Pulse,Blood Pressure and Cardiac Cycle by Prof. Liwayway Memije-Cruz Have you ever wondered why you sometimes experience a sudden change in your blood pressure? What about your pulse rate? Are these two involved with the cardiac cycle? Pulse The ventricles pump blood into the arteries regularly about 70 to 80 times a minute. The force of ventricular contraction starts a wave of increased pressure that begins at the heart and travels along the arteries. This wave is called pulse. It can be felt in any artery that is relatively close to the surface, particularly if the vessel can be pressed down against a bone. The pulse is most commonly obtained at the wrist where the radial artery passes over the bone on the thumb side of the forearm. The carotid artery in the neck and the dorsalis pedis on the top of the foot are the other vessels sometimes used for obtaining the pulse. Normal pulse rate is the same as the heart rate. If the heartbeat is abnormally weak or the artery is obstructed. the beat may not be detected as a pulse. It is important to use your second or third finger in checking the pulse of another person. Using your thumb may find you getting your own pulse. Various factors could influence the pulse rate. These are: 1.It is faster in small persons than in large persons and usually slightly faster in women than in men. 2.In a newborn infant the rate may be from 120 to 140 beats/minute. As the child grows, the rate tends to become slower. 3.It is influenced by muscular activity. 4.Emotional disturbances may increase pulse rate. 5.In many infections, the pulse rate increases with the increase in temperature. 6.An excessive amount of secretion from the thyroid gland may cause a rapid pulse. Blood Pressure Blood pressure is the force exerted by the blood against the inner walls of the blood vessels. It is the pressure within the large systemic arteries near the heart. It is also the force that keeps the blood continuously circulating even between heartbeats. Blood pressure is the product of the output of the heart and the resistance in the vessels. Blood pressure decreases as the blood flows from arteries into capillaries and finally into veins. The instrument used in measuring the blood pressure is called a sphygmomanometer. The two variables being measured in getting the ones blood pressure are the systolic and diastolic pressures. Cardiac Cycle Each heart has a sequence of muscle contractions and relaxation called the cardiac cycle. In each cycle, the four chambers of the heart go through paces of contractions called systole and relaxation called diastole. The timing of the pace is not the same for all the heart chambers; the atria contracts slightly before the ventricles contract. While the atria are relaxed and filing, the ventricles are also relaxed. As pressure rises in atria, the AV valves are forced open, so the ventricles start to fill even before the atria contract. Then the ventricles contract, causing the AV valve to snap shut and the pressure inside the ventricles rises sharply above the pressure in the vessel leading out from them. One of those vessels is the aorta; the other is the pulmonary artery. With the increased pressure, the semi lunar valves open and blood flows out of the heart. After the blood has been ejected, the ventricles relaxed and the cardiac cycle starts over. The blood and the heart movement during the cardiac cycle generate vibrations that produce a “lub-dup” sound. which can be heard at the chest wall. At each “dup”, the semi lunar valves are closing and the ventricles relaxed. The output of the heart is influenced by: 1. Strength of the contraction of the heart 2. Total blood volume, which controls the volume each, beat pushes out. The resistance in the vessels is affected by: 1. Vasomotor changes: a. Vasoconstriction increases resistance to flow. b. Vasodilation lowers resistance 2. Elasticity of blood vessels 3. Thickness of the blood or viscosity
Posted on: Sun, 17 Aug 2014 12:39:26 +0000
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