[Adjustment of the basal level of blood pressure]

Basically the circulation must satisfy three requirements: 1) to provide an adequate blood flow to the tissues for maintaining their energetic needs, 2) to sustain renal function (glomerular pressure and renal blood flow) for the precise homeostasis of body fluids, and 3) to grant cutaneous circulation for controlling body temperature. Therefore, the arterial circulation can be separated in oxygen dependent, filtration dependent and thermic dependent sectors. The blood flow distribution through these regions depends on the myogenic tone of the resistance vessels. The oxygen dependent section receives 70% of the cardiac output and settles the equivalent resistance of the arterial tree; so, it is the main one responsible for setting the blood pressure level. The total resistance of this section should be adjusted to maintain the ATP/ADP relationship. The mechanisms involved in the regulation of the myogenic tone are local metabolic products (pO2, pCO2, pH, etc.), vasoactive substances present in the vascular wall (EDRF, AgII, PGs, etc.) and intracellular variations (Na++, Ca++, PKC, IP3, etc.). The vascular resistance of this section, adjusted as an electronic module, settles the minimum blood pressure needed to maintain the energetic equilibrium, independently of the pressure required to achieve a normal renal function. Thus, the kidney to fulfill its function must modulate this previously established myogenic tone by employing the renin angiotensin system. Circulating AgII will increase the vascular tone in the oxygen dependent section overriding its local controls until the blood pressure reaches the necessary level to maintain an adequate renal function.