Mechanics of Breathing
Last Updated: 24th May 2018
- 1 The Lungs and Breathing
- 2 Inspiration
- 3 Passive Expiration
- 4 Forced Breathing
- 4.1 Forced inhalation
- 4.2 Forced expiration
- 5 Clinical relevance – Phrenic nerve palsy
The processes of inspiration and expiration are vital for providing oxygen to tissues and removing carbon dioxide from the body. Inspiration occurs via contraction of muscles such as the diaphragm, whereas expiration tends to be passive at rest. This process changes slightly when breathing is forced.
This article shall discuss the process of inspiration and expiration at rest, how this changes during forced breathing and relevant clinical conditions.
The Lungs and Breathing
The lungs are enveloped in parietal and visceral pleura and the space between the lungs and thoracic wall is called the pleural space. This is usually filled with pleural fluid which forms a pleural seal that holds the outer surface of the lungs against the inner surface of the thoracic wall. This ensures that when the thoracic cavity expands or reduces, the lungs move with it due to the surface tension of the pleural fluid forming the pleural seal.
Therefore, the contraction and relaxation of certain muscles during breathing causes movement of the lungs, changing the volume of air within the lungs.
Boyle’s law states that, when temperature is constant, the volume of gas is inversely proportional to pressure. Therefore, when the lungs expand increasing the volume of air within them, pressure declines. When the pressure of the air outside the lungs is greater than the air inside, air will rush into the lungs, and vice versa.
Inspiration allows air to be moved into the lungs and requires the contraction of various muscles:
- The diaphragm and external intercostal muscles contract
- When the diaphragm contracts, it flattens, pressing down on the abdominal contents and lifting the thoracic cavity. This leads to an increase in the volume of the thoracic cavity.
The diaphragm is the most important muscle in inspiration as it amounts to 60 to 80% of the work in ventilation.
- Contraction of the external intercostal muscles leads to an elevation of the ribs and sternum
- These actions cause an increase in the volume of the lungs
- According to Boyle’s law, an increase in the volume of air results in a decrease in the pressure of air within the lungs
- Pressure outside the lungs is larger compared to the inside and air rushes into the lungs as gas molecules move from an area of high pressure to low pressure
Expiration allows the removal of air from the lungs and at rest is passive, relying on elastic recoil:
- The diaphragm and external intercostal muscles relax and return to their resting position
- The elastic recoil of the stretched lungs causes them to recoil back to their original volume rather than due to an active movement
- Due to Boyle’s law, a reduced volumeof air leads to an increased pressure of air within the lungs
- When the pressure within the lungs is greater than the outside, air rushes out of the lungs as gas molecules move from an area of high pressure to low pressure
Forced breathing requires active inspiratory and expiratory effort with the help of accessory muscles.
This is similar to normal inspiration (diaphragm and external intercostals) but requires effort from the inspiratory accessory muscles such as scalenes, sternocleidomastoid, pectoralis major and minor, serratus anterior and latissimus dorsi.
Unlike normal expiration, this is an active process. It involves contraction of the abdominal muscles which forces the diaphragm upwards reducing the volume of the thoracic cavity. It also requires contraction of the internal intercostal muscles and innermost intercostal muscles which pull the ribs downwards. Both these actions contribute to a decreased thoracic volume and pressure is inversely proportional to volume. Therefore pressure within the lungs increases forcing the air out quicker than in normal expiration.
Clinical relevance – Phrenic nerve palsy
The phrenic nerve (C3, C4, and C5) innervates the diaphragm and if this nerve is damaged, the movements of the diaphragm are disrupted and in severe cases the diaphragm may be paralysed. Since it is the most important muscle in inhalation (60 to 80% of the effort), breathing is severely affected and patient may report symptoms of breathless and even respiratory distress.
Causes of this include:
- Mechanical trauma – ligation or damage to the nerve that typically occurs during surgery
- Compression – for example by a tumour in the chest cavity
- Myopathies – such as myasthenia gravis
- Neuropathies – such as diabetic neuropathy
If possible the underlying cause should be identified and treated, although this may not be possible in the case of surgical damage for example. Management also consists of providing symptomatic relief, which typically occurs through non-invasive ventilation, such as a CPAP (continuous positive airway pressure) machine.
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Mechanism Of Breathing
The action of breathing in and out is due to changes in pressure within the chest (thorax). This action is also known as external respiration and is created by the muscles of the chest and the diaphragm changing the size of the chest cavity (and air pressure). Here we explain the mechanics of breathing and how breathing is regulated at rest and during exercise.
