Mechanism of Ventilation

There are five muscles that make up the thoracic cage; the intercostals (external, internal and innermost), subcostals, and transversus thoracis. These muscles act to change the volume of the thoracic cavity during respiration.

There are some other muscles that do not comprise the thoracic wall, but do attach to it. These include the pectoralis major, minor, serratus anterior and the scalene muscles.

Intercostals

The intercostal muscles lie in the intercostal spaces between ribs. They are organised into three layers.

External Intercostals

There are 11 pairs of external intercostal muscles. They run inferoanteriorly from the rib above to the rib below, and are continuous with the external oblique of the abdomen.

  • Attachments: Originate at the lower border of the rib, inserting into the superior border of the rib below.
  • Actions: Elevates the ribs, increasing the thoracic volume.
  • Innervation: Intercostal nerves (T1-T11).

Internal Intercostals

These flat muscles lie deep to the external intercostals. Like the external intercostals, they run from the rib above to the one below, but in an opposite direction (inferoposteriorly). They are continuous with the internal oblique muscle of the abdominal wall.

  • Attachments: Originates from the lateral edge of the costal groove and inserts into the superior surface of the rib below.
  • Actions: The interosseous part reduces the thoracic volume by depressing the ribcage, and the interchondral part elevates the ribs.
  • Innervation: Intercostal nerves (T1-T11).

Innermost Intercostals

These muscles are the deepest of the intercostal muscles, and are similar in structure to the internal intercostals.

They are separated from the internal intercostals by the intercostal neurovascular bundle and are found in the most lateral portion of the intercostal spaces.

  • Attachments: Originates from the medial edge of the costal groove and inserts into the superior surface of the rib below.
  • Actions: The interosseous part reduces the thoracic volume by depressing the ribcage, and the interchondral part elevates the ribs.
  • Innervation: Intercostal nerves (T1-T11)

Transversus Thoracis

These muscles of the thoracic cage are continuous with transversus abdominis inferiorly.

  • Attachments: From the posterior surface of the inferior sternum to the internal surface of costal cartilages 2-6.
  • Actions: Weakly depress the ribs.
  • Innervation: Intercostal nerves (T2-T6).

Subcostals

The subcostal muscles are found in the inferior portion of the thoracic wall. They comprise of thin slips of muscle, which run from the internal surface of one rib, to second and third ribs below. The direction of the fibres parallels that of the innermost intercostal.

  • Attachments: These originate from the inferior surface of the lower ribs, near the angle of the rib. They then attach to the superior border of the rib 2 or 3 below.
  • Actions: Share the action of the internal intercostals
  • Innervation: Intercostal nerves

The Diaphragm

The diaphragm is a double-domed musculotendinous sheet, located at the inferior-most aspect of the rib cage. It serves two main functions:

  • **Separates the thoracic cavity from the abdominal cavity *(*the word diaphragm is derived from the Greek ‘diáphragma’, meaning partition).
  • Undergoes contraction and relaxation, altering the volume of the thoracic cavity and the lungs, producing inspiration and expiration.

The diaphragm is located at the inferior-most aspect of the ribcage, filling the inferior thoracic aperture. It acts as the floor of the thoracic cavity and the roof of the abdominal cavity. The attachments of diaphragm can be dividedinto peripheral and central attachments. It has three peripheral attachments:

  • Lumbar vertebrae and arcuate ligaments.
  • Costal cartilages of ribs 7-10 (attach directly to ribs 11-12).
  • Xiphoid process of the sternum.

The parts of the diaphragm that arise from the vertebrae are tendinous in structure, and are known as the right and left crura:

  • Right crus – Arises from L1-L3 and their intervertebral discs. Some fibres from the right crus surround the oesophageal opening, acting as a physiological sphincter to prevent reflux of gastric contents into the oesophagus.
  • Left crus – Arises from L1-L2 and their intervertebral discs.

The muscle fibres of the diaphragm combine to form a central tendon. This tendon ascends to fuse with the inferior surface of the fibrous pericardium. Either side of the pericardium, the diaphragm ascends to form left and right domes. At rest, the right dome lies slightly higher than the left – this is thought to be due to the presence of the liver.

Innervation and Vasculature

The halves of the diaphragm receive motor innervation from the phrenic nerve. The left half of the diaphragm (known as a hemidiaphragm) is innervated by the left phrenic nerve, and vice versa. Each phrenic nerve is formed in the neck within the cervical plexus and contains fibres from spinal roots C3-C5.

The majority of the arterial supply to the diaphragm is delivered via the inferior phrenic arteries, which arise directly from the abdominal aorta. The remaining supply is from the superior phrenic, pericardiacophrenic, and musculophrenic arteries. The draining veins follow the aforementioned arteries.

The Lungs and Breathing

The space between the outer surface of the lungs and inner thoracic wall is known as the pleural space. This is usually filled with pleural fluid, forming a seal which holds the lungs against the thoracic wall by the force of surface tension. This seal ensures that when the thoracic cavity expands or reduces, the lungs undergo expansion or reduction in size accordingly.

During breathing, the contraction and relaxation of muscles acts to change the volume of the thoracic cavity. As the thoracic cavity and lungs move together, this changes the volume of the lungs, in turn changing the pressure inside the lungs.

Boyle’s law states that the volume of gas is inversely proportional to pressure (when temperature is constant). Therefore:

  • When the volume of the thoracic cavity increases – the volume of the lungs increases and the pressure within the lungs decreases.
  • When the volume of the thoracic cavity decreases – the volume of the lungs decreases and the pressure within the lungs increases.

