Extracellular fluid osmolality

Except for respiratory and dermal insensible water-vapor losses, all remaining water lost by the body contains electrolytes, mainly sodium and chloride. The normal cation and anion constituent composition of the fluid spaces is given in Table IV. In the extracellular fluid space, sodium is the major cation and chloride the major anion. Those two ions constitute 95 of the extracellular fluid osmolality. Changes in plasma sodium concentration reflect changes in extracellular fluid volume. Potassium is the major cellular cation and phosphates and proteins comprise the major anions. The total cellular osmolality (175 + 135 = 310 mosraol/kg H2O) is equal to the total extracellular osmolality (155 + 155 = 310 mosmol/kg HaO) therefore, equal total osmotic concentrations are maintained between two fluid compartments of widely different ionic contents (Table IV). 

Astrocytes undergo rapid swelling in certain acute pathological conditions like ischemia and traumatic brain injury. Different mechanisms are involved in such swelling process of astrocytes. Some of these are, decreasing extracellular fluid osmolality, intracellular acidosis, formation of ammonia, increase in Na", K", 2CT co-transporter system, and due to drastically... 

Dialysis dysequilibrium is not seen often nowadays. It may occur in very uremic patients during the first dialysis, particularly with high-flux dialysis or very rapid blood flow. Its etiology is not clear, but is probably related to increased cerebrospinal fluid pressure and cerebral edema. The possible role of urea has already been discussed (see Section 3.1). Another hypothesis is that the brain cells of uremic patients produce idiogenic osmoles to prevent water loss to the hyperosmolar extracellular fluid. When high-flux dialysis rapidly decreases extracellular fluid osmolality, it may cause a water shift into the brain (B22). 

Excess effective osmoles in the extracellular fluid ° Hyperglycemic state or use of mannitol... 

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... 

Stimuli for Drinking. Thirst stimulation and the act of drinking are basic physiological responses. The three major circumstances known to stimulate thirst and drinking are (1) a deficit of body water (hypohydration and hypovolemia), (2) an increase in the osmolality of the extracellular fluid volume (hyperosmolality and hyperosmotemia), and (3) consumption of dry food (prandial thirst) (50). These three factors can function independently, but they are often interactive e.g., a hypovolemic subject is often hyperosmotic. In addition, the hormone angiotensin II acts as a stimulant for drinking (dipsogen) in animals, and possibly in man (51). 

Large doses of mannitol used in treating cerebral edema can alter extracellular fluid volume, osmolality, and composition and can lead under some circumstances to acute renal insufficiency, cardiac decompensation, and other complications (1). The patient s body habitus, age, total body water content relative to body weight, pretreatment plasma sodium concentration and plasma osmolality, and the presence of edema or ascites can influence the degree of extracellular fluid change and the rate of mannitol excretion to a significant degree. 

Mannitol, the most commonly employed osmotic diuretic, is a large polysaccharide molecule. It is often selected for use in the prophylaxis or treatment of oliguric ARF. It is not absorbed from the gastrointestinal tract and, therefore, is only administered i.v. with its elimination dependent on the GFR (within 30 to 60 min with normal renal function). Mannitol is distributed within the plasma and extracellular fluid spaces and produces an increase in the serum osmolality and expansion of the circulating volume. It is not generally used for the treatment of edema because any mannitol retained in the extracellular fluid can promote further edema formation. Furthermore, acute plasma volume expansion may challenge individuals with poor cardiac contractility and can precipitate pulmonary edema. Mannitol is commonly administered for the treatment of cerebral edema consequent to head trauma or to hypoxic-ischemic encephalopathy in neonatal foals. Because mannitol promotes water excretion, hypernatremia is a potential complication in patients that do not have free access to water (Martinez-Maldonado Cordova 1990, Wilcox 1991). 

Crystalloid solutions consist of electrolytes in water. Crystalloid solutions may be isotonic, hypertonic or hypotonic. Isotonic solutions have approximately the same osmolality as plasma and, therefore, may be given rapidly in large volumes into peripheral veins. Hypertonic solutions act to draw water into the extracellular fluid (ECF) from the intracellular fluid and represent a method of rapidly restoring circulating volume at the expense of tissue hydration. Hypotonic solutions are usually only used to correct plasma hypertonicity. Because true hypotonic solutions (e.g. sterile water) cause erythrolysis (Krumbhaar 1914), they can only be given slowly via a central vein (Worthley 1986). For this reason, isotonic solutions containing a metabolizable substrate, such as dextrose, and no electrolytes are usually used. 

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

Two-thirds of total body water is distributed intracellularly while one-third is contained in the extracellular space. Sodium and its accompanying anions, chloride and bicarbonate, comprise more than 90% of the total osmolality of the extracellular fluid (ECF), while intracellular osmolality is primarily dependent on the concentration of potassium and its accompanying anions (mostly organic and inorganic phosphates). The differential concentrations of sodium and potassium in the intra- and extracellular fluid is maintained by the Na+-K+-ATPase pump. Most cell membranes are freely permeable to water, and thus the osmolality of intra- and extracellular body fluids is the same. Symptoms in patients with hypo- and hypernatremia are primarily related to alterations in cell volume. It is therefore essential to understand the factors that cause changes in cell volume. 

