Exchange alveolar gas

Diffusing capacity for carbon monoxide (measurement of alveolar gas exchange)... 

Diffusion Carbon monoxide diffusing capacity Measurement of efficiency of alveolar gas exchange decreases with thickening of alveolar blood-air barrier... 

The control of airway dimensions the process by which the energy required to ventilate is kept at a minimum for any alveolar gas exchange requirement. 

When the alveolar gas exchanges are severely inadequate, the first manifestation is usually a drop in the oxygen tension of the blood. In most patients this is followed by increased CO2 tension. Only rarely are the oxygen tension of the blood decreased and normal CO2 levels maintained, except in perfusion or diffusion disturbances. In patients with chronic bronchitis, emphysema, and tuberculosis, the CO2 drops to 20-30 mm Hg and the Pcoj to 80-90 mm Hg. 

FIGURES. 17 Gas exchange beev/een alveolar and capillary compartmencs. 

Alveolar ventilation supplies O2 to the bloodstream while alveolar capillary perfusion provides alveolar gas with COj. Resting individuals consume approximately 250 mL 02/min and produce approximately 200 ml. COi/min because, stoichiometrically, metabolic processes require a greater supply of O, than the quantity of CO2 produced. Defining the respiratory exchange ratio, R, as... 

Alveolar gas transport Exchange of oxygen and carbon dioxide between al-... 

More specifically, the blood-gas interface consists of the alveolar epithelium, capillary endothelium, and interstitium. The alveolar wall is made up of a single layer of flattened type I alveolar cells. The capillaries surrounding the alveoli also consist of a single layer of cells — endothelial cells. In between the alveolar epithelium and capillary endothelium is a very small amount of interstitium. Taken together, only 0.5 pm separates the air in the alveoli from the blood in the capillaries. The extreme thinness of the blood-gas interface further facilitates gas exchange by way of diffusion. 

Alveolar ventilation. Alveolar ventilation is less than the total ventilation because the last portion of each tidal volume remains in the conducting airways therefore, that air does not participate in gas exchange. As mentioned at the beginning of the chapter, the volume of the conducting airways is referred to as anatomical dead space. The calculation of alveolar ventilation includes the tidal volume adjusted for anatomical dead space and includes only air that actually reaches the respiratory zone ... 

Dead space. Anatomical dead space is equal to the volume of the conducting airways. This is determined by the physical characteristics of the lungs because, by definition, these airways do not contain alveoli to participate in gas exchange. Alveolar dead space is the volume of air that enters unperfused alveoli. In other words, these alveoli receive airflow but no blood flow with no blood flow to the alveoli, gas exchange cannot take place. Therefore, alveolar dead space is based on functional considerations rather than anatomical factors. Healthy lungs have little or no alveolar dead space. Various pathological conditions, such as low cardiac output, may result in alveolar dead space. The anatomical dead space combined with the alveolar dead space is referred to as physiological dead space ... 

In a lung unit with low blood flow and high ventilation (alveolar dead space), the level of carbon dioxide is decreased and the level of oxygen is increased. The reduced carbon dioxide causes bronchoconstriction and a decrease in ventilation. The excess oxygen causes vasodilation and an increase in perfusion and, once again, the V/Q ratio is brought closer to one and gas exchange is improved. 

Parenchymal changes affect the gas-exchanging units of the lungs (alveoli and pulmonary capillaries). Smoking-related disease most commonly results in centrilobular emphysema that primarily affects respiratory bronchioles. Panlobular emphysema is seen in AAT deficiency and extends to the alveolar ducts and sacs. 

The volume of the airways in which no gas exchange occurs. It can be either anatomical or alveolar (ml). 

Start with the theoretical lungs shown in the figure and remember that each Vt has a component that is dead space ( Vd) and a remainder that must take part in gas exchange at the alveolus (Vt - Vd). This is the alveolar volume. 

Two different circulatory systems, the bronchial and the pulmonary, supply the lungs with blood [133], The bronchial circulation is a part of the systemic circulation and is under high pressure. It receives about 1% of the cardiac output and supplies the conducting airways, pulmonary blood vessels and lymph nodes [133], It is important for the distribution of systemically administered drugs to the airways and to the absorption of inhaled drugs from the airways [18]. The pulmonary circulation comprises an extensive low-pressure vascular bed, which receives the entire cardiac output. It perfuses the alveolar capillaries to secure efficient gas exchange and supplies nutrients to the alveolar walls. Anastomoses between bronchial and pulmonary arterial circulations have been found in the walls of medium-sized bronchi and bronchioles [18, 65, 67],... 

The situation is, however, different in the alveolar region of the lung where the respiratory gas exchange takes place. Its thin squamous epithelium is covered by the so-called alveolar surface liquid (ASL). Its outermost surface is covered by a mixture of phospholipids and proteins with a low surface tension, also often referred to as lung surfactant. For this surfactant layer only, Scarpelli et al. [74] reported a thickness between 7 and 70 nm in the human lung. For the thickness of an additional water layer in between the apical surface of alveolar epithelial cells and the surfactant film no conclusive data are available. Hence, the total thickness of the complete ASL layer is actually unknown, but is certainly thinner than 1 gm. 

Fig. 3.2 The blood-gas exchange system at the bronchioles and alveolar portions of the pulmonary system.

Particle deposition in the respiratory tract can initiate inflammatory responses. With repeated deposition, inflammation becomes chronic, and the site or sites of deposition beeome laden, not only with the particulates, but with several types of cells—fibroblasts, macrophages, leukocytes, and lymphocytes. These cells are normal constituents of the lung, an organ composed predominantly of connective tissue. Lung connective tissue forms the thin membrane that defines the functional alveolar-capillary unit. Inside this air sac and on the membrane are specialized eells required for gas exchange, maintenance, and repair (Fig. 3.6). 

The lower respiratory tract (pulmonary region or alveolar ducts and sacs) is the area where gas exchange occurs. Alveolar sacs, clusters of two or more alveoli, branch from alveolar ducts. It is generally considered that there is a total of approximately 300 million alveoli in the lungs of adult humans. The total alveolar surface area in the lungs of adult humans is... 

Respiration into the lung alveoli is the most important hazard for plant workers. The average lung has 300 million alveoli with a surface area of about 70 m, which is designed for rapid gas exchange. This alveolar surface area is about 40 times larger than the external skin area of a person. There has been a systematic study of the major air pollutants associated with the protection of industrial workers, as monitored by the OSHA and the ACGIH, and there are three well-documented measurements and databases ... 

Exposure of the lungs to xenobiotics may result in a number of disease conditions including bronchitis, emphysema, asthma, hypersensitivity pneumonitis, pneumoconiosis, and cancer. During repair, damaged lung alveolar epithelium may be replaced by fibrous tissue that does not allow for gas exchange, which intensifies the damage caused by the initial lesion. 

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