While on your rounds, you see two individuals. First is Alicia, a 28-year-old African American individual who comes in with progressive shortness of breath and cough. She also mentions that she lost weight in the past six months and that she had tuberculosis a few years ago. Examination reveals painful red skin lesions on each side of her nose and the anterior surface of both legs, along the tibia. The rest of the examination was normal. Next, you see a 65-year-old male named Richard, who presents with gradually progressive dyspnea on exertion and dry cough. He has no history of underlying lung disease or other relevant symptoms. On examination, there is nail clubbing but no other signs that could suggest a particular etiology, like pneumonia or COPD. Pulmonary function tests were performed in both cases, showing signs of a restricted pattern, including a significant reduction in forced vital capacity.
Both seem to have some type of restrictive lung disease. But first, a bit of physiology. The lung is compliant, meaning that it can expand and contract because its connective tissue is made up of proteins like elastin and collagen. Compliance is defined as the volume change produced by a change in the distending pressure, and is expressed as the ratio of ΔV, the change in volume, to ΔP, which is the change in pressure. In other words, the higher the compliance, the easier it is for the lungs to expand. In contrast, the lung’s tendency to collapse and push the air back out is called elastic recoil, which is balanced by the outward pull of the chest wall.
Now, remember that breathing also involves the structures around the lungs, like the ribs, intercostal muscles, diaphragm, or pleura. During inhalation, the diaphragm and intercostal muscles contract to pull the ribs up and out and expand the chest cavity. This creates a vacuum that pulls the lungs open to allow air in, which eventually reaches the alveoli and specifically, a thin membrane called the respiratory membrane, where gas exchange occurs. Air is then expelled by exhalation, when the diaphragm and intercostal muscles relax to allow the chest wall to fall and return the chest cavity to normal.