Inhalation and Exhalation

What Is Inhalation?

Inhalation is the active phase of breathing in which the breathing muscles enlarge the chest, lowering the pressure inside the lungs so that air flows in from the atmosphere. It is also called inspiration, and it brings oxygen-rich air down the airways to the alveoli, where gas exchange takes place.

Inhalation definition

In biology, inhalation is the movement of air into the lungs caused by contraction of the inspiratory muscles. The key muscle is the diaphragm, a dome-shaped sheet of skeletal muscle under the lungs, helped by the external intercostal muscles between the ribs. In quiet breathing the diaphragm does most of the work, moving roughly three-quarters of the air that enters the lungs.

Why air flows in

Air always flows from higher to lower pressure. When the chest enlarges, the lungs, which cling to the chest wall through the thin film of fluid in the pleural cavity, expand with it. The same amount of air now fills a bigger space, so its pressure drops, a relationship known as Boyle's law, and outside air rushes in until the pressures are equal. The physics of these pressure changes is covered in more depth on our mechanics of breathing page.

Explained simply

Picture the chest as a box with a stretchy floor. When the floor (the diaphragm) pulls down and the walls (the ribs) swing out, the box gets bigger inside. The air inside is spread thinner, so air from outside is pulled in to fill the space, much like air rushing into a syringe when you pull back the plunger.

What Is Exhalation?

Exhalation is the phase of breathing in which the chest becomes smaller, raising the pressure inside the lungs above atmospheric pressure so that air flows out. It is also called expiration, and it carries carbon dioxide, water vapor and leftover air out of the body.

During quiet breathing, exhalation is a passive process: no muscle has to contract. The lungs contain elastic fibers that were stretched during inhalation, and the fluid film lining the alveoli creates surface tension that also pulls inward. When the inspiratory muscles relax, this stored elastic energy springs the lungs and chest wall back to their resting size, a process called elastic recoil.

Exhalation becomes active when the body needs to move air faster or empty the lungs more fully, for example during heavy exercise, coughing, singing a long note or blowing out candles. Then the internal intercostal muscles and the abdominal muscles contract to push the air out, as described in the section on forced breathing below.

Inhalation Process: What Are the Steps of Inhalation?

The inhalation process runs in a fixed order: the brainstem sends a signal, the diaphragm and external intercostals contract, the chest enlarges, pressure in the lungs falls and air flows in. Each step takes only a fraction of a second, and a quiet breath in usually lasts about one to two seconds.

1. The brain sends the signal. Breathing centers in the medulla fire nerve impulses down the phrenic nerve to the diaphragm and down the intercostal nerves to the rib muscles. The control of breathing page explains how this rhythm is set.
2. The diaphragm contracts. Its central tendon is pulled down, so the dome flattens and the chest becomes taller.
3. The ribs lift. The external intercostals pull each rib upward and outward. The lower ribs swing out sideways like a bucket handle, and the sternum moves forward like a pump handle, widening and deepening the rib cage.
4. The lungs expand. Because the visceral pleura on the lungs and the parietal pleura on the chest wall are held together by pleural fluid, the lungs follow the chest wall outward.
5. Pressure falls. Pressure in the pleural cavity becomes more negative, and pressure in the alveoli drops about 1 mmHg below the air pressure outside.
6. Air flows in. Air moves through the airways into the alveoli until alveolar pressure again equals atmospheric pressure, which marks the end of the breath in.

In a resting adult, one quiet inhalation draws in about 500 mL of air, the tidal volume. About 150 mL of it stays in the airways, the anatomical dead space, and never reaches the alveoli.

What Happens During Exhalation?

During exhalation the inspiratory muscles relax, the elastic lungs and chest wall recoil inward, the pressure in the alveoli rises above atmospheric pressure, and air flows out until the pressures are equal again.

1. The signal stops. The medulla stops firing the inspiratory nerves at the end of the breath in.
2. The diaphragm relaxes. It is pushed back up into its dome shape by the recoil of the lungs and the pressure of the abdominal organs below.
3. The ribs drop. The rib cage falls back down and in under its own weight and elasticity.
4. The lungs recoil. Stretched elastic fibers and alveolar surface tension pull the lungs back toward their resting size.
5. Pressure rises. Alveolar pressure climbs about 1 mmHg above atmospheric pressure.
6. Air flows out. Air leaves through the airways until the pressures balance, and the lungs settle at their resting volume until the next breath.

Why the lungs do not collapse

Even at the end of a full exhalation, air remains in the lungs. The chest wall tends to spring outward while the lungs tend to pull inward, and the slightly negative pressure in the pleural cavity holds them together. Pulmonary surfactant lowers surface tension in the alveoli, which keeps the smallest air sacs from closing at the end of each breath out.

How long does exhalation take?

At rest, exhalation usually lasts a little longer than inhalation, often in a ratio of about 1 to 2. Because no muscle effort is needed, the end of a quiet breath out is a natural resting point, which is why there is often a short pause before the next inhalation begins.

Inhalation vs Exhalation: What Is the Difference?

