Anatomy of the Cardiovascular System

The cardiovascular system is built from three major components: the heart, the blood vessels, and the blood itself. This page covers the heart's structure and its own electrical timing system, the mechanics of the cardiac cycle, and the vessels that carry blood throughout the body — the anatomy and circuitry that everything else in this chapter builds on.

Key Takeaways

• The heart has four chambers (right/left atria and ventricles) and functions as two pumps in series: the right side drives pulmonary circulation, the left side drives systemic circulation.

• Blood's path through the heart: 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 the ventricular exits.

• The left ventricle's wall is thicker than the right because it must pump blood to the entire body, not just the nearby lungs.

• The cardiac conduction system fires in sequence: SA node → AV node (delay) → bundle of His → Purkinje fibers, producing the ECG's P wave, QRS complex, and T wave.

• The cardiac cycle alternates systole (ventricles contract, AV valves closed) and diastole (heart relaxes, AV valves open); cardiac output = heart rate × stroke volume, about 5 L/min at rest.

• Blood flows through arteries → arterioles → capillaries → venules → veins, with capillaries as the sole site of exchange and veins relying on valves and muscle contraction to return blood.

• Three portal systems route blood through two capillary beds in series: hepatic (digestive tract to liver), hypophyseal (hypothalamus to anterior pituitary), and renal (glomerulus to peritubular capillaries).

The Heart's Structure

The heart is a muscular, four-chambered organ that functions as a pump, keeping blood moving through the vasculature — the network of arteries, capillaries, and veins that reaches every tissue in the body.

Chambers and the Two Circuits

The heart functions as two pumps working in series. The right side pumps deoxygenated blood to the lungs, a loop called pulmonary circulation. The left side pumps oxygenated blood to the rest of the body, a loop called systemic circulation.

Each side of the heart has two chambers: an atrium, which receives incoming blood, and a ventricle, which pumps blood back out. That gives four chambers total — right atrium, right ventricle, left atrium, and left ventricle.

The Path of Blood Through the Heart

Blood returning from the body enters the right atrium through two large veins, the superior and inferior venae cavae. From there it flows through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary artery and on to the lungs, where gas exchange releases carbon dioxide and picks up oxygen.

Oxygenated blood returns to the heart through the pulmonary veins, entering the left atrium. It passes through the mitral valve into the left ventricle, which pumps it through the aortic valve into the aorta — the largest artery in the body — to deliver oxygen-rich blood to every tissue.

MCAT Callout — Path of Blood Through the Heart: venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta.

The heart contracts in a fixed sequence: the atria contract first, pushing blood into the ventricles, and then the ventricles contract, sending blood onward to the lungs or the body.

Heart Valves

Valves keep blood moving in one direction and prevent backflow, which matters because the pressure generated during each contraction is high enough to push blood the wrong way without them. There are two types.

AV (atrioventricular) valves sit between the atria and ventricles: the tricuspid valve on the right, and the bicuspid valve — also called the mitral valve — on the left.

Semilunar valves sit at the exits of the ventricles: the pulmonary valve between the right ventricle and the pulmonary artery, and the aortic valve between the left ventricle and the aorta.

MCAT Callout — AV Valves vs. Semilunar Valves: AV valves separate the atria from the ventricles (tricuspid on the right, bicuspid/mitral on the left); semilunar valves sit at the ventricular exits (pulmonary and aortic). During systole, AV valves close and semilunar valves open; during diastole, it's the reverse.

Why the Left Ventricle Wall Is Thicker

The left ventricle has a much thicker muscular wall than the right ventricle. It has to generate enough force to send blood through the entire body, while the right ventricle only needs to push blood to the nearby lungs, which offer much lower resistance.

The Cardiac Conduction System

The heart doesn't rely on external nerves to tell it when to beat — it has its own internal conduction system of specialized muscle cells that generate and time each heartbeat, making sure the atria contract before the ventricles.

The signal starts at the SA node (sinoatrial node), located in the wall of the right atrium. This is the heart's natural pacemaker, generating regular electrical impulses at about 60 to 100 signals per minute — which is why a normal resting heart rate falls in that same range.

From the SA node, the signal spreads through the atria, triggering their contraction, and reaches the AV node (atrioventricular node), located between the atria and ventricles. The AV node briefly delays the signal, giving the ventricles time to fill completely before they contract.

After the AV node, the signal travels down the bundle of His, a pathway running through the interventricular septum (the wall between the two ventricles), which branches into left and right bundle branches. The signal then reaches the Purkinje fibers, which spread throughout both ventricular walls and rapidly distribute the impulse, triggering a strong, coordinated ventricular contraction.

This same electrical activity produces the waves seen on an electrocardiogram (ECG):

• P wave — atrial depolarization (the atria contracting)

• QRS complex — ventricular depolarization (the ventricles contracting)

• T wave — ventricular repolarization (the ventricles resetting for the next cycle)

The Cardiac Cycle

One full heartbeat is called the cardiac cycle, made up of two phases: systole and diastole.

Systole is the phase when the ventricles contract. AV valves are closed and semilunar valves are open, allowing blood to be pumped out into the aorta or pulmonary artery. The first heart sound occurs when the AV valves close.

Diastole is the phase when the heart relaxes. Semilunar valves are closed and AV valves are open, letting the ventricles refill with blood for the next contraction. The second heart sound occurs when the semilunar valves close.

These phases connect directly to cardiac output — the total volume of blood pumped by one ventricle in one minute. It depends on heart rate (beats per minute) and stroke volume (blood pumped per beat):

Cardiac output = heart rate × stroke volume (CO = HR × SV)

For most adults at rest, cardiac output is around 5 liters per minute, rising during exercise as the heart beats faster and pumps more blood per contraction.

