Pharmacology for Nurses · Introduction to the Cardiovascular System

Introduction to the Heart, Circulation, and Blood Flow

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On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

The cardiovascular system is a closed loop of pipes and pumps: the heart pushes blood through a branching network of arteries, capillaries, and veins, delivering oxygen and nutrients to every tissue and removing carbon dioxide and waste. The heart is best understood as a double pump — two pumps fused side by side. The right heart sends deoxygenated blood to the lungs (the ); the left heart pumps oxygenated blood to the body (the ).

Every cardiovascular drug in later chapters works by changing one of the variables introduced here: heart rate, , , , conduction, or vessel tone. If you can trace a drop of blood, the drug chapters become variations on a theme.

Why this matters

The heart and circulation are the delivery system for every other organ — and the target of a huge share of medications.

Assessment depends on the circuit. A pedal pulse checks the far end of the systemic circuit; dependent edema is fluid backing up when the pump lags.
Drugs act on hemodynamic variables: a beta blocker slows heart rate, a diuretic reduces preload, a vasodilator reduces afterload.
Blood pressure is a product, not a mystery: BP = cardiac output × systemic vascular resistance, and nearly every antihypertensive changes one or both factors.

The college version

Core Concepts

The heart as a double pump

The heart has four chambers: two thin-walled receiving chambers (the atria) and two thick-walled pumping chambers (the ventricles). Functionally, think of two pumps:

Right pump (right atrium + right ventricle): sends deoxygenated blood to the lungs.
Left pump (left atrium + left ventricle): sends oxygenated blood to the body.

The left ventricle has the thickest wall because it must push blood through the entire systemic circuit; the right ventricle's job is easier (the lungs are close and low-resistance), so its wall is thinner.

Following one drop of blood around the loop

Left ventricle contracts → blood passes through the aortic into the aorta.
Aorta branches into arteries → arterioles → capillaries, where oxygen and nutrients diffuse out and wastes diffuse in; blood is now deoxygenated.
Capillaries merge into venules → veins, which carry blood back to the heart; one-way venous valves keep it moving against gravity.
Deoxygenated blood enters the right atrium via the vena cavae, through the tricuspid valve into the right ventricle.
Right ventricle contracts → blood passes through the pulmonary semilunar valve into the pulmonary arteries → lungs.
In lung capillaries, carbon dioxide is released and oxygen picked up; oxygenated blood returns via the pulmonary veins to the left atrium, through the mitral valve into the left ventricle.

Classic trap: pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood — vessels are named by direction, not oxygen content.

Valves: one-way doors

Four valves keep blood flowing forward. The atrioventricular (AV) valves — tricuspid on the right, mitral on the left — prevent backflow into the atria during ventricular contraction. The semilunar valves — pulmonary and aortic — prevent blood from falling back into the ventricles during relaxation. Valve closure produces the heart sounds: "lub" (AV valves) and "dub" (semilunar valves). Valve problems (stenosis = narrowed, regurgitation = leaky) disturb flow.

The vascular tree: arteries, capillaries, veins

Arteries carry blood away from the heart. Thick-walled and elastic, they damp the pressure surges of each heartbeat; smaller branches (arterioles) are the main site of resistance control — a primary lever for blood pressure drugs.
Capillaries are one-cell-thick exchange vessels where oxygen, nutrients, and waste move between blood and tissue. The big vessels only transport; capillaries are the delivery point.
Veins carry blood toward the heart; thinner-walled and more distensible than arteries, they hold most of the blood volume (the body's reservoir). One-way venous valves and skeletal muscle pumping return blood to the heart — why mobilization matters.

Cardiac output: the number that matters

is the volume pumped per minute:

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

Heart rate — beats per minute; sped up (positive chronotropes) or slowed (negative chronotropes) by drugs and autonomic tone.
Stroke volume — volume ejected per beat — depends on three things:
- Preload — ventricular stretch before contraction (roughly, venous return); more stretch = stronger contraction (Frank–Starling law).
- Afterload — the resistance the ventricle must overcome to eject (arterial pressure and tone); higher afterload = harder work.
- Contractility — the intrinsic strength of contraction; enhanced by inotropes, depressed by some drugs and by damage.

Blood pressure (BP) = CO × . Drugs lower BP by reducing CO (rate or contractility), reducing SVR (vasodilation), or reducing volume (diuretics → less preload).

