Which of the following statement(s) is/are true concerning the osmotic activity of body fluids?

a. Urea contributes to the osmolality of a solution but not its tonicity

b. The osmolality of the body remains fairly constant at approximately 289 mOsm/kg H2O

d. Serum sodium is the most valuable laboratory indicator of abnormal total body water content

Body fluids are aqueous solutions composed primarily of water and contained in different compartments of the body. The movement of water from these compartments depends on a number of physical properties, the most important of which is osmosis. According to the principles of osmosis, if two solutions are separated by semipermeable membrane, water moves across the membrane to equalize the concentration of the osmotically active particles. The osmotic activity across a semipermeable membrane is determined by the concentration of solutes on each side of the membrane. The body is capable of fine regulation of solute and water concentrations, so that osmolality remains fairly constant at an average of 289 mOsm/kg H2O. In response to small changes in cell volume, osmoreceptors in the paraventricular and supraoptic nuclei of the hypothalamus send signals to the neuronal centers that control the two primary regulators of water balance, thirst and antidiuretic hormone secretion. Changes in TBW are reflected by changes in extracellular solute concentration. Because sodium is the primary extracellular cation and potassium is the predominant intracellular cation, the serum sodium approximates the sum of the exchangeable total body sodium and exchangeable total body potassium divided by the TBW. Because total body solute content remains relatively stable over time, changes in TBW content result in inversely proportional changes in serum sodium. Thus, abnormalities in serum sodium are the indication of abnormal TBW content. In contrast to impermeable solutes that are excluded from the intracellular space, such as sodium, permeable solutes such as urea can freely cross the cell membranes. Although urea contributes to the osmolality of a solution, it has no effect on tonicity because it distributes equally across membranes, and as such does not contribute to the osmols that affects cell volume.