Cardiac Output (CO) is the volume of blood pumped by the heart per minute. It is a critical parameter that ensures adequate tissue perfusion. Cardiac output is determined by two key factors: heart rate (HR) and stroke volume (SV).

1. Heart Rate (HR): The number of heartbeats per minute. It is primarily regulated by the autonomic nervous system (ANS).

   • Sympathetic Nervous System: Increases heart rate via norepinephrine, which binds to beta-adrenergic receptors on cardiac cells.

   • Parasympathetic

     Nervous System: Decreases heart rate via the vagus nerve and the neurotransmitter acetylcholine.

2. Stroke Volume (SV): The amount of blood ejected by the left ventricle per beat. Stroke volume depends on three main factors:

   • Preload: The end-diastolic volume (EDV), which is influenced by venous return and ventricular filling.

   • Contractility: The intrinsic ability of cardiac muscle fibers to contract. It is affected by calcium ion availability and sympathetic stimulation.

   • Afterload: The resistance the left ventricle must overcome to eject blood. Increased systemic vascular resistance can decrease stroke volume.

The relationship between heart rate, stroke volume, and cardiac output is expressed by the formula:

Regulatory Mechanisms:

• Intrinsic Regulation: Frank-Starling law states that an increase in EDV leads to a more forceful contraction and a greater SV.

• Extrinsic Regulation: ANS, hormones (e.g., adrenaline), and other factors (e.g., blood volume, venous return).

Summary: Cardiac output is a vital measurement influenced by heart rate and stroke volume. Understanding the interplay of these factors and the regulatory mechanisms helps maintain proper cardiovascular function.