The main difference between inhalation and exhalation is that inhalation is an active process that enlarges the chest and draws air in, while quiet exhalation is a passive process in which the chest shrinks back and pushes air out. The two phases mirror each other in muscle action, chest size, pressure and the direction of airflow.

• Muscles: inhalation needs the diaphragm and external intercostals to contract; quiet exhalation only needs them to relax.
• Chest size: the thoracic cavity grows during inhalation and shrinks during exhalation.
• Pressure: alveolar pressure is slightly below atmospheric during inhalation and slightly above it during exhalation.
• Air: inhaled air is richer in oxygen; exhaled air carries more carbon dioxide and water vapor.
• Energy: inhalation always uses muscle energy; quiet exhalation uses the elastic energy stored during inhalation.

The terms inspiration and expiration mean the same as inhalation and exhalation. They should not be confused with respiration in the cellular sense, the use of oxygen inside cells, a distinction explained on our breathing vs respiration page. The table below follows one full breath, phase by phase.

How Is Quiet Breathing Different From Forced Breathing?

Quiet breathing uses only the diaphragm and external intercostals for inhalation and no muscles for exhalation, while forced breathing recruits extra accessory muscles for both phases to move larger volumes of air faster.

Forced inhalation

When the body needs more air, as during exercise, the diaphragm contracts more strongly and accessory muscles of the neck and chest join in. The scalene muscles and the sternocleidomastoid lift the upper ribs and sternum, and the pectoralis minor can help raise the ribs when the arms are braced. A deep breath in can draw in up to about 3 liters more than a quiet breath in an adult man, a reserve described on our lung volumes and capacities page.

Forced exhalation

Forced exhalation is active. The internal intercostal muscles pull the ribs down and in, and the abdominal muscles, including the rectus abdominis, the external and internal obliques and the transversus abdominis, squeeze the abdomen and push the diaphragm upward. This is what powers a cough, a sneeze, a shout or the hard breath out used in a breathing test such as spirometry.

The full list of breathing muscles, with their attachments and nerves, is on our muscles of respiration page. As breathing speeds up during exercise, both phases shorten, and exhalation can become as active as inhalation.

What Path Does Air Take During Inhalation and Exhalation?

During inhalation air travels from the nose or mouth through the pharynx, larynx, trachea, bronchi and bronchioles to the alveoli, and during exhalation it follows the same route in reverse. The full pathway of air has its own page, so here we focus on what each stage does to the air on the way in and out.

• Nose and nasal cavity: filter, warm and moisten the incoming air; on the way out they recover some of the heat and moisture.
• Pharynx: the shared passage for air and food behind the nose and mouth.
• Larynx: the vocal folds move apart slightly during inhalation, and exhaled air passing between them can produce the voice.
• Trachea and bronchi: cartilage keeps them open while mucus and cilia trap particles.
• Bronchioles: smaller airways whose smooth muscle adjusts their width; they widen slightly during inhalation as the lungs stretch.
• Alveoli: the tiny air sacs where oxygen enters the blood and carbon dioxide leaves it.

Breathing through the nose filters and conditions air better than breathing through the mouth, a comparison covered on our nose vs mouth breathing page.

How Is Inhaled Air Different From Exhaled Air?

Exhaled air contains less oxygen and much more carbon dioxide than inhaled air, and it is warmer and saturated with water vapor. Inhaled air is about 21% oxygen and 0.04% carbon dioxide, while exhaled air is about 16% oxygen and about 4% carbon dioxide.

The change happens in the alveoli, where the blood takes up part of the oxygen and releases carbon dioxide made by the body's cells during cellular respiration. Exhaled air is not pure alveolar air: it is mixed with the dead-space air that sat in the airways and never took part in the exchange, which is why it still holds a good deal of oxygen.

Nitrogen makes up about 78% of the air and passes in and out unchanged, because the body does not use it. The warmth and moisture added on the way in explain why breath forms a cloud on a cold day and fogs a mirror.

Common Conditions That Affect Inhalation and Exhalation

Inhalation and exhalation are affected by conditions that narrow the airways, stiffen the lungs, weaken the breathing muscles or let air or fluid into the pleural space. Our respiratory diseases section describes each condition, and anyone with breathing difficulty should see a doctor.

• Asthma: narrowed, inflamed airways make exhalation especially slow and can cause wheezing.
• COPD and emphysema: loss of elastic recoil and airway collapse trap air, so exhalation becomes slow and incomplete.
• Pulmonary fibrosis: scar tissue stiffens the lungs, so each inhalation takes more effort and moves less air.
• Pneumothorax: air in the pleural cavity breaks the link between the lung and chest wall, so the lung cannot expand normally.
• Pleural effusion: extra fluid in the pleural space presses on the lung and limits inhalation.
• Diaphragm weakness or paralysis: damage to the phrenic nerve or muscle disease reduces the main force of inhalation.

Conditions that block the airways tend to make exhalation harder, while those that stiffen the lungs or chest tend to make inhalation harder, the basis of the obstructive vs restrictive lung disease distinction.