Essay On Osmolality

Essay 13: How does the brain sense osmolality and regulate water balance? Before answering this question, it is necessary to understand the meaning of the term osmolality as well as its implications for human physiology. In chemistry, the osmolality of a solution is defined as the number of moles of solute (e.g. salt or sugar) per kilogram of solvent. When the solvent in question is water, osmolality is practically the same as the more familiar unit osmolarity, which is defined as moles of solute per liter of solvent. Since 1 liter of water has a mass of approximately 1 kilogram, the two values are virtually the same. In the case of the human circulatory system, approximately 55% of blood volume is composed of straw colored plasma while the remainder is occupied by the so called formed elements of blood: red blood cells, white blood cells, and platelets. Plasma itself is 90% water and contains many dissolved components: glucose, electrolytes (sodium, chloride, and bicarbonate ions); as well as a variety of proteins including albumin, clotting factors, and soluble antibodies. All of these plasma components contribute to the osmolality of blood, the most important ones being sodium and glucose. Physiologists have assigned a unit to blood osmolality called the

In a sense, they act as the brain's eyes and ears allowing it to quickly assess metabolic parameters and make necessary adjustments. In humans, two of these sensors are the subfornical organ (SFO) and the vascular organ of the lateral terminalis (OVLT), both located near the hypothalamus. These clusters of neurons contain specialized surface proteins called osmoreceptors capable of sensing changes in the concentration of sodium and chloride ions. If the blood becomes too concentrated, the SFO and OVLT activate hypothalamic neurons, ultimately culminating in the sensation of