1.1 Heart Structure and Layers

Heart Location and Orientation

The heart sits in the mediastinum, the central compartment of the thoracic cavity. Despite being thought of as a "left side" organ, it actually straddles the midline, with about two-thirds of its mass lying to the left of the midsternal line.

Position within the Thoracic Cavity

• Posterior to the sternum and costal cartilages, anterior to the vertebral column, and nestled between the two lungs
• The base (broad, superior portion) sits at roughly the level of the third costal cartilage
• The apex (inferior, pointed tip) rests at the fifth intercostal space, angled toward the left hip
• This gives the heart an oblique orientation rather than sitting straight up and down

Relationship to Surrounding Structures

• Rests on the diaphragm, which separates the thoracic and abdominal cavities
• Bordered laterally by the lungs and their mediastinal pleurae
• The great vessels (aorta, pulmonary trunk, superior and inferior venae cavae, pulmonary veins) emerge from the heart's superior aspect
• The esophagus and descending thoracic aorta lie directly posterior to the heart

Heart Wall Layers and Functions

The heart wall has three distinct layers. From outermost to innermost: epicardium, myocardium, and endocardium. Each has a different tissue composition and a specific job.

Epicardium

The epicardium is the outermost layer, made of mesothelium (simple squamous epithelium) overlying a thin layer of connective tissue. It's actually the same structure as the visceral layer of the serous pericardium, just named differently depending on context.

• Reduces friction between the beating heart and surrounding structures
• Contains blood vessels, lymphatics, and nerve fibers that supply the deeper heart tissue
• Also houses variable amounts of adipose tissue, especially along the coronary sulcus and interventricular grooves

Myocardium

The myocardium is the thick, muscular middle layer responsible for the heart's pumping action. It's composed of cardiac muscle cells (cardiomyocytes) arranged in spiral bundles that wring blood out of the chambers when they contract.

• Thickness varies by workload: the left ventricle wall is the thickest because it pumps blood into the high-pressure systemic circuit. The right ventricle is thinner (it only needs to push blood to the nearby lungs), and the atria are thinnest of all since they mainly move blood a short distance into the ventricles.
• This is a high-yield detail for exams: if you're asked why one chamber wall is thicker than another, the answer always comes back to how much pressure that chamber must generate.

Endocardium

The endocardium is the innermost layer, lining the interior of all four chambers. It consists of endothelium (simple squamous epithelium) resting on a thin connective tissue layer.

• Provides a smooth, non-thrombogenic surface so blood flows without clotting
• Continuous with the endothelial lining of every blood vessel entering and leaving the heart
• The heart valves (tricuspid, pulmonary, mitral, and aortic) are formed from folds of endocardium reinforced with dense connective tissue
• Endocardial cells can release signaling molecules like nitric oxide that help regulate myocardial contractility

Pericardium's Role in Protection

The pericardium is a layered sac that surrounds and protects the heart. Think of it as two nested bags: a tough outer bag (fibrous pericardium) and a slippery inner bag (serous pericardium) with a thin film of fluid between its two layers.

Fibrous Pericardium

• Outermost layer, made of dense irregular connective tissue
• Anchors the heart to the diaphragm, sternum, and vertebral column so it doesn't bounce around in the chest
• Acts as a physical barrier against infection and mechanical injury
• Resists overdistension, preventing the heart from overfilling with blood

Serous Pericardium

The serous pericardium has two layers:

• Parietal layer lines the inner surface of the fibrous pericardium
• Visceral layer (the epicardium) covers the outer surface of the heart itself
• Between these two layers is the pericardial cavity, which holds a small amount (15–50 mL) of serous fluid
• That fluid acts as a lubricant, allowing the heart to beat with minimal friction against surrounding tissues

Coronary vs. Systemic Circulation

The heart pumps blood to the entire body, but it also needs its own blood supply. That's where coronary circulation comes in. Systemic circulation, by contrast, is the large loop that serves every other tissue.

Coronary Circulation

The myocardium is too thick to receive adequate oxygen by diffusion from the blood inside its chambers. Instead, it relies on a dedicated network of coronary arteries and veins.

• The left and right coronary arteries branch off the base of the aorta, just superior to the aortic valve, in small dilations called the aortic sinuses
• The left coronary artery typically splits into the left anterior descending (LAD) artery and the circumflex artery, supplying most of the left ventricle and interventricular septum
• The right coronary artery supplies the right ventricle and, in most people, the inferior portion of the left ventricle
• After delivering oxygen, coronary veins collect deoxygenated blood and drain it into the coronary sinus, which empties into the right atrium
• Blockage of a coronary artery starves the downstream myocardium of oxygen, causing ischemia. Prolonged ischemia leads to a myocardial infarction (heart attack), where cardiac muscle cells die

Systemic Circulation

Systemic circulation is the high-pressure loop that delivers oxygenated blood to every tissue in the body and returns deoxygenated blood to the heart.

1. The left ventricle ejects oxygenated blood into the aorta
2. The aorta branches into progressively smaller arteries and then arterioles, which feed into capillary beds where gas and nutrient exchange occurs
3. Deoxygenated blood collects into venules, then veins
4. Veins converge into the superior vena cava (draining the head, neck, and upper limbs) and inferior vena cava (draining the trunk and lower limbs)
5. Both venae cavae empty into the right atrium, completing the circuit

The systemic circuit is responsible for delivering oxygen and nutrients to tissues and carrying away metabolic waste products like carbon dioxide. The right side of the heart then sends that deoxygenated blood through the pulmonary circuit to the lungs, but that's covered in detail later.