Key points Anaemia has little effect on pulmonary gas exchange but decreases oxygen carriage in the arterial blood in direct proportion to the reduction in haemoglobin concentration. Mechanisms that compensate for the reduced oxygen delivery include increased cardiac output, increased tissue oxygen extraction and a right shift of the oxyhaemoglobin dissociation curve. Older patients or those with poor cardiac reserve compensate less well when anaemic. Anaemia…

Key points Intracellular acidosis from anaerobic metabolism occurs soon after the onset of cellular hypoxia and is worse when there is a plentiful supply of blood and glucose to the cell. Lack of high-energy substrates such as adenosine triphosphate (ATP) and direct effects of hypoxia both inhibit the activity of ion channels, decreasing the transmembrane potential of the cell, leading to increased intracellular calcium levels. In…

Key points Hypocapnia occurs when alveolar ventilation is excessive relative to carbon dioxide production, and usually results from hyperventilation because of hypoxia, acidosis or lung disease. Hypercapnia most commonly occurs because of inadequate alveolar ventilation from a multitude of causes, or more rarely from increased carbon dioxide production. Arterial P co 2 affects the cerebral circulation—hypocapnia may cause potentially harmful vasoconstriction, whereas vasodilatation from hypercapnia may…

Key points All anaesthetic drugs reduce ventilation and impair the ventilatory response to both hypercapnia and hypoxia. Upper airway muscle function is inhibited by anaesthesia, leading to airway obstruction, usually at the level of the soft palate. Functional residual capacity is reduced within a few minutes of induction of anaesthesia as a result of altered respiratory muscle activity causing changes to the shape and volume of…

Key points Although becoming less popular, one-fifth of the UK population still smokes tobacco, and worldwide, the number of smokers is increasing. Smoking involves the regular inhalation of a variety of toxic compounds that stimulate airway irritant receptors and activate inflammatory pathways in the lung. The effects of passive smoking begin in utero, when lung development is impaired, leaving the infant susceptible to lower respiratory tract…

Key points Immersion in thermoneutral water activates protective airway reflexes, and aspiration does not occur until lung oxygen stores have been used up and hypoxia causes the airway to open. In cold water, the cold shock reflex causes gasping and hyperventilation under water, with inhalation of large quantities of water and rapid severe hypoxia. It is estimated that there are more than 360 000 victims of…

Key points Environments in which a closed atmosphere suitable for breathing is maintained include closed-circuit anaesthesia, submarines and space vehicles. Problems of maintaining acceptably low carbon dioxide concentrations and low levels of inhaled contaminants are common to all these environments. In the microgravity of space static lung volumes are reduced, ventilation and perfusion are better matched, and airway obstruction during sleep is uncommon. For atmospheric regeneration…

Key points When diving in water the increased density of inhaled gases and immersion in water cause an increase in the work of breathing, which can impair gas exchange during exercise. At greater than about 4 atmospheres absolute pressure nitrogen has anaesthetic effects, and divers must breathe helium, which also overcomes the problem of increased gas density. On ascent from a dive expansion of gases in…

Key points Low inspired oxygen partial pressure at altitude causes immediate hyperventilation, which increases further with acclimatization to produce hypocapnia and improve oxygen levels. The rate of ascent and altitude achieved are determinants of altitude-related illnesses, which vary from mild acute mountain sickness to potentially lethal high-altitude pulmonary oedema. High-altitude populations have adaptations to their environment such as lesser degrees of hyperventilation compensated for by a…

Key points Obese subjects have reduced static and dynamic lung volumes because of compression of the chest cavity by the mass of the chest wall and abdominal contents. Chest wall compliance is reduced by obesity, and the lower lung volume leads to a decrease in lung compliance and an increase in airway resistance, making airway closure more frequent. Hormonal changes in obesity such as increased leptin…

Key points During normal sleep tidal volume is reduced, with maximal reduction in ventilation occurring during rapid eye movement sleep when breathing also becomes irregular. Reduction in the speed and strength of pharyngeal muscle reflexes causes increased airways resistance, leading to snoring in many normal individuals. Sleep-disordered breathing describes a continuum of abnormalities ranging from occasional snoring to frequent periods of airway obstruction and hypoxia during…

Key points Oxygen consumption increases linearly with the power expended during exercise. The extra tissue oxygen requirement is provided by increases in cardiac output and blood oxygen extraction. To accommodate these changes ventilation also increases linearly with exercise—this response occurs the moment exercise begins. With increasing exercise intensity, lactate is produced from anaerobic muscle metabolism, and blood lactate levels increase, initially reaching a steady state but…

Key points Hormonal changes of pregnancy stimulate breathing, causing an increase in tidal volume and hypocapnia. In late pregnancy the enlarged uterus reduces lung volume, particularly in the supine position. Human lung development is incomplete at birth, with new alveoli continuing to form until around 3 years of age. Compared with adults, the respiratory system of a neonate has a very low compliance and a high…

Key points The entire cardiac output passes through the pulmonary circulation, so the lungs act as a filter, preventing emboli from passing to the left side of the circulation. The lungs constitute a huge interface between the outside environment and the body, requiring the presence of multiple systems for defence against inhaled biological and chemical hazards. In the pulmonary circulation there is active uptake and metabolism…

Key points Oxygen moves down a partial pressure gradient between the inspired gas and its point of use in the mitochondria, where the oxygen partial pressure may be only 0.13 kPa (1 mmHg). Significant barriers to oxygen transfer are between inspired and alveolar gas, between alveolar and arterial oxygen partial pressures and on diffusion from the capillary to the mitochondria. Each 100 mL of arterial blood…

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Key points For gas to transfer between the alveolus and haemoglobin in the red blood cell it must diffuse across the alveolar and capillary walls, through the plasma and across the red cell membrane. The reaction rate for oxygen with haemoglobin also affects the rate at which red blood cells become saturated with oxygen on passing through the pulmonary capillary. Transfer of oxygen and carbon dioxide…

Key points Both ventilation and perfusion are distributed preferentially to dependent regions of the lung as a result of gravity and lung structure, and vary with posture and lung volume. In healthy lungs ventilation and perfusion are closely matched, with little variation of the ventilation to perfusion ( ) ratio in different lung regions. Regions of lung with a ratio of 0 represent intrapulmonary shunting of…

Key points Pulmonary blood flow approximates to cardiac output and can increase several-fold with little change in pulmonary arterial pressure. Passive distension and recruitment of closed pulmonary capillaries, particularly in the upper zones of the lung, allow pulmonary vascular resistance to fall as blood flow increases. Active control of pulmonary vascular resistance has only a minor role in controlling pulmonary vascular resistance and involves intrinsic responses…

Key points Pharyngeal and laryngeal muscles display both tonic and phasic contraction to maintain airway patency and to regulate airflow. The diaphragm, the intercostal muscles and some neck muscles bring about inspiration by a complex combination of actions, varying with different postures. Expiration is normally passive, except during exercise or at minute volumes several times higher than normal, when intercostal and abdominal wall muscle contraction causes…