The complete cycle of contraction and relaxation of the heart is called the cardiac cycle. The
contraction phase is called systole, and the relaxation phase is called diastole.
Heart rate is the rate of contraction per minute, and the stroke volume is the amount of blood
pumped by the left ventricle during each contracion.
There are four heart valves and two atrioventricular (AV) valves between each atrium and
ventricle, which prevent the back flow of blood into the atria; there are also two semiulnar
valves-one located at the entrance to pulmonary artery and the second at the entrance to the
aorta.
Regulation of the Heartbeat
The sinoatrial (SA) node is the pacemaker of the heart. It is located in the upper wall of the right
atrium. It stes the rate at which cardiac muscle cells contract.
The AV or atrioventrical node, located in the
lower wall of the right atrium, delays the impulses from the SA node to allow the atria to
completely empty before the ventricles contract.
Cardiology
The heart is the main organ involved in the cardiovascular system. The heart is divided into four
chambers: two atria, the right and the left atrium(s), which revive venous blood; and two
ventricles, the right and the left ventricle(s), which send the blood into arteries. The right
ventricle pumps the blood to the lungs, where the blood becomes oxygenated; the left ventricle
pumps the now oxygenated blood out to the body.
There are two types of circulation that occur in the cardiovascular system: pulmonary and
systemic. Pulmonary circulation is the pathway the blood takes from the right ventricle of the
heart through the lungs and back into the left ventricle of the heart. Systemic circulation is when
oxygen rich blood is transported throughout the body.
The heart contains valves to keep the blood flowing in the right direction. The valves are the
pulmonary semilunar valve, the aortic semilunar valve, the tricuspid valve and the bicuspid
valve. These valves open and close allowing the blood to flow though the arteries and not back
flow in the wrong direction.
The cardiac cycle is the repeating pattern of contracting and relaxing of the heart. The cycle is
what we measure to figure blood pressure. Blood pressure is the measurement of the force
applied to the walls of the arteries as the heart pumps blood though the body. The pressure is
determined by the force and amount of blood pumped and the size and flexibility of the arteries.
Blood pressure is constantly changing depending on activity and exercise, temperature, diet,
emotions, posture, physical well being and medications. Blood pressure is regulated by blood
volume (stroke volume), total peripheral resistance and the cardiac rate.
Baroreceptor reflex consists of (1) aortic arch and carotid sinus Baroreceptor as sensors; (2) the
vasomotor and cardiac control center of the medulla oblongata as the integrating center; and (3)
parasympathetic and sympathetic axons to the heart and blood vessels as the effectors. Blood
pressure falls and the baroreceptors compensate to increase the cardiac output and total
peripheral resistance. If the blood pressure raises the sympathetic nerve activity declines while
the parasympathetic nerve activity increases. The baroreceptor reflex act to keep blood pressure
constant on a beat to beat basis.
“Arterial stretch reflexes are located in the atria of the heart. These receptors are activated by
increased venous return to the heart and in response stimulate reflex tachycardia, as a result of
increased sympathetic nerve activity, inhibit ADH release, resulting in the excretion of larger
volumes of urine and a lowering of blood volume; and promote increased secretion of artial
natriuretic peptide (ANP). ANP lowers blood volume by increasing urinary salts and water
excretion.
As a nurse I need to know how the cardiac cycle works because I may be called upon to help
with a cardiac emergency. It is also good for a nurse to know and understand this information as
it is vital to life. This is something a nurse measure on a regular basis and the doctor uses the
information to aid in diagnosis.