Step by Step
1
The core formula
Blood pressure = Cardiac Output (CO) × Systemic Vascular Resistance (SVR). Cardiac Output itself equals Heart Rate × Stroke Volume.
2
Systolic vs. diastolic pressure
Systolic pressure (the top number, normally 120) is the pressure during ventricular contraction. Diastolic pressure (the bottom number, normally 80) is the pressure during ventricular relaxation. Pulse pressure = systolic − diastolic (normally about 40 mmHg).
3
Mean arterial pressure (MAP)
MAP = diastolic + 1/3 of pulse pressure, giving a normal value of about 93 mmHg — this represents the average pressure driving blood through the circulatory system over the full cardiac cycle.
4
Long-term regulation: baroreceptors and RAAS
The baroreceptor reflex (via stretch receptors in the carotid sinus and aortic arch) provides fast, short-term adjustment of heart rate and SVR. The renin-angiotensin-aldosterone system (RAAS) provides slower, longer-term regulation: low blood pressure triggers renin release, leading to angiotensin II (which causes vasoconstriction) and aldosterone (which increases sodium and water retention, increasing blood volume and thus blood pressure).
Applied Walkthrough
1
A patient's blood pressure drops due to blood loss. Baroreceptors in the carotid sinus and aortic arch detect the reduced stretch and rapidly trigger increased heart rate and vasoconstriction to compensate.
2
If the low blood pressure persists, the slower-acting RAAS system activates: reduced blood flow to the kidneys triggers renin release.
3
Renin leads to the production of angiotensin II, which directly causes vasoconstriction (raising SVR), and also stimulates aldosterone release.
4
Aldosterone increases sodium and water retention in the kidneys, increasing blood volume — this combination of vasoconstriction and increased volume works together to raise blood pressure back toward normal, though on a slower timescale than the baroreceptor reflex.
Exam Application
Exams test whether you can apply the BP = CO × SVR formula, correctly calculate pulse pressure and MAP, and distinguish the fast-acting baroreceptor reflex from the slower-acting RAAS system.
⚠ Common Trap
The most common trap is confusing the timescales of the two regulatory systems — the baroreceptor reflex is a fast, short-term response (seconds), while RAAS is a slower, longer-term response (hours to days) that primarily works by adjusting blood volume rather than immediate vessel tone.
✓ Quick Self-Check
1. What is the formula for blood pressure?
BP = Cardiac Output (CO) × Systemic Vascular Resistance (SVR).
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2. What is pulse pressure, and how is it calculated?
Systolic pressure minus diastolic pressure; normally about 40 mmHg.
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3. What is the formula for mean arterial pressure (MAP)?
MAP = diastolic + 1/3 of pulse pressure.
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4. What triggers renin release in the RAAS system?
Low blood pressure (detected by the kidneys).
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5. What are the two main effects of RAAS activation?
Vasoconstriction (via angiotensin II) and increased sodium/water retention (via aldosterone), both raising blood pressure.
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