The primary factors that affect circulation - MABP, CO and SVR

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Introduction Blood flow is determined by the relative intensities of factors that drive and resist moving blood. • Cardiac output (CO) equals the mean arterial blood pressure (MABP, a driving force) divided by systemic vascular resistance (SVR, a resisting force). • Hormones and the central nervous system both influence and regulate MABP or SVR and impact CO. The primary factors that influence circulation arc: • Mean arterial blood pressure. • Systemic vascular resistance. • Cardiac output. • Blood flows from areas of higher pressure to areas of lower pressure. • The pressure gradient, determined by pressure differences in blood ejected from the heart (MABP) and that entering the right atrium (right atrial pressure), determines systemic blood flow. • Right atrial pressure is so low that blood flow is directly proportional to only MABP. • The greater the pressure gradient, the greater the flow. Cardiac output (CO) is: ■ Measure of blood ejected by ventricles over a given time. • Product of heart rate and stroke volume. • Influenced by venous return (preload), contractile strength of heart, and metabolic demands. The greater the cardiac output then the greater the blood flow. • Systemic vascular resistance is the natural dampening of blood flow. ■ The greater the resistance, the harder it is to move blood through the vessel. ■ Factors that increase resistance are: • Vessel radius - the smaller the radius, the greater the SVR. • Blood viscosity - the thicker the blood, the greater the SVR. ■ Vessel length - the greater the length, the greater the SVR. • The greater the resistance the greater cardiac output (and blood pressure) must be to overcome the resistance.

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