Cardiovascular physiology
Introduced| English | Meaning |
|---|---|
| cardiac output/ˈkɑːdiæk ˈaʊtpʊt/ | cardiac output |
| pressure–volume loop | pressure–volume loop |
| preload | preload |
| Frank–Starling law | Frank–Starling law |
| contractility | contractility |
| afterload | afterload |
A pump, pipes and a volume
- The circulation is a pump, a set of pipes and a volume of blood. Three equations run every shock, every heart failure and every blood-pressure clinic.
- This lesson is those equations, and the loop that shows what they hide.
Cardiac output
- Cardiac output 心输出量 is heart rate times stroke volume: $CO = HR \times SV$.
- At rest, 70 beats per minute × 70 mL = 4900 mL per minute, about 5 L per minute.
Heart rate is 80 per minute and stroke volume 60 mL. What is the cardiac output, in L per minute?
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80 × 60 = 4800 mL per minute = 4.8 L per minute.
Pressure, flow and resistance
- The circulation's version of Ohm's law: $MAP = CO \times SVR$, where SVR is the systemic vascular resistance (right atrial pressure is close to 0).
- Mean arterial pressure is not halfway between the readings: $MAP \approx DBP + \tfrac{1}{3}(SBP - DBP)$, so 120/80 gives about 93 mmHg.
Blood pressure is 140/80 mmHg. What is the mean arterial pressure, in mmHg?
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80 + (140 − 80)/3 = 80 + 20 = 100 mmHg.
The pressure–volume loop
- One beat of the left ventricle drawn as a loop: filling, isovolumetric contraction, ejection, isovolumetric relaxation. This is the pressure–volume loop 压力-容积环.
- Its width is the stroke volume; its area is the work the heart does on the blood.
Put the phases of the left ventricular cycle in order, starting with filling.
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Two phases change volume (filling, ejection) and two change only pressure, which gives the loop its four sides.
Three levers on output
- Preload 前负荷 (filling) stretches the muscle so it ejects more: the loop widens. This is the Frank–Starling law 弗兰克-斯塔林定律.
- Contractility 收缩力 lets the heart eject more from the same filling. Afterload 后负荷 (the pressure it must push against) narrows the loop.
Match each change to its effect on the loop.
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The loop shows the three levers that the equation CO = HR × SV hides inside SV.
Worked example: the arithmetic of shock
- Bleeding cuts filling, so CO falls from 5 to 3 L per minute. Reflex vasoconstriction raises SVR by two-thirds: $MAP \approx 3 \times 1.67 = 5$ times the old SVR, so pressure is back to normal.
- But flow to the organs is still 40% down. The cuff says "compensated"; the kidneys know the truth.
A normal blood pressure after bleeding means organ blood flow is normal.
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Vasoconstriction can hold MAP while cardiac output, and organ flow, are well below normal.
Worked example: a new murmur
- (Fictional case) A new systolic murmur after a heart attack: a damaged mitral valve now leaks backwards in systole.
- Part of each stroke goes back into the atrium, so the forward output falls, and the heart rate rises to make up for it. Timing (systole) plus volume load names the problem.
A leaking mitral valve lowers forward cardiac output because —
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The total stroke is split between forward flow into the aorta and backward flow into the atrium.
Three common slips
- Forgetting units. HR × SV in mL gives mL per minute; divide by 1000 for L.
- MAP as the average of the two readings. Diastole lasts longer, so MAP sits about a third of the way up.
- "Baroreceptors measure pressure." They sense stretch of the artery wall and reset within a day or two of a sustained change.
Recap
- $CO = HR \times SV$, about 5 L per minute; $MAP = CO \times SVR$.
- Loop width = stroke volume, area = work; preload widens it, afterload narrows it, contractility lets it eject more.
- Pressure can be normal while flow is low: trust organ perfusion over the cuff.