Common Confusions

Do not confuse	With	Difference
Arteries always carry oxygenated blood	The pulmonary arteries carry deoxygenated blood	Vessels are named by direction (away from/toward the heart), not oxygen content — the pulmonary circuit flips the pattern
Right heart	Left heart	Right heart pumps to the lungs (low pressure, thin-walled ventricle); left heart pumps to the body (high pressure, thick-walled ventricle)
Systole	Diastole	Systole = contraction/ejection (AV valves close = "lub"); diastole = relaxation/filling (semilunar valves close = "dub")
Veins	Arteries	Veins carry blood toward the heart, are low-pressure and distensible, hold most blood volume, and have one-way valves; arteries carry blood away under high pressure
Heart rate	Cardiac output	HR is one input to CO; CO also depends on stroke volume — a fast heart can still pump poorly
Preload	Afterload	Preload is the incoming stretch before contraction (venous return); afterload is the outgoing resistance during ejection (arterial tone)

Eli explains

The same idea, in plain words

Explain it like I’m 10

Your heart is like two pumps sharing a wall. The right pump sends blood to the lungs to pick up oxygen; the left pump sends the oxygen-filled blood to the rest of your body. One-way doors called valves keep blood from flowing backward, like turnstiles at a stadium. Each beat squeezes blood out to the body and refills.

Worked example

Ms. Nguyen has heart failure: her weakened left ventricle cannot fully eject its volume, so blood backs up into the lungs and she is short of breath. Her provider starts a diuretic. Walk the physiology: the diuretic removes excess fluid → less venous return → lower preload → less volume to push → less backup into the lungs → easier breathing. The chain — drug → preload → ventricular filling → dyspnea — is clinical pharmacology in action.

Key takeaways

Double pump: right heart → pulmonary circuit (lungs); left heart → systemic circuit (body).
Flow path: vena cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary arteries → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.
Pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood — vessels are named by direction, not oxygen content.
Valves: AV valves (tricuspid, mitral) prevent backflow into atria; semilunar valves (pulmonary, aortic) prevent backflow into ventricles. Closure = "lub-dub."
CO = HR × SV, and SV depends on preload, afterload, and contractility.
BP = CO × SVR — the master equation for antihypertensive pharmacology.
Drug levers: rate (beta blockers), contractility (inotropes), preload (diuretics/venodilators), afterload (vasodilators), conduction (antiarrhythmics).
Verify all drug information against current references, the formulary, and prescriber orders before clinical application.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

List the structures a drop of blood passes through from the right atrium to the left ventricle.
Show answer
Right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary arteries → lung capillaries → pulmonary veins → left atrium → mitral valve → left ventricle.
Why do the pulmonary arteries carry deoxygenated blood while systemic arteries carry oxygenated blood?
Show answer
Vessels are named by direction, not oxygen content: pulmonary arteries carry blood away from the heart to the lungs (deoxygenated); pulmonary veins carry it toward the heart from the lungs (oxygenated).
What do the AV valves and semilunar valves each prevent, and what heart sounds do they produce?
Show answer
AV valves (tricuspid, mitral) prevent backflow into the atria during ventricular contraction ("lub"); semilunar valves (pulmonary, aortic) prevent backflow into the ventricles during relaxation ("dub").
Write the equation for cardiac output and name the three determinants of stroke volume.
Show answer
CO = HR × SV. Stroke volume depends on preload (venous return/stretch), afterload (resistance to ejection), and contractility (intrinsic strength of contraction).
How does reducing preload (e.g., with a diuretic) help a patient with heart failure?
Show answer
The diuretic removes fluid, reducing venous return and therefore preload; the failing ventricle has less volume to push, reducing pulmonary backup and improving breathing.
Using BP = CO × SVR, explain two different ways a drug could lower blood pressure.
Show answer
A drug could lower BP by reducing CO (e.g., slowing heart rate with a beta blocker, or reducing volume with a diuretic) or by reducing SVR (e.g., arteriolar vasodilation). Many antihypertensives do both.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Pulmonary circuit: Loop from right heart → lungs → left heart
Systemic circuit: Loop from left heart → body → right heart
Atrioventricular (AV) valve: One-way valve between atrium and ventricle (tricuspid, mitral)
Semilunar valve: One-way valve at the ventricular outlet (pulmonary, aortic)
Preload: Ventricular stretch before contraction (roughly, venous return)
Afterload: The resistance the ventricle must overcome to eject blood
Contractility: Strength of contraction independent of stretch
Cardiac output (CO): Volume pumped per minute = HR × SV
Systemic vascular resistance (SVR): Total resistance of the systemic arterioles
Capillary: One-cell-thick exchange vessel

Sources & references

openstax.org — Pharmacology

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