The physiology of respiration (Advanced)

Week 2

Context

To assess the airway and breathing, recognise issues and manage them, requires a sound understanding of respiratory physiology.

It is not possible to replicate the teaching of physiology as required for most health care professionals within this short course, but rather to refresh what is known, or to provide links to learning materials outside of the course if needed.

Content

The content considered for the ABCDE assessment of a deteriorating patient is divided into three sections: ventilation; exchange of gasses; and the regulation of respiration.

Ventilation

To recap, the process of breathing is the supply of oxygen and the elimination of carbon dioxide, Chest expansion that takes place during breathing occurs as a result of muscular activity which is partly voluntary but mainly involuntary. There are 11 pairs of intercostal muscles occupying the spaces between the 12 pairs of ribs- spilt into the :

External Intercostal muscles – which extend downwards and forwards from the lower edge of the rib above to the upper edge of the rib below. These muscles are involved in inspiration.

Internal Intercostal muscles- extending downwards and backwards from the lower edge of the rib above to the upper edge of the rib below, crossing the external muscle fibres at right angles. These muslces are used for active exhalation.

Intercostal muscles are stimulated to contract by the intercostals nerves. The external intercostals muscles and the diaphragm contract simultaneously during inspiration, resulting in the enlargement of the thoracic cavity in all directions.

The diaphragm is the main muscle of inspiration, with a nerve supply from C3, C4 and C5 via the phrenic nerves. When stimulated the diaphragm moves downwards increasing the size of the thoracic cavity, which creates a negative pressure and draws air into the lungs. Note that spinal cord injured patients, breaks above C3 and C4 will affect the phrenic nerve and result in apnoea. The position of the diaphragm varies depending on posture, in an upright position the diaphragm flattens and the cross sectional area is increased, with a smaller movement required to achieve expansion, hence why patients with respiratory diseases find it more comfortable to sit upright.

In addition to the muscles described, in a deteriorating patient, we assess the use of accessory muscles in respiration. Use of the sternocleidomastoid muscle and scalene muscles in respiration indicates forced breathing is taking place and these muscles are supporting the patient during inspiration. This is a sign of respiratory distress or failure So far, we have identified that the muscles of respiration, the diaphragm and the chest wall all work together to expand the thorax during inspiration and inflate the lungs, then they reduce their effort causing the thorax to contract and the lungs to deflate during expiration.

Pulmonary ventilation (the process for moving gases in and out of the lungs) In a healthy patient the amount of air that can be accommodated will depend on the size of the lungs and thorax, related to size, age, ethnicity and sex- referred to as lung capacity. Some terms on lung volumes and capacities are as follow:

Lung volumes and capacities

Tidal Volume (VT, TV): volume of gas exchanged each breath; can change as ventilation pattern changes
Inspiratory Reserve Volume (IRV): maximum volume that can be inspired, starting from the end inspiratory position (potential volume increase at the end of inspiration)
Expiratory Reserve Volume (ERV): maximum volume that can be expired, starting from the end expiratory position (potential volume decrease at the end of expiration)
Residual Volume (RV): volume remaining in the lungs and airways following a maximum expiratory effort (note: lungs cannot empty completely because of (1) stiffness when compressed and (2) airway collapse and gas trapping at low lung volumes)
Vital Capacity (VC): maximum volume of gas that can be exchanged in a single breath VC = TV + IRV + ERV
Total Lung Capacity (TLC): maximum volume of gas that the lungs (and airways) can contain TLC = VC + RV = TV + IRV + ERV + RV
Functional Residual Capacity (FRC): volume of gas remaining in the lungs (and airways) at the end expiratory position FRC = RV + ERV
Inspiratory capacity (IC): maximum volume of gas that can be inspired from the end expiratory position IC = TV + IRV

Lung Volumes & Capacities

Measuring rates of airflow

There are two measures of this, which include Forced expiratory volume – measured using a spirometer (vitalograph) to ascertain the amount of air forcibly expelled from a maximal breath in 1 second. The second is PEFR (peak expiratory flow rate) commonly measured in respiratory patients using a peak flow meter.

