📐 Cardiovascular System Lesson

BP = CO × PVR: the blood pressure equation

This single equation explains nearly every cause of high or low blood pressure you will encounter clinically.

Result

Output

PVR

Resistance

Rate

Volume

📖 Full Breakdown

Breaking down the master equation piece by piece

Every blood pressure abnormality traces back to a change in one of these underlying variables.

BP = CO × PVR

The master equation

Blood pressure is the product of cardiac output and peripheral vascular resistance. Increasing either factor — without a compensating decrease in the other — raises blood pressure.

CO = HR × SV

Cardiac output

Cardiac output is heart rate multiplied by stroke volume — normally about 5 liters per minute at rest. Increasing either heart rate or stroke volume raises cardiac output.

Preload

Ventricular filling

The volume of blood filling the ventricle before contraction. Per the Frank-Starling law, more stretch from filling produces a stronger contraction, up to a physiological limit.

Afterload

Resistance to ejection

The resistance the ventricle must pump against to eject blood — increased afterload (as in hypertension) makes the heart work harder for the same stroke volume.

Baroreceptors

The feedback sensors

Located in the carotid sinus and aortic arch, these detect blood pressure changes and signal the medulla to adjust heart rate and vessel tone via the autonomic nervous system.

🩺 Clinical / Exam Application

A patient in hypovolemic shock from blood loss has a falling blood pressure. Baroreceptors in the carotid sinus and aortic arch detect the drop and trigger a reflex: increased heart rate (raising CO) and vasoconstriction (raising PVR) — both pushing blood pressure back up using exactly the two variables in the BP = CO × PVR equation. This is why a bleeding patient's heart rate rises before their blood pressure visibly drops — the compensation happens first.

⚠️ Exam Alert

A commonly tested clinical link: hypertension can result from an increase in CO (high heart rate or stroke volume) OR an increase in PVR (vasoconstriction) — exam questions often ask you to identify which side of the equation a given cause (e.g., a vasoconstrictor drug vs. a stimulant raising heart rate) is acting on.

🚧 Common Trap

Don't treat "cardiac output" and "blood pressure" as the same thing. A patient can have normal or even high cardiac output but low blood pressure if PVR (vessel resistance) has dropped significantly — as happens in septic shock, where vessels dilate.

✅ Quick Check

A medication causes vasodilation (widening blood vessels). Using the BP = CO × PVR equation, explain which variable it affects and what happens to blood pressure as a result, all else being equal.

📝 Exam Prep

Common Exam Questions

❓ What is the fundamental equation for blood pressure?

✅ Blood Pressure = Cardiac Output × Peripheral Vascular Resistance (BP = CO × PVR). Cardiac output itself equals Heart Rate × Stroke Volume.

❓ What do baroreceptors do and where are they located?

✅ Baroreceptors in the carotid sinus and aortic arch detect changes in blood pressure and signal the medulla to reflexively adjust heart rate and vessel tone to correct the pressure.