19.1 Heart Anatomy

The heart sits in the chest between the lungs and acts as a dual pump, moving blood through two separate circulatory loops simultaneously. Understanding its anatomy is the foundation for everything else in cardiovascular physiology, from how valves prevent backflow to why a blocked coronary artery causes a heart attack.

Heart Anatomy and Function

Position of heart in thorax

The heart is located in the mediastinum, the central compartment of the thoracic cavity. It sits between the left and right lungs, with about two-thirds of its mass lying to the left of the body's midline. It's roughly the size of your closed fist.

• Rests on the diaphragm inferiorly
• Protected by the sternum anteriorly, the vertebral column posteriorly, and the rib cage laterally
• The slight leftward positioning explains why you feel your heartbeat more on the left side of your chest

External and internal heart structures

The heart has several surface landmarks and internal features worth knowing:

• Apex: the pointed, inferior tip of the heart, directed to the left. This is where the heartbeat is most easily felt on the chest wall (around the 5th intercostal space).
• Base: the broad, superior and posterior portion, opposite the apex. The great vessels (aorta, pulmonary trunk, venae cavae) connect here.
• Coronary sulcus: an external groove encircling the heart that marks the boundary between the atria above and the ventricles below.

Internally, walls called septa divide the heart into left and right sides:

• Interatrial septum: separates the right and left atria
• Interventricular septum: separates the right and left ventricles
  • The lower portion is thick and muscular (muscular part)
  • The upper portion is thin (membranous part)

Two structures inside the ventricles work together to keep the AV valves from flipping inside out during contraction:

• Papillary muscles: cone-shaped muscle projections anchored to the ventricular walls
• Chordae tendineae: tough, fibrous cords (sometimes called "heart strings") that connect the papillary muscles to the flaps of the tricuspid and mitral valves

Layers of cardiac tissue

The heart wall has three layers, from outermost to innermost:

• Epicardium (visceral pericardium): the outermost layer, made of mesothelium and connective tissue. The coronary blood vessels run through this layer.
• Myocardium: the thick middle layer of cardiac muscle tissue. This is the layer that actually contracts and does the pumping work. It's much thicker in the ventricles than in the atria.
• Endocardium: the thin inner lining of endothelial cells and connective tissue. It's continuous with the endothelium that lines all blood vessels, creating one seamless inner surface.

Surrounding the entire heart is the pericardium, a protective sac with two components:

• Fibrous pericardium: tough outer layer that anchors the heart in the mediastinum and prevents overstretching
• Serous pericardium: a thinner, double-layered membrane. Between its two layers (parietal and visceral) sits a thin film of pericardial fluid that reduces friction as the heart beats.

Heart structure for pumping

The heart has four chambers arranged as two paired pumps working side by side.

Atria (upper chambers) receive blood returning to the heart:

• Right atrium: receives deoxygenated blood from the body via the superior and inferior vena cava
• Left atrium: receives oxygenated blood from the lungs via four pulmonary veins

Ventricles (lower chambers) pump blood out of the heart:

• Right ventricle: pumps deoxygenated blood to the lungs. Its wall is thinner because the lungs are close by and offer low resistance.
• Left ventricle: pumps oxygenated blood to the entire body. Its wall is noticeably thicker (about three times thicker than the right) because it must generate enough pressure to push blood through the systemic circulation.

Valves ensure blood flows in one direction only:

• Atrioventricular (AV) valves sit between the atria and ventricles:
  • Tricuspid valve: right atrium → right ventricle (three cusps)
  • Mitral (bicuspid) valve: left atrium → left ventricle (two cusps)
• Semilunar valves guard the exits from the ventricles:
  • Pulmonary valve: right ventricle → pulmonary trunk
  • Aortic valve: left ventricle → aorta

Systemic vs pulmonary circulation

These are the two loops that blood travels through, and the heart powers both simultaneously.

Pulmonary circulation (right side of the heart → lungs → left side):

• The pulmonary trunk carries deoxygenated blood from the right ventricle, splitting into left and right pulmonary arteries heading to each lung
• Gas exchange occurs in the lung capillaries: $CO_2$ is released, $O_2$ is picked up
• Four pulmonary veins (two from each lung) return oxygenated blood to the left atrium

Systemic circulation (left side of the heart → body → right side):

• The aorta, the body's largest artery, carries oxygenated blood from the left ventricle to organs and tissues throughout the body
• After delivering oxygen, blood returns as deoxygenated blood via the superior vena cava (from the head, neck, and upper limbs) and inferior vena cava (from the trunk and lower limbs) into the right atrium

Notice something that trips students up: pulmonary arteries carry deoxygenated blood, and pulmonary veins carry oxygenated blood. Arteries and veins are defined by direction of flow (away from or toward the heart), not by oxygen content.

Blood vessels of coronary circulation

The heart muscle can't absorb oxygen from the blood passing through its chambers. It needs its own blood supply, and that's what the coronary circulation provides.

Coronary arteries branch off the aorta just above the aortic valve (from the left and right aortic sinuses) and deliver oxygenated blood to the myocardium:

• Left coronary artery (LCA) splits into two major branches:
  • Left anterior descending (LAD) artery: supplies the anterior wall of both ventricles and the interventricular septum. Blockage here is sometimes called the "widow maker" because it affects such a large area of heart muscle.
  • Circumflex artery: wraps around to supply the left lateral and posterior walls
• Right coronary artery (RCA): supplies the right atrium, right ventricle, and portions of the posterior left ventricle

Coronary veins drain deoxygenated blood from the myocardium:

• Great cardiac vein: runs alongside the LAD artery
• Middle cardiac vein: runs in the posterior interventricular sulcus
• Small cardiac vein: runs along the right coronary artery
• These veins converge into the coronary sinus, a large venous channel on the posterior surface of the heart that empties directly into the right atrium

Blood flow through heart chambers

Here's the complete path of blood through the heart, step by step:

1. Deoxygenated blood enters the right atrium via the superior and inferior vena cava
2. Blood passes through the tricuspid valve into the right ventricle
3. The right ventricle contracts, pushing blood through the pulmonary valve into the pulmonary trunk, which splits into pulmonary arteries heading to the lungs
4. Gas exchange occurs in the lungs ($CO_2$ out, $O_2$ in)
5. Oxygenated blood returns via four pulmonary veins into the left atrium
6. Blood passes through the mitral valve into the left ventricle
7. The left ventricle contracts, pushing blood through the aortic valve into the aorta, which distributes it to the entire body

Cardiac function and regulation

A few key terms tie heart anatomy to heart function:

• Cardiac cycle: the complete sequence of events in one heartbeat, consisting of systole (contraction phase, when chambers pump blood out) and diastole (relaxation phase, when chambers fill with blood).
• Conduction system: specialized cardiac cells that generate and spread electrical impulses, coordinating the timing of atrial and ventricular contractions. This is what produces the electrical patterns seen on an ECG.
• Stroke volume (SV): the volume of blood ejected from the left ventricle in a single contraction, typically around 70 mL at rest.
• Cardiac output (CO): the total volume of blood the heart pumps per minute. It's calculated as:

$CO = HR \times SV$

At rest, with a heart rate of about 75 beats per minute and a stroke volume of 70 mL, cardiac output is roughly 5.25 L/min, meaning the heart pumps your entire blood volume approximately once every minute.