ADH increases the H20 permeability of the principal cells of the

late distal tubule.

- H20 is reabsorbed from the tubule until the osmolarity of distal tubular fluid equals that of the surrounding interstitial fluid in the renal cortex (300 mOsm/L).

- TF/Posm = 1.0 at the end of the distal tubule because osmotic equilibration occurs in the presence of ADH.

Decreases water reabsorption

Decreases water reabsorption

Figure 5-15. Mechanisms for producing hyperosmotic (concentrated) urine in the presence of antidiuretic hormone (ADH). Numbers indicate osmolarity. Heavy arrows indicate water reabsorption. The thick outline shows the water-impermeable segments of the nephron. (Adapted with permission from Valtin H: Renal Function, 2nd ed. Boston, Little, Brown, 1983, p 162.)

Figure 5-15. Mechanisms for producing hyperosmotic (concentrated) urine in the presence of antidiuretic hormone (ADH). Numbers indicate osmolarity. Heavy arrows indicate water reabsorption. The thick outline shows the water-impermeable segments of the nephron. (Adapted with permission from Valtin H: Renal Function, 2nd ed. Boston, Little, Brown, 1983, p 162.)

6. Collecting ducts—high ADH

- As in the late distal tubule, ADH increases the H20 permeability of the principal cells of the collecting ducts.

-As tubular fluid flows through the collecting ducts, it passes through the corticopapillary gradient (regions of increasingly higher osmolarity), which was previously established by countercurrent multiplication and urea recycling.

- H20 is reabsorbed from the collecting ducts until the osmolarity of tubular fluid equals that of the surrounding interstitial fluid.

- The osmolarity of the final urine equals that at the bend of the loop of Henle (1200 mOsm/L).

- TF/Posm >1.0 because osmotic equilibration occurs with the corticopapillary gradient in the presence of ADH.

C. Production of dilute urine (Figure 5-16)

- is called hyposmotic urine, in which urine osmolarity < blood osmolarity.

- is produced when circulating levels of ADH are low (e.g., water drinking, central diabetes insipidus) or when ADH is ineffective (nephrogenic diabetes insipidus).

1. Corticopapillary osmotic gradient—no ADH

- is smaller than in the presence of ADH because ADH stimulates both countercurrent multiplication and urea recycling.

2. Proximal tubule—no ADH

-As in the presence of ADH, 2/3 of the filtered water is reabsorbed isos-motically.

-TF/Posm = 1.0 throughout the proximal tubule.

Figure 5-16. Mechanisms for producing hyposmotic (dilute) urine in the absence of antidiuretic hormone (ADH). Numbers indicate osmolarity. Thick arrows indicate water reabsorption. The thick outline shows the water-impermeable segments of the nephron. (Adapted with permission from Valtin H: Renal Function, 2nd ed. Boston, Little, Brown, 1983, p 162.)

Figure 5-16. Mechanisms for producing hyposmotic (dilute) urine in the absence of antidiuretic hormone (ADH). Numbers indicate osmolarity. Thick arrows indicate water reabsorption. The thick outline shows the water-impermeable segments of the nephron. (Adapted with permission from Valtin H: Renal Function, 2nd ed. Boston, Little, Brown, 1983, p 162.)

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