Mechanics of breathing
When we inhale the intercostal muscles (between the ribs) and diaphragm contract to expand the chest cavity. The diaphragm flattens and moves downwards and the intercostal muscles move the rib cage upwards and out. This increase in size decreases the internal air pressure and so air from the outside (at a now higher pressure than inside the thorax) rushes into the lungs to equalise the pressures.
When we exhale the diaphragm and intercostal muscles relax and return to their resting positions. This reduces the size of the thoracic cavity, thereby increasing the pressure and forcing air out of the lungs.
The rate at which we inhale and exhale is controlled by the respiratory centre, within the Medulla Oblongata in the brain. Inspiration occurs due to increased firing of inspiratory nerves and so the increased recruitment of motor units within the intercostals and diaphragm. Exhalation occurs due to a sudden stop in impulses along the inspiratory nerves.
Our lungs are prevented from excess inspiration due to stretch receptors within the bronchi and bronchioles which send impulses to the Medulla Oblongata when stimulated.
Breathing rate is all controlled by chemoreceptors within the main arteries which monitor the levels of Oxygen and Carbon Dioxide within the blood. If oxygen saturation falls, ventilation accelerates to increase the volume of Oxygen inspired.
If levels of Carbon Dioxide increase a substance known as carbonic acid is released into the blood which causes Hydrogen ions (H+) to be formed. An increased concentration of H+ in the blood stimulates increased ventilation rates. This also occurs when lactic acid is released into the blood following high-intensity exercise.
Regulation of breathing
Respiration is controlled by the autonomic nervous system, which enables us to alter our breathing without thinking about it. The autonomic nervous system consists of two branches, the sympathetic nervous system (the pedals) and the parasympathetic nervous system (the breaks).
At rest, we inspire approximately 500 ml of air per breath and on average we breathe 12-15 times per minute. The volume of air we breathe in or out per breath is known as tidal volume, the volume of air we breathe in or out per minute is known as minute ventilation.
Our minute ventilation is calculated as follows:
Minute ventilation = tidal volume (500 ml) x breathing rate (15) = 7500ml/min (7.5 l/min)
Our respiration is coordinated by the respiratory centre in the medulla oblongata of the brain. The medulla oblongata could be considered “the boss” and controls many important functions in the body. During inspiration (breathing in), nerve impulses are sent via the phrenic and intercostal nerves which stimulates the inspiratory muscles, the external intercostal and diaphragm, causing them to contract, this stimulation lasts for approximately two seconds, after which, the inspiratory muscles relax and expiration occurs. Expiration is a passive process
Regulation of breathing at rest
- Medulla oblongata controls breathing
- Phrenic and intercostal nerves stimulate the external intercostal muscles and diagram
- Stimulation causes these muscles to contract
- Contraction of these muscles results in inspiration
- Stimulation ceases, muscles relax and expiration occurs
- Regulation of breathing during exercise
During exercise, a significant rise in minute ventilation occurs, this is due to an increased oxygen demand from the working muscles. Both tidal volume and breathing rate increase to compensate for an increased oxygen demand, therefore increasing minute ventilation.
Central to the increase in rate and depth of breathing during exercise are a series of receptors. Of particular importance are the chemoreceptors, which are found in the aortic arch and carotid arteries and detect blood acidity. Chemoreceptors detect blood acidity by monitoring the concentrations of oxygen and carbon dioxide. During exercise, the chemoreceptors detect a rise in carbon dioxide, a by-product of increased respiration, and a reduction in oxygen.
The chemoreceptors, send a nerve impulse to the medulla oblongata, which subsequently stimulates the sympathetic nervous system (the pedals) to increase breathing rate and depth.
Proprioceptors detect movement in the joints and muscles. During exercise, the proprioceptors detect a rise in movement and therefore oxygen demand, and send a nerve impulse to the medulla oblongata, which stimulates the sympathetic nervous system to increase breathing rate and depth.
During exercise, the depth of breathing is increased through the stimulation of three additional muscles. In addition to the external intercostal muscles and diaphragm, the sternocleidomastoid, scalene and pectoralis minor are stimulated to lift the ribs and sternum further, increase the volume of the thoracic cavity, allowing an increase in the depth of breathing.
Breathing frequency is also increased during exercise due to the expiratory centre being activated and stimulating the expiratory muscles, the abdominals and internal intercostal muscles, making expiration an active process and increasing the rate of expiration. Stretch receptors prevent over-inflation of the lungs, so if lungs are excessively stretched, the expiratory centre sends impulses to induce expiration.
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The function of the human respiratory system is to transport air into the lungs and to facilitate…
VO2 max is the measure of the peak volume of Oxygen (VO2) you can consume and use in a minute and…