Cardiac

Refers to the amount of blood discharged by the heart in one minute.
Calculated as: \text{Cardiac Output (CO)} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}
Related concepts include stroke volume, heart rate, conduction pathway, and the cardiac cycle.

Sinoatrial Node (SA Node): The primary pacemaker of the heart. It stimulates the atrial myocardium through gap junctions.
- Function: Initiates heartbeats leading to depolarization.
Atrial Diastole: A state when the atrial myocardium is relaxed before contraction.
Atrioventricular Node (AV Node): The electrical gateway from atria to ventricles.
- Function: Pauses electrical impulse (AV delay of about 100 \text{ ms}) to allow atrial emptying before ventricular contraction. Conduction is slowest here.
Atrioventricular Bundle (Bundle of His): After the AV node, the impulses travel here.
- Divides into right and left bundle branches that descend through the interventricular septum to the apex of the heart.
Purkinje Fibers: Extensions from the bundle branches that spread impulses quickly through the ventricular myocardium. Conduction is fastest here, ensuring near-simultaneous contraction of the ventricles.

Ventricular Systole: The phase when the ventricles contract, creating tension to pump blood.
Blood pressure values used for illustration: (120) \text{ over } (80) \text{ mm Hg} or (110) \text{ over } (70) \text{ mm Hg}
When the ventricles discharge, they create a bolus of blood that causes arteries to expand and contract, which is perceived when taking blood pressure.
Systolic Value: The maximum pressure during ventricular contraction.

Definition: The complete sequence of events in the heart from one heartbeat to the next, primarily focused on pressure differentials and blood movement.
Phases of the Ventricular Cycle:
1. Ventricular Filling (includes Atrial Contraction): Blood flows passively/actively into the ventricles.
2. Isovolumetric Contraction: Rapid pressure increase with all valves closed.
3. Ventricular Ejection: Semilunar valves open, blood is expelled.
4. Isovolumetric Relaxation: Ventricles relax, pressure drops dramatically with all valves closed, leading into the next filling phase.
Snapshot of the Right Side of the Heart:
- Blood returns from the body via the inferior and superior vena cavae, filling the right atrium.
- Tricuspid Valve (TCV) opens when atrial pressure exceeds ventricular pressure.
- Atrial Contraction: Increases the pressure in the right atrium, pushing the final 20\%-30\% of blood into the right ventricle.
- End Diastolic Volume (EDV): The volume of blood in the ventricles when relaxed, filling them to about two-thirds (around 120 \text{ ml} typical resting value).
- Ventricular Systole Activation: Ventricles create tension leading to increased intraventricular pressure above atrial pressure, causing closure of the AV valves.

Isovolumetric Contraction: A phase when the ventricles contract without volume change, as AV valves are closed but semilunar valves haven't opened yet.
Ejection Phase: Once ventricular pressure exceeds that of the aorta and pulmonary trunk, the semilunar valves open, allowing blood to be ejected into systemic and pulmonary circulation.
End Systolic Volume (ESV): The amount of blood remaining in the ventricles post-ejection (typically around 50 \text{ ml}).

Heart Rate (HR): Controlled by chronotropic agents.
- Positive Chronotropic Agents: Increase HR (e.g., stimulants, drugs, sympathetic activity via norepinephrine).
- Negative Chronotropic Agents: Decrease HR (e.g., parasympathetic activity via acetylcholine, certain medications).
Stroke Volume (SV): The volume of blood ejected with each heartbeat, influenced by:
- Preload: How much blood fills the ventricles before contraction (end-diastolic volume).
- Influenced by the Frank-Starling Mechanism: Greater venous return (higher EDV) results in a larger initial stretch of the ventricular wall muscle fibers, leading to a stronger force of contraction and increased stroke volume.
- Contractility: Strength of ventricular contraction, influenced by calcium availability and muscle elasticity (often mediated by positive inotropic agents).
- Afterload: The resistance the ventricles must overcome to eject blood during systole. Increased systemic pressure or constricted arteries increase afterload, reducing stroke volume.

Typical stroke volume is (70 \text{ ml}) (SV) at a resting heart rate of (75 \text{ beats/min}) (HR).
Resulting in an average cardiac output (CO) of approximately (5.2 \text{ liters/min}), equating to the total blood volume circulated in one minute for an adult.