The Heart: More Than a Pump
The cardiovascular system — comprising the heart, blood vessels, and blood — is the body's primary distribution network. It delivers oxygen and nutrients to every cell, carries away carbon dioxide and waste, transports hormones, regulates temperature, and plays a critical role in immune defense. Understanding its anatomy and physiology is foundational to every health profession.
Anatomy of the Heart
The heart is a hollow, muscular organ about the size of a fist, located slightly left of center in the chest cavity. It is divided into four chambers:
- Right atrium — receives deoxygenated blood from the body via the superior and inferior vena cava
- Right ventricle — pumps deoxygenated blood to the lungs via the pulmonary artery
- Left atrium — receives oxygenated blood returning from the lungs via the pulmonary veins
- Left ventricle — pumps oxygenated blood to the entire body via the aorta; has the thickest walls of any chamber
The Cardiac Cycle
Each heartbeat consists of two phases: systole (contraction) and diastole (relaxation). During systole, the ventricles contract and eject blood. During diastole, the ventricles relax and fill with blood from the atria. This cycle repeats about 60–100 times per minute at rest.
The characteristic "lub-dub" sounds of the heartbeat are produced by the closing of heart valves — the AV valves (mitral and tricuspid) produce the first sound; the semilunar valves (aortic and pulmonary) produce the second.
Pulmonary vs. Systemic Circulation
Blood travels two circuits simultaneously:
- Pulmonary circulation: right heart → lungs → left heart. Gas exchange occurs in the pulmonary capillaries, where CO₂ is released and O₂ is absorbed.
- Systemic circulation: left heart → body tissues → right heart. Oxygen and nutrients are delivered; CO₂ and wastes are collected.
Blood Pressure and Cardiac Output
Cardiac output (CO) = Heart Rate (HR) × Stroke Volume (SV). A resting adult has a cardiac output of approximately 5 liters per minute — meaning the entire blood volume circulates roughly once per minute. During vigorous exercise, this can increase fivefold.
Blood pressure is the force exerted by blood on vessel walls. Normal resting blood pressure is around 120/80 mmHg. The systolic number (120) reflects ventricular contraction; the diastolic (80) reflects relaxation. Consistently elevated blood pressure — hypertension — damages vessel walls and is a leading risk factor for heart attack and stroke.
Clinical Relevance
Cardiovascular disease remains the leading cause of death in the United States. Conditions like coronary artery disease, heart failure, arrhythmias, and valvular disorders are all rooted in disruptions of the anatomy and physiology we study here. Understanding normal function is the essential prerequisite for understanding — and preventing — what goes wrong.
Takeaway for Students
Master the cardiac cycle, the two circuits of circulation, and the pressure-volume relationships of the heart. These concepts appear on every board exam and underpin clinical reasoning across every specialty that touches cardiovascular health — which, given its centrality to all body systems, is nearly every specialty there is.