Exchange of Gasses

Exchange of gases occurs across a semi-permeable membrane by way of diffusion. Remember that diffusion means moving something from an area of high concentration to an area of low concentration. Diffusion of oxygen and carbon dioxide depends on pressure differences, for example between atmospheric air and the blood, or blood and the tissues. There are two occurrences of gaseous exchange: external and internal respiration:

External Respiration. This refers to the process that takes place between the alveolar and the alveolar capillaries. Each alveolar wall is one cell thick and and a network of capillaries surrounds them, making the huge surface area for the diffusion to take place. The venous blood from the body contains high levels of CO₂ and low levels of O₂. Therefore the CO₂ moves across the membrane into the alveoli from a high concentration to a low concentration, whilst the O₂, which is in high concentration in the alveoli moves across into the capillaries for distribution back to the body tissues. The flow of blood through the capillaries is relatively slow allowing more time for the diffusion to take place.

Internal Respiration. Internal respiration is a metabolic process and refers to the exchange of gases between the blood and the individual cells within the body tissue. Oxygen is delivered to the cells through diffusion across the capillary wall. At this point the O₂ concentration is higher in the capillaries so moves across via diffssion into the cells, at the same time the high CO₂ concentration in the cells move across into the capillaries. Note that gaseous exchange cannot take place from arteries as the walls are too thick.

Regulation of Respiration

Control of respiration is partly voluntary but mainly involuntary. Both the rate and depth of breathing are controlled by homeostatic mechanisms. The respiratory centre, located in the medulla, is formed from a group of nerves which control the repiratory pattern. Activity is informed by nerves in the Pons, in reponse to input from other parts of the brain. Motor implulses initiated in the brain pass through the phrenic and intercostal nerves to the diaphragm, and the intercostal muscles.

Chemo receptors are receptors that respond to changes in the partial pressure of CO₂ and O₂ in the blood and Cerebro-Spinal Fluid (CSF). Chemoreceptors increase activity when an increase in carbon dioxide is present, such as in hypoxia, which then in turn stimulates a change to the rate and depth of breathing to restore partial pressures of oxygen and carbon dioxide to the usual levels. Adversley, too much ventilation will decrease the partial pressure of carbon dioxide and reduce the amount of chemoreceptor activity and ventilation.

There are two types of chemos receptors in the body:

Central – located in the medulla in the brain and respond to changes in partial pressure of carbon dioxide.

Peripheral/Arterial – monitor changes to partial pressure of oxygen and carbon dioxide in arterial blood.

Central Chemoreceptors are surrounded by CSF. When arterial PaCO₂ rises they respond by stimulating the repiratory centre to increase the respiratory rate in order to decrease the excess CO₂. A small reduction in PaO₂ will have a similar effect but less pronounced. Peripheral Chemoreceptors located in the carotid body and aorta are more responsive to small rises in arterial PaCO₂ than small decreases in PaO₂. Central Chemoreceptors generate impulses sent along the glossopharyngeal and vagus nerves to the medulla, which in turn increases the rate and depth of breathing.

References and Further Reading

Rubin, B.K. 2002. Physiology of airway mucus clearance. Respiratory Care 47: 761–8.

Warrell, D., Cox, T., Firth, J. 2012. Oxford textbook of Medicine 5th Edition Oxford: Oxford University Press.

Marieb E.N, Hoehn K (2013) Human Anatomy and Physiology (9ed) London: Pearson Education

Tortora.G, Derrikson B (2013) Essentials of Human Anatomy and Physiology Chichester: Wiley

Cuthbertson S and Kelly M, 2007 Support of respiratory function in ACCCNs Critical Care Nursing eds Elliott D Allen L and Chaboyer W Elsievier Marrickville pp 263-306

Francis, C. 2006. Respiratory Care: Essential clinical skills for nurses Oxford: Blackwell.