🦴 Anatomy introductory Lesson 2 of 8 2 min read

The Cardiovascular System

The heart, blood vessels, and blood form a closed transport loop that delivers oxygen and nutrients to every tissue and carries waste away for disposal.

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What you'll learn

Trace the path of blood through the four chambers and two circuits of the heart.
Describe the phases of the cardiac cycle and relate them to the heart sounds.
Outline the cardiac conduction system from SA node to Purkinje fibers.
Compare the structure and function of arteries, capillaries, and veins.
Explain how blood pressure is determined and why hypertension is dangerous.

Overview

The cardiovascular system consists of the heart, blood vessels (arteries, capillaries, veins), and blood. Its primary function is to deliver oxygen and nutrients to tissues and remove metabolic waste (CO₂, urea).

The Heart

The heart is a four-chambered muscular pump, located in the mediastinum, surrounded by the pericardium.

Chamber	Function
Right atrium	Receives deoxygenated blood from systemic veins (SVC, IVC)
Right ventricle	Pumps to lungs via pulmonary artery
Left atrium	Receives oxygenated blood from pulmonary veins
Left ventricle	Pumps to systemic circulation via aorta (thickest wall — highest pressure)

Blood moves through two circuits: the pulmonary circuit (right heart → lungs → left heart) picks up oxygen, and the systemic circuit (left heart → body → right heart) delivers it. This connects the cardiovascular system directly to the respiratory system at the alveoli.

Cardiac Cycle

One complete heartbeat (systole + diastole) ~0.8 s at rest (75 bpm):

Atrial systole — atria contract, fill ventricles fully (~0.1 s)
Ventricular systole — ventricles contract; AV valves close (S1 sound); semilunar valves open; blood ejected
Ventricular diastole — ventricles relax; semilunar valves close (S2 sound); AV valves open; passive filling

Ejection fraction (EF) = stroke volume / end-diastolic volume × 100%. Normal EF ≥ 55%. HFrEF (heart failure with reduced EF) is EF < 40%.

Conduction System

Electrical impulse pathway: SA node (pacemaker, ~70 bpm) → AV node (delays 0.1 s) → Bundle of His → left and right bundle branches → Purkinje fibers → ventricular myocardium

Blood Vessels

Type	Wall composition	Function
Arteries	Thick tunica media (smooth muscle + elastin)	Carry blood away from heart; withstand high pressure
Capillaries	Single-cell endothelium	Exchange of gases, nutrients, waste
Veins	Thin walls; valves prevent backflow	Return blood to heart; capacitance vessels

Blood

Blood is a connective tissue: plasma (fluid) carries red cells (oxygen transport via hemoglobin), white cells (immune defense), and platelets (clotting). It links nearly every organ system, carrying hormones from the endocrine glands and delivering wastes to the kidneys.

Blood Pressure

BP = Cardiac output × Total peripheral resistance.
Cardiac output = Heart rate × Stroke volume.

Normal: < 120/80 mmHg. Hypertension (≥130/80 mmHg) increases risk of stroke, MI, and CKD.

Clinical Relevance

Coronary artery disease results from atherosclerotic plaque buildup in coronary arteries, reducing blood supply to myocardium. Rupture of vulnerable plaque triggers thrombosis → acute MI (heart attack). Classic symptoms include crushing chest pain, which may radiate to the left arm or jaw, with sweating and shortness of breath. Treatment involves reperfusion (PCI, thrombolytics) and secondary prevention (statins, antiplatelets).

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The Frank–Starling mechanism

Within physiologic limits, the force of ventricular contraction rises as end-diastolic volume increases, because greater sarcomere stretch improves the overlap of actin and myosin and enhances myofilament calcium sensitivity. This intrinsic property lets the heart automatically match its output to venous return and keeps the outputs of the right and left ventricles balanced beat to beat. In heart failure the Frank–Starling curve is depressed and shifted rightward, so the ventricle needs much higher filling pressures to generate the same stroke volume, contributing to pulmonary congestion.

The cardiac action potential and refractory period

Ventricular myocytes have a five-phase action potential: rapid Na+ influx (phase 0), transient K+ efflux (phase 1), a plateau where L-type Ca2+ influx balances K+ efflux (phase 2), repolarization by delayed-rectifier K+ channels (phase 3), and the resting potential (phase 4). The long plateau produces an extended absolute refractory period that prevents tetany and lets the chamber refill. Pacemaker cells of the SA node instead show phase-4 spontaneous depolarization driven by the funny current (I_f) and Ca2+ channels, giving the heart its automaticity.

Key terms

Myocardium: The thick muscular middle layer of the heart wall that contracts to pump blood.
Systole: The phase of the cardiac cycle when a heart chamber contracts and ejects blood.
Diastole: The phase when a heart chamber relaxes and fills with blood.
SA node: The sinoatrial node, the heart's natural pacemaker, which sets the resting heart rate.
Stroke volume: The volume of blood ejected by a ventricle in one beat.
Ejection fraction: The percentage of blood in a filled ventricle that is pumped out with each beat; normally 55% or more.
Capillary: A microscopic vessel with walls one cell thick where gases, nutrients, and wastes are exchanged with tissues.
Atherosclerosis: The buildup of fatty plaque within artery walls that narrows the vessel and can trigger clots.

Check your understanding

5 questions · answers reveal instantly.

1.Which heart chamber pumps oxygenated blood into the systemic circulation via the aorta?
2.Which structure is the heart's natural pacemaker?
3.The first heart sound (S1) is produced by closure of which valves?
4.Which vessel type has the thinnest wall, allowing exchange of gases and nutrients?
5.Cardiac output is best defined as:

Citations & References

Links open publicly available educational and peer-reviewed sources.