Fluid regulation osmolality

The major cation in extracellular fluid is sodium (Na+). Since sodium has a strong influence on osmotic pressure, it plays a major role in fluid regulation. As sodium is absorbed, water usually follows by osmosis. In fact, sodium levels are regulated more by fluid volume and the osmolality of body fluids than by the amount of sodium in the body. As stated earlier, ANH and aldosterone control fluid levels by directly influencing the reabsorption or excretion of sodium. 

The potent antidiuretic hormone AVP orchestrates the regulation of free water absorption, body fluid osmolality, cell contraction, blood volume, and blood pressure through stimulation of three G-protein-coupled receptor subtypes Vi-vascular types a and b, V2-renal, and V3-pituitary. Increased AVP secretion is the trademark of several pathophysiological disorders, including heart failure, impaired renal function, liver cirrhosis, and SIADH. As a consequence, these patients experience excess water retention or inadequate free-water excretion, which results in the dilution of sodium concentrations, frequently manifesting as clinical hyponatremia (serum sodium concentration <135mmol/L). This electrolyte imbalance increases mortality rates by 60-fold. Selective antagonism of the AVP V2 receptor promotes water... 

The kidneys play an important role in maintaining a proper environment for the cells in the body. By regulating the excretion of water, salts and metabolic end products, the kidneys control the plasma osmolality (i.e., the concentration of ions in the blood), the extracellular fluid volume, and the proportions of various blood solutes. The kidneys are also involved in the production of a set of hormones that make the blood vessels (arterioles) contract in the kidneys as well as in other parts of the body. These hormones can give rise to changes in the vascular structure, and... 

The composition and volume of extracellular fluid are regulated by complex hormonal and nervous mechanisms that interact to control its osmolality, volume, and pH. 

AVP is a pituitary peptide hormone that plays an important role in regulation of renal water and solute excretion. AVP secretion is linked directly to changes in plasma osmolality, thus attempting to maintain body fluid homeostasis. The physiologic effects of AVP are mediated through the Via and V2 receptors. Via receptors are located in vascular smooth muscle and in myocytes, where their stimulation by AVP results in vasoconstriction and increased cardiac contractility, respectively. V2 receptors are located in the collecting duct of the kidney, where AVP stimulation causes reabsorption of free water. 

Arginine vasopressin (AVP) regulates renal water loss and thus causes changes in the osmolality of body fluid compartments. 

Body fluid osmolality is controlled by a homeostatic mechanism that adjusts both the rate of water intake and the rate of solute-free water excretion by the kidneys—i.e., water balance. Abnormah-ties in this homeostatic system can result from genetic diseases, acquired diseases, or drugs, and may cause serious and potentially life-threatening deviations in plasma osmolality. Arginine vasopressin is the main hormone that regulates body fluid osmolality. 

Fig. 4.10. Body fluid homeostasis (constant body water balance). Intake is influenced by availability of fluids and food, thirst, hunger, and the ability to swallow. The rates of breathing and evaporation and urinary volume influence water loss. The body adjusts the volume of urinary excretion to compensate for variations in other types of water loss and for variations in intake. The hormones aldosterone and antidiuretic hormone (ADH) help to monitor blood volume and osmolality through mechanisms regulating thirst and sodium and water balance.

If cells are to survive and function normally, the fluid medium in which they live must be in equilibrium. Fluid and electrolyte balance, therefore, implies constancy, or homeostasis. This means that the amount and distribution of body fluids and electrolytes are normal and constant. For homeostasis to be maintained, the water and electrolytes that enter (input) the body must be relatively equal to the amount that leaves (output). An imbalance of osmolality, the amount of force of solute per volume of solvent (measured in miliosmoles per kilogram—mOsm/kg or mmol/ kg), of this medium can lead to serious disorders or even death. Fortunately, the body maintains homeostasis through a number of self-regulating systems, which include hormones, the nervous system, fluid-electrolyte balance, and acid-base systems. Els... 

Ruid regulation depends on the sensing of the osmolality, or solute concentration, of the blood. As more water is retained in the body solutions, the osmolaUty is decreased and can result in hypo-osmolar fluid that has a lower amount of solute than water. When water is lost from the body, the osmolality of body fluids increases and can result in hyperosmolar fluid that has a higher amount of solute than water. The body responds to an increase in osmolality by stimulating the release of ADH, which causes the retention of fluid and lowers the osmolality of body fluids. 

Ruid volume also plays a part in regulation of fluid levels in the body. Several mechanisms, in addition to ADH, respond to the sensation of low or high fluid volumes and osmolality. Neural mechanisms, through sensory receptors, sense low blood volume in the blood vessels and stimulate a sympathetic response resulting in constriction of the arterioles, which, in turn, result in a decrease in blood flow to... 

Ruid regulation is based on osmolality and volume triggers. As fluid is lost from the body,... 

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