Chapter 156

### BLOOD PRESSURE

Blood pressure is the force exerted by blood against vessel walls. More specifically, blood pressure refers to the pressure of blood within the circulatory system’s arteries. Arterial blood pressure is determined by the cardiac cycle’s systole and diastole. During ventricular contraction, or systole, blood exits the heart’s right and left ventricle into the pulmonary artery and aorta, causing pressures in these arteries to rise steeply. Systolic blood pressure (SBP) is the maximum pressure achieved during ventricular contraction. When ventricles relax during diastole, they fill with blood in preparation for the next contraction, and arterial blood pressure drops. Diastolic blood pressure (DBP) is the blood pressure following contraction of the heart, during heart chamber refilling, and represents the lowest arterial pressure prior to the next contraction cycle. The difference between systolic and diastolic pressures is the pulse pressure.

However, the primary pressure that drives blood flow in organs is the mean arterial pressure (MAP), which is determined from systolic and diastolic pressures. Mean arterial pressure drives blood flow to organs and tissues, and is the average pressure of several heartbeats over time. It can be determined by the following equation:

Diastole counts twice as much as systole since approximately two-third of the cardiac cycle is spent in diastole. The usual healthy range of MAP is 70-110, and an MAP of 60 is necessary to perfuse the body’s vital organs and prevent ischemia.

Alternatively, cardiac output (CO), systemic vascular resistance (SVR), and central venous pressure (CVP) determine MAP, according to the equation:

MAP = (CO × SVR) + CVP

Mean arterial pressure is proportional to the above variables. If CO and SVR change reciprocally, yet proportionately, MAP will remain the same.

### PRESSURE, FLOW, AND RESISTANCE

Hemodynamics refers to the study of blood flow, and explains the physical laws that determine blood flow in vessels. Flow (Q) through a blood vessel is primarily determined by two factors: (1) the pressure gradient that pushes blood through the vessel (ΔP), and (2) the resistance of the vessel to blood flow (R). The pressure gradient (ΔP) is expressed as the difference between arterial and venous pressures. Flow is determined by the pressure gradient (ΔP) divided by resistance (R).

Between the pressure gradient and resistance, flow is more dependent on resistance, since arterial and venous blood pressures are largely maintained within a narrow range. There are several factors that determine resistance to flow, including characteristics of blood (density or viscosity), blood flow (laminar or turbulent), vessels (length, radius), and vessel network organization (series or parallel).

The primary factors that determine resistance to blood flow are vessel length, vessel radius, and blood viscosity. The relationship is seen in the following equation, which ...

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