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Respiration and pulse oximetry

Definition

The function of the respiratory system is to ensure that the cells and tissues of the human body have oxygen supplied to them and carbon dioxide removed, in order that they can continue to carry out their functions (Patton and Thiobodeau 2009). Respiration consists of four processes: ventilation; external respiration or gaseous exchange; transport; and internal respiration (Marieb and Hoehn 2010) (see Chapter 10).

Anatomy and physiology

For effective respiration to occur, effective functioning and interaction between the various body systems are required, including the circulatory system, nervous system and musculoskeletal system (Patton and Thiobodeau 2010). The organs and structures of the respiratory system can be split into two groups: the conducting zone and the respiratory zone (Marieb and Hoehn 2010). The conducting zone consists of the respiratory passages through which air passes to get to the area of gaseous exchange, such as the nasal cavity and the bronchi; this area provides conduits through which air can pass and also be warmed, humidified and cleansed (Marieb and Hoehn 2010). The respiratory zone consists of the bronchioles, alveolar ducts, alveoli and the microscopic anatomy which provide the site for gaseous exchange (Marieb and Hoehn 2010). The respiratory muscles (the diaphragm and intercostal muscles) promote ventilation by causing volume and pressure changes within the respiratory system (Marieb and Hoehn 2010).

The mechanism of breathing

The key mechanism of pulmonary ventilation is that changes of volume within the respiratory system lead to changes in pressure and these in turn lead to a flow of gases (Marieb and Hoehn 2010). Indeed, the vital principle of respiration is that the flow of air will always go down a pressure gradient; this means that air will flow from an area of higher pressure to an area of lower pressure (Patton and Thiobodeau 2009). The two main pressures are intrapulmonary, or intraalveolar, pressure, which varies with breathing but which always eventually is equal to atmospheric pressure (the pressure of gases or air outside the body) (Marieb and Hoehn 2010), and intrapleural pressure, which also varies with breathing but which is generally about 4 mmHg less than the intrapulmonary pressure (Marieb and Hoehn 2010). Boyle’s Law describes how at a constant temperature, the pressure of a gas has an inverse relationship to its volume (Patton and Thiobodeau 2009). Therefore, in a larger container the pressure of gas is less than in a smaller one. The relationship of these pressures can be seen in Figure 12.13.

Figure 12.13 Pressure changes in pulmonary ventilation.

Reproduced from Tortora and Derrickson (2009).

Inspiration

Inspiration occurs because the phrenic nerve stimulates the diaphragm to flatten and descend; this movement results in the greatest volume change in the lungs (Davies and Moores 2003). The intercostal muscles lift the rib cage and sternum, causing the ribs to broaden outwards and increasing the diameter of the thoracic cavity, both from side to side and front to back (Patton and Thiobodeau 2009). This change in volume of the thoracic cavity causes the volume of the lungs to increase; therefore, according to Boyle’s Law the pressure of gas within the lungs is less. This drop in pressure causes air to move in from outside the body, from an area of higher pressure to one of lower pressure (Marieb and Hoehn 2010), thereby causing inspiration.

Expiration

The inspiratory muscles relax, causing the thoracic cavity to return to its normal size, the lungs recoil and this decrease in size compresses the alveoli, causing an increase in intrapulmonary pressures and forcing gases to flow out of the lungs into the atmosphere (Marieb and Hoehn 2010). Therefore, in normal, gentle breathing the process of expiration is largely passive. The mechanism of inspiration and expiration in relation to pulmonary volume can be seen in Figure 12.14.

Figure 12.14 Changes in thoracic volume