Human physiology: study guide

Key ideas

Physiology questions often come down to a flow multiplied by a rate. Cardiac output, ventilation and blood pressure are the standard examples, and each has a simple formula.

• Cardiac output is heart rate times stroke volume: CO = HR × SV. Resting values are about 70 beats per minute and 70 mL, giving about 5 L/min.
• Mean arterial pressure is MAP = DBP + (SBP − DBP) ÷ 3, which is closer to the diastolic value because the heart spends longer in diastole.
• Ejection fraction is the share of the end-diastolic volume ejected per beat: EF = SV ÷ EDV × 100, normally about 55 to 70 %.
• Minute ventilation is tidal volume times breathing rate. BMI is mass in kilograms divided by height in meters squared.

Key equations

• CO = HR × SV
• MAP = DBP + (SBP − DBP) ÷ 3
• EF = SV ÷ EDV × 100
• minute ventilation = tidal volume × rate
• BMI = mass ÷ height²

Worked example

A heart rate of 70 beats per minute and a stroke volume of 70 mL give CO = 4900 mL/min = 4.9 L/min. For a blood pressure of 120/80 mmHg, MAP = 80 + (120 − 80) ÷ 3 = 93 mmHg. With an end-diastolic volume of 120 mL and the stroke volume of 70 mL, EF = 70 ÷ 120 = 58 %.

Common mistakes

• Taking MAP as the simple average of systolic and diastolic pressure.
• Mixing mL and L when quoting cardiac output.
• Using the end-systolic volume instead of the end-diastolic volume in the ejection fraction.

Practice

Test yourself with practice questions on this topic: Human physiology