Cardiovascular
Medical-Surgical Nursing
Lesson 1 — The Heart: Blood Flow, Cardiac Function & Cardiac Output. A complete Clinical Compass prototype lesson.
The Heart: Blood Flow, Cardiac Function & Cardiac Output
How the heart moves blood — and why it matters at the bedside
Clinical Compass is a supplemental learning-support platform — not a replacement for your nursing program. This lesson clarifies and reinforces what you have already started learning in class.
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The Heart: Blood Flow, Cardiac Function & Cardiac Output
The one idea behind everything
The heart is a pump that moves blood through the lungs and body to maintain tissue perfusion.
Sends blood to the lungs to pick up oxygen.
Sends oxygen-rich blood out to the body.
Keep blood moving one direction — not backward.
Hold this picture in mind — every section below just fills in the details.
Follow the Drop
Trace one drop of blood through the entire circuit. Watch the oxygen change — and learn why the pulmonary artery is the one artery that carries oxygen-poor blood.
Body (tissues)
Tissues have used the oxygen. Blood returns carrying CO₂.
Right side → lungs (oxygen-poor) · Left side → body (oxygen-rich). The pulmonary artery is the only artery carrying oxygen-poor blood; the pulmonary vein is the only vein carrying oxygen-rich blood.
Meet the Pump
Not every anatomical detail — only the structures a nurse needs to understand cardiac function. For each: what it is, what it does, and why a nurse cares.
A Clinical Compass original labeled heart illustration will appear here. For now, this placeholder shows the layout: right heart (top), left heart (bottom), valves between chambers and vessels.
Right Atrium
ChamberWhat is it?
Upper-right receiving chamber.
What does it do?
Collects oxygen-poor blood returning from the body.
Why does a nurse care?
Problems here show up as backed-up systemic signs (edema, JVD) when the right side fails.
Right Ventricle
ChamberWhat is it?
Lower-right pumping chamber.
What does it do?
Pumps blood to the lungs.
Why does a nurse care?
Weak pumping raises pressure in the lungs and body differently than left-sided failure.
Left Atrium
ChamberWhat is it?
Upper-left receiving chamber.
What does it do?
Collects oxygen-rich blood from the lungs.
Why does a nurse care?
Pressure here reflects what is happening in the lungs.
Left Ventricle
ChamberWhat is it?
Lower-left pumping chamber — the thickest wall.
What does it do?
Pumps oxygen-rich blood to the whole body.
Why does a nurse care?
The left ventricle does the heaviest work, so it is the most common site of pump failure.
Tricuspid Valve
ValveWhat is it?
Three-flap valve between right atrium and right ventricle.
What does it do?
Prevents blood from flowing back into the right atrium.
Why does a nurse care?
A leak here changes the sounds you hear and the signs you see.
Pulmonic Valve
ValveWhat is it?
Valve between right ventricle and pulmonary artery.
What does it do?
Prevents backflow from the pulmonary artery into the right ventricle.
Why does a nurse care?
Narrowing or leaking changes flow to the lungs.
Mitral Valve
ValveWhat is it?
Two-flap valve between left atrium and left ventricle.
What does it do?
Prevents blood from flowing back into the left atrium.
Why does a nurse care?
The most commonly diseased valve — a key source of murmurs.
Aortic Valve
ValveWhat is it?
Valve between left ventricle and aorta.
What does it do?
Prevents backflow from the aorta into the left ventricle.
Why does a nurse care?
Narrowing forces the left ventricle to work much harder.
Vena Cava (SVC + IVC)
VesselWhat is it?
The largest veins returning blood to the heart.
What does it do?
Deliver oxygen-poor blood from the body to the right atrium.
Why does a nurse care?
High pressure here (JVD) signals right-heart strain.
Pulmonary Artery
VesselWhat is it?
The vessel carrying blood from the right ventricle to the lungs.
What does it do?
Carries oxygen-poor blood to the lungs.
Why does a nurse care?
Rising pressure here (pulmonary hypertension) strains the right heart.
Pulmonary Veins
VesselWhat is it?
Vessels returning oxygen-rich blood from the lungs.
What does it do?
