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

# Preload, afterload and contractility

- Concept
- Exam priority: medium

How much each beat pumps depends on how full the heart is, how hard it must push, and how strong the muscle is.

Updated 2026-11-01

Also called: Inotropy, Determinants of stroke volume

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

## Key points

- Preload ≈ end-diastolic volume (or wall stress at end-diastole); rises with venous return, blood volume, atrial kick; falls with venodilators, diuretics, standing.
- Afterload ≈ ventricular wall stress during ejection; mostly set by aortic pressure (SVR) and aortic valve area; LaPlace: σ = P × r / 2h.
- Contractility (inotropy) = force independent of loading; ↑ by sympathetic β1, Ca²⁺, digoxin; ↓ by ischemia, acidosis, hypoxia, beta blockers, many anesthetics.
- Hypertrophy (thicker wall) is the ventricle's response to high afterload: it lowers wall stress but stiffens the chamber.
- SV ↑ with preload and contractility; SV ↓ with afterload.

## What it is

Stroke volume has three determinants. Preload is the stretch of the ventricle at the end of filling; afterload is the load the ventricle must overcome to eject; contractility is the strength of contraction at a given preload and afterload. Each can be changed by physiology, disease and drugs, and each shifts the pressure–volume loop in its own way.

### Left-ventricular pressure–volume loop

**EDV** 120 mL **ESV** 65 mL **SV** 55 mL **EF** 46% **ESP** 139 mmHg

#### ↑ Afterload

↑ Afterload (e.g. hypertension): the Ea line is steeper and the aortic valve opens at a higher pressure; the loop gets taller and narrower: ESV rises and SV falls.

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)

Switch between the preload, afterload and contractility presets to compare how each moves the loop.

## Equations

**Law of LaPlace (wall stress)**σ = P × r / (2h)

**σ**
wall stress

**P**
intraventricular pressure

**r**
chamber radius

**h**
wall thickness

A dilated ventricle (large r) faces more afterload at the same pressure; a thick wall faces less.

## The three determinants side by side

| Determinant | Best index | Raised by | Lowered by |
| --- | --- | --- | --- |
| Preload | End-diastolic volume | Fluids, venoconstriction, lying down | Bleeding, diuretics, venodilators, standing |
| [Afterload](https://durgical.com/en/conditions/aortic-valve-stenosis/) | Wall stress in systole ≈ aortic pressure | Hypertension, aortic stenosis, vasoconstriction | Arterial vasodilators, low SVR (sepsis) |
| [Contractility](https://durgical.com/en/conditions/heart-failure/) | Slope of end-systolic P–V line (ESPVR) | Sympathetic β1, Ca²⁺, digoxin | Ischemia, acidosis, beta blockers, heart failure |

## Clinical notes

- Afterload reduction (ACE inhibitors, vasodilators) increases forward stroke volume in failing ventricles and in mitral regurgitation.
- In aortic stenosis the afterload sits at the valve, so lowering SVR does not relieve the ventricle and can drop pressure dangerously.

## Memory aid

Preload is how full the balloon is, afterload is how tight the nozzle is, contractility is how hard you squeeze.

## Test yourself

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

## Why does a dilated ventricle face higher afterload than a normal one at the same arterial pressure?

By the law of LaPlace, wall stress rises with chamber radius; a larger radius means more stress per unit of wall for the same pressure.

## Which determinant does a venodilator mainly change, and how does stroke volume respond in a healthy heart?

Preload falls because blood pools in the veins; on the steep part of the Starling curve, stroke volume falls.

## How does concentric hypertrophy help, and what does it cost?

A thicker wall lowers wall stress (afterload) at high pressure, but the stiff chamber fills poorly, raising diastolic pressure (diastolic dysfunction).

## Why is digoxin called a positive inotrope?

By inhibiting the Na⁺/K⁺ pump it raises intracellular Na⁺, slows Ca²⁺ removal by the Na⁺/Ca²⁺ exchanger, and increases SR Ca²⁺ stores, so contraction is stronger.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/preload-afterload-and-contractility/)

## See it in 3D

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

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

## Related topics

- [The Frank–Starling mechanism](https://durgical.com/en/physiology/the-frank-starling-mechanism/)
- [The ventricular pressure–volume loop](https://durgical.com/en/physiology/the-ventricular-pressure-volume-loop/)
- [Cardiac output and what sets it](https://durgical.com/en/physiology/cardiac-output-and-what-sets-it/)
- [Heart failure physiology](https://durgical.com/en/physiology/heart-failure-physiology/)
- [Cardiac excitation–contraction coupling](https://durgical.com/en/physiology/cardiac-excitation-contraction-coupling/)

## Related conditions

- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Aortic Valve Stenosis](https://durgical.com/en/conditions/aortic-valve-stenosis/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Mitral regurgitation](https://durgical.com/en/conditions/mitral-regurgitation/)

## In Pharmacology

- [ACE inhibitors](https://durgical.com/en/pharmacology/classes/ace-inhibitors/)
- [Digoxin](https://durgical.com/en/pharmacology/digoxin/)
- [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); [Heart Failure (Congestive Heart Failure)](https://www.ncbi.nlm.nih.gov/books/NBK430873/) (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 9). 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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Source: https://durgical.com/en/physiology/preload-afterload-and-contractility/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/preload-afterload-and-contractility/