13.6 Physiology of Urine Formation: The Vertical Osmotic Gradient and Altering Urine Concentration

As mentioned in Section 13.0, one of the main functions of the kidneys is osmoregulation, which is maintaining the osmolarity (solute concentration) of the blood. The kidneys maintain plasma solute concentration by varying urine concentration and volume. The concentration of urine produced varies throughout a day depending on the body’s needs. To understand this, consider these extremes:

• when a person is dehydrated and water needs to be conserved, the kidneys produce a small volume of concentrated urine; and
• when a person is overhydrated and excess water needs to be excreted, the kidneys produce a large volume of dilute urine.

The kidneys’ ability to vary urine concentration and volume relies on the vertical osmotic gradient in the interstitial fluid of the kidney (Figure 13.6.1.)

Countercurrent Mechanisms and Establishment of the Vertical Osmotic Gradient

The vertical osmotic gradient of the interstitial fluid of the kidney is created and maintained countercurrent mechanisms occurring in the Loops of Henle and the vasa recta of juxtamedullary nephrons. The term “countercurrent” refers to opposite fluid flow in adjacent tubes. Two countercurrent systems function in the nephron:

• the countercurrent multiplier system of the Loops of Henle of juxtamedullary nephrons; and
• the countercurrent exchanger system of the vasa recta surrounding the Loops of Henle.

The ascending limb actively pumps out salts, mainly NaCl, which adds to the osmolarity of the interstitial fluid. The countercurrent flow in and the different properties of the descending and ascending limbs establish a positive feedback cycle by which this pumping action of the ascending limb is multiplied. The cycle can be summarized as follows:

1. the ascending limb pumps NaCl out of the filtrate, which increases the interstitial fluid osmolarity
2. the increased interstitial fluid osmolarity causes water to move out of the descending limb via osmosis
3. as water leaves the filtrate in the descending limb, its concentration increases, providing salt to be pumped out of the ascending limb (back to step 1)

Formation of Concentrated or Dilute Urine

As discussed above, NaCl and urea are the main substances contributing to the increased osmolarity of the interstitial fluid in the medulla. This osmotic gradient is vital to the kidneys’ ability to form concentrated urine when necessary.The process of concentrating the filtrate (soon to be urine) occurs in the collecting duct. Recall that the DCT and collecting duct are nearly impermeable to water without sufficient ADH. The filtrate entering the DCT is about 100 mOsM, so if excretion of a dilute urine is necessary, this can be done. As ADH levels increase, more water is reabsorbed from the DCT and collecting duct, resulting in more water being retained in the body and in the excretion of a more concentrated urine (Figure 13.6.4). Note that while ADH increases the permeability of the DCT and collecting ducts to water, the reason water moves out via osmosis is the vertical osmotic gradient: as filtrate flows through the collecting duct, water is reabsorbed all along its length due to the increasing osmolarity of the surrounding interstitial fluid. Without the vertical osmotic gradient, humans would not be able to produce concentrated urine. ADH will be discussed further in Chapter 14, but in summary, the main stimuli for increased ADH release include increased blood osmolarity and/or decreased blood volume:

• • if the blood becomes hyperosmotic, ADH levels increase and more water is reabsorbed and a smaller volume of concentrated urine is excreted
  • if the blood becomes hyposmotic, ADH levels decrease and less water is reabsorbed and a larger volume of dilute urine is excreted

Image Descriptions

Figure 13.6.1. This diagram illustrates the countercurrent multiplier mechanism in the loop of Henle, showing how the kidney generates and maintains a medullary osmotic gradient. The image depicts the descending and ascending limbs of the loop of Henle, with the cortex at the top (shown in red) and the pelvis at the bottom (shown in yellow). Osmolarity values are labeled along both limbs, starting at 300 mOsm at the cortex and progressively increasing to 1200 mOsm at the bottom of the loop near the pelvis, then decreasing back to 100 mOsm as fluid ascends. The descending limb (left side) is permeable to water (H2O) as indicated by outward arrows, causing the tubular fluid to become progressively more concentrated as it descends. The ascending limb (right side) shows active transport of sodium and chloride (Na+ and Cl-) out of the tubule, as indicated by arrows and symbols, while being impermeable to water. Blue arrows within the tubule indicate the direction of fluid flow down the descending limb and up the ascending limb. A vertical osmolarity scale on the right shows the gradient from cortex to medulla, with a large red arrow indicating increasing osmolarity toward the pelvis. The interstitial fluid osmolarity values (300, 400, 600, 800, 1000, 1200 mOsm) are shown in the space between the limbs, demonstrating the hyperosmotic medullary interstitium. This countercurrent multiplication system enables the kidney to produce concentrated urine by establishing and maintaining the osmotic gradient necessary for water reabsorption in the collecting duct. [Return to Figure 13.6.1]