Process of Inspiration

Inspiration is the phase of ventilation in which air enters the lungs. It is initiated by contraction of the inspiratory muscles:

  • Diaphragm – flattens, extending the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – elevates the ribs and sternum, extending the anterior/posterior dimension of the thoracic cavity.

The action of the inspiratory muscles results in an increase in the volume of the thoracic cavity. As the lungs are held against the inner thoracic wall by the pleural seal, they also undergo an increase in volume.

As per Boyle’s law, an increase in lung volume results in a decrease in the pressure within the lungs. The pressure of the environment external to the lungs is now greater than the environment within the lungs, meaning air moves into the lungs down the pressure gradient.

Process of Passive Expiration

Expiration is the phase of ventilation in which air is expelled from the lungs. It is initiated by relaxation of the inspiratory muscles:

  • Diaphragm – relaxes to return to its resting position, reducing the superior/inferior dimension of the thoracic cavity.
  • External intercostal muscles – relax to depress the ribs and sternum, reducing the anterior/posterior dimension of the thoracic cavity.

The relaxation of the inspiratory muscles results in a decrease in the volume of the thoracic cavity. The elastic recoil of the previously expanded lung tissue allows them to return to their original size.

As per Boyle’s law, a decrease in lung volume results in an increase in the pressure within the lungs. The pressure inside the lungs is now greater than in the external environment, meaning air moves out of the lungs down the pressure gradient.

Forced Breathing

Forced breathing is an active mode of breathing which utilises additional muscles to rapidly expand and contract the thoracic cavity volume. It most commonly occurs during exercise.

Active Inspiration

Active inspiration involves the contraction of the accessory muscles of breathing (in addition to those of quiet inspiration, the diaphragm and external intercostals). All of these muscles act to increase the volume of the thoracic cavity:

  • Scalenes – elevates the upper ribs.
  • Sternocleidomastoid – elevates the sternum.
  • Pectoralis major and minor – pulls ribs outwards.
  • Serratus anterior – elevates the ribs (when the scapulae are fixed).
  • Latissimus dorsi – elevates the lower ribs.

Active Expiration

Active expiration utilises the contraction of several thoracic and abdominal muscles. These muscles act to decrease the volume of the thoracic cavity:

  • Anterolateral abdominal wall – increases the intra-abdominal pressure, pushing the diaphragm further upwards into the thoracic cavity.
  • Internal intercostal – depresses the ribs.

Innermost intercostal – depresses the ribs.

Babies can only breathe via abdominal breathing Newborn ribs more horizontal so can't use pump/bucket handle movements ➢ Intercostals weak • Abdominal breathing is done by contracting the diaphragm • As the diaphragm is located horizontally between the thoracic and abdominal cavities, air enters the lungs, and the thoracic cavity expands • Reliance on the diaphragm for breathing means there is a high risk for respiratory failure if the diaphragm is not able to contract

Respiratory Distress – Obligate nasal breathers until 4 – 6 wks
Neonates:
– Short neck & shorter, narrow airways – more
susceptible to airway obstruction / respiratory distress
– Tongue is larger in proportion to the mouth - more likely to obstruct airway if child unconscious
– Smaller lung capacity and underdeveloped chest muscles
– Have a higher respiratory rate – newborns ~60 breaths/min, early teens ~20-30 breaths/min

Use of accessory muscles in the neck whilst at rest can be a sign of respiratory distress. This is when the lungs fail to provide enough oxygen. Symptoms include Blue-coloured extremities, rapid & shallow breathing and rapid heart rate.

Neonatal Respiratory Distress Syndrome (NRDS)

  • Affects premature babies, if they are born before their lungs are fully developed and capable of working properly
  • Not enough surfactant
  • The more premature the baby, the more likely it is that s/he will have respiratory distress syndrome
  • Approximately half of all babies born before 28 weeks of pregnancy will develop NRDS
  • Leading cause of death in newborns (accounts for 20% of deaths)

Acute Respiratory Distress Syndrome (ARDS)

  • Fluid / proteins leak from the blood vessels into the alveoli (air sacs) Lungs become stiff and so don’t work normally
    Breathing becomes difficult
  • Mainly affects people over age of 75
  • Approximately 1 in 6,000 people per year affected in England
  • Common causes are an infection in the lungs e.g. pneumonia
  • Lung clots or injury (e.g. from a car crash) can also trigger the condition

Asthma

In an asthma exacerbation, the already narrowed airways (due to mucosal inflammation and smooth muscle hypertrophy) are further constricted due to increased smooth muscle tone.

This can decrease the diameter of the airways significantly, causing resistance to airflow to become very high. This means the patient must work harder to overcome the increased resistance. This can lead to turbulent flow, causing the characteristic wheeze of an asthma attack.

Beta-receptor agonists such as salbutamol can be given to reverse the constriction. This acts on beta-2 receptors in the airway, acting on Gs G-proteins to open potassium channels. Allowing potassium channels to open hyperpolarises the cell, making contraction of the smooth muscle less likely and effectively causing bronchodilation.

Emphysema

In emphysema, there is destruction of elastin fibres within alveoli. Therefore, there is less elastic recoil holding open the smaller airways, and thus reduced radial traction. This means that during expiration, when the intrathoracic pressure is greater, the smaller airways collapse very easily, trapping an increased volume of air.

In Chronic Obstructive Pulmonary Disease (COPD) the airway obstruction is compounded by chronic bronchitis. This causes additional narrowing of airway lumens. People with COPD often exhale through pursed lips in an effort to maintain a high intrapulmonary pressure and prevent premature collapse of the small airways.