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The Cardiac Cycle and Blood Pressure

One complete heartbeat involves the complex coordination of the actions of all four chambers of the heart, during which both of the atria contract simultaneously (while the ventricles are relaxing), followed by the contraction of both ventricles (while the atria are relaxed). These events are called the cardiac cycle, and can be traced electrically using an electrocardiogram (ECG). The heart is able to “excite itself”, i. e., no external nervous stimulus is necessary to initiate the depolarization of the myocardium. Instead, a specialized region of the heart called the sinoatrial (S-A) node initiates impulses at a rate of 70-80 times/minute in an adult. Since it is responsible for the generation of the heartbeat, the S-A node, located in the right atrium, is often referred to as the pacemaker. The depolarization spreads across the muscle cells of the atria, causing them to contract, and another critical region, the atrioventricular (A-V) node, is stimulated. The A-V node is the only electrical connection between the ventricles and the atria; by the time it transmits the “message” to the cells of the ventricles signaling them to contract, the atria have relaxed. An ECG tracing of the cardiac cycle graphically displays these events in detail.

An electrocardiogram (ECG) tracing of a single heartbeat


Although stimulation from the nervous system is not necessary to initiate the heartbeat, it is important in the regulation of the heartrate. This rate must change, for example, during physical exertion. The cardiac center of the brain is located in the medulla oblongata of the brainstem; it is able to analyze the levels of carbon dioxide and oxygen in the blood indirectly by sensing blood pH. The heartrate can subsequently be affected through parasympathetic stimulation (decreasing the rate) or sympathetic stimulation (increasing the rate) of the S-A node by motor nerves as is appropriate. (Similar mechanisms usually cause an accompanying change in the rate of respiration.) Additionally, certain hormones have an effect on the heartrate, most notably epinephrine, which causes the rate to increase in response to stressful situations.
As noted earlier, the contraction of the left ventricle forces blood into the aorta, and is responsible for the pressure of blood against the walls of the arteries. This force is what we usually refer to as blood pressure, although the pressure of blood can be measured anywhere in the circulatory system. Traditionally, the blood pressure is measured at an artery in the arm by a device that expresses the pressure in terms of its ability to raise a column of mercury (in units of mm Hg). Blood pressure is usually expressed as two numbers: one represents the systolic pressure, the highest pressure which occurs as the ventricles contract, with the other representing the diastolic pressure, the lowest pressure occurring as the ventricles relax. Normal blood pressure is considered to be 120/80 in an adult male, although many factors can influence an individual’s blood pressure at a given time. Chronic high blood pressure, or hypertension, can be a significant health threat as it puts undue stress on the cardiovascular system.

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