Osmolality and osmolarity

The experimentally derived osmotic pressure is frequently expressed as the osmolality which is the mass of solute which, when dissolved in 1 kg of water, will exert an osmotic pressure, H, equal to that exerted by 1 mole of an ideal unionised substance dissolved in 1 kg of water. The unit of osmolality is the osmole (abbreviated as osmol), which is the amount of substance that dissociates in solution to form one mole of osmotically active particles, thus 1 mole of glucose (not ionised) forms 1 osmole of solute, whereas 1 mole of NaCl forms 2 osmoles (1 mole of Na+ and 1 mole of 

In practical terms, this means that a 1 molal solution of NaCl will have (approximately) twice the osmolality (osmotic pressure) as a 1 molal solution of glucose. 

According to the definition, = n/fT. The value of If may he obtained from equation (3.59) hy noting that for an ideal unionised substance v = 0 = I, and since m is also unity, equation (3.59) becomes 

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Based on the theory of colligative properties and the principles of osmometry, it is understood that osmometer will read osmolalities and not osmolarities because colligative properties are directly proportional to the total solute concentration expressed in molality [see Eqs. (1)-(16)]. The relationship between osmolality and osmolarity and its significance can be found in the Remington s Pharmaceutical Sciences and in a review article by Deardorff. However, it is more convenient to use osmolarity because it is based on weight/volume rather than on weight/weight as in... 

B. Serum osmolality and osmolar gap. Serum osmolality may be measured in the laboratory with the freezing-point-depression osmometer or the heat-of-vaporization osmometer. Under normal circumstances the measured semm osmolality is approximately 290 mOsm/L and can be calculated from the results of the sodium, glucose, and BUN tests. The difference between the calculated osmolality and the osmolality measured in the laboratory is the osmo-lal gap, more commonly referred to as the osmolar gap (Table 1-22). 

B. Other useful laborafory sfudies include electrolytes, glucose, BUN, creatinine, semm osmolality and osmolar gap, and arterial blood gases or oximetry. 

B. Other useful laboratory studies include electrolytes, BUN, creatinine, serum osmolality and osmolar gap (magnesium may elevate the osmolar gap), calcium, and ECG. 

B. Other useful laboratory studies include electrolytes, glucose, BUN, creatinine, calcium, ammonia, liver transaminases, bilirubin, prothrombin time (PT), amylase, serum osmolality and osmolar gap (see p 32 serum levels > 1500 mg/L may increase the osmolar gap by 10 mOsm/L or more), arterial blood gases or oximetry, and EGG monitoring. Valproic acid may cause a falsepositive urine ketone determination. 

Osmolar gap The difference between the measured serum osmolality and the calculated serum osmolality. 

Urea concentration in the medulla plays an important role maintaining the high osmolarity of the medulla and in the concentration of urine. ADH secretion is regulated by serum osmolality and by volume status. A new class of drugs, the vaptans (see under Agents That Alter Water Excretion), are ADH antagonists. 

The osmotic activity of a dissolved substance is characterized by its osmolarity and/or osmolality and the osmotic pressure. Osmolarity and osmolality are defined as follows ... 

B. Other useful laboratory studies include electrolytes, lactate, ethanol, glucose, BUN, creatinine, calcium, hepatic transaminases, urinalysis (for crystals and Wood s lamp examination), measured osmolality, arterial blood gases, and ECG monitoring. Serum beta-hydroxybutyrate levels may help distinguish ethylene glycol poisoning from alcoholic ketoacidosis, which may also cause increased anion and osmolar gaps. (Patients with alcoholic ke-... 

The process of transport of water through a semiperme-able membrane due to a concentration difference is called osmosis and the final pressure difference between both sides of the membrane is called osmotic pressure. Basically, the osmotic pressure should be expressed in Pascal, but in practice the words osmolality or osmolarity are used to indicate osmotic pressure, both with osmole as unit. 

Low Osmolality Contrast Media. An ideal intravascular CM possesses several properties high opacity to x-rays, high water solubihty, chemical stabihty, low viscosity, low osmolahty, and high biological safety. Low cost and patentabihty are also important for commercial agents. The newer nonionic and low osmolar agents represent an advanced class of compounds in the development of x-ray contrast media. 

