Cardiovascular Anatomy, Circulation, and Hemodynamics

Circulation: Pulmonary, Systemic, and Coronary

Blood moves through the body in two primary circuits that work in series:

The Pulmonary Circuit

The right ventricle pumps deoxygenated blood through the pulmonary valve into the pulmonary arteries, which lead to the lungs. In the pulmonary capillaries, gas exchange occurs, and the blood becomes oxygenated. This circuit operates at low resistance and low pressure.

The Systemic Circuit

Oxygenated blood returns via the pulmonary veins to the left atrium, passes through the mitral valve into the left ventricle, and is then ejected through the aortic valve into the aorta. The left ventricle must generate enough force to overcome the high resistance of the systemic circuit to deliver oxygen and nutrients to all body tissues.

Coronary Circulation

The myocardium (heart muscle) requires its own dedicated blood supply via the coronary arteries, which branch directly from the aorta. Crucially, most left ventricular coronary perfusion occurs during diastole (when the heart relaxes). If oxygen demand—driven by heart rate, contractility, or afterload—exceeds this supply, ischemia occurs.

The Cardiac Cycle and Electrical Conduction

The rhythmic contraction of the heart is coordinated by a specialized electrical conduction system:

1. SA Node: The natural pacemaker that initiates the impulse and causes atrial depolarization.
2. AV Node: Provides a brief delay to allow the ventricles to fill with blood before they contract.
3. Bundle of His and Bundle Branches: Carry the impulse through the interventricular septum.
4. Purkinje Fibers: Distribute the impulse throughout the ventricular myocardium to ensure coordinated contraction.

This electrical activity drives the mechanical cardiac cycle, which consists of two phases:

• Diastole: The phase where the ventricles relax and the AV valves (tricuspid and mitral) open to allow the chambers to fill. During this phase, coronary perfusion increases.
• Systole: The phase where the ventricles contract. The AV valves close (creating the S1 heart sound), and the semilunar valves (pulmonary and aortic) open to eject blood.

Hemodynamics: Cardiac Output and Stroke Volume

To understand how well the heart is perfusing the body, we must look at hemodynamics. The most critical measure is Cardiac Output (CO), which is the volume of blood pumped by the heart per minute. It is calculated as:

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

Stroke volume (the amount of blood ejected per beat) is shaped by three primary factors:

1. Preload: The degree of ventricular stretch at the end of diastole (filling). If preload is too low, stroke volume decreases; if it is too high, it can lead to congestion.
2. Afterload: The resistance the ventricle must overcome to eject blood. High systemic vascular resistance (hypertension) or aortic stenosis increases afterload.
3. Contractility: The intrinsic strength of the myocardial contraction, which is independent of preload.

Exam Tip: Remember that blood pressure is a product of both Cardiac Output and Systemic Vascular Resistance (SVR). If CO drops, the body may increase SVR (vasoconstriction) to maintain pressure, but this increases the workload (afterload) on the heart.

Clinical Correlations: Ischemia and Failure

When the balance between oxygen supply and demand is disrupted, clinical syndromes emerge.

Coronary Artery Disease (CAD) and Angina

CAD often begins with endothelial injury, leading to atherosclerosis. This can manifest as angina, which is myocardial ischemia:

• Stable Angina: Predictable chest pain triggered by exertion that improves with rest.
• Unstable Angina: New, worsening, or pain at rest, suggesting plaque disruption and a high risk of myocardial infarction.
• Vasospastic Angina: Caused by temporary coronary spasms, often occurring at rest.

Heart Failure

Heart failure occurs when the heart cannot meet the body's metabolic demands due to impaired ejection or impaired filling:

• Systolic Dysfunction (Reduced Ejection Fraction): The heart muscle is too weak to contract effectively, often following a myocardial infarction.
• Diastolic Dysfunction (Preserved Ejection Fraction): The ventricle is too stiff or thick to fill properly, often due to chronic pressure overload (hypertension).

Symptoms of heart failure are categorized into congestion (backward failure) and low output (forward failure):

• Congestion: Results in fluid backup. Left-sided failure causes pulmonary edema (crackles, dyspnea); right-sided failure causes systemic edema (JVD, peripheral edema, ascites).
• Low Output: Results in poor tissue perfusion. This manifests as fatigue, cool extremities, weak pulses, low urine output, and altered mental status.

Clinical Note: While the body uses compensatory mechanisms like the Sympathetic Nervous System and the RAAS to support perfusion in the short term, these responses eventually become harmful. Chronic tachycardia increases oxygen demand, and fluid retention increases preload and afterload, creating a cycle of worsening failure.