

When your bladder is two-thirds full, you start to feel really uncomfortable. When you want to start urinating, the sphincter muscle at the top of your urethra relaxes, opening the urethra and letting the urine out.

Your kidneys do an amazing job of concentrating your wastes. For every 125 milliliters of fluid that leaves your blood every minute, only one milliliter of fluid leaves your kidneys to enter your bladder. The rest of the fluid is recycled back to your blood!

For the following questions, use the terms that follow to match each structure with the function it performs. Some structures may perform more than one function.

a. Filtration

b. Reabsorption

c. Secretion

d. Excretion

  1. Bowman’s capsule

  2. Proximal convoluted tubule

  3. Loop of Henle

  4. Distal convoluted tubule

  5. Collecting duct

The following are the answers to the practice questions:

  1. The answer is a. Filtration.

  2. The answer is b. Reabsorption and c. Secretion.

  3. The answer is b. Reabsorption.

  4. The answer is b. Reabsorption and c. Secretion.

  5. The answer is b. Reabsorption and d. Excretion.

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