Mechanics and Regulation of Breathing

The mechanics of inspiration and expiration

Air moves because of a pressure difference. Anything that increases the volume of the thoracic cavity lowers the pressure inside it below atmospheric pressure, and air flows in. Anything that decreases the volume raises the pressure and air flows out.

Inspiration at rest is an active process. The diaphragm contracts and flattens, the external intercostals contract to lift the ribs up and outwards, the thoracic cavity increases in volume, the pressure inside falls below atmospheric pressure and air rushes into the lungs.

Expiration at rest is passive. The diaphragm and external intercostals relax, the elastic tissue of the lungs and chest wall recoils, the thoracic cavity decreases in volume, the pressure inside rises above atmospheric pressure and air is forced out. No muscles are needed for quiet expiration.

The respiratory muscles and when each is used

Cambridge names seven muscles. Learn which phase each one belongs to. Three of them are the main muscles used at rest, and the other four are recruited only when breathing becomes deeper or more forceful, during exercise.

During exercise, the same muscles work harder and faster, and expiration becomes active: the internal intercostals pull the ribs down and in, and the rectus abdominis contracts to press the abdominal contents upwards against the diaphragm, forcing air out quickly.

During recovery, breathing rate and depth fall gradually. Active expiration continues for a short time while the demand for oxygen is still above resting level, then the accessory muscles switch off and quiet, passive expiration returns.

Breathing rate, tidal volume and minute ventilation

Breathing rate is the number of breaths taken in one minute, measured in breaths per minute. Tidal volume is the volume of air inspired or expired in one breath at rest, measured in litres. Minute ventilation is the volume of air inspired or expired in one minute, measured in litres per minute.

Minute ventilation is calculated with the same pattern as cardiac output:

Minute ventilation = breathing rate × tidal volume → MV = BR × TV

Both breathing rate and tidal volume rise during exercise, so minute ventilation rises faster than either one alone. Learn the values in these tables with their units; you will be asked to compare an untrained individual with a trained athlete at the same intensity.

• Trained athlete, rest: breathing rate 10–12 breaths per minute, tidal volume 0.5 litres, minute ventilation 5.0–6.0 litres per minute
• Trained athlete, sub-maximal intensity: breathing rate 15–20, tidal volume 2.0–2.5 litres, minute ventilation 30–50 litres per minute
• Trained athlete, maximal intensity: breathing rate 50–60, tidal volume 3.0–4.0 litres, minute ventilation 150–200 litres per minute
• The trained athlete's minute ventilation at rest is slightly lower than the untrained performer's, because a lower breathing rate multiplied by the same tidal volume gives a smaller total

Worked calculations

Use the equation in all three directions and keep the units consistent: breathing rate in breaths per minute and tidal volume in litres give minute ventilation in litres per minute.

• Untrained at rest: BR 15 breaths/min, TV 0.5 l → MV = 15 × 0.5 = 7.5 l/min
• Untrained at maximal intensity: BR 45 breaths/min, TV 2.5 l → MV = 45 × 2.5 = 112.5 l/min (inside the untrained maximal range 80–150 l/min)
• Trained at maximal intensity: BR 55 breaths/min, TV 3.5 l → MV = 55 × 3.5 = 192.5 l/min (inside the trained maximal range 150–200 l/min)
• Rearranged, to find tidal volume: MV 40 l/min at a breathing rate of 20 breaths/min → TV = 40 ÷ 20 = 2.0 litres
• Rearranged, to find breathing rate: MV 45 l/min with a tidal volume of 1.5 litres → BR = 45 ÷ 1.5 = 30 breaths per minute

The other lung volumes and how to read a spirometer trace

The remaining quantities are volumes the lungs can hold and move. Inspiratory reserve volume (IRV) is the extra volume of air that can be inspired above tidal volume with a maximal inspiration. Expiratory reserve volume (ERV) is the extra volume of air that can be expired below tidal volume with a maximal expiration. Residual volume is the volume of air that remains in the lungs after a maximal expiration and cannot be expired. Vital capacity is the maximum volume of air that can be expired after a maximal inspiration. Total lung volume is the total volume of air the lungs can hold.

• Vital capacity = tidal volume + inspiratory reserve volume + expiratory reserve volume
• Total lung volume = vital capacity + residual volume
• Residual volume cannot be read from a simple spirometer trace, because the air never leaves the lungs
• On a trace at rest, tidal volume appears as small, regular waves above and below the resting level
• On the same trace, a deep inspiration shows the IRV above the tidal wave and a forced expiration reaches down to the ERV below it

Regulation of breathing

Breathing is controlled by the respiratory control centre in the medulla oblongata of the brain. It contains an inspiratory centre and an expiratory centre, and it sets the rate and depth of breathing by sending impulses to the diaphragm and the intercostal muscles. Normally these impulses are involuntary, which is why you keep breathing while asleep.

The main stimulus is carbon dioxide. Chemoreceptors detect an increase in the carbon dioxide concentration and a fall in pH in the blood and send impulses to the respiratory control centre, which increases both the rate and the depth of breathing to remove the extra carbon dioxide. Adrenaline also raises breathing rate and depth as part of hormonal control.

• Chemoreceptors — detect changes in carbon dioxide, oxygen and pH and are the main driver of breathing during exercise
• Thermoreceptors — detect changes in temperature; a rise in temperature increases breathing rate
• Mechanoreceptors — detect the movement and stretch of muscles and joints, contributing to the anticipatory rise before exercise
• Stretch receptors — detect over-inflation of the lungs and trigger the Hering–Breuer reflex
• Hering–Breuer reflex: when the lungs are stretched, the stretch receptors send impulses that inhibit the inspiratory centre so that inspiration stops and expiration begins. This prevents over-inflation of the lungs
• Hormonal control: adrenaline increases breathing rate and depth before and during exercise
• Sympathetic nervous system — increases breathing rate and depth during exercise; the parasympathetic nervous system — reduces them during rest and recovery

Responses to exercise and changes caused by long-term training

Immediately before exercise there is an anticipatory rise in breathing rate and depth, caused by adrenaline and by impulses from mechanoreceptors as the muscles start to work.

During exercise breathing rate and tidal volume both rise, so minute ventilation rises sharply from around 6–7.5 l/min at rest to 80–150 l/min in an untrained performer at maximal intensity. The deeper and faster breathing increases the diffusion gradient at the alveoli, bringing more oxygen in and removing more carbon dioxide.

During recovery breathing rate and depth fall back towards resting values. The speed of recovery is faster in a trained athlete.

• Long-term training increases the strength of the respiratory muscles, so a larger tidal volume can be achieved with less effort
• Respiratory muscle strength also means active expiration is more effective, so more air can be expired per breath
• Trained athletes breathe less frequently at rest (10–12 breaths per minute) but much more deeply at maximal intensity (tidal volume 3.0–4.0 litres)
• Maximum minute ventilation rises to 150–200 l/min, so more oxygen can be delivered to the muscles and more carbon dioxide removed
• Vital capacity increases slightly and recovery of breathing rate is faster after exercise