Extracellular fluid osmolarity

Osmotic pressure from high concentrations of dissolved solutes is a serious problem for cells. Bacterial and plant cells have strong, rigid cell walls to contain these pressures. In contrast, animal cells are bathed in extracellular fluids of comparable osmolarity, so no net osmotic gradient exists. Also, to minimize the osmotic pressure created by the contents of their cytosol, cells tend... 

The kidneys also regulate the osmolarity of extracellular fluid, in particular plasma osmolarity. The maintenance of plasma osmolarity close to 290 mOsm prevents any unwanted movement of fluid into or out of the body s cells. An increase in plasma osmolarity causes water to leave the cells, leading to cellular dehydration a decrease in plasma osmolarity causes water to enter the cells, leading to cellular swelling and possibly lysis. Plasma osmolarity is regulated primarily by altering the excretion of water in the urine. 

The process of tubular reabsorption is essential for the conservation of plasma constituents important to the body, in particular electrolytes and nutrient molecules. This process is highly selective in that waste products and substances with no physiological value are not reabsorbed, but instead excreted in the urine. Furthermore, reabsorption of many substances, such as Na+, H+, and Ca++ ions, and water is physiologically controlled. Consequently, volume, osmolarity, composition, and pH of the extracellular fluid are precisely regulated. 

Osmotic diuretics such as mannitol act on the proximal tubule and, in particular, the descending limb of the Loop of Henle — portions of the tubule permeable to water. These drugs are freely filtered at the glomerulus, but not reabsorbed therefore, the drug remains in the tubular filtrate, increasing the osmolarity of this fluid. This increase in osmolarity keeps the water within the tubule, causing water diuresis. Because they primarily affect water and not sodium, the net effect is a reduction in total body water content more than cation content. Osmotic diuretics are poorly absorbed and must be administered intravenously. These drugs may be used to treat patients in acute renal failure and with dialysis disequilibrium syndrome. The latter disorder is caused by the excessively rapid removal of solutes from the extracellular fluid by hemodialysis. 

Regulation of the osmolarity of extracellular fluid, including that of the plasma, is necessary in order to avoid osmotically induced changes in intracellular fluid volume. If the extracellular fluid were to become hypertonic (too concentrated), water would be pulled out of the cells if it were to become hypotonic (too dilute), water would enter the cells. The osmolarity of extracellular fluid is maintained at 290 mOsm/1 by way of the physiological regulation of water excretion. As with sodium, water balance in the body is achieved when water intake is equal to water output. Sources of water input include ... 

The osmoreceptors of the hypothalamus monitor the osmolarity of extracellular fluid. These receptors are stimulated primarily by an increase in plasma osmolarity they then provide excitatory inputs to the thirst center and the ADH-secreting cells in the hypothalamus. The stimulation of the thirst center leads to increased fluid intake. The stimulation of the ADH-secreting cells leads to release of ADH from the neurohypophysis and, ultimately, an increase in reabsorption of water from the kidneys and a decrease in urine output. These effects increase the water content of the body and dilute the plasma back toward normal. Plasma osmolarity is the major stimulus for thirst and ADH secretion two additional stimuli include ... 

Pharmacology Normal osmolarity of the extracellular fluid ranges between 280 to 300 mOsm/L it is primarily a function of sodium and its accompanying ions, chloride, and bicarbonate. Sodium chloride is the principal salt involved in maintenance of plasma tonicity. One gram of sodium chloride provides 17.1 mEg sodium and 17.1 mEq chloride. 

Several mechanisms have evolved to prevent this catastrophe. In bacteria and plants, the plasma membrane is surrounded by a nonexpandable cell wall of sufficient rigidity and strength to resist osmotic pressure and prevent osmotic lysis. Certain freshwater protists that live in a highly hypotonic medium have an organelle (contractile vacuole) that pumps water out of the cell. In multicellular animals, blood plasma and interstitial fluid (the extracellular fluid of tissues) are maintained at an osmolarity close to that of the cytosol. The high concentration of albumin and other proteins in blood plasma contributes to its osmolarity. Cells also actively pump out ions such as Na+ into the interstitial fluid to stay in osmotic balance with their surroundings. 

