Answer:
The distribution of water in one or another compartment is due to forces that drive the movement of the liquid. These forces depend on the concentration of solutes (particles, molecules and ions dissolved in the water of a compartment) present in the compartments because the water moves to the compartment where the concentration of solutes is higher. This force that determines the movement of water is the osmotic pressure.
In organisms the most important electrolytes from the point of view of their concentration, therefore, of their osmotic effect, are the sodium and potassium ions. But they predominate in different compartments. Potassium is the most important electrolyte in the cell compartment while sodium is in the extracellular compartment. The concentration of these cations in the compartments indicated is expressed as a characteristic called osmolarity (number of osmoles per liter).
It follows from the foregoing that the control of volume homeostasis depends on the control of osmolarity.
The regulation of volume homeostasis then allows a normal circulatory and blood function condition that is vital for normal cell functioning. If a situation of hypovolemia (decrease in blood volume) occurs due to dehydration or blood loss, a series of physiological and behavioral mechanisms appear to correct the imbalance. Hypovolemia is detected by baroreceptors present in the sasnguine vessels (aotic arch, carotid sinus, afferent renal arterioles). The first to respond are those of the aortic arch and the carotid sinus which send signals to the nucleus of the solitary tract, located in the brainstem. Signals from this nucleus reach the hypothalamus and act on the supraoptic and paraventricular nuclei. These nuclei, which produce the hormone arginine-vasopressin (or vasopressin or antidiuretic hormone), increases its release. This hormone acts on the kidney causing an increase in water reabsorption, resulting in decreased urine flow.
Normal volemia is thirst. Hypovolemia not only represents a decrease in plasma volume but also an increase in the osmolarity of the extracellular compartment. A change in this parameter is a very efficient signal about thirst behavior, described as an intense motivation to seek, obtain and consume water. An increase in plasma osmolarity between 1-4% induces thirst behavior. The increase in osmolarity seems to act on specific cells sensitive to this type of stimuli, the osmorreceptors, which have been located in the vascular organ of the terminal lamina, in the anterior hypothalamus. Other hyperosmolarity sensitive neurons are located in the subfornical organ, in the medial preoptic nucleus and also in the magnocellular cells.
But also hypovolemia stimulates the secretion of renin by the kidneys. This enzyme causes the formation in the blood plasma of a substance, angiotensin I, which is transformable in another molecule, angiotensin II. This is a peptide that acts as a potent vasoconstrictor, but at the same time stimulates the secretion of aldosterone, a hormone from the adrenal cortex that also acts on the kidney. In this organ it causes the reabsorption of Na +, which passes to the plasma where it acts as a water retention factor, contributing, therefore, to the restoration and / or conservation of plasma volume.
In the case of hyperosmolarity, another strategy that the body uses in addition to drinking and water retention, is to eliminate excess Na +. This is achieved through the participation of other hormones such as atrial natriuretic factor, which is synthesized by the heart at the atrial level. This peptide is released by the mechanical stimulus that means the atrial distention and the actions on the kidney stimulating the loss of sodium chloride.
