The Cardiac Cycle Overview

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.