Physiology of the Heart

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Notes

Mechanical Physiology of Heart Function

Process	Action
Cardiac cycle	The complete cycle of mechanical pumping of blood through the heart in one heartbeat (contraction and relaxation).
Systole	The contraction phase of the cardiac cycle, which generally refers to ventricular contraction.
Diastole	The relaxation phase of the cardiac cycle, when the ventricles are filling. This phase lasts much longer than systole.
Stroke volume (SV)	The amount of blood ejected from either ventricle with a single contraction. Stroke volume is determined and affected by three factors: Preload-The pressure under which the ventricle fills. This pressure is influenced by the amount of venous blood return. A feature of myocardial muscle is that the more it is stretched (up to a limit), the greater its force of contraction. Therefore, stroke volume can be increased considerably by increasing the blood volume that fills the ventricles and thus increasing the amount of myocardial muscle fiber stretch. This concept is known as Starling’s law of the heart. Afterload-The resistance against which the ventricles contract, also influences stroke volume. Afterload is determined by systemic arterial resistance. Cardiac Contractility-The intrinsic state of the heart muscle’s force of contraction, also called the heart’s contractile, or inotropic, state.
Cardiac output (CO)	Amount of blood pumped through the cardiovascular system per min. CO = SV × Heart rate (HR). An increase in stroke volume alone can improve the cardiac output. However, heart rate also has great impact. Rate increases in the healthy heart can improve the cardiac output up to threefold.

Properties of Cardiac Cells

Property	Ability
Automaticity	The ability to generate an electrical impulse independently of stimulation by the nervous system or any other source. This property is unique to certain cardiac cells called pacemaker cells.
Excitability	The ability of cells to respond to electrical stimulation.
Conductivity	The ability to pass or propagate an electrical impulse from cell to cell through the heart.
Contractility	Shortens in response to electrical stimulation.

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