| The Cardiac Cycle Posted: 01 May 2011 05:49 AM PDT The Cardiac Cycle A complete cardiac cycle occurs with each audible 'lub-dub' that is heard with a
stethoscope. During this heartbeat, both atria simultaneously contract followed soon
after by the contraction of the ventricles. Systole is the contractile phase of each
chamber while diastole is the relaxation phase. During the cardiac cycle, the atria and
the ventricles each have periods of both systole and diastole.
The purpose of the cardiac cycle is to effectively pump blood. The right heart delivers
deoxygenated blood to the lungs. Here oxygen is picked up and carbon dioxide is
breathed off. The left heart delivers oxygenated blood to the body. Normally, the
volume of blood ejected by the right ventricle to the lungs is about the same as the
volume ejected by the left ventricle. A mismatch in volumes ejected by the ventricles
(i.e. right ventricle pumps more blood than the left ventricle) can result in heart failure.
De-oxygenated blood enters the right side of the heart via the vena cava and is ejected through
to the lungs where oxygen is replenished and carbon dioxide diffuses out to the lungs.
Oxygenated blood enters the left side of the heart and is subsequently delivered to the body.
The synchronized actions of the atria and the ventricles are coordinated to maximize
pumping efficiency. This sequence of events is worth considering. Rhythm
disturbances can greatly impair this synchrony, resulting in a less effective cardiac cycle.
For simplicity, we'll consider the events that lead to the ejection of blood from the right
ventricle into the lungs beginning at the end of atrial diastole. These events mirror
those of the left heart.
The tricuspid valve closes during ventricular systole – otherwise, it remains open. At
end atrial diastole and ventricular diastole, an open tricuspid valve provides a channel
between the right atrium and the right ventricle. As a result, blood flows into both the
right atrium and the right ventricle simultaneously. The ventricle receives up to 85% of
its blood volume during this period.
Prior to ventricular systole, the atrium contracts. Since the atrium is about 1/3 the size
of the ventricle, an atrial contraction only contributes an additional15-35% of blood
volume to the ventricle. This 'topping up' of the ventricle by the atrium is called atrial
kick. Note that the conclusion of atrial systole coincides with the end of ventricular
diastole.
After ventricular end-diastole, the ventricle enters systole and contracts forcefully, . As
the pressure within the ventricle increases, the tricuspid valve closes to ensure forward
blood flow. Very soon after, the pulmonic valve opens as pressure within the ventricle
becomes greater than pulmonary artery pressure. Blood is then ejected into the
pulmonary arteries.
As blood is ejected, ventricular pressure falls. When ventricular pressure is below the
pulmonary artery pressure, the pulmonic valve closes to prevent back flow of blood
into the right ventricle. As mentioned in chapter one, the closure of the AV valves
(tricuspid and mitral valves) normally produces the S1 heart sound. The closure of the
semilunar valves (pulmonic and aortic valves) produces the S2 heart sound.
While ventricular systole ejects blood into either the pulmonary or systemic vascular
systems, ventricular diastole is at least as important. Without a sufficient period of
diastole, systole is ineffective. During diastole, the ventricles relax. But in relaxing, the
ventricles open to regain their pre-contractile size, effectively dropping the chamber
pressure below that of the vena cava. As a result, blood is drawn into the ventricle
during ventricular (and atrial) diastole. Then the cardiac cycle begins again.
And this cardiac cycle is repeated over 100,000 times daily! Remarkable. source The Cardiac Cycle http://www.skillstat.com/mce.htm 
 |
Tidak ada komentar:
Posting Komentar