The heart cycle
The heart cycle consists of two main phases –
the systole and diastole. Both of them explain three very important
phenomena: pressure, volume and blood flow. The pressure represents potential
energy that is transformed into the flow (kinetic energy) that supplies all
cells of the organism with oxygen and nutrients. To put it more clearly, blood
volume and pressure, which provide the tissue perfusion are the basis (the
essence) of cardiovascular system (CVS) when the maintenance of life is
concerned.
Time of duration of the heart cycle calculated for the frequency of 75 beats per
minute amounts to 0.8 sec, out of which 0.3 sec belongs to systole and 0.5 sec
to diastole.
Systole chamber (0.3 sec) consists of two phases (times):
Phase I (time) of isovolumetric contraction (0.05 sec)
Phase II ejection (EF) (0.25 sec)
1. Phase of fast ejection lasts for about 0.10 sec
2. Phase of slow ejection of blood lasts for 0.15 sec.
Diastole chamber (0.5 sec)
Phase III of isovolumetric or isometric relaxation (0.08 sec)
It is preceded by protodiastolic phase (0.04 sec)
Phase IV of filling – passive
1. Phase of (passive) filling
2. Phase of slow filling (diastasis) (0.19 sec)
Phase V of filling – active (presystolic fast filling) (0.1 sec)
Blood pressure produced by retained blood at the
systole end in the left chamber is called ventricular end-diastolic pressure
(VEDP), and in the left chamber it is lower that 12 mmHg, and in the right
chamber it is higher than 5 mmHg .
Normally, blood volume in a chamber immediately prior to the start of its
contraction, the ventricular end-diastolic volume (VEDV), ranges between
120 – 140 ml. Volume of blood that remains in the chamber at the end of ejection
(ejection of blood from the chamber,) represents the ventricular end-systolic
volume, ranging normally between 40 – 70 ml. It is also often called the
residual volume.
Striking volume means the quantity of blood ejected into the blood
circulation by a heart chamber at each normal systole, and in a normal heart, it
always corresponds to VEDV decreased by VESV, amounting to 70 – 80 ml.
Dividing the striking volume by VEDV we obtain the ejection fraction (EF)
ranging normally between 60% - 80%. It can be considerably decreased if the
chamber function is weakened. The decrease indicates the level of damage of the
heart muscle. It plays an important role in the estimation of the functional
stability of the heart. With patients suffering from heart chamber dysfunction,
and in case there are indications for possible open heart surgery, low EF values
point to the surgery risk.
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