Question

Q5. Describe the role of ADH in affecting the concentration of urine (including a description of the countercurrent exchange) [10pts].

Total Body Water = 42L

Extracellular Fluid = 14L

Osmolarity = 300mOsmoles/L

Intracellular Fluid = 28L

Answer 1

The most important variable regulating antidiuretic hormone secretion is plasma osmolarity, or the concentration of solutes in blood. Osmolarity is sensed in the hypothalamus by neurons known as anosmoreceptors, and those neurons, in turn, stimulate secretion from the neurons that produce antidiuretic hormone.When plasma osmolarity is below a certain threshold, theosmoreceptors are not activated and secretio of antidiuretic hormone is suppressed. When osmolarity increases above the threshold, the ever-alert osmoreceptors recognize this as their cue to stimulate the neurons that secrete antidiuretic hormone. As seen the the figure below, antidiuretic hormone concentrations rise steeply and linearly with increasing plasma osmolarity.Osmotic control of antidiuretic hormone secretion makes perfect sense. Imagine walking across a desert: the sun is beating down and you begin to lose a considerable amount of body water through sweating. Loss of water results in concentration of blood solutes - plasma osmolarity increases. antidiuretic hormone is secreted, allowing almost all the water that would be lost in urine to be reabsorbed and conserved.The most common disease of man and animals related to antidiuretic hormone is diabetes insipidus.

Counter Current exchange -

The creation of a hypertonic medulla is facilitated by the renal tubules and vasa recta (blood vessels surrounding the tubules in the renal medulla) as follows:
1) Active absorption of urea from the collecting tubule under the influence of ADH.
2) Passive absorption of water without NaCl in the descending limb of the loop of Henle, which helps to concentrate NaCl in the tubular lumen as it enters the ascending limb. In the vasa recta surrounding the descending limb of the loop of Henle, NaCl is absorbed into plasma, whereas water leaves the plasma and enters the interstitium in response to the high interstitial sodium chloride and urea concentrations created by the renal tubules .
3) In the vasa recta surrounding the ascending limb, the reverse occurs because the plasma is now hypertonic to the medullary interstitium. This results in absorption of water into the blood and return of the absorbed solute to the interstitium. This is referred to as the countercurrent exchange mechanism.