Water-fluid balance is characterized by –
Correct Answer: Maintained by endogenous water loss
Description: Water and electrolyte homeostasis Total body water (TBW) is approximately 60% of body weight in an adult male, although the propoion is somewhat more for infants and less for women. In a 70 kg man TBW is therefore about 40 L. Approximately 25 L is located inside cells (the intracellular fluid or ICF), while the remaining 15 L is in the extracellular fluid (ECF) compament (Fig. 14.1). Most of the ECF (approximately 12 L) is interstitial fluid, which is within the tissues but outside cells, whereas the remainder (about 3 L) is in the plasma compament. The ion composition between the main body fluid compaments intracellularly and extracellularly is illustrated in Figure 14.1. The dominant positively charged ion (cation) within cells is potassium, whereas phosphates and negatively charged proteins constitute the major intracellular negatively charged ions (anions). In the ECF the dominant cation is sodium, while chloride and, to a lesser extent, bicarbonate are the most impoant ECF anions. An impoant difference between the intravascular (plasma) and interstitial compaments of the ECF is that only plasma contains significant concentrations of protein. The major force maintaining the difference in cation concentrations between the ICF and ECF is the sodium-potassium pump (Na,K-activated adenosine triphosphatase (ATPase)), which is present in all cell membranes. Maintenance of these gradients is essential for many cell processes, including the excitability of conducting tissues such as nerve and muscle. The difference in protein content between the plasma and the interstitial fluid compament is maintained by the impermeability of the capillary wall to protein. This protein concentration gradient (the colloid osmotic, or oncotic, pressure of the plasma) contributes to the balance of forces across the capillary wall that our fluid retention within the plasma compament. The concentration of sodium in the ECF plays a pivotal role in determining plasma osmolality and thereby controlling intracellular volume through changes in water balance between the intracellular and extracellular space. In contrast, plasma volume is largely controlled by total body sodium, which determines volume change. Therefore, disturbances in water homeostasis typically present with biochemical abnormalities such as hyponatraemia or hypernatraemia, whereas disturbances in sodium homeostasis present with hypervolaemia or hypovolaemia as the result of expansion or contraction of ECF volume, respectively. Ref Davidson 23rd edition pg 349
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