3- Cardiac physiology

Myocardium

Heart muscle that comprises 95% of the heart’s mass.

Cardiomyocytes

Heart muscle cells that are striated, branched, and contain 1-2 central nuclei.

Desmosomes

Structures in intercalated discs that provide structural support to cardiomyocytes.

Gap junctions

Connections that allow ion flow between cardiomyocytes for coordinated contraction.

Sinoatrial (SA) Node

The pacemaker of the heart that fires at 100 bpm and is influenced by parasympathetic input to slow it down.

Atrioventricular (AV) Node

Delays the impulse slightly; can act as a pacemaker firing at 40-60 bpm.

Purkinje Fibers

Fibers that spread the impulse through the ventricles and trigger contraction.

Action Potential

The change in electrical potential associated with the passage of an impulse along the membrane of a muscle cell or nerve cell.

Depolarization

Phase of action potential involving sodium ion influx.

Plateau Phase

Phase of maintained depolarization due to calcium influx and potassium efflux.

Repolarization

Phase where potassium efflux continues, returning to resting potential.

Refractory Period

Duration longer than the contraction, preventing tetany (sustained contraction).

Electrocardiogram (ECG/EKG)

A graphic representation of the electrical activity of the heart.

P Wave

Electrocardiogram wave representing atrial depolarization.

QRS Complex

Electrocardiogram complex representing ventricular depolarization.

T Wave

Electrocardiogram wave representing ventricular repolarization.

Systole

Contraction phase of the cardiac cycle.

Diastole

Relaxation phase of the cardiac cycle.

End Diastolic Volume (EDV)

Volume of blood in a ventricle before contraction, approximately 120 mL.

Stroke Volume (SV)

Amount of blood ejected per beat; calculated as EDV - ESV.

Cardiac Output (CO)

Amount of blood ejected by the heart per minute; defined by the formula CO = HR × SV.

Cardiac Reserve

The difference between resting CO and maximum CO.

Preload

Degree of stretch of the heart muscle before contraction.

Afterload

Resistance to ejection of blood from the ventricle.

Sympathetic Stimulation

Neurotransmitters like norepinephrine increase heart rate.

Parasympathetic Stimulation

Acetylcholine decreases heart rate.