PATHOPHYSIOLOGY TERMINOLOGY
1. Define hypoxia, hypercapnia, cyanosis, and asphyxia.
2. What is pulmonary surfactant?
3. What characterizes pneumonia, emphysema, and atelectasis?
Pulmonary surfactant is a surface tension-reducing substance produced by the alveolar epithelial cells.
The alveolar surface is compressed during expiration, which concentrates surfactant at the surface. The concentration of surfactant reduces the surface tension and makes beginning inspiration easier. At the end of inspiration surfactant is spread out because of enlargement of the alveoli and surface tension increases, which assists expiration.Anoxia literally means without oxygen and should not be used when the condition is one of decreased oxygen. In such a case, hypoxia is more appropriate. Hypoxia is a decrease below normal Po2 in air, blood, or tissue, short of anoxia. Hypoxemia is a decrease in the oxygen concentration of the arterial blood. Four types of hypoxia are recognized
1. Ambient hypoxia: the arterial blood is insufficiently saturated with oxygen because of a low Po2 in the atmosphere being breathed. This occurs naturally at high altitudes.
2. Anemic hypoxia: there is a decrease in the oxygen capacity of the blood because of a shortage of functioning hemoglobin. The Po2 in arterial blood and the percentage saturation of hemoglobin are normal. Oxygen delivery to the tissues may be inadequate. Anemic hypoxia occurs after hemorrhage, in various anemias, and when some of the hemoglobin is changed to methemoglobin or is combined with carbon monoxide.
3. Stagnant hypoxia, also known as ischemic hypoxia: blood flow through the whole body or a tissue is diminished. The oxygen content of arterial blood is normal, but the tissues fail to receive enough oxygen because of diminished blood flow.
4. Histotoxic hypoxia: the cells are unable to use the oxygen that is supplied.
The amount of oxygen in arterial blood is normal, but because the cells are unable to use it, the amount is above normal in venous blood.Hypercapnia and hypocapnia are increased and decreased Paco2 respectively, in arterial blood and is indicative of hypoventilation and hyperventilation.
Cyanosis refers to a bluish or purplish coloration of the skin and mucous membranes. The intensity of the color is a result of the degree of deoxygenation of hemoglobin. As observed systemically, it relates to inadequate oxygenation of blood. When seen locally, it is probably caused by blood flow obstruction.
Asphyxia is a condition of hypoxia combined with hypercapnia. Hypoxia and hypercapnia can occur as separate entities, but only their combination results in asphyxia. Breathing into a closed space is an example resulting in what is commonly called suffocation.
Three pathologic conditions often referred to when discussing respiratory physiology are emphysema, pneumonia, and atelectasis. Emphysema is a condition in which destruction of alveolar membranes has occurred, resulting in a smaller area available for gas diffusion. It is often coupled with other conditions, such as chronic bronchitis, that increase the positive pressure within alveoli that is needed for the expiratory phase of the respiratory cycle. Pneumonia is an acute inflammatory condition of the lungs in which the alveoli fill with fluid and cell debris. Final resolution involves liquefaction of the cellular debris, its removal, and regeneration of the alveolar epithelium. Atelectasis is failure of the alveoli to open or remain open and usually involves one or more relatively small areas of lung. The usual cause of atelectasis is occlusion of the bronchus or bronchiole that supplies the area. This results most often from plugs of mucus or purulent exudate closing the bronchus or bronchiole with the trapped air contained in the alveolus at the time being absorbed; the airless alveoli.collapse because of the surrounding pressure.
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