FIGURE 49-4. Diagnostic and treatment algorithm for hypernatremia. D5W, 5% dextrose in water ECF, extracellular fluid H2O, water Na, sodium Uosm, urine osmolality Uvol, daily urine volume. See text for guidelines regarding calculations of infusion rates for intravenous solutions. 

The rise in the plasma sodium concentration and osmolality causes acute water movement from the intracellular to the extracellular fluid. In the brain this decrease in volume can cause rupture of the cerebral veins, leading to focal intracerebral and subarachnoid hemorrhages and possible irreversible neurologic damage. 

Finally, hyperosmolality results in enhanced movement of potassium from the cell into the extracellular fluid. This occurs most likely because of the associated cell shrinkage and water loss, which increases the intracellular-to-extracellular potassium gradient." This is seen most commonly in conditions such as diabetic ketoacidosis. Conversely, hypo-osmolality does not seem to affect potassium distribution. 

Another use for mannitol and urea is in the treatment of dialysis disequilibrium syndrome. Too rapid removal of solutes from the extracellular fluid by hemo- or peritoneal dialysis reduces the osmolality of the extracellular fluid. Consequently, water moves from the extracellular compartment into the intracellular compartment, causing hypotension and CNS symptoms (i.e., headache, nausea, muscle cramps, restlessness, CNS depression, and convulsions). Osmotic diuretics increase the osmolality of the extracellular fluid compartment and thereby shift water back into the extracellular compartment. 

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 test for osmolality measures the concentration of particles dissolved in blood. Sodium is a major contributor to osmolality in extracellular fluid. Serum osmolality... 

Sodium is the most abundant cation in the extracellular fluid and is the major factor in extracellular osmolality (the concentration of particles dissolved in blood). Sodium commonly moves with water, and water moves with sodium thus, as a determinant of osmolality, the concentration of sodium has an impact on the flow of water across the cell membrane. Additionally, the concentration of sodium and volume of water play a critical role in blood pressure. 

It has been estimated that the blood plasma normally contains about 140 mEq/L of sodium, which is higher than that found in other extracellular fluids, causing sodium to contribute greatly to the osmolality of body fluids. 

In hyponatremia, fluid moves from the extracellular fluid into the cells, moving from a lower osmolality with low sodium concentrations to a higher osmolality and high sodium concentrations. This results in tissue swelling or edema in many body, areas and organs including the brain (Fig. 5-1). 

Sodium is the primary positive ion in extracellular fluid and is a major determinant of fluid concentration or extracellular osmolality. Sodium is present in the body in a variety of forms and is stored in the bones and, more prevalently, in body fluids. Sodium is important for blood pressure maintenance, nerve impulse conduction, and circulation of nutrients into the cell. Thus sodium imbalance (outside the 135-145 mEq/L range) can result in fluid imbalance, as well as other electrolyte imbalances. 

The human body contains 60% water, which is an excellent solvent for electrolytes and plasma proteins [8], Because all cell membranes are freely permeable to water, intra- and extracellular fluids are generally considered to be in osmotic equilibrium. Therefore, the osmolality (the total molality of individual ionic and neutral solute species that contribute to the osmotic pressure) of the extracellular fluid is approximately equal to that of the intracellular fluid (ICF) and hence plasma osmolality is a guide to intracellular osmolality. Normal osmolality in plasma is —0.30 osmoles (kg water) F This is contributed mainly by sodium, chloride, potassium, urea and glucose, and additionally by other ions and substances in the blood. Most of the body fluids are isotonic ( of equal osmotic pressure , when only imperme-ant solutes are taken into account), with the notable exceptions of urine, sweat and saliva [9]. 

Dehydration may result from primary water deficiency, usually because of decreased intake, but in some instances (e.g., diabetes insipidus) it may result from increased losses of water. In general, the term dehydration implies intracellular and interstitial fluid depletion, in contrast to volume depletion, which implies extracellular, and particularly intravascular, sodium and water loss. In the case of primary water deficit, cell dehydration occurs, with delayed circulatory failure from decreased circulatory volume with ongoing losses. Initially, the patient may be thirsty and possibly have some mental status changes, such as confusion. If the cellular dehydration occurs slowly, intracellular substances, referred to as idiogenic osmols, develop that firnit progressive comphcations (e.g., cerebral edema or coma). With combined water and salt deficiencies, such as might occur with gastrointestinal... 

Water distributes between the different fluid compartments according to the concentration of solutes, or osmolality, of each compartment. The osmolality of a fluid is proportionate to the total concentration of all dissolved molecules, including ions, organic metabolites, and proteins (usually expressed as milliosmoles (mOsm)/kg water). The semipermeable cellular membrane that separates the extracellular and intracellular compartments contains a number of ion channels through which water can freely move, but other molecules cannot. Likewise, water can freely move through the capillaries separating the interstitial fluid and the plasma. As a result, water will move from a compartment with a low concentration of solutes (lower osmolality) to one with a higher concentration to achieve an equal osmolality on both sides of the membrane. The force it would take to keep the same amount of water on both sides of the membrane is called the osmotic pressure. 

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. 

Hypotonic fluids have a lower concentration of solutes (hypo-osmolality) than is found inside the cells, which causes fluid to flow into cells and out of the extracellular spaces. This causes cells to swell and possibly burst. 

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