The Cardiac Cycle Overview

Study Notes

The Cardiac Cycle

The cardiac cycle encompasses the sequence of events in a single heartbeat, a complex interplay of electrical and mechanical phenomena.
It's divided into two phases: systole (contraction) and diastole (relaxation).
These phases are further categorized into distinct periods for both the atria and ventricles.

Cardiac Cycle Phases

Atrial Systole: The atria contract, forcing blood into the ventricles. This phase is relatively brief.
Atrial Diastole: The atria relax allowing them to be filled with blood from the veins.
Ventricular Systole: The ventricles contract powerfully. This is a more prolonged phase, pushing blood through the pulmonary and systemic circuits.
- Isovolumetric ventricular contraction: Immediately after ventricular contraction, the valves close, producing an abrupt increase in pressure without any change in blood volume.
Ventricular Diastole: The ventricles relax, allowing them to passively refill with blood returning from the atria and then from the veins.
- Isovolumetric ventricular relaxation: The ventricles relax, the pressure falls below atrial pressure, and the AV valves open, initiating the passive filling phase of the ventricles.

Electrical Conduction System

The heart's rhythmic contractions are initiated and coordinated by a specialized electrical conduction system.
The sinoatrial (SA) node, located in the right atrium, acts as the heart's natural pacemaker, spontaneously generating electrical impulses.
These impulses spread through the atria, causing them to contract simultaneously, and then through the atrioventricular (AV) node, which delays the impulse before transmission to the ventricles.
The bundle of His and its branching Purkinje fibers ensure rapid and coordinated ventricular contractions.

Heart Sounds

Heart sounds, heard through a stethoscope, are produced by the closure of heart valves.
The "lubb" sound corresponds to the closing of the AV valves, signifying the beginning of ventricular systole.
The "dubb" sound corresponds to the closing of the semilunar valves, signaling the end of ventricular systole.

Factors Affecting Cardiac Output

Cardiac output (CO), the volume of blood pumped by each ventricle per minute, is determined by heart rate (HR) and stroke volume (SV).
Heart rate is influenced by autonomic nervous system input, primarily from the sympathetic and parasympathetic branches.
Stroke volume is affected by preload (filling pressure), afterload (pressure resisting ejection), and contractility (force of contraction).
Preload is the degree of stretch on the ventricles before they contract.
Afterload is the resistance that the ventricles must overcome to eject blood.
Contractility is influenced by factors such as hormones and the sympathetic nervous system.

Regulation of Cardiac Function

Neural control: The autonomic nervous system (ANS), comprised of the sympathetic and parasympathetic nervous systems, regulates heart rate and contractility.
Sympathetic stimulation increases heart rate and contractility.
Parasympathetic stimulation decreases heart rate.
Hormonal control: Hormones such as epinephrine and norepinephrine can affect the heart's pace and contraction strength.
Feedback mechanisms are crucial for maintaining a consistent and functional cardiac response to the body's needs (such as during exercise or stress).

Blood Flow and Cardiac Output

Cardiac output is the overall measure of blood flow through the circulatory system.
Adequate cardiac output is critical for delivering nutrients and oxygen to tissues while removing waste products.
Variations in cardiac output maintain homeostasis in various physiological conditions.

Description

Explore the intricate events of the cardiac cycle, which are essential for heart function. The quiz covers key phases including systole and diastole, as well as the roles of the atria and ventricles. Test your understanding of this vital physiological process.