Figure 6.1. Compositions and concentrations of intracellular and extracellular fluids of selected marine animals having widely different total osmolarities. M = intracellular fluids of muscle tissue PI = plasma or hemolymph TMAO = trimethylamine-A-oxide Bet = glycine betaine FAA = free amino acids. (Data compiled from various sources for a comprehensive list of osmolyte compositions and concentrations in diverse species, see Kirschner, 1991 Somero and Yancey, 1997 Yancey et al., 1982.)...

Q7 An excess of vasopressin produces a hypo-osmolar condition with excessive water retention. This greatly dilutes the sodium content of plasma and causes an overall dilution of the extracellular fluid (ECF), which can lead to tissue swelling, for example in the brain. Mental symptoms such as confusion, irritability, seizures and coma can occur when ECF sodium falls below 120 mEq l-1. 

Sodium ions are primarily responsible for maintaining the osmolarity of the extracellular fluid in contrast, potassium ions participate in maintaining the osmolarity of the intracellular fluid. The potassium content of plasma is too low to be able to influence osmoregulation. There are 7-150 mEq of potassium per liter in the intracellular fluid, and only 4.5 mEq/ liter in the plasma fluid. Consequently, 95% of the body potassium is intracellular, and the bulk of the potassium is found in muscle. 

Systemic edema may be the consequence of water retention, salt retention, or hypoproteinemia. When sodium is retained, the osmotic pressure of the body fluids increases as a result, the secretion of the antidiuretic hormone is stimulated, and water is retained until iso-osmolarity is reestablished and water accumulates in the extracellular fluid. A drop in the plasma... 

A damaged kidney cannot eliminate all the water that is ingested and produced through metabolism. Water retention has a triple effect—it turns off ADH secretion, reduces the osmolarity of the plasma and the extracellular fluid, and increases the plasma volume. The increase in plasma volume triggers the volume receptors and stimulates diuresis. If water diuresis does not compensate for the water intake and production, the hypotonicity is corrected by sodium retention, and the vicious circle is closed. 

Factors Regulating Movement. The body requires water. To ensure that this requirement is fulfilled, the sensation of thirst creates a conscious desire for water. The sensation of thirst is caused by nerve centers in the hypothalamus of the brain which monitors the concentration primarily of sodium In the blood. When the sodium concentration, and hence the osmolarity of the blood, increases above the normal 310 to 340 mg/100 ml (136 to 145 mEq/liter), cells in the thirst center shrink. They shrink because the increased osmotic pressure of the blood pulls water out of their cytoplasm. This shrinking causes more nervous impulses to be generated in the thirst center, thus creating the sensation of thirst. Increased osmolarity of the blood is primarily associated with water loss from the extracellular fluid. As water is lost the sodium concentration of the remaining fluid increases. When water is drunk, it moves across the membrane lining the gut into the blood thereby decreasing the sodium concentration—osmolarity—of the blood. In turn, the cells of the hypothalamus take on water and return to their normal size. This time water moves back into these cells via osmosis in the opposite direction. 

Hypotonic The concentration is less than the concentration of intracellular fluid (hypo-osmolar range less than 240 mOsm/L). Moves fluid from extracellular space into inside cells. 

Fluids move through the body continuously. The heart pumps the blood, pressure is exerted on the vessels from outside the body, and muscles relax and contract to help the heart move the fluid through the vascular system. Fluid moves into and out of the cells and the extracellular spaces by osmotic pressure. This is the pressure exerted by the flow of water through a semipermeable membrane separating two solutions with different concentrations of solute. Osmotic pressure is determined by the concentration of the electrolytes and other solutes in water and is expressed as osmolarity or osmolality. However, the terms are used interchangeably. 

Fluids and electrolytes are stored in two compartments intracellular (inside the cell) and extracellular (outside the cell). The amount of electrolytes in fluid is called a concentration. There are three types of fluid concentrations iso-osmolar (same concentration), hypo-osmolar (low concentration), and h) er-osmolar (high concentration). These concentrations are used to describe IV solutions as isotonic (iso-osmolar), hypotonic (hypo-osmolar), and hypertonic (hyper-osmolar). 

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