Recording-2025-03-12T03:30:33.480Z

Objectives Overview

This note provides an overview of two distinct physiological functions of the heart: the Electrical system and the Blood flow/mechanical system.

Electrical System of the Heart

Function:

The heart's electrical system functions akin to wiring, providing the signals that regulate heartbeat contractions through a network of specialized cells.

Key Components:

1. SA Node (Sinoatrial Node):initiate the heart electrical impulses

   • Acts as the primary pacemaker, located in the right atrium.

   • Generates electrical signals that initiate heartbeats. The SA nodes cells posses automaticity, meaning they spontaneously depolarized and generate action potential at regular intervals.

   • These action potential spread across the atria, causing them to contract and push blood into the ventricles.

   • The SA node typically sets the heart rate at 60 to 100 beats per minute.

   • As the heart‘s natural pacemaker, the SA node ensures that the heart maintains a regular rhythm, known as sinus rhythm. The spread of these electrical signals through the atrial myocardium is represented as the P wave on the ECG.

   • When malfunctioning, can lead to abnormal heart rates, potentially requiring artificial pacemakers to maintain rhythmic heart function.

2. AV Node (Atrioventricular Node):

   • Positioned between the atria and ventricles.

   • Serves as a relay point to spread electrical signals throughout the heart.

   • Introduces a slight delay in the transmission of signals, ensuring proper timing for ventricles to contract after atrial contraction.

   • This delay essential because it allows the Atria to fully contract and empty their blood into the ventricles’s before the ventricles began to contract.

   • With all this delay, the atria and ventricles would contract simultaneously, which would severely impair the efficiency of blood flow. The an AV node can also act as a back up pacemaker, although it generates impulses at a slow rate 40 to 60 bpm if the SA node fails.

3. Bundle of His:

   • A collection of heart muscle cells that transmit electrical impulses from the AV node to the ventricles.

   • The bundle of His quickly splits into the left and right bundle branches, which carry the electrical signal down the inter ventricular septum (the wall separated the left, and right ventricles.)

   • The bundle branches ensure that the electrical signal is eventually distributed to both ventricles, allowing for simultaneous contraction. This synchronized ventricular contraction is vital for effective pumping, as it maximize the amount of blood ejected from the heart during each beat.

   • Critical for coordinating the timing of heartbeats.

4. Purkinje Fibers:

   • The Purkinje fibers are the final part of the conduction system, located deep within the ventricles.

   • These fibers are highly specialized for rapid conduction, allowing electrical impulses to spread quickly through the ventricular myocardium.

   • This rapid propagation ensures that the ventricles contract from the apex of the heart upward, efficiently, pushing blood out through the aorta (from the left ventricle) and the pulmonary artery (from the right ventricle).

   • Specialized fibers that spread electrical signals throughout the ventricles, ensuring coordinated contraction.

   • Facilitate rapid and synchronized contractions of the ventricles for efficient blood ejection.

   • The synchronized contractions of the ventricles, also known as ventricular depolarization, is represented by the QRS complex on an ECG.

   • This ensures that blood is pumped effectively into the systemic and pulmonary circulation, providing oxygenated blood to tissues and removing carbon dioxide.

Electrical System Steps:

1. Depolarization:

   • Initiated when sodium and calcium ions rapidly enter cardiac cells, triggering contractions.

2. Action Potential:

   • Generated by depolarization; results in synchronized contraction of heart cells.

3. Gap Junctions:

   • Specialized connections between heart cells that allow for rapid transmission of impulses, facilitating coordinated contractions similar to a Wi-Fi network.

4. Repolarization:

   • Occurs as potassium exits the cells, helping to restore the resting state after contraction.

Electrocardiogram (ECG/EKG) Reading Components:

• P Wave: THE START: Represents atrial depolarization, initiate by the SA node. initiating atrial contraction.

• PR Interval: The duration of the delay before ventricular contraction begins, essential for ensuring proper timing between atria and ventricles.

• QRS Complex: Indicates Depicts ventricular depolarization; where the signal travels through the bundle branches and Purkinje fibers, causing the ventricles to contract.

• reflects the rapid spike in electrical activity as the ventricles contract.

• T Wave: Corresponds to ventricular repolarization, allowing the heart to reset before the next contraction.

