Regulation of Flow and Pressure. A. Arterial Pressure (overview) 1. Arterial pressure pulse 2. Mean arterial pressure MAP = mean arterial pressure, P s = systolic pressure, P d = diastolic pressure.
For regulation, you only get to change two things: the cardiac output (heart rate, stroke volume) and the resistance of the vasculature
c. Blood Pressure Regulation: “reactive hyperemia”.
iii. Differentiation between “capacitance” and “ resistance” vessels:
Regulation of Blood Pressure: arterial an venous control in the short term, blood volume in the long term.
f. chemoreceptors: same Aortic/carotid sinuses receptors that regulate respiratory rate have some effect on the vasoconstriction of VSM, via the medulla. (when competing with the baroreceptors, though, they lose).
Note competition between intrinsic and extrinsic regulation of blood flow.
The more vital/intolerant to hypoxia an organ is, the more intrinsic control it gets over its blood flow (brain/heart…)
Dominant Intrinsic control: brain, heart, ACTIVE skeletal muscle
Dominant Extrinsic control: skin, digestive, RESTING skeletal muscle
The system as a big circuit
A. Veins are very compliant, not very resistive, but there is still a venomotor tone. Cerebral ischemia triggers venous constriction to bring up mean blood pressure.
B. Unidirectional valves
C. Large Blood Volume (60% of total)
D. Venous Volume at the Vena Cava determines how much blood will be available for the next stroke volume, but that volume is a function of the stroke volume.
Because of the “wiring”, the cardiac output drives the venous volume.
Pretty Good approximation in the real world, but CAREFUL: venous volume has an effect on the cardiac output too! The heart will not pump any more blood out than what it receives.
(consider what happens with bleeding, edema, dehydration ... )
Vascular function curves: vary cardiac output artificially, what happens to venous pressure.
reflects lag between cardiac output and venous pressure right after diastole Note the difference in compliance between arteries and veins:
Arteriolar responses: constriction, dilation … Think of them in terms of changing resistance in the circuitry.
BUT, stroke volume is a function of heart rate too, via myocardial contractility.
i. Renal function (though vagal response to cardiovascular BP)
ii. Fluid intake
iii. Other factors related to fluid retention. Bleeding, vomiting, hormones, sweating …