The Heart as a Pump: Overview, Position, and Gross External Anatomy

The Heart as a Pump: Overview, Position, and Gross External Anatomy

Why This Matters for EKG Technicians

Before you place a single electrode, you need a clear mental map of the heart. Every waveform you record reflects electrical events happening inside a specific chamber or layer of cardiac tissue. When you understand where the heart sits and how it is built, you interpret tracings with confidence and recognize when something looks wrong.

Where the Heart Lives

The heart is located in the mediastinum, the central compartment of the thoracic cavity. It sits between the lungs, behind the sternum (breastbone), and above the diaphragm. Roughly two-thirds of the heart's mass lies to the left of the body's midline. The base of the heart (the wider, upper portion) points toward the right shoulder, while the apex (the lower, pointed tip) angles toward the left hip and is typically found near the fifth intercostal space at the midclavicular line. This is why you can feel and hear the apical pulse there.

Knowing this position matters when you place precordial (chest) leads. An electrode placed too high or too far to the right will record a distorted signal, potentially leading to a misread tracing.

Layers of the Heart Wall

The heart wall is made of three distinct tissue layers, and it is wrapped by a protective sac—four structures in all.

Pericardium

The pericardium is a double-walled fibroserous sac that encloses the heart. The tough outer layer (fibrous pericardium) anchors the heart to surrounding structures and prevents overstretching. The inner layer (serous pericardium) has two sub-layers with a small amount of fluid between them that lubricates the heart's movement. Clinical note: When fluid accumulates abnormally in this space—a condition called pericardial effusion—it can compress the heart and impair filling, a medical emergency called cardiac tamponade. As an EKG technician you may record a classic finding called electrical alternans (alternating QRS height) in tamponade; always report abnormal-looking tracings promptly.

Epicardium

The epicardium is the outermost layer of the heart wall itself. It is the visceral layer of the serous pericardium and is a thin, smooth membrane that covers the external surface of the myocardium. The coronary arteries and veins that supply the heart muscle run within and just beneath the epicardium.

Myocardium

The myocardium is the thick, muscular middle layer responsible for the pumping action of the heart. It is composed of specialized cardiac muscle cells (cardiomyocytes) that contract in a coordinated wave triggered by electrical impulses—the very impulses your EKG records. The left ventricle has the thickest myocardium because it must pump blood to the entire body.

Endocardium

The endocardium is the smooth inner lining of all four chambers and covers the heart valves. Its smooth surface helps blood flow without clotting. Infection of this layer is called endocarditis.

The Four Chambers

The heart has four hollow chambers arranged in two side-by-side pumps.

Right Atrium

Receives oxygen-poor blood returning from the body via the superior and inferior vena cavae, and from the heart itself via the coronary sinus. It passes blood through the tricuspid valve into the right ventricle.

Right Ventricle

Pumps deoxygenated blood through the pulmonary valve into the pulmonary arteries and on to the lungs for gas exchange. Its wall is thinner than the left ventricle because the pulmonary circuit is a low-pressure system.

Left Atrium

Receives freshly oxygenated blood from the lungs via four pulmonary veins. It passes blood through the mitral (bicuspid) valve into the left ventricle.

Left Ventricle

Pumps oxygenated blood through the aortic valve into the aorta and out to the entire body. It has the thickest muscular wall of all four chambers. On an EKG, the electrical activity of this large chamber dominates many waveforms.

The Cardiac Cycle in Plain Terms

The heart works as a dual pump: the right side handles the pulmonary (lung) circuit and the left side handles the systemic (body) circuit. Both atria contract together, then both ventricles contract together. Systole is the contraction phase; diastole is the relaxation and filling phase. The EKG records the electrical events that trigger these mechanical events—this link is fundamental to everything you will do as a technician.

Safety and Scope of Practice Reminders

• Always confirm patient identity and explain the procedure before attaching electrodes.
• If a patient reports chest pain, difficulty breathing, or you observe a potentially life-threatening tracing pattern, do not interpret or diagnose—notify the supervising nurse or physician immediately.
• Proper electrode placement depends on accurate knowledge of thoracic landmarks covered in this lesson; poor placement produces artifact and can mimic pathology.