Understanding the Human Heart: Anatomy, Function, and Disorders

A key anatomical distinction is the thickness of the muscular walls. The left ventricle possesses a significantly thicker muscular wall than the right ventricle because it must generate enough force to overcome the high systemic resistance required to pump blood to the entire body.

Cardiac Output and Blood Pressure Regulation

To meet the metabolic demands of the body, the heart must maintain an adequate volume of blood flow. This is measured as Cardiac Output (CO), which is the volume of blood pumped by each ventricle per minute. It is calculated using the following formula:

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

Blood pressure (BP) is the force exerted by the blood against the arterial walls. A normal reading is approximately $120/80 \text{ mm Hg}$. The body regulates BP by adjusting cardiac output and through endocrine mechanisms. When the heart walls are stretched due to high blood volume or high pressure, the heart releases specialized peptides to restore balance:

• Atrial Natriuretic Peptide (ANP): Released by the atria.
• Brain Natriuretic Peptide (BNP): Produced in the ventricles.

Both peptides act to lower blood pressure by increasing the excretion of sodium ($\text{Na}^{+}$) and water in the kidneys and by inhibiting the Renin-Angiotensin-Aldosterone System (RAAS).

Exam Tip: If a question asks how the heart responds to increased blood volume, remember the "Stretch $\rightarrow$ Release $\rightarrow$ Salt/Water Out" logic. Increased stretch triggers ANP/BNP, which promotes sodium and water loss to reduce volume and pressure.

Unidirectional Flow: The Valve System

To ensure that blood moves in only one direction, the heart utilizes four specialized valves. These valves open and close based on pressure differences, and the "lub-dub" sound heard during auscultation is caused by their closing.

1. Atrioventricular (AV) Valves: Located between the atria and ventricles.
   • Tricuspid Valve: Between the right atrium and right ventricle.
   • Bicuspid (Mitral) Valve: Between the left atrium and left ventricle.
2. Semilunar Valves: Located at the exits of the ventricles.
   • Pulmonary Valve: Between the right ventricle and the pulmonary artery.
   • Aortic Valve: Between the left ventricle and the aorta.

Electrical Conduction and the Cardiac Cycle

The heart is myogenic, meaning it exhibits auto-rhythmicity; it generates its own electrical impulses to initiate contraction. The cardiac cycle is the sequence of atrial and ventricular contraction (systole) and relaxation (diastole) that occurs in a single heartbeat.

The electrical impulse follows a specific pathway to ensure coordinated contraction:

1. Sinoatrial (SA) Node: The primary pacemaker, located in the right atrium, which sets the normal sinus rhythm (60–100 beats/min).
2. Atrioventricular (AV) Node: Receives the impulse from the SA node. There is a slight delay here to allow the ventricles to fill with blood before they contract.
3. Atrioventricular Bundle (Bundle of His): Carries the impulse toward the ventricles.
4. Bundle Branches: Distributes the impulse through the interventricular septum.
5. Purkinje Fibers: Rapidly distribute the impulse throughout the ventricular myocardium, triggering contraction.

Coronary Circulation: Feeding the Heart

While the heart pumps blood to the entire body, it requires its own dedicated blood supply to power the myocardium. This is provided by the coronary arteries, which arise from the base of the aorta:

• Left Coronary Artery: Branches into the left circumflex and the left anterior descending (interventricular) arteries. It supplies the left atrium and much of the left ventricle.
• Right Coronary Artery: Supplies the right atrium and most of the right ventricle.

Deoxygenated blood from the myocardium is collected by cardiac veins, which merge into the coronary sinus, eventually emptying into the right atrium. Notably, coronary blood flow to the left ventricle is greatest during diastole (relaxation), as the muscular contraction during systole can compress the vessels.

Cardiovascular Disorders

Cardiovascular disorders can be categorized by the specific component of the system that fails: rhythm/conduction, coronary blood supply, valve function, pumping strength, or pressure regulation.

A primary driver of many of these issues is atherosclerosis, the buildup of plaque within the arterial walls.

Common Pathologies:

• Coronary Artery Disease (CAD): Occurs when the coronary arteries become narrowed or blocked by plaque, reducing oxygen delivery to the heart muscle. This can manifest as angina (chest pain) or lead to a myocardial infarction (heart attack).
• Cardiac Arrhythmia: A failure in the electrical conduction system, resulting in an irregular heartbeat. For example, if the SA node fails, the AV node may take over as an "escape pacemaker," but at a much slower rate (around 45–50 beats/min).
• Heart Failure: A condition where the heart cannot pump blood effectively enough to meet the body's needs. This can be categorized into systolic failure (pumping strength) or diastolic failure (filling/pressure issues).
• Valvular Disorders: Problems with the opening or closing of the heart valves.

Clinical Correlation: A patient presenting with dizziness and an ECG showing a regular heart rate of 45 bpm without P waves is likely experiencing a junctional rhythm, where the AV node has taken over as the pacemaker because the SA node is not functioning.