The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [475]
Oxygen
The peripheral chemoreceptors, found in the aortic arch and carotid arteries, are responsible for the sensing the PO2 in the body (Marieb and Hoehn 2010). Normally, decreasing levels of oxygen only affect respiratory rate by causing the peripheral chemoreceptors to have increased sensitivity to carbon dioxide (Marieb and Hoehn 2010). Arterial PO2 must drop from the normal level of 100 mmHg to at least 60 mmHg before it stimulates respiratory function; at this point the central receptors become suppressed as a result but the peripheral receptors stimulate the respiratory centres and ventilation is increased (Marieb and Hoehn 2010). This mechanism usually only works as an emergency measure, because the level of oxygen has to drop significantly before stimulation of the chemoreceptors occurs (Patton and Thiobodeau 2009).
Other ways in which respiration is controlled
There are a number of irritant receptors in the lungs which can cause constriction of the air passages, and when stimulated in the bronchi or trachea, a cough is initiated (Marieb and Hoehn 2010). These have a protective mechanism to prevent obstruction or aspiration of food or liquids (Patton and Thiobodeau 2009). There are also stretch receptors present in the conducting passages and the visceral pleura which are stimulated when the lungs inflate; these then signal the respiratory centres to end inspiration (Marieb and Hoehn 2010).
Some of the mechanisms through which breathing is controlled are summarized in Figure 12.15.
Figure 12.15 Factors influencing rate and depth of breathing.
Related theory
Respiratory volumes
The quantity of air that is breathed in and out of the lungs varies depending on the condition of inspiration and expiration (Marieb and Hoehn 2010). Therefore, information about the different volumes of the lungs and combinations or respiratory capacities of these various volumes can give an indication about the respiratory condition of the individual (Marieb and Hoehn 2010). A summary of these volumes can be seen in Figure 12.16. See Table 12.3 for a summary of respiratory volumes and capacities for males and females.
Figure 12.16 Spirogram of lung volumes and capacities. The average values for a healthy average male and female are indicated, with the values for a female in parenthesis. Note that the spirogram is read from right (start of record) to left (end of record).
Reproduced from Tortora and Derrickson (2009).
Table 12.3 Summary of respiratory volumes and capacities for males and females
Gaseous exchange
Gaseous exchange occurs during external respiration where oxygen and carbon dioxide diffuse into or out of the blood at the lungs (Marieb and Hoehn 2010). This occurs as there is a flow of gases from areas of higher pressure to areas of lower pressure, so oxygen diffuses from the alveoli into the blood, as the pressure of oxygen in the alveoli is higher than in the blood, and the reverse happens with carbon dioxide (Marieb and Hoehn 2010). Internal respiration takes place where the same gases move into or out of the tissues of the body by diffusion, so oxygen moves into the tissues and carbon dioxide moves out of them (Marieb and Hoehn 2010) (see Chapter 10 for further information).
Transport through the blood
1.5% of the oxygen is transported through our blood by being dissolved in the plasma; the other 98.5% is bound to haemoglobin, forming oxyhaemoglobin, and carried in the red blood cells (Marieb and Hoehn 2010). Carbon dioxide is transported in the blood chiefly as a bicarbonate ion within the plasma (70% is transported via this method), 20% is bound to haemoglobin forming carbaminohaemoglobin, and the other 7–10% is dissolved in the plasma (Marieb and Hoehn 2010). This process is summarized in Figure 12.17; see also Chapter 10.
Figure 12.17 Summary of chemical reactions that occur during gas exchange.
Reproduced from Tortora and Derrickson (2009).
Hypercapnia and hypoxia
Hypercapnia is an elevated level of carbon dioxide level in the blood. Signs include tachypnoea (eventually