Water excretion, as with sodium excretion, is regulated in partnership with the CV system. Central goals in regulating both salt and water excretion are to: 1).preserve vascular volume and 2).maintain plasma osmolality at a level that is healthy for tissue cells. The main regulators of water excretion, not surprisingly, relate to osmolality and volume.
Quantitatively, renal water excretion is determined by 2 values: 1).the amount of solute in the urine and 2).the osmolality of the urine.
Urine water excretion = urine solute excretion/urine osmolality
Excreted solute consists mostly of organic waste and excess electrolytes. In a given metabolic state, the rate of organic waste excretion is more or less constant, and is not altered for purposes of controlling water excretion. Electrolyte excretion is highly regulated, but more to achieve balance of individual substances like sodium and potassium than to control water excretion per se. Given that solute excretion is so variable, the main way the body controls water excretion in normal circumstances is to control urine osmolality. In other words, given a certain amount of solute that is excreted, the body controls the amount of water accompanying it by controlling urine osmolality.
When the body excretes urine that is more dilute than plasma (osmolality below 285 mOsm/kg H2O), the body is excreting "free water" (like adding pure water to otherwise isosmotic urine). Conversely, when the excreted urine is more concentrated than plasma, there is "negative free water" excretion. It is as if the body has reclaimed pure water from otherwise isosmotic urine.
The kidneys first generate hypo-osmotic tubular fluid in the loop of Henle. Then, as the fluid subsequently flows through the collecting duct system, variable amounts of water are reabsorbed by allowing the tubular fluid to equilibrate to varying degrees with the surrounding interstitum. The final urine osmolality, and hence final volume, depends on the peak medullary osmolality and how closely the tubular osmolality approaches that value. We also know that equilibration with the interstitium is a function of water permeability in the collecting ducts under the control of the hormone antidiuretic hormone (ADH). Therefore the regulation of water excretion, that is independent of solute excretion, focuses on control over ADH secretion.
Osmoreceptor Control of ADH Secretion
Plasma osmolality is one of the most tightly regulated variables in the body. Plasma osmolality is set mainly by the ratio of ECF sodium (plus its assocaited anions) to water. Other solutes (e.g., glucose and potassium) make some contribution, but those other solutes are regulated for reasons other than plasma osmolality. Thus, except under unusual circumstances such as severe hyperglycemia, variations in plasma osmolality mostly reflect variations in sodium concentration. If the body keeps the inputs and outputs of sodium and water matched in lock step, osmolality remains constant. But inputs are often not matched. The major effect of gaining or losing water or salt without corresponding changes in the other is a change in the osmolality of the body fluids. When osmolality deviates from normal, strong reflexes come into play to change ADH secretion, and thus change the excretion of water.
Baroreceptor Control of ADH Secretion
There is another major influence on ADH secretion. This originates in systemic baroreceptors. A decreased extracellular volume or major decrease in arterial pressure reflexively activates increased ADH secretion. The response is mediated by neural pathways originating in cardiopulmonary baroreceptors, and if arterial pressure decreases, from arterial baroreceptors.
Decreased CV pressures cause less firing by the baroreceptors, which relieves inhibition of stimulatory pathways and results in more ADH secretion. In effect, the low CV pressures are interpreted as low volume, and the response of increased ADH appropriately serves to minimize loss of water. Conversely, baroreceptors are stimualted by increased CV pressures, interpreted as excess volume, and this causes inhibition of ADH secretion. The decrease in ADH results in decreased reabsorption of water in the collecting ducts, and more excretion. The adaptive value of these baroreceptor reflexes is to help stabilize ECF volume and, hence, blood pressure.
There is a second adaptive value to this reflex: Large decreases in plasma volume, as might occur after a major hemorrhage, elicit such high concentrations of ADH – much higher than those needed to produce maximal antidiuresis – that the hormone is able to exert direct vasoconstrictor effects on arteriolar smooth muscle. The result is increased total peripheral resistance, which helps restore arterial blood pressure independently of the slower restoration of body fluid volumes. Renal arterioles and mesangial cells also participate in this constrictor response, and so a high plasma concentration of ADH, quite apart form its effect on water permeability and sodium reabsorption in the distal nephron, promotes retention of both sodium and water by lowering GFR.
The cells that synthesize ADH in the hypothalamus also receive synaptic input from many other brain areas. Thus, ADH secretion and, hence, urine flow can be altered by pain, fear, and a variety of other factors, including drugs such as alcohol, which inhibits ADH release. However, this complexity should not obscure the generalization that ADH secretion is determined over the long term primarily by the states of body fluid osmolality and plasma volume.
We have described 2 different major afferent pathways controlling the ADH-secreting hypothalamic cells: 1 from baroreceptors and 1 from osmoreceptors. These hypothalamic cells are, therefore, true integrators, whose activity is determined by the total synaptic input to them. Thus, a simultaneous increase in plasma volume and decrease in body fluid osmolality causes strong inhibition of ADH secretion. Conversely, a simultaneous decrease in plasma volume and increase in osmolality produces very marked stimulation of ADH secretion. However, what happens when baroreceptor and osmoreceptor inputs oppose each other? In general, because of the high sensitivity of the osmoreceptors, the osmoreceptor influence predominates over that of the baroreceptors when changes in osmolality and plasma volume are small to moderate. However, a dangerous reduction in plasma volume will take precedence over decreased body fluid osmolality in influencing ADH secretion; under such conditions, water is retained in excess of solute even though the body fluids become hypo-osmotic (for the same reason, plasma sodium concentration decreases). In essence, when blood volume reaches a life-threatening low level, it is more important for the body to preserve vascular volume and thus ensure an adequate cardiac output than it is to preserve normal osmolality.
Thirst and Salt Appetite
Deficits of salt and water cannot be corrected by renal conservation, and ingestion is the ultimate compensatory mechanism. The subjective feeling of thirst, which drives one to obtain and ingest water, is stimulated both by reduced plasma volume and by increased body fluid osmolality. Note that these are precisely the same changes that stimulate ADH production, and the receptors – osmoreceptors and the nerve cells that respond to the CV baroreceptors – that initiate the ADH-controlling reflexes are near those that initiate thirst. The thirst response, however, is significantly less sensitive than the ADH response.
There are also other pathways controlling thirst. For example, dryness of the mouth and throat causes profound thirst, which is relieved by merely moistening them. Also, when animals such as the camel (and humans, to a lesser extent) become markedly dehydrated, they will rapidly drink just enough water to replace their previous losses and then stop. What is amazing is that when the stop, the water has not yet had time to be absorbed from the gastrointestinal tract into the blood.