Interactive changes in V and R determine long term arterial pressure
Arterial blood pressure is determined by the arterial blood volume (V) relative to the capacity of the arterial space (R)
The body sodium content determines the arterial blood volume.
When dietary salt increases, the body salt content increases, and this in turn increases the arterial blood volume. Blood pressure would rise if the capacity of the arterial space did not increase at the same time.
When dietary salt is reduced, arterial blood volume falls. This would lower blood pressure if the capacity of the arterial space did not contract at the same time.
Thus, the key to maintaining a normal level of BP in the face of changing levels of body salt is reciprocal changes in the capacity of the arterial space.
PRA (R) affects the capacity of the arterial space.
Arteriolar smooth muscles contract or relax as the plasma angiotensin II (Ang II) concentration rises or falls.
PRA determines plasma Ang II levels and PRA is tightly regulated and reciprocally related to body sodium-volume (V).
Relationship between PRA (R) and arterial blood volume (V). BP = V x R
Renin is secreted from the kidneys
The secretion rate of renin is largely determined by the interrelated factors of body salt and arterial pressure.
PRA falls as salt intake rises. PRA rises as salt intake falls.
These changes adjust the capacity of the arterial space to stabilize BP.