# 5.10: Summary of Renal Water Handling

The kidney is the most important organ in the regulation of water balance in the body. Under normal circumstances, the sensitive hypothalamic osmoreceptors detect any change in extracellular tonicity and respond by altering secretion of ADH from the posterior pituitary. The volume receptors are much less sensitive and really function as a backup sensor. Most water intake is not due to thirst. The thirst mechanism functions as a backup effector mechanism.

### The kidney is the effector organ for body water balance.

Glomerular filtration rate (GFR) is very large (180 l/day) in comparison to the amount of urine that is typically produced. Most of the water in the filtrate is reabsorbed because of renal processes which are independent of ADH action.

The diagram below summarises the percentages of water reabsorbed in the various renal segments. The two extreme examples of absence of ADH and maximal ADH production are outlined. The kidney adjusts the water reabsorption between these two extremes (under the influence of ADH) in order to maintain a constant plasma osmolality. The minimum (or obligatory) urine volume is determined by the size of the daily solute load and the maximal urine osmolality that can be achieved. The maximum urine osmolality decreases with increasing age in adults so the obligatory urine volume is higher for a given solute load in the elderly.

### All water reabsorption in the kidney is passive.

Water moves in response to osmotic gradients. These osmotic gradients are all directly or indirectly due to the reabsorption of solute particularly sodium. There are no water pumps in the body.

## Quantitative Summary of Renal Water Handling

GFR (180 l/day)

65% Reabsorbed Proximal Tubule

15% Reabsorbed Loop of Henle (thin descending limb)

20% of filtrate Enters Distal Tubule

$$8 \% \text {Reab in CD} > 19 \% \text {Reab in CD}$$
$$12 \% \text {of filtered } H_{2}O < 1 \% {of filtered } H_{2}O$$