Study Notes on Cardiac System and Cycle
Cardiac System Overview
Heart Structure:
Specialized muscle that contracts to eject blood.
Located slightly left of the sternum, apex points toward left hip.
Apex sits at the fourth intercostal space; base at the third rib.
Heart Cycle:
Systole: Contraction of the heart; ejects blood.
Diastole: Relaxation of the heart; fills with blood.
Electrical Conduction:
Follows an ECG pattern identifiable by peaks (P wave, QRS complex, T wave).
SA Node: Initiates depolarization; spreads through atria, resulting in P wave.
AV Node: Delays signal; QRS complex represents ventricular depolarization.
T wave indicates ventricular repolarization.
Mechanical Phases:
Diastole: Atria fill ventricles passively.
Atrial Contraction: Actively fills ventricles post-passive filling.
Isovolumetric Contraction: Ventricles contract, AV valves close (no blood ejection yet).
Ventricular Ejection: Blood moves out through semilunar valves; AV valves closed.
Isovolumetric Relaxation: Ventricles relax; semilunar valves close.
Heart Sounds:
S1 (first heart sound) heard during isovolumetric contraction.
S2 (second heart sound) heard during isovolumetric relaxation.
Electrical Events Correlation:
Atrial contraction starts halfway through P wave.
Ventricular contraction starts halfway through QRS complex.
Cardiac System Overview
Heart Structure:
Specialized cardiac muscle tissue (myocardium) that exhibits automaticity and rhythmicity, contracting involuntarily to eject blood.
Contains intercalated discs with gap junctions, facilitating rapid spread of electrical signals and synchronized contraction.
Located slightly left of the sternum in the mediastinum, within the thoracic cavity, protected by the rib cage.
Its apex, the pointed inferior part, points toward the left hip and rests on the diaphragm.
The apex typically sits at the fifth intercostal space (midclavicular line) for auscultation, while the broader base (superior aspect) is located at the level of the third rib.
Composed of four chambers: two atria (collecting chambers) and two ventricles (pumping chambers).
Contains four valves: tricuspid (right AV), mitral or bicuspid (left AV), pulmonary (right semilunar), and aortic (left semilunar), ensuring unidirectional blood flow.
Heart Cycle:
Systole: The contraction phase of the heart chambers (atrial and ventricular systole) during which blood is ejected into the arteries.
Ventricular systole involves a sharp increase in intraventricular pressure to overcome arterial pressure.
Diastole: The relaxation phase of the heart chambers (atrial and ventricular diastole) during which the heart fills with blood.
Ventricular diastole involves a drop in intraventricular pressure, allowing blood to flow from the atria.
Electrical Conduction:
The electrical activity of the heart follows a precise sequence, generating an ECG (Electrocardiogram) pattern identifiable by distinct peaks:
SA Node (Sinoatrial Node): Located in the right atrium, it is the primary pacemaker of the heart, initiating electrical impulses (depolarization) at a rate of 60-100 beats/min.
The impulse spreads rapidly through the atrial muscle, causing atrial contraction and resulting in the P wave on the ECG, which signifies atrial depolarization.
AV Node (Atrioventricular Node): Located near the interatrial septum, it receives the signal from the atria and introduces a crucial delay of approximately 0.1 seconds.
This delay allows for complete ventricular filling before ventricular contraction begins.
After the delay, the signal passes through the Bundle of His (AV bundle), bundle branches (left and right), and Purkinje fibers, rapidly spreading depolarization throughout the ventricles.
The QRS complex on the ECG represents ventricular depolarization, leading to ventricular contraction. Atrial repolarization also occurs during this complex but is usually masked by the larger ventricular event.
The T wave indicates ventricular repolarization, during which the ventricles relax and reset electrically.
Mechanical Phases:
These phases closely follow the electrical events, orchestrating blood flow:
Diastole (Ventricular Filling): The ventricles are relaxed; AV valves (mitral and tricuspid) are open, and semilunar valves are closed.
Approximately 70-80% of ventricular filling occurs passively as blood flows from the atria due to a pressure gradient.
Atrial Contraction (Atrial Systole): Occurs after passive filling, initiated by the SA node.
The atria contract, actively pushing the remaining 20-30% of blood into the ventricules. This final push is known as the "atrial kick."
Isovolumetric Contraction: Ventricles begin to contract, initiated by ventricular depolarization.
Ventricular pressure rapidly increases, exceeding atrial pressure, causing the AV valves to snap shut.
For a brief moment, all four heart valves are closed, so ventricular blood volume remains constant (isovolumetric) while pressure rises sharply.
Ventricular Ejection: When ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves (aortic and pulmonary) are forced open.
Blood is then rapidly ejected from the ventricles into the great arteries. The AV valves remain closed.
Isovolumetric Relaxation: Ventricles begin to relax (repolarization).
Ventricular pressure falls rapidly below arterial pressure, causing the semilunar valves to close, preventing backflow.
For a brief period, all four valves are again closed, and ventricular volume remains constant (isovolumetric) as pressure continues to drop, preparing for the next filling phase.
Heart Sounds:
S1 (First Heart Sound): Often described as a "lubb" sound, it is heard during isovolumetric contraction.
It is caused by the sudden closure of the AV valves (mitral and tricuspid) as ventricular pressure rises, preventing backflow into the atria.
S2 (Second Heart Sound): Often described as a "dupp" sound, it is heard during isovolumetric relaxation.
It is caused by the sudden closure of the semilunar valves (aortic and pulmonary) as ventricular pressure drops below arterial pressure, preventing backflow from the great arteries.
Additional heart sounds, S3 and S4, can sometimes be heard, often indicating abnormalities but can be normal in some individuals (e.g., S3 in children or trained athletes).
Electrical Events Correlation:
Atrial contraction (mechanical event) begins approximately halfway through the P wave (electrical event).
Ventricular contraction (mechanical event) begins approximately halfway through the QRS complex (electrical event).