• Initiates ventricular repolarization, as the heart prepares for the next cycle

• ST Segment: represents the time during particular contraction, when there is no net electrical movement.

• QT Intervals: Reflects The total time for ventricular depolarization and repolarization

Regulation of Heart Function

Nervous System Control:

• Sympathetic Nervous System:

  • Activates the 'fight or flight' response, significantly increasing heart rate and contractility to prepare the body for action.

  • The sympathetic nervous system increases the heart rate and contractibility during times of stress, exercise, or excitement by releasing norepinephrine or epinephrine

  • These neurotransmitters bind to beta-Adrenalin receptors in the SA node, AV node, and myocardium, accelerating the rate of depolarization and enhancing the strength of ventricular contraction

  • The results in increased cardiac output, which is crucial for delivering more oxygen and nutrients to active tissues during the heighten activity.

• Parasympathetic Nervous System:

  • Regulates the 'rest and digest' response, reducing heart rate to conserve energy and promote digestive functions.

  • Via the vagus nerve, decreases heart rate during periods of rest and relaxation.

  • It releases acetylene, which binds to muscadine receptors on the SA node, slowing the heart rate by reducing the speed of depolarization.

  • This parasympathetic activity dominates during sleep or rest, prompting energy conservation lowering the heart rate.

Hormonal Control:

• Epinephrine: (released by the adrenal medulla during her stress or exercise)Released during stress, it increases heart function and energy availability.

• Thyroid Hormones: Enhance The sensitivity of the heart to sympathetic stimulation, prompting increased heart rate and cardiac output.

• Affect overall metabolism and heart rate; heightened hormone levels correlate with increased heart activity, influencing rate and strength of contractions.

Mechanical Pumping Mechanism

Blood Flow Pathway:

1. Deoxygenated Blood:

   • Blood enters the heart via the superior and inferior venae cavae into the right atrium.

2. Atria Contract:

   • These contractions push blood through the tricuspid valve into the right ventricle.

3. Ventricles Contract:

   • The right ventricle contracts, propelling blood through the pulmonary artery towards the lungs for oxygenation.

4. Oxygenated Blood:

   • Returns to the heart via pulmonary veins into the left atrium.

5. Left Ventricular Contraction:

   • From the left atrium, blood flows through the aortic valve into the aorta, supplying oxygen-rich blood to the entire body.

Key Functions:

• Systole: The phase of contraction of the heart when blood is pumped out of the chambers.

• Diastole: The phase of relaxation where the heart fills with blood, critical for efficient pumping.

• Stroke Volume: The volume of blood ejected from the heart with each contraction, a key indicator of cardiac output.

• Preload: Refers to the degree of stretching of the ventricles before contraction; influences the heart's contraction strength.

• Refers to the degree of stretch in the ventricles at the end of diastole, determined by the volume of blood, returning to the heart, higher venous return increases, ventricular, filling, and preload.

• Afterload: The resistance faced by the ventricles during contraction when pumping blood into the arteries, influencing overall cardiovascular efficiency.

• Refers to the resistance, the ventricles must overcome to eject blood into the arteries, for the left ventricle, it’s the pressure in the aorta, and for the right ventricle it’s a pressure in the pulmonary artery.

• Increased afterload due to high blood pressure or aortic stenosis, makes it harder for the heart to eject blood, potentially reducing stroke volume.

• Frank-Starling Law: States that the more the ventricles are stretched by blood volume during filling the stronger, they contract, ensuring that the heart adjust to varying levels of Venus return.

• heart fills with blood during diastole, the greater the force of contraction during systole, enhancing stroke volume.

Heart Sounds:

• First Sound (Lub): Occurs due to the closure of the atrioventricular (AV) ( tricuspid and mitral) valves during ventricular contraction. At the beginning of systole.

• Second Sound (Dub): Results from the closure of the semilunar valves ( aortic and pulmonary) after ventricular systole, marking the end of the heartbeat. At the beginning of diastole.

Summary of Heart Function

The heart operates through a complex interplay of electrical signals for coordinating contractions, and mechanical mechanisms that ensure the continuous circulation of blood throughout the body. Understanding these systems is crucial for comprehending overall cardiac function and health.