PubMed1977
Arterial blood pressure varies very little among human beings and most other mammals for that matter. This suggests that a powerful control scheme is at work; it becomes more apparent when we break the various feedback loops and observe the excursions of blood pressure in the absence of any control. Two important control loops are found in the baroreceptor reflexes operating over the short term and the kidneys operating over the long term. The aortic and carotid baroreceptors stabilize pressure, preventing short-term fluctuations; when this control loop is surgically removed, lability increases with little change in the average pressure. Over the long term, the kidneys determine the average level of arterial pressure; when they are removed, pressure slowly drifts up and down as fluid is inadvertently accumulated or lost. There are several possible connections between the function of the kidneys and arterial pressure, including the release of vasoactive endocrines by the kidney and the adjustment of body fluids via salt and water excretion. Because salt excretion and water excretion often change in parallel, it has been difficult to identify the individual importances of each. However, we found that increasing the sodium stores of the body while holding volume constant does not produce hypertension, expanding fluid volume while maintaining or actually decreasing sodium concentration does lead to hypertension. Hence, when the kidneys are normal, long-term stability results from this loop: fluid volumes alter arterial pressure; pressure alters renal excretion; excretion alters fluid volumes.