Synopsis
Apply pressure-flow relationships, cardiac-cycle mechanics, ventricular function and vascular control to calculate haemodynamic changes and explain compensatory responses.
- Flow through a vascular bed equals its pressure difference divided by resistance; resistance is strongly influenced by vessel radius, while serial resistances add and parallel beds lower total resistance.
- Cardiac output equals heart rate multiplied by stroke volume; stroke volume reflects preload, contractility, afterload and the mechanical constraints of filling and ejection.
- The Frank–Starling mechanism increases force as ventricular filling stretches myocardium within the physiological range, helping match right and left ventricular outputs.
Reasoning priorities
Connect heart rate and stroke volume to systemic flow.
Multiply compatible units and test plausibility; tachycardia can raise output only until shortened filling or pathology reduces stroke volume.
Worked reasoning
A model circulation has heart rate 80 beats/min, end-diastolic volume 130 mL, end-systolic volume 50 mL and arterial pressure 120/75 mmHg.
- Calculate stroke volume as 130 minus 50, giving 80 mL per beat.
- Multiply 80 mL/beat by 80 beats/min to obtain 6400 mL/min, or 6.4 L/min cardiac output.
- Calculate pulse pressure as 120 minus 75, giving 45 mmHg; add one-third of 45 to diastolic pressure.
- The final approximate mean arterial pressure is 90 mmHg at an ordinary heart rate.
- Verify units and arithmetic, then check the physiological identity: output uses stroke volume rather than end-diastolic volume and the mean-pressure result lies between systolic and diastolic values.