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INTRODUCTION

Mean arterial blood pressure (MABP) is the average pressure measured in the aorta or a large artery during a cardiac cycle. MABP can be estimated from the systolic and diastolic arterial pressures:

MABP = (systolic minus diastolic pressure)/3 + diastolic pressure. If systolic pressure is 120 mm Hg and diastolic pressure is 80 mm Hg, then MABP = (40/3) + 80 = 93. MABP can also be calculated as MABP = [(2DP) + SP]/3 where DP = diastolic pressure and SP = systolic pressure. If SP = 120 and DP = 80, then MABP = [2(80) + 120]/3 = 93.

MABP is the driving force for blood flow through the systemic vasculature. The counterpart to MABP for the pulmonary circulation is the mean pulmonary artery pressure. All flow is dependent on a pressure gradient, ΔP. The actual pressure gradient for systemic blood flow is MABP – RAP, where RAP = right atrial pressure. Since RAP is approximately 0, we often express the pressure gradient, ΔP, as simply the MABP. Clinically, patients commonly have a measured RAP of 5 to 8 mm Hg or even higher but this elevation may be due to cardiopulmonary disease, renal function, intravenous fluid administration, or positive pressure ventilation.

The pressure gradient for pulmonary blood flow is MPAP – LAP where MPAP = mean pulmonary artery pressure and LAP = left atrial pressure. LAP is generally greater than 0 and therefore flow to the pulmonary circulation is expressed as CO = (PAP – LAP)/PVR where PVR = pulmonary vascular resistance.

Since MABP is the driving force for blood flow to all organs except the lungs, it is essential that it be regulated within a fairly limited range. Regulation of MABP is the result of both short-term mechanisms (the baroreceptor reflex) and long-term mechanisms (blood volume and blood composition—osmolarity and sodium concentration—as regulated by the kidney). Multiple circulating factors such as vasopressin, prostaglandins, angiotensin II, or the tissues’ metabolism (thyroid hormone, chorionic gonadotropin) can affect the blood flow to tissues by altering vascular tone and thus blood pressure. The second-to-second regulation of blood pressure is, however, normally under control of the baroreceptor reflex.

The baroreceptor reflex consists of five components: a sensor or detector (the baroreceptors); afferent nerve pathways; a comparator or controller (the brain); with efferent pathways and the effector organs—the heart, arterioles, and veins. Control of MABP is via changes in SV and/or HR and/or SVR since MABP = CO × SVR = SV × HR × SVR.

The baroreceptors for MABP regulation are located in the carotid sinus and the aortic arch. The baroreceptors are mechanoreceptors or stretch receptors that increase their nerve firing rate (number of action potentials) with increases in stretch induced by increases in MABP (Figure 19-1). The maximum sensitivity, that is, the maximum change in nerve firing rate for a given change ...

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