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Illustration labels parts of a nephron and their function. The nephron begins at the glomerulus, a spherical structure that filters small solutes from the blood. The filtrate then enters a winding proximal convoluted tubule, which reabsorbs ions, water, and nutrients, and removes toxins and adjusts the filtrate pH. The proximal convoluted tubule empties into the descending loop of Henle. Aquaporins in the descending loop allow water to pass from the filtrate to the interstitial fluid. The descending loop of Henle turns into the ascending loop of Henle. Both the descending loop and ascending loop are thin at the bottom, and turn thick about a third of the way up. In the ascending loop of Henle, sodium and chlorine ions are reabsorbed from the filtrate into the interstitial fluid. The ascending loop of Henle empties into the distal convoluted tubule, which selectively secretes and absorbs ions to maintain blood pH and electrolyte balance. The distal convoluted tubule empties into a collecting duct, which reabsorbs water and solutes from the filtrate. The collecting duct travels down, toward the middle of the kidney.
Each part of the nephron performs a different function in filtering waste and maintaining homeostatic balance. (1) The glomerulus forces small solutes out of the blood by pressure. (2) The proximal convoluted tubule reabsorbs ions, water, and nutrients from the filtrate into the interstitial fluid, and actively transports toxins and drugs from the interstitial fluid into the filtrate. The proximal convoluted tubule also adjusts blood pH by selectively secreting ammonia (NH 3 ) into the filtrate, where it reacts with H + to form NH 4 + . The more acidic the filtrate, the more ammonia is secreted. (3) The descending loop of Henle is lined with cells containing aquaporins that allow water to pass from the filtrate into the interstitial fluid. (4) In the thin part of the ascending loop of Henle, Na + and Cl - ions diffuse into the interstitial fluid. In the thick part, these same ions are actively transported into the interstitial fluid. Because salt but not water is lost, the filtrate becomes more dilute as it travels up the limb. (5) In the distal convoluted tubule, K + and H + ions are selectively secreted into the filtrate, while Na + , Cl - , and HCO 3 - ions are reabsorbed to maintain pH and electrolyte balance in the blood. (6) The collecting duct reabsorbs solutes and water from the filtrate, forming dilute urine. (credit: modification of work by NIDDK)

Glomerular filtration

Glomerular filtration filters out most of the solutes due to high blood pressure and specialized membranes in the afferent arteriole. The blood pressure in the glomerulus is maintained independent of factors that affect systemic blood pressure. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, except for macromolecules like proteins, pass through by passive diffusion. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys. GFR is regulated by multiple mechanisms and is an important indicator of kidney function.

Tubular reabsorption and secretion

Tubular reabsorption occurs in the PCT part of the renal tubule. Almost all nutrients are reabsorbed, and this occurs either by passive or active transport. Reabsorption of water and some key electrolytes are regulated and can be influenced by hormones. Sodium (Na + ) is the most abundant ion and most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na + is actively transported out of the tubule, water follows it to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT.

In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb.

Art connection

A U-shaped tube represents the loop of Henle. Filtrate enters the descending limb, and exits the ascending limb. The descending limb is water-permeable, and water travels from the limb to the interstitial space. As a consequence, the osmolality of the filtrate inside the limb increases from 300 milliosmoles per liter at the top to 1200 milliosmoles per liter at the bottom. The ascending limb is permeable to sodium and chloride ions. Because the osmolality inside bottom part of the limb is higher than the interstitial fluid, these ions diffuse out of the ascending limb. Higher up, sodium is actively transported out of the limb, and chloride follows.
The loop of Henle acts as a countercurrent multiplier that uses energy to create concentration gradients. The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so the concentration of solutes inside the limb increases as it descends into the renal medulla. At the bottom, the concentration of solutes is higher inside the loop than in the interstitial fluid. Thus, as filtrate enters the ascending limb, Na + and Cl - ions exit through ion channels present in the cell membrane. Further up, Na + is actively transported out of the filtrate and Cl - follows. The concentration of solutes is given in units of milliosmoles per liter.

By the time the filtrate reaches the DCT, most of the water and solutes have been reabsorbed. If the body requires additional water, more of it can be reabsorbed at this point. Further reabsorption is controlled by hormones, which will be discussed in a later section. Excretion of wastes occurs due to lack of reabsorption combined with tubular secretion. Undesirable products like metabolic wastes, urea, uric acid, and certain drugs, are excreted by tubular secretion. Most of the tubular secretion happens in the DCT, but some occurs in the early part of the collecting duct. Kidneys also maintain an acid-base balance by secreting excess H + ions.

Career connection

Nephrologist

A nephrologist studies and deals with diseases of the kidneys—both those that cause kidney failure (such as diabetes) and the conditions that are produced by kidney disease (such as hypertension). Blood pressure, blood volume, and changes in electrolyte balance come under the purview of a nephrologist.

Nephrologists usually work with other physicians who refer patients to them or consult with them about specific diagnoses and treatment plans. Patients are usually referred to a nephrologist for symptoms such as blood or protein in the urine, very high blood pressure, kidney stones, or renal failure.

Nephrology is a subspecialty of internal medicine. To become a nephrologist, medical school is followed by additional training to become certified in internal medicine. An additional two or more years is spent specifically studying kidney disorders and their accompanying effects on the body.

Section summary

The kidneys are the main osmoregulatory organs in mammalian systems; they function to filter blood and maintain the correct concentration of solutes in body fluids. They are made up internally of three distinct regions—the cortex, medulla, and pelvis.

The blood vessels that transport blood into and out of the kidneys arise from and merge with the aorta and inferior vena cava, respectively. The renal arteries branch out from the aorta and enter the kidney where they further divide.

The nephron is the functional unit of the kidney, which actively filters blood and generates urine. The nephron is made up of the renal corpuscle and renal tubule. The nephron filters and exchanges water and solutes with two sets of blood vessels and the tissue fluid in the kidneys.

There are three steps in the formation of urine: glomerular filtration, which occurs in the glomerulus; tubular reabsorption, which occurs in the renal tubules; and tubular secretion, which also occurs in the renal tubules.

Art connections

[link] Which of the following statements about the kidney is false?

  1. The renal pelvis drains into the ureter.
  2. The renal pyramids are in the medulla.
  3. The cortex covers the kidney.
  4. Nephrons are in the renal cortex.

[link] C

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[link] Which of the following statements about the nephron is false?

  1. The collecting duct empties into the distal convoluted tubule.
  2. The Bowman’s capsule surrounds the glomerulus.
  3. The loop of Henle is between the proximal and distal convoluted tubules.
  4. The loop of Henle empties into the distal convoluted tubule.

[link] A

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[link] Loop diuretics are drugs sometimes used to treat hypertension. These drugs inhibit the reabsorption of Na + and Cl - ions by the ascending limb of the loop of Henle. A side effect is that they increase urination. Why do you think this is the case?

[link] Loop diuretics decrease the excretion of salt into the renal medulla, thereby reducing its concentration of solutes. As a result, less water is excreted into the medulla by the descending limb, and more water is excreted as urine.

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