Cardiac physiology

Intercellular connective tissue also helps to strongly bind the cells together, in order to withstand the forces of contraction.

It seems that it moves in a radial way, but Bachmann's bundle and coronary sinus muscle play a role in conduction between the two atria, which have a nearly simultaneous systole.

[5][6][7] While in the ventricles, the signal is carried by specialized tissue called the Purkinje fibers which then transmit the electric charge to the myocardium.

[8] If embryonic heart cells are separated into a Petri dish and kept alive, each is capable of generating its own electrical impulse followed by contraction.

A fully developed adult heart maintains the capability of generating its own electrical impulse, triggered by the fastest cells, as part of the cardiac conduction system.

The SA node is a specialized grouping of cardiomyocytes in the upper and back walls of the right atrium very close to the opening of the superior vena cava.

The contractile cells then begin contraction from the superior to the inferior portions of the atria, efficiently pumping blood into the ventricles.

Because of this connection, each papillary muscle receives the impulse at approximately the same time, so they begin to contract simultaneously just prior to the remainder of the myocardial contractile cells of the ventricles.

The Purkinje fibers have a fast inherent conduction rate, and the electrical impulse reaches all of the ventricular muscle cells in about 75 ms.

The total time elapsed from the initiation of the impulse in the SA node until depolarization of the ventricles is approximately 225 ms.[1] Action potentials are considerably different between conductive and contractive cardiomyocytes.

[1] At this point, calcium channels open and Ca2+ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV.

This phenomenon accounts for the long refractory periods required for the cardiac muscle cells to pump blood effectively before they are capable of firing for a second time.

In both cases, when stimulated by an action potential, voltage-gated channels rapidly open, beginning the positive-feedback mechanism of depolarization.

This rapid influx of positively charged ions raises the membrane potential to approximately +30 mV, at which point the sodium channels close.

The repolarization lasts approximately 75 ms. At this point, membrane potential drops until it reaches resting levels once more and the cycle repeats.

Their influx through slow calcium channels accounts for the prolonged plateau phase and absolute refractory period.

The SA node, without nervous or endocrine control, would initiate a heart impulse approximately 80–100 times per minute.

Although each component of the conduction system is capable of generating its own impulse, the rate progressively slows from the SA node to the Purkinje fibers.

Depending upon the specific individual, as rates fall much below this level, the heart would be unable to maintain adequate flow of blood to vital tissues, initially resulting in decreasing loss of function across the systems, unconsciousness, and ultimately death.

Both the atria and ventricles undergo systole and diastole, and it is essential that these components be carefully regulated and coordinated to ensure blood is pumped efficiently to the body.

At the start of atrial systole, the ventricles are normally filled with approximately 70–80 percent of their capacity due to inflow during diastole.

It may be conveniently divided into two phases, lasting a total of 270 ms. At the end of atrial systole and just prior to ventricular contraction, the ventricles contain approximately 130 mL blood in a resting adult in a standing position.

[1] Initially, as the muscles in the ventricle contract, the pressure of the blood within the chamber rises, but it is not yet high enough to open the semilunar (pulmonary and aortic) valves and be ejected from the heart.

[10] Murmurs are graded by volume, from 1) the quietest, to 6) the loudest, and evaluated by their relationship to the heart sounds and position in the cardiac cycle.

It is also influenced by central factors through sympathetic and parasympathetic nerves[3]: 116–22  of the two paired cardiovascular centres of the medulla oblongata.

During rest vagal stimulation normally predominates as, left unregulated, the SA node would initiate a sinus rhythm of approximately 100 bpm.

Among these receptors are various proprioreceptors, baroreceptors, and chemoreceptors, plus stimuli from the limbic system which normally enable the precise regulation of heart function, via cardiac reflexes.

[3] Cardiac output (CO) is a measurement of the amount of blood pumped by each ventricle (stroke volume, SV) in one minute.

There are several important variables, including size of the heart, physical and mental condition of the individual, sex, contractility, duration of contraction, preload or EDV, and afterload or resistance.

Positive or negative inotropic factors via sympathetic and parasympathetic stimulation respectively, can increase or decrease the force of contractions.