Deliver oxygen-rich blood to the left atrium.
Why does a nurse care?
Backup here reflects lung problems before left-heart problems.
Aorta
VesselWhat is it?
The largest artery in the body.
What does it do?
Carries oxygen-rich blood from the left ventricle to the body.
Why does a nurse care?
Pulses here reflect left-heart output; changes signal serious emergencies.
Septum
WallWhat is it?
The muscular wall dividing the right and left sides of the heart.
What does it do?
Keeps oxygen-poor and oxygen-rich blood from mixing.
Why does a nurse care?
A hole here changes oxygenation dramatically — seen as cyanosis.
Myocardium
MuscleWhat is it?
The heart muscle itself.
What does it do?
Contracts to squeeze blood out of the ventricles.
Why does a nurse care?
Damage (ischemia/infarct) weakens the pump — the root of most cardiac emergencies.
One Heartbeat
Before any sounds or rhythms, understand the mechanical cycle: filling then ejection. The heart sounds (S1, S2) make sense only after you see the cycle.
open
closed
Ventricles relaxed — filling through open AV valves.
Ventricles relax and fill. The AV valves (tricuspid + mitral) are open. The semilunar valves (pulmonic + aortic) are closed.
Now the sounds make sense
Stroke Volume & Cardiac Output
Stroke Volume (SV) is the amount of blood ejected by a ventricle with each beat. Not all the blood in the ventricle leaves — the heart squeezes out a portion. That portion is the stroke volume.
Cardiac Output (CO) is the total blood the heart pumps in one minute. It is simply:
Heart Rate (beats/min) × Stroke Volume (mL/beat) = Cardiac Output (mL/min)
Cardiac Output
5,250 mL/min
≈ 5.3 L/min
The real question: What determines how much blood the heart ejects with each beat? That is stroke volume — and three things shape it: preload, contractility, and afterload.
Fill — Squeeze — Push
Three intuitive words unlock the three determinants of stroke volume: Preload, Contractility, Afterload. No advanced math — just what each means and why it changes the output.
How much the ventricle fills before contraction.
Preload and contractility raise stroke volume; afterload resists it. All three combine to determine how much blood each beat actually moves.
Cardiac Output Becomes Perfusion
The chain that runs through nursing assessment
Everything you assess about a sick patient — blood pressure, pulses, skin, mental status, urine output — traces back to this chain. The heart is not an isolated organ. It is the engine of perfusion.
Think first: If cardiac output decreases, what findings might you expect?
Picture a patient whose heart is moving less blood than before. What would you see, feel, or measure? Reason it out — then reveal.
Think Like a Nurse
A short reasoning interaction. You will not diagnose — you will read what the patient's body is telling you and reason toward the underlying problem.
Heart Rate
122 bpm
Blood Pressure
86/52 mmHg
Extremities
Cool to touch
Urine Output
Declining
Do not diagnose the patient yet. What is the patient's body telling you?
Reason out loud. Do not jump to a disease name — describe what the body is doing.
Break the System
How disruption of each normal piece points to a future cardiac topic. These are previews — not today's lesson.
Knowledge Check
5 prototype interactions
Each has an educational rationale. No large question bank — just a check on the big ideas.
Score
0/0
Q1 · Sequencing
Place these structures in the order a drop of oxygen-poor blood passes through them.
Q2 · Concept check
Which chamber pumps oxygen-rich blood to the entire body?
Q3 · Concept check
A patient's blood pressure is high and sustained. Which determinant of stroke volume is the left ventricle now working harder against?
Q4 · Calculation
A patient has a heart rate of 80 bpm and a stroke volume of 70 mL. What is the cardiac output (in mL/min)?
Q5 · Clinical reasoning
A patient has a decreased cardiac output. Which finding is most directly explained by reduced tissue perfusion?
Before You Leave
Before You Leave
Five ideas to carry to the bedside
- Blood travels through the heart and lungs in a continuous circuit.
- Diastole is mainly ventricular filling; systole is ventricular ejection.
- Stroke volume is the amount of blood ejected with each beat.
- Preload, contractility, and afterload influence stroke volume.
- Cardiac output matters because blood flow is required for tissue perfusion.