Our first article in this essential section regards our physiological control of cardiac output. Cardiac output is defined as stroke volume multiplied by heart rate. This is therefore affected by stroke volume and sympathetic/parasympathetic output to the heart. Stroke volume is affected by how much the heart fills in diastole and how easy it is for blood to be expelled in systole. This article shall consider components of stroke volume, Starling’s law, its regulation and clinical conditions that may occur when it is inadequate to meet the body’s needs.
Our second article in this section considers the control of our heart rate. The heart rate is established by the Sinoatrial Node (SAN) – the pacemaker of the cardiac muscle. In the absence of any influences the SAN pacing rate would be 100 bpm, however heart rate and cardiac output must be able to vary in response to the needs of the body. By influencing the cells in the SAN, nerve impulses and hormones can affect the speed at which the SAN generates electrical impulse. This affects the heart rate (or chronotrophy), which in turn affects the cardiac output. In this article we will discuss how hormones and nerve impulses work to control the rate of the heart.