SME
Human Anatomy & Physiology
Cardiovascular and respiratory systems
Use this for “A heart beats 70 times a minute, 70 mL a beat, at 120/80 mmHg. What are the cardiac output and the mean arterial pressure?”
Diagram
CO = HR × SV = 70 min⁻¹ × 70 mL
CO = 4.90 L/min
MAP
=
DBP
+
SBP − DBP
3
=
80
+
40
3
=
93.3
mmHg.
TPR
=
MAP
CO
=
19.0
mmHg·min/L.
This is an example. Type your own values to start a new problem.
CO (L/min)
=
HR (min⁻¹)
×
SV (mL)
MAP (mmHg)
=
DBP (mmHg)
+
SBP (mmHg)
−
DBP (mmHg)
3
TPR
Total peripheral resistance
calculated · mmHg·min/L
Formulas
CO
=
HR
×
SV
1,000
4.9
=
70
×
70
1,000
MAP
=
DBP
+
SBP − DBP
3
93.3
=
80
+
120 − 80
3
TPR
=
MAP
CO
19
=
93.3
4.9
Assumptions
•
Cardiac output is the blood the left ventricle pumps in a minute: the beats a minute times the stroke volume.
•
The heart spends about two thirds of each beat relaxed, so the mean pressure sits one third of the way from diastolic to systolic.
•
Total peripheral resistance is the mean pressure the vessels need for each liter a minute of flow (venous pressure taken as 0).
Step-by-step
WE KNOW
HR = 70 min⁻¹, SV = 70 mL, SBP = 120 mmHg, DBP = 80 mmHg
FIND
cardiac output (CO), mean arterial pressure (MAP), total peripheral resistance (TPR)
Step 1 · Find cardiac output (CO)
Use CO = HR × SV ÷ 1,000
Each beat pushes out one stroke volume, HR times a minute; 1,000 mL make 1 L.
CO
=
70
×
70
1,000
CO
=
4,900
1,000
CO = 4.9 L/min
Step 2 · Find mean arterial pressure (MAP)
Use MAP = DBP + (SBP − DBP) ÷ 3
Start at the diastolic pressure and add one third of the pulse pressure, SBP − DBP.
MAP
=
80
+
120 − 80
3
MAP
=
80
+
40
3
MAP = 80 + 13.33
MAP = 93.3 mmHg
Step 3 · Find total peripheral resistance (TPR)
Use TPR = MAP ÷ CO
Resistance is the pressure that drives the flow divided by the flow.
TPR
=
93.3
4.9
TPR = 19 mmHg·min/L
Check
Put the numbers back in. Both sides should match.
4.9
=
70
×
70
1,000
✓
93.3
=
80
+
120 − 80
3
✓
19
=
93.3
4.9
✓
Topic check
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