Renal-Body Fluid System for Arterial Pressure Control
The renal–body fluid system for arterial pressure control is a simple one: When the body contains too much extracellular fluid, the blood volume and arterial pressure rise. The rising pressure in turn has a direct effect to cause the kidneys to excrete the excess extracellular fluid, thus returning the pressure back toward normal.
In the phylogenetic history of animal development, this renal–body fluid system for pressure control is a primitive one. It is fully operative in one of the lowest of vertebrates, the hagfish. This animal has a low arterial pressure, only 8 to 14 mm Hg, and this pressure increases almost directly in proportion to its blood volume. The hagfish continually drinks sea water, which is absorbed into its blood, increasing the blood volume as well as the pressure. However, when the pressure rises too high, the kidney simply excretes the excess volume into the urine and relieves the pressure. At low pressure, the kidney excretes far less fluid than is ingested. Therefore, because the hagfish continues to drink, extra-cellular fluid volume, blood volume, and pressure all build up again to the higher levels.
Throughout the ages, this primitive mechanism of pressure control has sur-vived almost exactly as it functions in the hagfish; in the human being, kidney output of water and salt is just as sensitive to pressure changes as in the hagfish, if not more so. Indeed, an increase in arterial pressure in the human of only a few millimeters of mercury can double renal output of water, which is calledpressure diuresis, as well as double the output of salt, which is called pressure natriuresis.
In the human being, the renal–body fluid system for arterial pressure control, just as in the hagfish, is the fundamental basis for long-term arterial pressure control. However, through the stages of evolution, multiple refinements have been added to make this system much more exact in its control in the human being. An especially important refinement, as we shall see, has been addition of the renin-angiotensin mechanism.
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