---
title: "Henry’s Law | Anatomy and Physiology I"
description: "Henry’s law says gas dissolved in a liquid rises with the gas’s partial pressure, which explains oxygen and carbon dioxide exchange in Anatomy and Physiology I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/henrys-law"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 22"
---

# Henry’s Law | Anatomy and Physiology I

## Definition

Henry’s law says the amount of gas dissolved in a liquid depends on the gas’s partial pressure above it. In Anatomy and Physiology I, that idea explains how oxygen and carbon dioxide move between alveoli and blood.

## What It Is

Henry’s law is the rule that a gas dissolves in a liquid in proportion to the partial pressure of that gas above the liquid. In Anatomy and Physiology I, that liquid is usually blood plasma or the moisture lining the respiratory membranes, and the gas is usually oxygen or carbon dioxide.

The simple idea is this: if the partial pressure of a gas is higher in the air than in the liquid, more of that gas will move into the liquid and stay dissolved. If the partial pressure is lower, less gas remains dissolved. So Henry’s law is about equilibrium, not just motion, it tells you how much gas can be held in solution at a given pressure.

This matters in the lungs because inhaled air in the alveoli has a high oxygen partial pressure compared with deoxygenated blood arriving at the lung capillaries. Oxygen diffuses across the alveolar-capillary membrane and dissolves into the blood, where it can then be carried by hemoglobin. Carbon dioxide works the other direction, because its partial pressure is higher in venous blood than in alveolar air, so it moves into the alveoli to be exhaled.

Henry’s law is not the same thing as diffusion itself. Diffusion is the movement down a concentration or partial-pressure gradient, while Henry’s law describes how much gas will actually dissolve once it reaches a liquid. In other words, diffusion gets the gas to the liquid, and Henry’s law helps explain the dissolved amount you end up with.

You can see the same idea in a scuba example. At higher pressure, more nitrogen dissolves into blood and tissues. If pressure drops too fast, that dissolved gas can come out of solution as bubbles, which is why pressure changes matter so much for the body.

## Why It Matters

Henry’s law is one of the cleanest ways to explain gas exchange in the respiratory system. If you know how partial pressure affects gas solubility, the movement of oxygen into blood and carbon dioxide out of blood stops feeling like memorization and starts looking like a pressure problem.

It also connects the respiratory system to homeostasis. Your cells need oxygen for aerobic metabolism, and they produce carbon dioxide as waste. Henry’s law helps explain how the lungs keep blood oxygenated and keep carbon dioxide from building up, which is part of maintaining normal blood pH and stable body chemistry.

This term also shows up when you compare normal breathing with disease or impaired ventilation. If the alveoli are poorly ventilated or the respiratory membrane is damaged, gas exchange gets less efficient because the pressure relationships and solubility conditions are no longer working as well. That makes Henry’s law a useful bridge between anatomy, physiology, and clinical reasoning.

## Connections

### Partial Pressure

Henry’s law depends on partial pressure, because the amount of gas that dissolves changes with the pressure of that specific gas. In gas exchange questions, you usually compare oxygen or carbon dioxide partial pressures on each side of the respiratory membrane to predict the direction of movement.

### Gas Exchange

Gas exchange is the bigger process Henry’s law helps explain. Oxygen and carbon dioxide move across the alveoli and capillaries because of pressure differences, and Henry’s law tells you how much of that gas will stay dissolved in blood once it crosses.

### Respiratory Membranes

Respiratory membranes are the thin surfaces where gases cross between alveolar air and blood. Henry’s law matters there because the gases must first dissolve in the thin moist layer before they can diffuse across the membrane efficiently.

### [Carbonic Anhydrase](/anatomy-physiology/key-terms/carbonic-anhydrase)

Carbonic anhydrase helps blood handle carbon dioxide by speeding the conversion between CO2 and carbonic acid. That works alongside Henry’s law, since CO2 has to move into and out of solution in blood as it is transported and later exhaled.

## On the AP Exam

A quiz question may give you oxygen or carbon dioxide partial pressures and ask which way the gas will move, or why more gas dissolves at higher pressure. Your job is to connect the pressure gradient to solubility, then tie that to alveolar exchange. If you see a lung, blood, or scuba scenario, Henry’s law is usually the clue that the question is about gas dissolving in liquid, not just air moving in and out. In lab or class discussions, you might also explain why rapid pressure changes can cause gas bubbles to form in tissues.

## Henry’s law vs Diffusion

Henry’s law and diffusion are related, but they are not the same thing. Diffusion is the movement of gas from high to low partial pressure, while Henry’s law describes how much gas dissolves in a liquid at a given partial pressure. Diffusion drives the movement, and Henry’s law describes the dissolved result.

## Key Takeaways

- Henry’s law says the amount of gas dissolved in a liquid depends on the gas’s partial pressure above that liquid.
- In Anatomy and Physiology I, it helps explain how oxygen enters blood and carbon dioxide leaves blood in the lungs.
- The law is about solubility at equilibrium, not the physical act of moving across a membrane.
- Higher partial pressure means more gas can dissolve, which is why pressure changes matter in breathing and diving.
- When you connect Henry’s law to gas exchange, alveoli, and blood, you can trace the whole path from inhaled air to transported gases.

## FAQs

### What is Henry’s law in Anatomy and Physiology I?

Henry’s law says that the amount of a gas that dissolves in a liquid depends on the partial pressure of that gas above the liquid. In Anatomy and Physiology I, that explains why oxygen dissolves into blood in the lungs and why carbon dioxide leaves the blood to be exhaled.

### How does Henry’s law relate to gas exchange?

Gas exchange depends on partial pressure differences between alveolar air and blood. Henry’s law explains how those pressure differences affect how much oxygen or carbon dioxide can dissolve in the fluid lining the respiratory membranes and in blood plasma.

### Is Henry’s law the same as diffusion?

No. Diffusion is the movement of gas down a partial-pressure gradient, while Henry’s law describes gas solubility in a liquid. They work together in respiration, but they answer different questions.

### Why does pressure matter for dissolved gases in the body?

When pressure increases, more gas can stay dissolved in blood and tissues. When pressure drops too quickly, dissolved gas can come out of solution, which is why pressure changes matter in situations like diving or other rapid pressure shifts.

## Related Study Guides

- [22.4 Gas Exchange ](/anatomy-physiology/unit-22/4-gas-exchange/study-guide/c3RYm6yAnVDxt6Al)

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