Define a cardiac cycle and the cardiac output.

A cardiac cycle refers to the sequence of events that occur during one complete heartbeat, including the contraction and relaxation of the heart chambers and the opening and closing of heart valves. The cardiac cycle consists of systole (contraction) and diastole (relaxation) phases of the atria and ventricles, along with the opening and closing of the atrioventricular (AV) and semilunar valves. It encompasses the entire process of blood flow through the heart, including the filling and ejection of blood from the atria and ventricles.

The cardiac cycle can be divided into several phases:

1. Atrial Systole: The atria contract, pushing blood into the ventricles through the open AV valves (mitral and tricuspid valves). This phase corresponds to the P wave on an electrocardiogram (ECG).

2. Ventricular Systole: The ventricles contract, generating pressure that closes the AV valves and opens the semilunar valves (aortic and pulmonic valves). Blood is ejected from the ventricles into the pulmonary artery and aorta. This phase corresponds to the QRS complex on an ECG.

3. Isovolumetric Relaxation: Following ventricular systole, the ventricles relax, and the semilunar valves close, preventing blood from flowing back into the ventricles. The AV valves remain closed during this phase, resulting in no change in ventricular volume.

4. Ventricular Diastole: The ventricles continue to relax, and pressure in the ventricles drops below that in the atria, causing the AV valves to open. Blood flows passively from the atria into the ventricles, filling them with blood. This phase corresponds to the T wave on an ECG.

The cardiac cycle repeats continuously, with each heartbeat driving blood through the systemic and pulmonary circulations to deliver oxygen and nutrients to tissues and organs and remove waste products.

Cardiac output refers to the volume of blood ejected by the heart per unit of time, typically expressed in liters per minute (L/min). It is calculated by multiplying stroke volume (the volume of blood ejected from the left ventricle with each heartbeat) by heart rate (the number of heartbeats per minute). Mathematically, cardiac output can be expressed as:

Cardiac Output (CO)=Stroke Volume (SV)×Heart Rate (HR)Cardiac Output (CO)=Stroke Volume (SV)×Heart Rate (HR)

Cardiac output is a vital physiological parameter that reflects the heart's efficiency in pumping blood and meeting the body's metabolic demands. It is influenced by factors such as heart rate, stroke volume, preload (ventricular filling), afterload (resistance to ventricular ejection), and contractility (force of ventricular contraction). Alterations in cardiac output can have significant implications for cardiovascular health and may occur in various conditions, including heart failure, shock, and exercise.