11 The respiratory system’s function is to ensure that oxygen enters the arterial blood so that it can be transported to tissues and organs. Oxygen (O2) is required for the breakdown of glucose into the energy essential for cellular function. This process of aerobic metabolism occurs in the mitochondria of the cell, producing energy in the form of adenosine triphosphate (ATP). Carbon dioxide (CO2) is the waste product of this metabolism and is removed by the lungs in the process of ventilation. Some cells can cope with periods of anaerobic metabolism (without O2) (e.g. skeletal muscle), but even short periods without O2 is detrimental for other cells (brain/myocardium). The principles described in this chapter can seem a little complex, but if understood can greatly enhance understanding of the factors that influence effective oxygen delivery to the tissues. Acutely unwell patients experience physiological stress, often with increased oxygen requirements that require prompt treatment, to prevent clinical deterioration. Atmospheric air is a mixture of gases (Table 11.1) with each gas exerting a partial pressure in kilopascals (kPa) contributing to the total atmospheric pressure of 101 kPa at sea level. Atmospheric air moves through the upper and lower airways on inspiration, arriving at the alveoli, where gaseous exchange takes place. Alveolar air composition alters due to mixing with expired CO2 and the addition of water vapour in the moist lung environment. Gases move down a pressure gradient by diffusion; this is from an area of higher partial pressure, to an area of lower partial pressure. Oxygen moves from the atmosphere to the alveoli by bulk flow, then diffuses into the blood and body cells (Figure 11.1). The partial pressure of O2 in venous blood (PvO2), returning via the right side of the heart to the pulmonary capillary, is lower than the alveolar partial pressure of oxygen (PAO2). Oxygen diffuses across the alveolar capillary membrane until equilibrium is reached. CO2
Respiratory physiology: oxygenation
Transport of oxygen
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