The respiratory system serves to ensure that all tissues receive the O2 they need and can dispose of the CO2 they produce. Lungs exchange gases with atmosphere via the nasal cavity, which filters, warms and humidifies the air in preparation for efficient gas exchange. Blood carries gases to and from tissues, and has the intrinsic capacity to pick up O2 and lose CO2 if exposed to the right gaseous environment, and the lungs expose the blood to this type of environment.
Breathing
At rest we breathe at a respiratory rate of, on average, 12 breathes per minute, each with a volume of 0.5 L (with strenuous exercise this can rise to 120 L/min). The active inhalation phase lasts 2 s and the passive exhalation phase lasts 3 s. This normal respiration rate, depth and rhythm is called eupnoea. Breathing is therefore tidal, the last air to be inhaled is the first air to be exhaled. One-third of the air inhaled remains in the conducting airways at the end of inhalation and cannot take part in gas exchange. This relative proportion of ‘dead space’ decreases with deeper breaths as the conducting airways contribute the same absolute volume. Thus deeper breaths are more efficient in moving air in and out of alveoli for gas exchange, but require more muscular effort. Two other critical internal factors that affect the distribution of inhaled gas and hence the efficiency of breathing, are airway resistance and lung compliance (Figure 24.1).
Airway resistance and conductance
Airway resistance is the opposition to airflow in the respiratory tree. Conductance (the reciprocal of resistance) is the change in flow for a unitary change in pressure, and usually limits exhalation. Airway resistance and conduction depend on friction and cross-sectional area. The major source of non-elastic resistance to air flow is friction in the respiratory passageways. Resistance in the respiratory tree is determined mostly by the diameters of the conducting tubules (i.e. trachea, bronchi, bronchioles, etc.). Gas flow stops at the terminal bronchioles, where airways have small diameters, but this is not a problem, as here diffusion is the main force driving gas movements. Therefore, the greatest resistance to gas flow occurs in the medium-sized bronchi and bronchioles. Although individual cross-sectional areas decrease with deeper penetration into the respiratory tree, total cross-sectional area increases. This means that there has to be very extensive damage to the small airways for ventilation to be affected.
Factors increasing airways resistance include: decreasing total lung volume; and increasing bronchomotor tone, age, mucus secretion and disease. Posture (through the effect of gravity) also affects airway resistance, as the height relative to heart affects blood vessel diameter and pressure on airways, while the abdominal contents either push up or pull down on the diaphragm altering lung volume by up to 1 L. Smooth muscle in the walls of the bronchioles has sensitive neural parasympathetic reflexes in response to inhaled irritants to prevent their deeper penetration (overreaction leads to asthma and can contribute to anaphylaxis).
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