The volume of air that is breathed in and out of the lungs varies depending on the conditions of inspiration and expiration (Marieb and Hoehn [110]). As such, several respiratory volumes can be described, combinations of which (termed ‘respiratory capacities’) are measured to gain information about a person's respiratory status (Marieb and Hoehn [110]). A summary of these volumes can be seen in Figure 14.31. See Table 14.6 for a summary of respiratory volumes and capacities for males and females.

Gaseous exchange occurs during external respiration where oxygen diffuses from the air in the alveoli of the lungs into blood in the pulmonary capillaries, and carbon dioxide diffuses from the blood into the alveolar air (Patton [155]). This occurs as there is a flow of gases from areas of higher pressure to areas of lower pressure; oxygen diffuses from alveolar air, where the partial pressure is higher than that in the capillary blood, and carbon dioxide diffuses from the blood, where the partial pressure is higher than that in the alveolar air (Tortora and Derrickson [199]).

Internal respiration, or systemic gas exchange, takes place where the same gases move into or out of the cells of the body by diffusion, so oxygen moves into the tissues and carbon dioxide moves out of them (Tortora and Derrickson [199]). See Figure 14.32 for the changes that occur in the partial pressures of oxygen and carbon dioxide during internal and external respiration. See Chapter c12: Respiratory care, CPR and blood transfusion for further information.

Oxygen does not dissolve easily in water and therefore only 1.5% of the oxygen is transported in the blood by being dissolved in the plasma; the other 98.5% is bound to haemoglobin in the red blood cells, forming oxyhaemoglobin (Hb–O₂) (Tortora and Derrickson [199]). The majority of carbon dioxide is transported in the blood as bicarbonate ions (HCO₃⁻) within the plasma (70%); approximately 20–23% is bound to haemoglobin, forming carbaminohaemoglobin (Hb–CO₂); and the remaining 7–10% is dissolved in the plasma (Marieb and Hoehn [110]). This process is summarized in Figure 14.33; see also Chapter c12: Respiratory care, CPR and blood transfusion.

Hypoxia is defined as inadequate oxygen delivery to the tissues, which can have various causes. Based on these causes, it can be classified into four types (Marieb and Hoehn [110], Tortora and Derrickson [199]):

Signs of hypoxia include tachypnoea, dyspnoea, tachycardia, restlessness and confusion, headache, mild hypertension and pallor. In its severe stages, the symptoms will worsen, leading to slow, irregular breathing, cyanosis, hypotension, altered level of consciousness, blurred vision and eventually respiratory arrest (Sprigings and Chambers [189]).

Hypercapnia is an elevated level of carbon dioxide in the blood. Signs include tachypnoea (eventually becoming bradypnoea as it worsens), dyspnoea, tachycardia, hypertension, headaches, vasodilation, drowsiness, sweating and a red coloration (Sprigings and Chambers [189]). Patients with hypercapnia require urgent medical attention and close monitoring as hypercapnia causes respiratory acidosis (Cleave [45]). Patients with chronic hypercapnia, such as those who have chronic obstructive pulmonary disease, will have at least partially adapted to the chronically high levels of carbon dioxide, which means their primary respiratory drive will be a low PaO₂ rather than a high PaCO2. Oxygen therapy therefore needs to be administered with caution in these patients, often aiming for lower than usual peripheral oxygen saturations (88–92%) (O'Driscoll et al. [147]).

Normal oxygen saturation sits at 96% or above; however, patients with chronic respiratory conditions may have adjusted to lower oxygen saturation levels, hence oxygen saturations should be targeted to be as near to the patient's normal range as possible (O'Driscoll et al. [147]). In general terms, a level below 91% is of concern, but the trend of oxygen saturations may be of more importance than individual readings as this gives an indication of whether the patient is responding to therapy or deteriorating (O'Driscoll et al. [147]). Supplementary oxygen may be required to achieve saturations of 94–98% in an acutely ill adult or 88–92% in patients with a chronic respiratory disease and those at risk of hypercapnic respiratory failure (O'Driscoll et al. [147]). Oxygen therapy should be regarded as a drug and be prescribed in line with local policy (BNF [22]). In addition, a sudden drop of greater than 3% in oxygen saturation suggests a thorough clinical assessment should be performed (O'Driscoll et al. [147]).