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[Physiology](https://durgical.com/en/physiology/) · [Cardiovascular](https://durgical.com/en/physiology/#s-cardio)

# Cardiac output and what sets it

- Regulation
- Exam priority: high

Cardiac output is how much blood the heart pumps each minute: beats per minute times the volume of each beat.

Updated 2026-11-01

Also called: Stroke volume, Cardiac index, Fick principle

**See it in 3D:** [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/) · [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/)

## Key points

- CO = HR × SV ≈ 70/min × 70 mL ≈ 5 L/min; cardiac index = CO / body surface area ≈ 2.5–4 L/min/m².
- SV rises with preload and contractility and falls with afterload.
- Raising HR helps only up to ≈ 150–180/min; above that, diastole is too short to fill and SV falls.
- Fick principle: CO = O₂ consumption ÷ (arterial − mixed venous O₂ content).
- In steady state, CO = venous return; tissue metabolism (local vasodilation) is the main long-term controller.

## What it is

Cardiac output (CO) is the volume each ventricle pumps per minute, about 5 L/min in a resting adult. It equals heart rate times stroke volume; stroke volume depends on preload, afterload and contractility. Over minutes and hours, the heart pumps what returns to it, so venous return and tissue needs largely set CO.

### Left-ventricular pressure–volume loop

**EDV** 120 mL **ESV** 40 mL **SV** 81 mL **EF** 67% **ESP** 121 mmHg

#### ↑ Contractility

↑ Contractility (sympathetic drive or an inotrope): the ESPVR gets steeper and the LV empties more: ESV falls, SV and EF rise.

Tap a corner of the loop (MC, AO, AC, MO) to see the valve event.

- Current loop
- Normal loop
- ESPVR (contractility, Ees)
- EDPVR (passive filling)
- Ea (arterial afterload)
- Stroke volume (SV)

Compare stroke volume (loop width) with the normal, preload and afterload presets.

## Equations

**Cardiac output**CO = HR × SV

**HR**
heart rate

**SV**
stroke volume = EDV − ESV

**Fick principle**CO = V̇O2 / (CaO2 − CvO2)

**V̇O 2**
O₂ consumption (≈ 250 mL/min at rest)

**CaO 2 − CvO 2**
arteriovenous O₂ content difference (≈ 50 mL/L)

250 ÷ 50 ≈ 5 L/min.

**Hemodynamic Ohm's law**MAP − RAP = CO × SVR

**MAP**
mean arterial pressure

**RAP**
right atrial pressure

**SVR**
systemic vascular resistance

## Reference values (resting adult)

| Parameter | Value | Note |
| --- | --- | --- |
| Cardiac output | ≈ 4–8 L/min | ≈ 5 L/min typical |
| Cardiac index | ≈ 2.5–4 L/min/m² | |
| Stroke volume | ≈ 60–100 mL | |
| Maximal CO in a trained athlete | up to ≈ 30–35 L/min | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Regulation

- *+* **Sympathetic nerves (β1)**: Raise heart rate and contractility.
- *+* **Venous return / preload (Frank–Starling)**: More filling gives a larger stroke volume.
- *−* **Afterload (high arterial pressure, aortic stenosis)**: Lowers stroke volume.
- *−* **Vagus nerve**: Lowers heart rate (little effect on ventricular contractility).
- *−* **Hypoxia, acidosis, ischemia**: Reduce contractility.

## Clinical notes

- High-output states: anemia, sepsis (early), hyperthyroidism, pregnancy, arteriovenous fistula, beriberi.
- Low output with high SVR and cold skin points to cardiogenic or hypovolemic shock.

## Memory aid

Output is a bus line: buses per hour (rate) times passengers per bus (stroke volume); too many buses too close together leave no time to board.

## Test yourself

Think of the answer, then tap the question to see it.

## A person consumes 300 mL O₂/min, with arterial O₂ content 200 mL/L and mixed venous 140 mL/L. What is the cardiac output?

5 L/min: 300 ÷ (200 − 140) = 300 ÷ 60 = 5 L/min (Fick principle).

## Why does cardiac output fall when heart rate exceeds about 180/min in an adult?

Diastole shortens so much that the ventricle cannot fill; end-diastolic volume and stroke volume fall more than the rate rises.

## Over the long term, what mainly sets cardiac output in a healthy person?

Venous return, which depends on the tissues' metabolic needs (local vasodilation); the heart, through Frank–Starling, pumps what returns.

## Why does severe anemia raise cardiac output?

Low O₂ content causes tissue vasodilation and lower blood viscosity, so SVR falls and venous return rises; sympathetic activity also increases.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/cardiac-output-and-what-sets-it/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)
- [Right ventricle](https://durgical.com/en/anatomy/right-ventricle/)

## Related topics

- [The Frank–Starling mechanism](https://durgical.com/en/physiology/the-frank-starling-mechanism/)
- [Preload, afterload and contractility](https://durgical.com/en/physiology/preload-afterload-and-contractility/)
- [Venous return and the Guyton curves](https://durgical.com/en/physiology/venous-return-and-the-guyton-curves/)
- [Muscle blood flow and the cardiovascular response to exercise](https://durgical.com/en/physiology/muscle-blood-flow-and-the-cardiovascular-response-to-exercise/)
- [Shock physiology](https://durgical.com/en/physiology/shock-physiology/)

## Related conditions

- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Hypovolaemic shock](https://durgical.com/en/conditions/hypovolaemic-shock/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

## In Pharmacology

- [Beta blockers](https://durgical.com/en/pharmacology/classes/beta-blockers/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [19.4 Cardiac Physiology](https://openstax.org/books/anatomy-and-physiology-2e/pages/19-4-cardiac-physiology) (OpenStax (Rice University), 2022); [Hypovolemia and Hypovolemic Shock](https://www.ncbi.nlm.nih.gov/books/NBK513297/) (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 20). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/