Anatomy of the Cardiovascular System
High-Yield Summary
- Heart = 4 chambers, 2 pumps in series: right side drives pulmonary circulation, left side drives systemic circulation.
- Blood path: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta.
- AV valves (tricuspid, bicuspid/mitral) sit between atria and ventricles; semilunar valves (pulmonary, aortic) sit at ventricular exits.
- Left ventricle wall is thicker than the right — it pumps blood to the whole body, not just the nearby low-resistance lungs.
- Cardiac conduction: SA node (pacemaker, 60–100 bpm) → AV node (delay) → bundle of His → Purkinje fibers. Produces ECG's P wave, QRS complex, T wave.
- Cardiac cycle: systole (ventricles contract, AV valves closed) and diastole (heart relaxes, AV valves open). Cardiac output = HR × SV, ~5 L/min at rest.
- Blood flows: arteries → arterioles → capillaries (sole site of exchange) → venules → veins (rely on valves + muscle contraction to return blood).
Path of Blood Through the Heart
- 1Venae cavae → right atrium.
- 2Tricuspid valve → right ventricle.
- 3Pulmonary valve → pulmonary artery → lungs (gas exchange).
- 4Pulmonary veins → left atrium.
- 5Mitral (bicuspid) valve → left ventricle.
- 6Aortic valve → aorta → systemic circulation.
AV Valves vs. Semilunar Valves
| AV Valves (tricuspid, bicuspid/mitral) | Semilunar Valves (pulmonary, aortic) |
|---|---|
| Sit between atria and ventricles | Sit at the exits of the ventricles |
| Closed during systole (1st heart sound) | Open during systole |
| Open during diastole | Closed during diastole (2nd heart sound) |
The Vasculature
- Arteries
- Carry blood away from the heart under high pressure; thick, elastic, muscular walls.
- Arterioles
- Small branching arteries with muscular walls; regulate flow/resistance — "resistance vessels."
- Capillaries
- Single endothelial cell layer thick; site of gas/nutrient/waste exchange; slow flow, fragile.
- Veins
- Carry blood toward the heart under low pressure; rely on valves + skeletal muscle contraction to return blood.
Cardiac Conduction System
- 1SA node (right atrial wall) — natural pacemaker, fires 60–100 signals/min → P wave.
- 2Signal spreads through atria, triggering atrial contraction.
- 3AV node — briefly delays the signal, letting ventricles fill completely.
- 4Bundle of His (interventricular septum) → left/right bundle branches → QRS complex.
- 5Purkinje fibers — rapidly distribute impulse through ventricular walls → coordinated ventricular contraction, then T wave (repolarization).
Cardiac Output
CO = HR × SV
- CO = Cardiac output — blood pumped by one ventricle per minute (~5 L/min at rest)
- HR = Heart rate (beats per minute)
- SV = Stroke volume (blood pumped per beat)
Common MCAT Trap
- Only 3 portal systems route blood through two capillary beds in series: hepatic (digestive tract → liver), hypophyseal (hypothalamus → anterior pituitary), renal (glomerulus → peritubular capillaries).
- Pulmonary veins carry oxygenated blood and pulmonary arteries carry deoxygenated blood — the reverse of the systemic norm (same exception noted in the Respiratory System chapter).
Quick Recall
Why is the left ventricle wall thicker than the right?
What is the order of the cardiac conduction system?
What is the equation for cardiac output?
Name the three portal systems.