The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [358]
3 Transport of respiratory gases: moving oxygen and carbon dioxide between the lungs and the body tissues. Transport is affected by the cardiovascular system and uses the blood as a carrying mechanism.
4 Cellular respiration: oxygen delivery and carbon dioxide uptake between the systemic blood and tissue cells.
The respiratory system is composed of the following structures.
The two respiratory centres in the medulla oblongata and pons of the brain.
The nose, mouth and connecting airways.
The trachea, main bronchus, bronchioles and alveoli.
The respiratory muscles: the diaphragm and the intercostal muscles.
The respiratory nerves: the subphrenic nerve and the intercostal nerves.
The bone structure of the thorax: the ribs, vertebrae and sternum.
The lung parenchyma.
The pleura.
Alteration, damage or blockage to any of the structures listed above may result in either respiratory impairment or respiratory failure. It is essential when considering respiratory function to remember the close association and dependence between the cardiovascular, neurological, musculoskeletal and respiratory systems.
Tissue oxygenation
All the cells of the body require a continuous supply of oxygen to ensure growth and repair of tissues and optimum metabolism. Oxygen is drawn into the body through the nose and mouth; it then travels down the trachea and into the smaller airways and alveoli of the lungs. Once it has reached the alveoli, oxygen in solution is able to transfer into the network of capillaries and from there travels via the venous network to all cells of the body. This tissue oxygenation is known as cellular oxygenation. Low oxygen levels are called hypoxia. In low oxygen conditions, anaerobic cellular oxygenation will occur, generating the waste product lactic acid. If the low oxygen state is allowed to continue lactic acid will accumulate, leading to a metabolic acidosis and cell death (Berne 2004, Bersten et al. 2009, Guyton and Hall 2006, Hess 2000, Kumar and Clark 2009, Marieb et al. 2010, Pierson 2000, Tortora and Derrickson 2009, West 2008).
There are three components to oxygenation: oxygen uptake, oxygen transportation and oxygen utilization. Oxygen uptake is the process of extracting oxygen from the environment. Oxygen transportation is the mechanism by which the uptake of oxygen results in the delivery of oxygen to the cells. Oxygen utilization is the metabolic need for molecular oxygen by the cells of the body (Marieb et al. 2010, Tortora and Derrickson 2009).
In order for oxygenation to take place there needs to be an adequate cardiac output.
Oxygen uptake
The air that we breathe in during normal conditions from the atmosphere is composed of the following gases:
oxygen 21%
carbon dioxide 0.03%
nitrogen 79%
rare gases 0.003%.
Inspired air at sea level has a total atmospheric pressure of 760 mmHg. According to Dalton’s Law, where there is a mixture of gases each gas exerts its own pressure as if there were no other gases present. The pressure of an individual gas in a mixture is called the partial pressure and is denoted as P, which is then followed by the type of gas, so that the partial pressure of oxygen is written PO2 (Tortora and Derrickson 2009).
Oxygen 0.21 × 760 = 159 mmHg (21 kPa).
Carbon dioxide 0.03 × 760 = 22.8 mmHg (3.0 kPa).
Nitrogen 0.79 × 760 = 600 mmHg (80 kPa).
The partial pressure of gases controls the movement of oxygen and carbon dioxide through the body between the atmosphere and the lungs, the lungs and the blood and finally the blood and the cells.
Gaseous exchange
Movement of gases is by diffusion. Diffusion is the movement of gas molecules from an area of relatively high partial pressure to one of lower partial pressure (Tortora and Derrickson 2009).
Diffusion of oxygen takes place from the alveolus into the pulmonary capillaries and movement of carbon dioxide from the capillary into the alveolus. From the alveolus, the oxygen diffuses from the capillaries into the tissues and mitochondria of the cells (Figure 10.1).
Figure 10.01 Gas movement