Extracellular fluid volume is directly proportional to the total amount of Na+ in the body. The plasma Na+ concentration is not an indicator of total body Na+ but of water balance. Changes in plasma Na+ concentration occur only when water loss or gain from the body exceeds the capacity to regulate plasma osmolality by controlling water drinking through thirst and water excretion by the kidneys. A typical American diet contains 8-15 g NaCl, equivalent to 150-250 mmol of NaCl. An equal amount must be excreted to maintain a constant and normal extracellular fluid volume. About 25,000 mmol of Na+ are filtered per day, but only 150-250 mmol are usually excreted; thus, more than 99% of the filtered Na+ is reabsorbed by the nephron.
The large veins and atria have stretch receptors that increase their rates of firing with changes in filling. Arterial baroreceptors are also stretch receptors that respond to changes in blood pressure and pulse pressure, and relay information about cardiac output. The kidney responds to changes in volume sensed by stretch receptors through three basic mechanisms: changes in the glomerular filtration rate (GFR), regulation of aldosterone secretion, and secretion of natriuretic hormones. Reabsorption of water and salt in the proximal tubule normally is a constant fraction of the filtered amount irrespective of the GFR. This is referred to as glomerulotubular balance. However, a small change in GFR results in a relatively large change in NaCl delivery to the
Essential Medical Physiology, Third Edition
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