The duration of systole is more fixed.
Tachycardia is accompanied mainly by shortening of the diastole – if more than 180/min –
insufficient filling – critical frequency (HR) for adults.
Functions of the valves
The AV – valves prevent backflow of blood from V to A during systole
The semilunar valves prevent backflow from the aorta and pulmonary artery to V during
diastole.
All valves close and open passively – by pressure gradient.
The Electrical Activity of the Heart
Resting membrane potential (RMP): myocardial fibers
approximately – 90 mV
SA node: -55 to –60 mV
Conductive tissue: - 90 to –100 mV
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RMP depends on differences in concentration of K
+
K
i
+
150 mmol/l; K
e
+
- 5 mmol/l = 30 x
RMP don´t allow to K
+
to equalize concentrations.
Na
i
+
= 5-10 mmol/l; Na
e
+
= 140 mmol/l
Depolarization: Firing level –65 mV
Initial – is due to an increase in Na
+
permeability
(through fast Na
+
channels)
Following – a slower increase in Ca
2+
permeability
(through slow Ca
2+
channels) – plateau (!)
Repolarization is due to a delayed increase in K
+
permeability.
The excitation in the conductive system cells
Lower RMP (-60 mV) firing level – 35 mV
Fast Na
+
channel is not activated.
Unstable RMP – open slow (nonspecific channel) – pacemaker potential = prepotential – due to
a steady decrease in K
+
permeability
Effect of heart nerves on prepotential:
-
vagus – acetylcholine – increase in K
+
permeability – the slope of
prepotentials in decreased
-
sympathetic nerves – opposite effect - decrease in K
+
permeability ...
Prepotential in
SA node
has the slope increased in comparison to one in AV node –
-
primary center
. Gradient of automaticity. The slope of the prepotential determines HR.