Inspiration, Expiration and Pressure-Driven Breathing

Air movement requires a pressure difference

Air does not enter the lungs because the lungs actively suck it in like a pump. Air flows whenever pressure at the airway opening differs from pressure in the alveoli. If alveolar pressure is slightly lower than atmospheric pressure, air moves inward; when it becomes slightly higher, air moves outward. The diaphragm and chest-wall muscles alter the volume of the thoracic cavity, which changes lung volume and therefore pressure. This relationship is an application of the general tendency for gas pressure to decrease when the volume available to it increases.

Inspiration begins with muscle contraction

During a normal quiet inspiration, the dome-shaped diaphragm contracts and descends, increasing the vertical dimension of the thorax. The external intercostal muscles also contract, lifting the ribs and sternum and increasing the front-to-back and side-to-side dimensions of the chest. The parietal and visceral pleurae move together through their fluid coupling, transmitting the change to the lungs. Increased lung volume reduces alveolar pressure just below ambient atmospheric pressure, creating a small inward flow of air.

This pressure difference may be small but is sufficient to ventilate the healthy lung because the conducting airways are relatively open. At the end of a normal inspiration, alveolar pressure returns close to atmospheric pressure and airflow pauses, even though the lungs remain expanded. The muscle work is therefore used to change volume and overcome elastic recoil and airway resistance. The lungs contain elastic tissues that tend to recoil towards a smaller volume; inspiration must oppose this tendency.

Quiet expiration usually occurs by recoil

During an ordinary relaxed expiration, the diaphragm and external intercostal muscles relax. The stretched lung and chest-wall structures recoil, the diaphragm moves upward towards its dome shape, thoracic and lung volumes decrease, and alveolar pressure rises slightly above atmospheric pressure. Air then flows out. Because elastic recoil supplies much of the driving force, quiet expiration is usually described as passive, whereas quiet inspiration requires active muscle contraction. At the end of quiet expiration, airflow stops when the pressure difference disappears.

Forced expiration is different. When blowing hard, coughing or during strenuous exercise, abdominal muscles and internal intercostal muscle groups can contribute to reducing thoracic volume. Forced inspiration can recruit additional muscles of the upper chest and neck. The broad school-level distinction is active inspiration versus predominantly passive quiet expiration, but it should not be interpreted as meaning that no muscle can contribute to any expiration.

Pressure inside the pleural cavity

The pressure in the pleural space is normally below atmospheric pressure through most of a resting breathing cycle because of opposing tendencies of lung and chest wall. This negative intrapleural pressure helps keep the lungs expanded. When the diaphragm contracts, pleural pressure typically becomes more negative and the lungs expand. Alveolar pressure and intrapleural pressure are not interchangeable terms: the former determines airflow relative to atmosphere while the latter influences lung expansion.

A pneumothorax illustrates the importance of pleural coupling. If air enters the pleural cavity, the normal pressure relationship can be disrupted and the affected lung may collapse partly or fully. The key lesson is that chest expansion moves the lungs indirectly through the pleural arrangement, not through a skeletal muscle wrapped around individual alveoli. This clinical example should be used to understand mechanics, not as a substitute for medical assessment of sudden breathlessness.

Breathing rate, tidal volume and minute ventilation

A typical healthy adult at rest breathes roughly 12–16 times per minute in the textbook illustration, although normal rates vary with age and circumstances. A commonly used tidal volume is about 500 mL per breath. Multiplying these gives a minute ventilation of approximately 6000–8000 mL per minute, or 6–8 litres of total air moved per minute. This is total inspired or expired volume, not necessarily the amount of fresh air reaching alveoli because part of each breath occupies conducting airways.

During exercise, carbon dioxide production rises and oxygen consumption increases. Ventilation may increase through changes in breathing depth, frequency or both. This does not mean that increased breathing always increases blood oxygen content proportionally: in a healthy person, haemoglobin saturation is already high at rest. Ventilation also serves to regulate carbon dioxide and thus acid–base balance, an important link to neural control studied later.

Predict airflow from a pressure-volume statement

When a problem states that thoracic volume has increased, do not immediately label the entire event inspiration without checking the direction of the pressure gradient and what the respiratory muscles are doing. During an active inhalation, the expansion lowers alveolar pressure, causing inward flow until pressures equalise. During a passive exhalation the reverse sequence occurs. Identifying cause then effect—muscle action, volume, pressure, airflow—is the most reliable way to explain breathing.

Quick Quiz

Model Answers

1. Diaphragm/external intercostals contract → thoracic volume increases → lung expansion lowers alveolar pressure below atmospheric → air enters until pressures equalise.

2. Inspiratory muscles relax and elastic recoil reduces lung volume. Alveolar pressure briefly exceeds atmospheric pressure, so air leaves; most quiet expiration requires little additional muscle contraction.

3. Forced expiration recruits abdominal and internal intercostal muscles to reduce thoracic volume more strongly, whereas quiet expiration relies mainly on elastic recoil.

4. Minute ventilation = 14 × 500 mL = 7000 mL/min = 7 L/min. This is total ventilated air, not identical to fresh alveolar ventilation.

5. Alveolar pressure determines airflow into and out of lung air spaces; intrapleural pressure describes the pressure between pleural layers and influences lung expansion relative to chest-wall motion.