What is Blood Pressure (Hemodynamics)?
Blood pressure is the force blood exerts on vessel walls, driven by how much blood the heart pumps and how much resistance the vessels offer. Hemodynamics describes this relationship mathematically, linking cardiac output and vascular resistance to the pressure that keeps blood flowing.
Mean arterial pressure equals cardiac output multiplied by total peripheral resistance: MAP = CO × TPR. Raising either the heart's output or vessel resistance raises blood pressure.
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Step-by-step worked examples
A patient has a cardiac output of 5 L/min and total peripheral resistance of 18 mmHg·min/L. Find their mean arterial pressure.
MAP = CO × TPR MAP = 5 × 18 MAP = 90 mmHg
During exercise, cardiac output rises to 8 L/min while resistance drops to 10 mmHg·min/L (vessels dilate). Find the new MAP.
MAP = CO × TPR MAP = 8 × 10 MAP = 80 mmHg (Even though CO rose, vasodilation kept MAP from rising too much.)
A patient in shock has TPR fall sharply to 6 mmHg·min/L while CO stays at 5 L/min. What happens to MAP, and why is this dangerous?
MAP = CO × TPR MAP = 5 × 6 MAP = 30 mmHg This is far below the normal range (~70–100 mmHg), so organs may not get enough blood flow — a hallmark of circulatory shock.
Flashcards
Quick quiz
Q1.CO = 6 L/min, TPR = 15 mmHg·min/L. What is the MAP?
Q2.If TPR increases while CO stays constant, blood pressure will…
Q3.What is the unit of total peripheral resistance in this formula?
Q4.Vasodilation of arterioles primarily affects blood pressure by…
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Common mistakes
Thinking blood pressure depends only on the heart. — Correct: It depends on both cardiac output AND vessel resistance — MAP = CO × TPR.
Confusing systolic/diastolic pressure with mean arterial pressure. — Correct: MAP is a weighted average of the two, roughly DBP + 1/3(SBP − DBP), not a simple average.
Assuming higher cardiac output always means higher blood pressure. — Correct: If TPR drops enough (e.g., vasodilation during exercise), MAP can stay stable or even fall.
Mixing up the units of TPR. — Correct: TPR is in mmHg·min/L so the units correctly cancel to give MAP in mmHg.
FAQ
What is the formula for blood pressure?
Mean arterial pressure: MAP = CO × TPR, cardiac output times total peripheral resistance.
What are examples of blood pressure calculations?
A resting CO of 5 L/min and TPR of 18 mmHg·min/L gives MAP = 90 mmHg — a normal reading.
How do you calculate blood pressure step by step?
Multiply cardiac output (L/min) by total peripheral resistance (mmHg·min/L) to get MAP in mmHg.
What raises blood pressure the most, heart output or vessel resistance?
Both matter equally in the formula — a rise in either CO or TPR raises MAP proportionally.