Fig. 2. Schematic representation of relevant electrolyte transport through the renal tubule, depicting the osmolar gradient ia medullary iaterstitial fluid ia ywOj yW where represents active transport, —passive transport, hoth active and passive transport, and passive transport of H2O ia the presence of ADH, ia A, the cortex, and B, the medulla. An osmole equals a mole of solute divided by the number of ions formed per molecule of the solute. Thus one mole of sodium chloride is equivalent to two osmoles, ie, lAfNaCl = 2 Osm NaCl. ADH = antidiuretic hormone.

Uremia results in increased permeability of the blood-brain barrier to sucrose and insulin K+ transport is enhanced whereas Na+ transport is impaired. There is an increase in brain osmolarity in acute renal failure due to the increase in urea concentrations. However, in contrast to acute renal failure, the increase in osmolarity in chronic renal failure results from the presence of idiogenic osmoles in addition to urea. CBF is increased in uremic patients but CMR02 and CMR are decreased. In the brains of rats with acute renal failure, ATP, phosphocreatine and glucose are increased whereas AMP, ADP and lactate are decreased, most probably as a result of decreased energy demands. 

What is the osmolarity of 0.9% w/v NaCl injection with a reported osmolality of 287 mOsm/kg and a density of 1.0046 ... 

Currently available contrast agents can be classified into three different groups, high-osmolar compounds with osmolalities in the order of 1500 mosm kg low-osmolar agents with 600 - 700 mosm kg and isotonic substances with osmolaH-ties similar to that of blood (300 mosm kg ). Data for individual contrast agents are summarized in Table 2. 

This calculated value is normally 280-290 mOsm/L. Ethanol and other alcohols may contribute significantly to the measured serum osmolality but, since they are not included in the calculation, cause an osmolar gap ... 

The development of low osmolar non-ionic X-ray contrast agents has resulted in a distinct reduction in the toxicity and the observed side-effects in patients. However, as already mentioned the osmotic activity of MRI contrast agents is less important in view of the smaller injection volumes which are used. All the formulations of extracellular gadolinium chelates are hypertonic when compared with blood. But the overall increase in osmolality after injection of even 0.3 mmol/kg body weight is insignificant. Osmololatiy-induced adverse reactions have been observed rarely not only because of the relatively small injection volumes but also because of the rapid dilution of the injected agent in the blood. 

Other substances that can increase the osmolar gap include acetone, mannitol, and magnesium. Note Most laboratories use the freezing point method of determining osmolality. However, if the vaporization point method is used, the alcohols may be driven off and their contribution to osmolality will be lost. ... 

Living cells, among them the red blood cells, are surrounded by semipermeable membranes. The osmolarity of most cells is 0.30 osmol. For example, a 0.89% w/v NaCl solution, normally referred to as physiological saline solution, has an osmolarity of 0.30. Thus when a cell is put in physiological saline solution, the osmolarity on both sides of the membrane is the same and therefore no osmotic pressure is generated across the membrane. Such a solution is called isotonic. On the other hand, if a cell is put in water (pure solvent) or in a solution which has lower osmolarity than the cell, there will be a net flow of water into the cell driven by the osmotic pressure. Such a solution is called hypotonic. A cell placed in a hypotonic solution will swell and eventually may burst. If that happens to a red blood cell, the process is called hemolysis. In contrast, a solution with higher osmolarity than the cell is called a hypertonic solution. A cell suspended in a hypertonic solution will shrivel there is a net flow of water from the cell into the surroundings. When that happens to a red blood cell, the process is called crenation. 

The colligative properties, described in Section 3.4.1 to Section 3.4.3, have been used to determine the molar mass of unknown chemical compounds. Pharmaceutical scientists and pharmacists may apply this concept in the preparation of isotonic (meaning of equal tone) solution dosage forms. These solution dosage forms can be applied to sensitive and delicate organs such as the eye, nose, or ear or directly injected into the body (i.e., blood vessels, muscles, lesions, etc.). They should have, when administered, the same osmotic pressure as body fluids. Otherwise, transport of body fluids inside and outside the cell tissues will occur, causing discomfort and damage to the tissue. Osmolarity of body fluids is approximately 0.307 osmol/L or 307 mosmol/L. 

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