Anatomy of the Heart
Heart Description
The heart is a hollow, muscular organ that pumps blood to supply tissues with nutrients and remove metabolic wastes. It is the central pump of the cardiovascular system, which also includes the blood and blood vessels.
Location of the Heart
The heart lies in the mediastinum — the medial cavity of the thorax between the lungs. It is slightly tilted, with approximately two-thirds lying to the left of the sternum and one-third to the right.
Directional Terminology
| Term | Description |
|---|---|
| Base | Upper portion formed by the left atrium and a small amount of right atrium; approximately at the level of the 2nd rib within the midclavicular line; lies in front of the esophagus and descending aorta |
| Apex | Lower portion formed by the tip of the left ventricle; lies above the diaphragm between the 5th/6th rib within the midclavicular line |
| Right Border | Faces the right lung; extends from the inferior surface to the base |
| Left Border | Faces the left lung; extends from the base to the apex |
| Inferior Surface | Between the apex and the right border; rests mostly on the diaphragm |
| Anterior Surface | Deep to the sternum and the ribs (front of the heart) |
Size and Shape
- Length: 5 inches
- Width: 3.5 inches
- Depth: 2.5 inches
- Weight: Females ~8 oz, Males ~10 oz
Size and shape are influenced by age, body weight and build, frequency of physical exercise, and heart disease.
Pericardium
The pericardium is a double-walled sac that encloses the heart, protects it, reduces friction, and secures it in place.
Fibrous Pericardium (Outer Layer)
- Composed of tough, inelastic, dense irregular connective tissue
- Anchors the heart in the mediastinum
- Prevents overstretching and movement with changes in body position
Serous Pericardium (Inner Layer)
A thinner, double-walled sac:
- Parietal pericardium — outer layer, fused to the fibrous pericardium
- Visceral pericardium (epicardium) — inner layer, adheres to the surface of the heart
Pericardial Cavity
The space between the visceral and parietal layers contains approximately 20 mL of serous fluid — slippery secretions produced by pericardial cells that reduce friction as the heart beats.
Layers of the Heart Wall
| Layer | Description |
|---|---|
| Epicardium (outer) | Contains blood capillaries, lymph capillaries, nerve fibers, and fat; main coronary arteries lie on its surface; first area to receive blood from coronary arteries |
| Myocardium (middle) | Cardiac muscle tissue — striated like skeletal muscle, involuntary like smooth muscle; branching "stair step" pattern with gap junctions; thickest layer (95% of heart wall); responsible for pumping action; ventricles thicker than atria, LV is thickest |
| Endocardium (inner) | Thin layer of endothelium over connective tissue; lines chambers, valves, chordae tendineae, and papillary muscles; minimizes surface friction |
Heart Chambers
Atria (Upper Chambers)
Thin-walled chambers that receive blood from the body.
Anatomical Features:
- Auricles — wrinkled pouch-like structures that allow a small increase in blood volume
- Interatrial septum — cartilaginous wall between the two atria
Right Atrium (RA):
- Receives deoxygenated blood
- Walls are 2–3 mm thick
- Average pressure: 0–8 mmHg
- Receives blood from the superior vena cava (head/upper extremities), inferior vena cava (lower body), and coronary sinus (drains coronary vessels)
Left Atrium (LA):
- Receives oxygenated blood from the lungs via the pulmonary veins
- Walls are 2–3 mm thick
- Average pressure: 1–10 mmHg
Ventricles (Lower Chambers)
Thick-walled chambers that pump blood out of the heart.
Anatomical Features:
- Trabeculae carneae — ridges/raised bundles of cardiac muscle fibers that house some of the conduction system
- Papillary muscles — cone-shaped extensions of the trabeculae carneae that connect to the chordae tendineae
- Chordae tendineae — string-like connective tissue attached to the AV valves and papillary muscles
Right Ventricle (RV):
- 4–5 mm thick
- Pressure: 0–8 mmHg (diastole), 15–30 mmHg (systole)
- Pumps deoxygenated blood to the lungs via the pulmonary artery
Left Ventricle (LV):
- 13–15 mm thick (thickest part of the heart)
- Pressure: 3–12 mmHg (diastole), 100–140 mmHg (systole)
- Pumps oxygenated blood to the body via the aorta
- Rotates forward while contracting, causing the point of maximal impulse (PMI) — the site where the LV is most strongly felt
External Separation of Chambers
Sulci are grooves on the external surface of the heart that contain coronary arteries and fat, marking the external boundaries between chambers.
| Sulcus | Location | Separates |
|---|---|---|
| Coronary sulcus | Horizontal groove | Atria from ventricles |
| Anterior interventricular sulcus | Vertical groove on front | Right and left ventricles |
| Posterior interventricular sulcus | Vertical groove on back | Right and left ventricles |
Internal Separation of Chambers
Interatrial Septum (IAS):
- Separates the RA and LA
- Contains the fossa ovalis — an oval depression marking the location of the fetal foramen ovale
Interventricular Septum (IVS):
- Separates the RV and LV
Fibrous Skeleton
The fibrous skeleton consists of dense rings of connective tissue that:
- Surround the heart valves
- Fuse the four valves together
- Provide structural support to the valves
- Prevent overstretching of the valves
- Serve as the insertion point for the Bundle of His
- Act as an electrical insulator, preventing premature conduction of electrical signals from the atria to the ventricles