---
title: "External Intercostals | Anatomy and Physiology I"
description: "External intercostals are the rib muscles that lift the thorax during inspiration, expanding chest volume and helping air move into the lungs."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/external-intercostals"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 11"
---

# External Intercostals | Anatomy and Physiology I

## Definition

External intercostals are the muscles between the ribs that contract during inspiration to lift the rib cage up and out. In Anatomy and Physiology I, they are part of the mechanics of breathing.

## What It Is

External intercostals are the outer layer of muscles between each rib in Anatomy and Physiology I. Their main job is to help you inhale by lifting the rib cage up and outward, which increases the size of the thoracic cavity.

When these muscles contract, the ribs do not just move straight up. They also swing outward, which increases both the front-to-back and side-to-side dimensions of the chest. That extra space lowers pressure inside the lungs compared with the outside air, so air flows in.

This works with the diaphragm. The diaphragm flattens downward while the external intercostals stabilize and lift the ribs, so the chest cavity gets larger from two directions at once. If you picture breathing as a pressure change problem, the external intercostals are part of the muscle group that creates the pressure drop for inspiration.

You will usually see this muscle group described as active during quiet inhalation and even more noticeable during deeper breathing. If you are taking a lab or looking at a diagram, the external intercostals sit between adjacent ribs and run in a downward and forward direction. That fiber direction matches their action, because when the muscle shortens it helps raise the rib cage.

A common mistake is mixing them up with the internal intercostals. The external intercostals mainly help with inspiration, while internal intercostals are more associated with forced expiration in many textbook models. So if a question asks which muscle helps expand the thoracic cavity during normal breathing, external intercostals are the answer.

They matter because breathing is not just about one muscle moving one bone. It is a coordinated mechanical process that changes thoracic volume, shifts pressure, and moves air toward the alveoli where gas exchange can happen.

## Why It Matters

External intercostals show up anywhere you have to explain how air gets into the lungs. In Anatomy and Physiology I, they connect muscular anatomy to respiratory physiology, which is one of the big course themes. You are not just naming a muscle, you are tracing a cause-and-effect chain: muscle contraction, rib movement, thoracic expansion, pressure drop, airflow.

That chain also helps you make sense of why breathing is an active process during inspiration. The lungs do not inflate by themselves. They respond to changes in the volume of the thoracic cavity, and the external intercostals are one of the muscles that creates that volume change.

This term also helps when you compare quiet breathing to forced breathing. During exercise, coughing, or other conditions that increase demand, the body recruits more muscles and changes the pattern of rib movement. If you know what external intercostals do at rest, it is easier to spot when that normal pattern is being pushed harder or replaced by other mechanics.

The concept shows up in respiratory diagrams, muscle charts, and questions about rib cage movement. It also connects to clinical thinking, because anything that limits chest wall expansion can make ventilation less efficient. So this term is a small anatomy label with a big physiology payoff.

## Connections

### Inspiration

External intercostals are one of the main muscles used in inspiration. When they contract, they raise the ribs and help expand the thoracic cavity, which lowers pressure in the lungs and draws air inward. If a question asks what happens during inhalation, external intercostals are part of the mechanism, not just a label on a diagram.

### Expiration

Expiration is the phase students often compare with external intercostals because the muscle’s main action is not to push air out during quiet breathing. In a normal breath out, the muscles relax and the thoracic cavity recoils. In forced expiration, other muscles do more of the work, so this term helps you separate passive exhalation from active chest wall movement.

### Intercostal Muscles

External intercostals are one member of the broader intercostal muscle group. That larger category includes muscles between the ribs that support rib motion and breathing mechanics, but the external layer has a specific fiber direction and a specific role in lifting the rib cage. Knowing the group name helps you place the muscle in the thorax instead of treating it as a standalone structure.

### [Alveolar Air](/anatomy-physiology/key-terms/alveolar-air)

External intercostals do not perform gas exchange themselves, but they help create the airflow that refreshes alveolar air. By expanding the chest and pulling air into the lungs, they help maintain the oxygen and carbon dioxide gradients needed at the alveoli. That makes them part of the setup for gas exchange, even though the exchange happens later at the alveolar-capillary membrane.

## On the AP Exam

A labeling question may ask you to identify the muscles between the ribs on a chest diagram, especially if the arrows point to the outer intercostal layer. A short-answer item may ask which muscle group helps raise the rib cage during inhalation, or why thoracic volume increases when you breathe in. In a lab practical, you might need to point to the muscle location and explain how its fiber direction matches its action. If a respiratory case asks why breathing is shallow or painful, you may connect impaired chest wall movement to reduced ventilation. The key move is to trace structure to function, then function to airflow.

## External Intercostals vs Internal Intercostals

These two muscle layers are easy to mix up because both sit between the ribs. External intercostals are the outer layer and mainly help with inspiration by lifting the rib cage. Internal intercostals are the deeper layer and are usually taught as helping with forced expiration. If you remember outer layer equals inhale, deeper layer equals forced exhale, you will avoid a common test mistake.

## Key Takeaways

- External intercostals are the outer muscles between the ribs, and their main job in Anatomy and Physiology I is to help with inhalation.
- When they contract, the ribs move up and out, which increases thoracic volume and lowers pressure inside the lungs.
- They work with the diaphragm to make inspiration efficient, especially during normal breathing.
- They are different from internal intercostals, which are usually associated with forced expiration rather than quiet inhalation.
- If you can trace the path from muscle contraction to rib movement to pressure change, you know the concept well.

## FAQs

### What is external intercostals in Anatomy and Physiology I?

External intercostals are the outer muscles between the ribs that help you inhale. They contract to lift the rib cage up and outward, increasing thoracic cavity volume and helping air move into the lungs. In A&P, they are part of the mechanics of breathing.

### What do the external intercostals do during inhalation?

During inhalation, they contract and raise the ribs. That widens the chest cavity in multiple directions, which lowers intrapulmonary pressure so air flows into the lungs. They work with the diaphragm instead of replacing it.

### How are external intercostals different from internal intercostals?

External intercostals are the more superficial layer and are mainly tied to inspiration. Internal intercostals are deeper and are usually associated with forced expiration in textbook anatomy. The names are easy to mix up, so layer and action are the two best clues.

### Why do external intercostals matter for breathing?

They help create the pressure change that pulls air into the respiratory system. Without rib cage elevation, the thoracic cavity would not expand as effectively, and ventilation would be less efficient. That makes them a core part of the breathing mechanism, not just a muscle identification term.

## Related Study Guides

- [11.4 Axial Muscles of the Abdominal Wall, and Thorax ](/anatomy-physiology/unit-11/4-axial-muscles-abdominal-wall-thorax/study-guide/CisJDZEKv0MQe4ra)
- [22.4 Gas Exchange ](/anatomy-physiology/unit-22/4-gas-exchange/study-guide/c3RYm6yAnVDxt6Al)
- [22.3 The Process of Breathing ](/anatomy-physiology/unit-22/3-process-breathing/study-guide/oe2Sz6Yq9gIM9Ca3)

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