Extracellular osmotic activity

Sodium is the most abundant extracellular cation in the body and is the major osmotically active ion in the extracellular fluid. Sodium concentration determines the distribution of... 

Osmotically active laxatives (C) are soluble but nonabsorbable particles that retain water in the bowel by virtue of their osmotic action. The osmotic pressure (particle concentration) of bowel contents always corresponds to that of the extracellular space. The intestinal mucosa is unable to maintain a higher or lower osmotic pressure of the luminal contents. Therefore, absorption of molecules (e.g., glucose, NaQ) occurs isoosmotically, i.e., solute molecules are followed by a corresponding amount of water. Conversely, water remains in the bowel when molecules cannot be absorbed. 

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. 

Further support comes from the studies relating cell wall biosynthesis and amino acid accumulation capacity in vitamin B6-deficient cells, since it is difficult to account for these observations without attributing considerable osmotic activity to the accumulated amino acids. Any description of accumulation which invokes amino acid attachment to intracellular binding sites, whose affinity can be reduced by a vitamin B6 deficiency, must account for the stimulation of uptake that accompanies the synthesis of essentially extracellular cell wall material. If the reduction in affinity occurs because the cell interior becomes overhydrated (a reasonable postulate which follows from the osmotic experiments), the beneficial effect of wall synthesis is not readily explicable, since vitamin B6-deficient cells have a swollen appearance which is not significantly altered after wall synthesis has been stimulated. Thus, the existing overhydration within the cell probably is not reversed by this change. In contrast, the deposition of additional wall substance would prevent further unfavorable consequences of swelling such as membrane distention, and, in this way, forestall the premature cessation of amino acid accumulation. 

Maltose is used in oral and parenteral pharmaceutical formulations and is generally regarded as an essentially nontoxic and nonirritant material. However, there has been a single report of a liver transplantation patient with renal failure who developed hyponatremia following intravenous infusion of normal immunoglobulin in 10% maltose. The effect, which recurred on each of four successive infusions, resembled that of hyperglycemia and was thought to be due to accumulation of maltose and other osmotically active metabolites in the extracellular fluid. ... 

The fluid spaces in the body occupy about 60% of the total body mass. The two main spaces are the intracellular fluid (ICF) and the extracellular fluid (ECF), with the ECF subdivided into the intravascular space (plasma), the interstitial space (lymph), and transcellular fluids such as pleural, cerebrospinal, pericardial, peritoneal, and gastrointestinal fluids. The ECF and ICF spaces are normally in an osmotic equilibrium in which body water moves under osmotic pressure between ICF and ECF, governed by the osmotically active molecules in each space. The electrolyte constituents of ICF and ECF are different, particularly for sodium, which is higher in ECF than ICF, and potassium, which is higher in ICF compared to ECF. 

The sodium ion concentrations in body fluids are listed in Table 1 [2]. From a physiological viewpoint, Na provides the bulk of osmotically active solutes in plasma with its associated anions, thus affecting the distribution of the body water significantly. An excess translocation of Na into cells or a loss of it from the body results in a decrease of extracellular fluid volume, affecting circulation, renal function, and nervous system function. 

Cautious intravenous infusion of isotonic sodium chloride solution improves the patient s condition. Although sodium chloride solution is neutral, the provision of the principal extracellular electrolytes provides the necessary osmotically active material to return the extracellular fluid volume towards normal. This switches the drive from mechanisms for retention of electrolyte to those for correction of metabolic alkalosis. The body puts its own house in order so far as the acid-base problems are concerned and the situation is saved. 

Sodium and potassium are distributed characteristically unevenly in the organism Na+ in extracellular fluids, especially in the interstitial spaces K" " in the cells. The intracellular K+ concentration is less than the extracellular Na" " concentration since the cells contain relatively much osmotically active material, osmotic equilibrium would not be ensured othenvise. In some organs (nerve, muscle) this uneven distribution is the basis of their function, i.e. of excitability. 

Aldosterone acts on the distal tubule of the nephron to increase sodium reabsorption. The mechanism of action involves an increase in the number of sodium-permeable channels on the luminal surface of the distal tubule and an increase in the activity of the Na+-K+ ATPase pump on the basilar surface of the tubule. Sodium diffuses down its concentration gradient out of the lumen and into the tubular cells. The pump then actively removes the sodium from cells of the distal tubule and into the extracellular fluid so that it may diffuse into the surrounding capillaries and return to the circulation. Due to its osmotic effects, the retention of sodium is accompanied by the retention of water. In other words, wherever sodium goes, water follows. As a result, aldosterone is very important in regulation of blood volume and blood pressure. The retention of sodium and water expands the blood volume and, consequently, increases mean arterial pressure. 

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. 

---