Sw Dp Sv

Any increase in DP must result in a reduction in stroke volume unless there is an increase in stroke work.

Preload and afterload are precise terms used in muscle physiology. Preload refers to the load used to stretch muscle before a contraction, and afterload is the tension at which the load is lifted by the contraction. Clinically, preload refers to the degree of ventricular filling (i.e. the degree of stretch of the myocytes during diastole). Afterload is the resistance against which blood is expelled (arterial pressure). Arterial pressure may also influence cardiac function via extrinsic mechanisms.

A number of reflexes are involved in the regulation of the cardiovascular system and their precise mechanisms are beyond the scope of this brief review. The baroreflex is important in regulating blood pressure and heart rate. It depends on baroreceptors, which are tonically active stretch receptors found in the aortic arch and carotid sinuses. Their afferents travel with the vagus nerve and the glossopharyngeal nerve respectively to the brainstem, terminating in the nucleus tractus solitarius. The efferent limb consists of both vagal and cardiac sympathetic nerves to the heart. An increase in arterial pressure will produce increased baroreceptor afferent activity and increased vagal efferent activity. This results in a bradycardia. At the same time there will be a reduction in sympathetic outflow to the heart. This causes a reduction in cardiac contractility and arterial pressure. Sympathetic outflow to the peripheral vasculature will also be reduced, causing a fall in peripheral vascular resistance and thus a fall in arterial pressure.

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