Osmotic balance

The body s normal daily sodium requirement is 1.0 to 1.5 mEq/kg (80 to 130 mEq, which is 80 to 130 mmol) to maintain a normal serum sodium concentration of 136 to 145 mEq/L (136 to 145 mmol/L).15 Sodium is the predominant cation of the ECF and largely determines ECF volume. Sodium is also the primary factor in establishing the osmotic pressure relationship between the ICF and ECF. All body fluids are in osmotic equilibrium and changes in serum sodium concentration are associated with shifts of water into and out of body fluid compartments. When sodium is added to the intravascular fluid compartment, fluid is pulled intravascularly from the interstitial fluid and the ICF until osmotic balance is restored. As such, a patient s measured sodium level should not be viewed as an index of sodium need because this parameter reflects the balance between total body sodium content and TBW. Disturbances in the sodium level most often represent disturbances of TBW. Sodium imbalances cannot be properly assessed without first assessing the body fluid status. 

Metals in biological systems function in a number of different ways. Group 1 and 2 metals operate as structural elements or in the maintenance of charge and osmotic balance (Table 1.2). Transition metal ions that exist in single oxidation states, such as zinc(II), function as structural elements in superoxide dismutase and zinc fingers, or, as an example from main group +2 ions, as triggers for protein activity—that is, calcium ions in calmodulin or troponin C... 

The concentrations of sodium, potassium (and chloride) ions in the body are high and make the largest contribution to the electrical charge of cells hence they are known as electrolytes. They have two important roles maintenance of the total solute concentration in the cell which prevents excessive movement of water into or out of cells through osmosis and the controlled movement of these ions across cell membranes acts as a signalling mechanism (e.g. the action potential in neurones and muscle. Chapter 14). Severe disruption of sodium or potassium levels in the body interferes with this signalling mechanism and with osmotic balance in cells. 

Calcium Substitution Experiments. Saline solutions utilized in this senes of experiments contained calcium chloride at 0.25X, 0.5X, 2X, 3X and 4X the normal level. The osmotic balance of the saline was maintained by adjusting the NaCl concentrations. 

On the other hand, the cornea is located between two hypertonic grounds tears and the aqueous humor. The rules of osmotic balance are a good explanation for the passive move of water toward the front and back sides of the cornea, so contributing in the maintenance of the relative dehydration of the stroma. 

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 2-13 Effect of extracellular osmolarity on water movement across a plasma membrane. When a cell in osmotic balance with its surrounding medium (that is, in an isotonic medium) (a) is transferred into a hypertonic solution (b) or hypotonic solution (c), water moves across the plasma membrane in the direction that tends to equalize osmolarity outside and inside the cell. 

Depletion of glutamate in the glutamine synthetase reaction may have additional effects on the brain. Glutamate and its derivative y-aminobutyrate (GABA see Fig. 22-29) are important neurotransmitters the sensitivity of the brain to ammonia may reflect a depletion of neurotransmitters as well as changes in cellular osmotic balance. ... 

Ammonia is also the major nitrogenous end product in some of the simpler aquatic and marine animal forms, such as protozoa, nematodes, and even bony fishes, aquatic amphibia, and amphibian larvae. Such animals are called am-monotelic. But in many animals, NH3 is toxic, and its removal by simple diffusion is difficult. Thus, in terrestrial snails and amphibia, as well as in other animals living in environments in which water is limited, urea is the principal end product (fig. 22.6). Urea formation also helps to maintain osmotic balance with seawater in cartilagenous fishes. In such animals, most of the urea secreted by the kidney glomerulus is reabsorbed by the tubules. Indeed, the amount of nitrogen excreted by the kidneys of fishes is small com-... 

Na+, K+ Very weak Osmotic balance Charge neutralization Gradients and control mechanisms Structure stabilization (K+) Enzyme activation (K+)... 

The physiological functions of isofloridoside in algae have been shown to be (a) a carbohydrate reserve, and (b) a factor involved in controlling the osmotic balance in, for example, Ochromonas malha-men.st.s.242 244-247-249 Ochromonas is a green-brown alga that lacks a cell... 

Analogous to the development of the non-ionic X-ray contrast agents many years ago neutral chelates have been sythesized to reduce the osmotic pressure of the aqueous solutions. However, for MRI, injection volumes are used that are much smaller than for X-ray contrast agents. That means that the osmolality of the solutions is of minor importance for MRI contrast agents. The increase in blood osmolality after intraveneous injection of up to 0.3 mmol/kg Gd-DTPA will not cause disturbance of the organism s osmotic balance. 

Trimethylamine oxide (TMO) is one of the compounds retained by elasmobranchs to assist their osmotic balance (reviewed by Love, 1970). This compound is also used by eels when they are transferred from fresh water to salt. The fish were not fed, but TMO levels increased for about 24 h. The compound is probably synthesized from choline via trimethylamine, and synthesis of the enzymes responsible for the system appears to begin immediately after transfer to the sea water (Daikoku and Sakaguchi, 1990). 

Salinity affects microbial activity, in part, because it controls water availability. The higher the salinity, the more energy an organism must expend to maintain a favorable osmotic balance. Salts, of course, have effects on living organisms beyond water availability. For example, salts can be both a source of essential nutrients as well as a source of toxic heavy metals. Also, sulfate salts appear to be more favorable for life than chloride salts see the discussion in Sect. 5.1.2 (Aqueous Saline Environments). However, in this section on salinity, the focus will be on salinity as a control on water availability. 

There is no obvious correlation between MAA and scytonemin concentrations in free-living or symbiotic cyanobacteria. This lack of covariance suggests that although both compounds have UV-absorbing properties, they may respond to different stimuli. It has been suggested that scytonemin and MAAs have different UV photoreceptors.168 It is also possible that primary or secondary functions may differ. In addition to UV absorption, MAAs may have physiological functions related to antioxidant activity, regulation of reproductive processes, or maintenance of osmotic balance. Scytonemin is more likely to provide only radiation protection, and, therefore, there may be little reason to expect concentrations of these compounds to be correlated. 

As well as being essential for certain reactions Na+ and, to a lesser extent, K+ are important in maintaining the osmotic balance in the cell. The sodium ion concentration is maintained about isotonic largely by the NaCl in the BSS but in Earle s BSS this is supplemented with NaHC03 and to a lesser extent with other salts (see Appendix 1). If the Na+ concentration is increased from the normal 120 mM up to 220 mM it leads to a dissociation of polysomes with concomitant inhibition of protein synthesis and cell growth (Fig. 5.1) (Saborio et al., 1974). 

J 2 Jullander, J. Studies on nitrocellulose including the construction of an osmotic balance. Arkiv Kemi, Mineral. Geol. 20 A, Nr. 8 (1945). 

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