Cardiovascular System: Electrical Conduction, Cardiac Cycle, and Blood Pressure

Cardiovascular System

Overview

The cardiovascular system is responsible for the transport of blood, nutrients, gases, and wastes throughout the body. It consists of the heart, blood vessels, and blood. This section focuses on the physiology of the heart and blood vessels, including electrical conduction, the cardiac cycle, and blood pressure regulation.

Electrical Conduction in Myocardium

Autorhythmic Cells and Action Potentials

• Autorhythmic cells in the heart (primarily in the SA node) spontaneously generate action potentials without external stimulation.

• These action potentials spread to adjacent contractile cells via gap junctions in intercalated discs, ensuring coordinated contraction.

• The electrical current generated by the SA node initiates the heartbeat and sets the pace for cardiac rhythm.

• Action potentials in autorhythmic cells have a characteristic shape, with a rapid depolarization and repolarization phase.

Example: The SA node acts as the heart's natural pacemaker, initiating each heartbeat.

The Conducting System of the Heart

• Electrical signaling begins in the SA node (sinoatrial node), located in the right atrium.

• The signal spreads through the atria to the AV node (atrioventricular node), then down the AV bundle (bundle of His), bundle branches, and Purkinje fibers to the ventricles.

• This conduction pathway ensures that atrial contraction precedes ventricular contraction, optimizing blood flow.

• The depolarization wave spreads upward from the apex of the heart, resulting in coordinated ventricular contraction.

Additional info: The delay at the AV node allows the ventricles to fill completely before they contract.

Electrocardiogram (ECG)

Principles of ECG

• An electrocardiogram (ECG or EKG) records the electrical activity of the heart as detected on the body surface.

• The ECG reflects the sum of all electrical activity generated by the heart's nodes and muscle cells during each heartbeat.

• Key components of the ECG waveform include the P wave (atrial depolarization), QRS complex (ventricular depolarization), and T wave (ventricular repolarization).

Einthoven's Triangle and ECG Leads

• Einthoven's triangle is formed by placing electrodes on the right arm, left arm, and left leg, creating three standard limb leads (I, II, III).

• Each lead records the electrical potential difference between two points, providing different views of the heart's electrical activity.

Interpreting the ECG

• Heart rate can be determined by measuring the time between two consecutive P waves or Q waves.

• Tachycardia: abnormally fast heart rate; Bradycardia: abnormally slow heart rate.

• Regularity of the rhythm and the presence of normal waves (P, QRS, T) are assessed to detect arrhythmias.

• Each P wave should be followed by a QRS complex; the PR interval should be consistent.

Example: A missing or abnormal QRS complex may indicate a conduction block.

The Cardiac Cycle

Phases of the Cardiac Cycle

• The cardiac cycle includes all events associated with blood flow through the heart during a single heartbeat.

• It consists of alternating periods of systole (contraction and pumping) and diastole (relaxation and filling).

• One cycle includes atrial systole and diastole, followed by ventricular systole and diastole.

Cardiac Output

Definition and Calculation

• Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.

• It is calculated as:

$\text{Cardiac Output} = \text{Heart Rate} \times \text{Stroke Volume}$

• Typical resting values: Heart rate ≈ 70 beats/min, Stroke volume ≈ 70 mL/beat, so CO ≈ 4,900 mL/min (≈ 5 L/min per ventricle).

• During strenuous exercise, cardiac output can increase up to 40 L/min.

Factors Affecting Cardiac Output

• Changes in heart rate (HR) and stroke volume (SV) both influence cardiac output.

• Heart rate is modulated by the autonomic nervous system:

  • Sympathetic stimulation (norepinephrine) increases HR.

  • Parasympathetic stimulation (acetylcholine) decreases HR.

  • Epinephrine from the adrenal medulla also increases HR.

• Stroke volume is affected by:

  • Preload: The degree of stretch of cardiac muscle cells before contraction (related to end-diastolic volume).

  • Afterload: The resistance the left ventricle must overcome to eject blood (often related to arterial pressure).

  • Contractility: The strength of contraction, independent of preload and afterload.

Frank-Starling Law: The heart pumps all the blood that returns to it; increased venous return increases stroke volume.

Blood Pressure

Definition and Measurement

• Blood pressure (BP) is the force exerted by circulating blood on the walls of blood vessels.

• It is highest in the arteries and lowest in the veins.

• Systolic pressure: Pressure during heart contraction; Diastolic pressure: Pressure during heart relaxation.

• Pulse pressure = Systolic pressure − Diastolic pressure.

• Mean arterial pressure (MAP) represents the average pressure in the arteries:

$\text{MAP} = \text{Diastolic Pressure} + \frac{1}{3}(\text{Pulse Pressure})$

• Blood pressure is measured using a sphygmomanometer and reported in mmHg.

Blood Flow and Resistance

• Blood flows from areas of higher pressure to lower pressure, driven by a pressure gradient (ΔP).

• Flow is opposed by resistance (R) in the blood vessels, which depends on vessel radius, length, and blood viscosity.

• The relationship is given by:

$\text{Flow} = \frac{\Delta P}{R}$

• Flow rate is usually expressed in liters or milliliters per minute.

• Velocity of blood flow is inversely related to the total cross-sectional area of the vessels.

Regulation of Blood Pressure

• Blood pressure is regulated by neural, hormonal, and local mechanisms.

• The medulla oblongata in the brainstem coordinates cardiovascular reflexes.

• Baroreceptor reflex: Stretch-sensitive mechanoreceptors in the carotid arteries and aorta detect changes in blood pressure and adjust heart rate and vessel diameter accordingly.

• Sympathetic nervous system increases vascular tone and heart rate; parasympathetic system decreases heart rate.

• Vasoconstriction and vasodilation alter resistance and thus blood pressure.

Table: Key Parameters in Cardiovascular Physiology

Parameter	Definition	Typical Value (Resting Adult)
Heart Rate (HR)	Beats per minute	70 bpm
Stroke Volume (SV)	Blood pumped per beat	70 mL
Cardiac Output (CO)	Blood pumped per minute	5 L/min
Systolic BP	Pressure during contraction	~120 mmHg
Diastolic BP	Pressure during relaxation	~80 mmHg
MAP	Mean arterial pressure	~93 mmHg

Summary

• The heart's electrical conduction system ensures coordinated contraction and efficient blood flow.

• ECG is a valuable tool for assessing cardiac function and diagnosing arrhythmias.

• Cardiac output and blood pressure are tightly regulated by neural and hormonal mechanisms to maintain tissue perfusion.