The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [359]
The alveolar oxygen partial pressure is higher than the arterial oxygen partial pressure in order to push the oxygen through the alveolar membrane into the interstitial spaces and from there into the pulmonary capillaries. Oxygen continues to diffuse from the capillaries into the tissues then to the mitochondria of the cells for metabolism (Berne 2004, Bersten et al. 2009, Carpenter 1991, Esmond 2001, Guyton and Hall 2006, Marieb et al. 2010, Pierce 1995, Tortora and Derrickson 2009).
As inspired air enters the respiratory tract, it encounters water vapour present in the upper airways which warms and humidifies it. Water vapour exerts its own partial pressure of 47 mmHg. The partial pressure of the water vapour must be subtracted from the total atmospheric pressure to give a corrected atmospheric pressure and partial pressure of each gas (Carpenter 1991, Marieb et al. 2010).
Corrected total atmospheric pressure 760 − 47 = 713 mmHg.
Oxygen 0.21 × 713 = 150 mmHg (20 kPa).
Carbon dioxide 0.03 × 713 = 21 mmHg (2.8 kPa).
As oxygen continues to pass down the respiratory tract to the alveolus, it encounters carbon dioxide leaving the respiratory tract which also exerts a partial pressure, equal to 40 mmHg. This in turn must be subtracted to determine the correct values. Oxygen has a corrected value of 150 − 40 = 110−100 mmHg (14.6−13.3 kPa).
Oxygen transportation
Oxygen is carried in the blood in two ways.
Dissolved in the plasma (serum): only 2–3% is carried in this way as oxygen is not very soluble (Ahrens and Tucker 1999, Marieb et al. 2010). This is measured as the PaO2. There is 0.003 mL of blood for each 1 mmHg partial pressure oxygen. At 100 mmHg partial pressure, only 0.3 mL of oxygen would be carried per 100 mL of plasma.
Bound to haemoglobin in the red blood cells: 95–98% of oxygen is carried in this way and is measured as the percentage of oxygen saturated (SaO2). Each gram of haemoglobin can carry 1.34 mL of oxygen per 100 mL blood.
Haemoglobin is composed of haem (iron) and globulin (protein). Each haemoglobin molecule has four binding sites, each able to carry one molecule of oxygen. A haemoglobin molecule is said to be fully saturated with oxygen when all four haem sites are attached to oxygen. When fewer than four are attached the haemoglobin is said to be partially saturated.
The bond between haemoglobin and oxygen is affected by various physiological factors that shift the oxygen dissociation curve to the right or left (Figure 10.2) (Marieb et al. 2010, Tortora and Derrickson 2009).
Figure 10.2 Oxyhaemoglobin dissociation curve. With a PaO2 of 8 kPa and more, saturations will remain high (flat portion of curve). NB: The middle red line is the normal position of the curve.
Oxyhaemoglobin curve shift to the right
When a shift occurs to the right there is reduced binding of oxygen to haemoglobin and oxygen is given up more easily to the tissues. The saturation will be lower (Pierson 2000).
Factors that cause the curve to shift to the right are an increase in:
body temperature due to infection, sepsis
hydrogen ion content (acidaemia), known as the Bohr effect, due to infection, sepsis or other shock conditions
carbon dioxide due to sepsis, pulmonary disease, postoperatively
2-3-DPG (an enzyme found in the red blood cells that affects haemoglobin and oxygen binding).
Oxyhaemoglobin curve shift to the left
When a shift occurs to the left there is an increase in the binding of oxygen to the haemoglobin, oxygen is given up less easily to the tissues and cellular hypoxia can occur (Pierson 2000).
Factors that cause the curve to shift to the left are a decrease in:
body temperature due to exposure, near drowning, trauma
hydrogen ion content (alkalaemia)
carbon dioxide
2-3-DPG.
Oxygen utilization
The relationship