Figure 13.6.2. This diagram illustrates the ionic concentrations and urea recycling mechanism in the loop of Henle and collecting duct. The image shows a simplified nephron structure with the loop of Henle on the left and the collecting duct on the right (shown in pale orange/tan). At the top, a gray box lists the normal plasma concentrations: Na+ 142 mEq/L, K+ 5 mEq/L, Urea 5 mM, Glucose 5 mM, and Cl- 108 mEq/L. Another box shows ADH (antidiuretic hormone) presence with +/- indicators. Throughout the tubule segments, black boxes with white text indicate the concentrations of Na+, K+, and Cl- at different points, showing how these values change as fluid moves through the system. The descending limb shows Na+ 285, K+ 10, and Cl- 285, while the ascending limb shows Na+ 35, K+ 75, and Cl- 60. At the bottom of the loop, concentrations peak with Na+ and Cl- at 285 mEq/L. Orange arrows indicate the movement of urea, demonstrating urea recycling between the collecting duct and the loop of Henle. A text box at the bottom explains: “Urea recycling: Water reabsorption from the collecting duct concentrates the urea, some of which diffuses back into the filtrate in the ascending loop of Henle.” Additional labels show “ADH +” near the collecting duct, indicating where ADH acts to increase water reabsorption, and “1200 mOsm” indicating the maximum osmolarity achieved at the tip of the loop. [Return to Figure 13.6.2]

Figure 13.6.3. This diagram illustrates the countercurrent multiplier system in the loop of Henle, showing the movement of sodium (Na+) and water (H2O) between the descending and ascending limbs. The image depicts a U-shaped tubule with the cortex at the top (shown in orange) and the papilla at the bottom (shown in yellow). Osmolarity values are labeled at different levels: 300 mOsm in the cortex, progressing to 600 mOsm in the outer medulla, and reaching 1200 mOsm at the deepest point near the papilla. The vasa recta (blood vessels) are shown on both sides of the loop. Blue arrows indicate water (H2O) movement out of the descending limb, while pink/purple arrows show active sodium (Na+) transport out of the ascending limb. The descending limb is permeable to water but relatively impermeable to solutes, allowing water to move into the increasingly concentrated medullary interstitium. The ascending limb actively pumps Na+ and Cl- into the interstitium while being impermeable to water. Large red arrows on the outer edges point downward toward the medulla on the left and upward from the medulla on the right, representing the direction of blood flow in the vasa recta and the maintenance of the osmotic gradient. The color gradient from orange (cortex) through red (medulla) to yellow (papilla) visually represents the increasing osmolarity from the cortex to the inner medulla, demonstrating how this countercurrent system concentrates urine. [Return to Figure 13.6.3]

Figure 13.6.4. This diagram compares the function of the collecting duct in the presence and absence of antidiuretic hormone (ADH). The image is split into two panels showing the collecting duct extending from the medulla (top, shown in orange/red gradient) to the pelvis (bottom, shown in yellow). A large red arrow labeled “ADH” separates the two conditions. The left panel labeled “no ADH” shows the collecting duct with osmolarity measurements ranging from 100 mOsm at the top to 1200 mOsm at the medullary depth, but the tubular fluid remains at approximately 100 mOsm throughout its length, indicated by the pale yellow color within the duct. Small amounts of H2O (water) movement are shown with minimal arrows. This results in dilute urine production. The right panel labeled “ADH” shows the same collecting duct structure, but with ADH present, the tubular fluid osmolarity increases progressively from 300 mOsm at the top to 1200 mOsm at the bottom, matching the surrounding medullary interstitium osmolarity (shown by the deepening yellow color within the duct). Multiple blue arrows indicate increased H2O reabsorption from the duct into the hyperosmotic medullary interstitium. The interstitial osmolarity values are marked at regular intervals (100, 300, 500, 700, 900, 1100, 1200 mOsm) along both ducts. This demonstrates how ADH increases water permeability of the collecting duct by inserting aquaporin channels, allowing water to be reabsorbed and producing concentrated urine, while the absence of ADH results in an impermeable collecting duct that produces dilute urine. [Return to Figure 13.6.4]

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