Long-term arterial pressure control: an analysis from animal experiments and computer and graphic models

Long-term arterial pressure control is very different from acute control, because many of the acute control systems are overridden by a single long-term mechanism that has little to do with short-term control. This is the renal fluid volume mechanism for pressure control. It is based on a simple functional property of the kidney: as the arterial pressure rises, the kidney output of water and electrolytes increases dramatically. When the output rises above the net intake of water and electrolytes, negative body fluid balance occurs, causing both the body fluid volume and the pressure to decrease. This decrease continues until the kidney fluid output exactly balances the net fluid intake. Conversely, if the pressure falls below the exact level for balance, intake becomes greater than output; then fluid builds up in the body and the pressure rises until intake and output again exactly balance each other. This fluid mechanism for pressure control has been known from the beginning of blood pressure research. However, its overpowering importance was not appreciated until a mathematical computer analysis in 1966 demonstrated the renal-fluid feedback mechanism to have infinite feedback gain for long-term pressure control. This is the principal topic of the present review.