The Vasculature

To distribute blood throughout the body, the cardiovascular system relies on a branching network of vessels — arteries, arterioles, capillaries, venules, and veins — all lined with endothelial cells, a thin, smooth layer that maintains vessel integrity, regulates flow, and allows substances to pass in and out of the bloodstream.

Arteries, Capillaries, and Veins

Arteries carry blood away from the heart under high pressure; their thick, elastic, muscular walls absorb the force of each heartbeat and help maintain pressure during diastole. As arteries branch and narrow, they become arterioles, which still have muscular walls and regulate blood flow and pressure by changing diameter — earning them the name "resistance vessels," since they direct blood toward the tissues that need it most.

Capillaries are the smallest and thinnest vessels, with walls just one endothelial cell thick. That structure allows efficient exchange of gases, nutrients, and waste with surrounding tissue. Blood flow through capillaries is slow, giving substances time to diffuse, and capillaries are fragile — rupture leads to bruising or bleeding.

Veins carry blood toward the heart under much lower pressure than arteries, so they rely on one-way valves and on skeletal muscle contractions (especially in the legs) to keep blood moving, particularly against gravity. Long periods of inactivity — a long flight, bed rest — raise the risk of a clot forming, called deep vein thrombosis (DVT); if that clot travels to the lungs, it can cause a pulmonary embolism.

Vessel Wall Structure

Arteries and veins share the same three wall layers — tunica externa, tunica media, and tunica intima — but the layers are thinner in veins. Capillaries, by contrast, are built from just a basement membrane and a single layer of endothelium.

The Complete Circulation Pathway

Putting it all together: blood returning from the body enters the right atrium through the venae cavae, passes through the tricuspid valve into the right ventricle, and is pumped through the pulmonary valve into the pulmonary artery toward the lungs, where gas exchange occurs. Oxygenated blood returns through the pulmonary veins into the left atrium, flows through the mitral valve into the left ventricle, and is pumped through the aortic valve into the aorta and out into systemic circulation — arteries, then arterioles, then capillaries (where exchange happens with tissues), then venules, then veins, back to the right atrium through the venae cavae. This loop — heart to lungs, lungs to heart, heart to body, body to heart — repeats continuously.

Portal Systems

In most cases, blood passes through only one capillary bed before returning to the heart. But the body has three exceptions, called portal systems, where blood flows through two capillary beds in series before completing the loop:

• The hepatic portal system routes blood from the digestive tract to the liver before it returns to the heart.

• The hypophyseal portal system carries hormones from the hypothalamus to the anterior pituitary.

• The renal portal system routes blood through the glomerulus and then a second capillary network in the kidneys.

These systems allow for extra processing, regulation, or exchange at specific sites before blood rejoins general circulation.

Common MCAT Mistakes

• Mixing up AV valves and semilunar valves. AV valves (tricuspid, bicuspid/mitral) sit between the atria and ventricles; semilunar valves (pulmonary, aortic) sit at the ventricular exits. During systole, AV valves close and semilunar valves open — not the reverse.

• Assuming the two ventricles are structurally identical. The left ventricle's wall is far thicker than the right's, because it must generate enough pressure to drive blood through the entire body (systemic circulation), while the right ventricle only pushes blood to the nearby, low-resistance lungs.

• Reversing systole and diastole. Systole is the contraction phase (AV valves closed, semilunar valves open, blood ejected); diastole is the relaxation/filling phase (semilunar valves closed, AV valves open). Mixing these up flips which valves are doing what at each point in the cycle.

• Misordering the cardiac conduction pathway. The signal always travels SA node → AV node (delay) → bundle of His → Purkinje fibers. The AV node's delay is what lets the ventricles finish filling before they contract — it doesn't fire before the SA node.

MCAT-Style Concept Check

Question: The left ventricle's wall is far thicker than the right ventricle's, even though both chambers pump roughly the same volume of blood with each heartbeat. Which of the following best explains why, in terms of the two circuits the heart drives?

• A) The left ventricle pumps a much larger volume of blood per beat than the right ventricle

• B) The left ventricle must generate higher pressure to overcome the greater resistance of systemic circulation, while the right ventricle only drives the lower-resistance pulmonary circuit

• C) The left ventricle contracts more frequently than the right ventricle

• D) The right ventricle's wall is thin because it pumps deoxygenated blood, which requires less muscular force

Answer: B

Explanation: Wall thickness tracks the pressure a chamber must generate, not the volume it pumps. Systemic circulation reaches every tissue in the body and offers much higher resistance to flow than pulmonary circulation, which only has to move blood to and from the lungs. To overcome that higher resistance and maintain adequate pressure throughout the systemic vasculature, the left ventricle needs substantially more muscle mass than the right ventricle, even though both chambers eject the same volume of blood per beat (since the two circuits are in series, one after the other).

FAQ

What is the path of blood through the heart?

Venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta.

What's the difference between AV valves and semilunar valves?

AV valves (tricuspid on the right, bicuspid/mitral on the left) sit between the atria and ventricles. Semilunar valves (pulmonary and aortic) sit at the ventricular exits. AV valves close and semilunar valves open during systole; it's the reverse during diastole.

Why is the left ventricle's wall thicker than the right ventricle's?

Because the left ventricle drives systemic circulation, pumping blood to the entire body against high resistance, while the right ventricle only pumps blood to the nearby, low-resistance lungs.

What produces the P wave, QRS complex, and T wave on an ECG?

The P wave is atrial depolarization (atria contracting), the QRS complex is ventricular depolarization (ventricles contracting), and the T wave is ventricular repolarization (ventricles resetting for the next cycle) — all generated by the heart's own conduction system (SA node → AV node → bundle of His → Purkinje fibers).