1.1 Cardiac Anatomy & Blood Flow Pathway

Cardiac Anatomy & Blood Flow Pathway

The cardiovascular system operates as a continuous, closed-loop, dual-pump circuit. A profound understanding of its anatomy and hemodynamics is foundational for any cardiographic technician.

1. Cardiac Layers and the Pericardium

The heart wall consists of three primary tissue layers, encased in a protective sac known as the pericardium.

• Endocardium: The innermost, smooth endothelial layer lining the heart chambers and valves. It is continuous with the endothelium of the major blood vessels. Infection here (endocarditis) typically affects the valves.
• Myocardium: The thick, muscular middle layer responsible for contraction. The left ventricular myocardium is significantly thicker (approx. 9-15 mm) than the right ventricle (3-5 mm) because it must overcome high systemic vascular resistance to pump blood to the entire body.
• Epicardium: The thin, outermost layer of the heart wall, which also serves as the visceral layer of the serous pericardium.

The Pericardial Sac

The heart sits within the pericardium, a double-walled sac that anchors the heart in the mediastinum, prevents over-distension, and reduces friction.

1. Fibrous Pericardium: The tough, outer connective tissue layer.
2. Serous Pericardium: The inner double layer, composed of:
   • Parietal Layer: Lines the inside of the fibrous pericardium.
   • Visceral Layer (Epicardium): Covers the surface of the heart. Between these two serous layers lies the pericardial cavity, normally containing 10 to 50 mL of ultrafiltrate fluid which acts as a lubricant. An abnormal accumulation of fluid here is called a pericardial effusion, which, if rapidly accumulating, can lead to cardiac tamponade—a life-threatening compression of the heart.

2. Chambers and Valves

The heart features four chambers and four valves that ensure unidirectional blood flow.

Atria (Receiving Chambers)

• Right Atrium (RA): Receives deoxygenated blood from the Superior Vena Cava (SVC), Inferior Vena Cava (IVC), and the Coronary Sinus. Normal mean pressure is 2-8 mmHg.
• Left Atrium (LA): Receives oxygenated blood from the four pulmonary veins. It has a slightly thicker wall than the RA. Normal mean pressure is 2-12 mmHg.

Ventricles (Pumping Chambers)

• Right Ventricle (RV): Pumps deoxygenated blood into the low-pressure pulmonary circulation. Normal systolic pressure is 15-25 mmHg.
• Left Ventricle (LV): Pumps oxygenated blood into the high-pressure systemic circulation. Normal systolic pressure is 100-140 mmHg.

Heart Valves

Valves open and close passively in response to pressure gradients.

Exam Trap: Remember that AV valves (Tricuspid, Mitral) have chordae tendineae and papillary muscles, while Semilunar valves (Pulmonic, Aortic) do not. A ruptured papillary muscle will cause massive AV valve regurgitation, not semilunar regurgitation.

3. The Blood Flow Pathway

Tracing a single drop of blood through the cardiovascular system is a crucial clinical skill. The pathway is a figure-eight circuit combining the pulmonary and systemic loops.

Step-by-Step Pathway of Blood

1. Deoxygenated blood from the body enters the Superior/Inferior Vena Cava and Coronary Sinus.
2. Blood flows into the Right Atrium.
3. Passes through the open Tricuspid Valve.
4. Enters the Right Ventricle.
5. During systole, the RV contracts, pushing blood through the Pulmonic Valve.
6. Blood enters the Main Pulmonary Artery, which bifurcates into the left and right pulmonary arteries, carrying deoxygenated blood to the Lungs.
7. Gas exchange occurs in the pulmonary capillaries (oxygen picked up, carbon dioxide released).
8. Oxygenated blood returns via the Four Pulmonary Veins.
9. Enters the Left Atrium.
10. Passes through the open Mitral Valve.
11. Enters the Left Ventricle.
12. During systole, the LV contracts, pushing blood through the Aortic Valve.
13. Blood enters the Aorta and is distributed to the Systemic Circulation (the entire body).

4. Coronary Anatomy

The myocardium requires its own continuous blood supply, delivered by the coronary arteries. These vessels originate at the base of the aorta, just above the aortic valve leaflets in pouches called the Sinuses of Valsalva.

Crucial Physiological Fact: Unlike systemic arteries that fill during systole, coronary arteries fill primarily during ventricular diastole, when the aortic valve is closed and the myocardium is relaxed.

Left Coronary Artery System

The Left Main Coronary Artery (LMCA) quickly bifurcates into two major branches:

1. Left Anterior Descending (LAD): Travels down the anterior interventricular sulcus. It supplies the anterior wall of the LV, the anterior two-thirds of the interventricular septum, and the bundle branches. Often referred to clinically as the "widow maker" because proximal occlusions are frequently fatal.
2. Left Circumflex (LCX): Travels in the left atrioventricular groove. It supplies the lateral wall of the LV, the Left Atrium, and in about 10-15% of the population, it supplies the AV node.

Right Coronary Artery System

The Right Coronary Artery (RCA) travels in the right atrioventricular groove.

• Supplies the Right Atrium, Right Ventricle, and the inferior wall of the Left Ventricle.
• In roughly 90% of people (right-dominant), the RCA gives rise to the Posterior Descending Artery (PDA), which supplies the inferior wall and the posterior one-third of the septum.
• The RCA also supplies the SA node in ~60% of the population and the AV node in ~90% of the population. Therefore, an inferior wall myocardial infarction (RCA occlusion) is strongly associated with bradycardia and AV heart blocks.