6.1 Structure and Functions of the Lungs

During asthma, the airway narrows due to smooth muscle constriction. How can the body respond to maintain ventilation of the alveoli? Why, sometimes, do people die from severe asthma?

A decrease in the diameter of the airways will increase airway resistance, but the individual can overcome this extra resistance by creating a greater transpulmonary pressure gradient (inhaling and expiring more deeply). However, there are limits to how much pressure can be generated by the respiratory system. If the airway constriction is too great, the individual cannot compensate, and airflow to the alveoli will be too low to supply sufficient oxygen to the body.

Explain how the structure of the alveoli facilitate their function in gas exchange.

Alveoli are tiny air-filled spheres, surrounded by a very thin membrane, encircled by capillaries. The alveoli are the site of diffusive gas exchange in the lungs with the blood. Diffusion is faster when the surface area is greater, and the diffusion distance is short. Because they are very tiny (remember surface:volume ratio is high in small spheres), and there are many of them, the combined total surface area of alveoli is huge (roughly the size of a tennis court). This extensive surface area and the thinness of the barrier permits the rapid exchange of large quantities of oxygen and carbon dioxide by diffusion.

Explain the effect of holding your breath or hyperventilation on alveolar and blood PO2 and PCO2.

Holding your breath will cause oxygen to be depleted from the lung and carbon dioxide to accumulate. The PO2 in the alveoli and arteries will decline, while the PCO2 will rise. Hyperventilation has the opposite effect. PO2 will rise in the alveoli and arteries, while PCO2 will fall.

Explain the pathology and treatment of respiratory distress syndrome in infants.

In some cases, especially when the baby is born prematurely, the newborns' type II alveoli cells are underdeveloped, leading to a low amount of surfactant being produced within the lungs. Since the alveoli are coated with water that is not diluted with surfactant, the surface tension of the water produces a force that decreases lung compliance making it much more difficult to expand the lungs during inspiration. The varying surface tension between neighboring alveoli of differing radii also leads to collapse of the alveoli with a smaller radius. Respiratory distress syndrome is treated by inserting artificial surfactants into the lungs, and mechanically ventilating the lungs.

A collapsed lung, known as pneumothorax, is a condition that occurs when air enters the space between the chest wall and the lung (in the pleural space) due to a hole in the chest. Why does this cause the lung to collapse and what could you do to treat this condition?

Normally there is a negative intrapleural pressure that keeps the lungs expanded. A hole in the chest wall allows air to flow in through the hole into the intrapleural space, eliminating this negative pressure. The elastic filaments in the lungs then cause the lung to collapse. One way to treat pneumothorax would be by blocking the hole so no air can pass through. Longer-term, it may be necessary to remove the air from the intrapleural space with needle aspiration.

Person A, panting: 60 breaths per minute, tidal volume = 200 ml, anatomical dead space = 100 ml

Person A, alveolar ventilation rate = 60 * (200-100) = 6,000 ml per minute.

Person B, after exercise: 30 breaths per minute, tidal volume = 1100 ml, anatomical dead space = 100 ml

Person B, alveolar ventilation rate = 30 * (1100 - 100) = 30,000 ml per minute.

What is pulmonary edema and diffuse interstitial fibrosis? If these cause ventilation-perfusion inequality, how will the lungs deal with this in terms of local homeostatic response?

Pulmonary edema is where, some of the alveoli may become filled with fluid. Edema is the accumulation of fluid in tissues; in the alveoli, this increases the diffusion barrier for gases. Diffuse interstitial fibrosis is where the alveolar walls become severely thickened with connective tissue (fibrotic) due to infection, autoimmune disease, hypersensitivity to inspired substances, exposure to toxic airborne chemicals, or many other causes. A decrease in ventilation within a group of alveoli leads to a decrease in alveolar PO2 and the area around it, including the capillaries and arterioles. This may cause a local homeostatic response within the lungs that leads to local vasoconstriction, diverting blood flow away from the poorly ventilated area. This local adaptive effect, unique to the pulmonary arterial blood vessels, ensures that blood flow is directed away from diseased areas of the lung toward areas that are well ventilated.

What is a pleura, and what is the significance of the intrapleural fluid?

The pleura is a epithelial layer. The visceral pleura covers the lungs, while the parietal pleura covers the inner lining of the thorax around the lungs. The intrapleural fluid is the fluid in between the visceral and parietal pleura. It lubricates the sliding of the two pleura on each other.

Which of the following effects will occur in case of a severe pulmonary edema? (Select all that apply.) a. Alveolar PO2 will rise b. Arterial PO2 will fall c. Alveolar PCO2 will fall d. Arterial PCO2 will rise

a. Alveolar PO2 will rise b. Arterial PO2 will fall c. Alveolar PCO2 will fall d. Arterial PCO2 will rise

Select the TRUE statements about pulmonary surfactants? (Select all that apply.) a. They are secreted by alveolar cells b. They lower the surface tension at the alveolar surface c. They reduce airway resistance d. They are made of lipids and proteins

a. They are secreted by alveolar cells b. They lower the surface tension at the alveolar surface d. They are made of lipids and proteins

Calculate the partial pressure of oxygen at a barometric pressure of 500 mm Hg if oxygen is 20% of the air. a. 500 mmHg b. 100 mmHg c. 20 mmHg d. 160 mmHg e. 10,000 mm Hg

b. 100 mmHg

Which structures are restricted, causing an asthma attack? a. Trachea b. Bronchioles c. Alveoli d. Capillaries

b. Bronchioles

Select the statement that is TRUE during expiration? a. Transpulmonary pressure (Ptp)= 0 b. Alveoli pressure (Palv)− atmospheric pressure (Patm)= 0, and Palv = Patm c. Palv is greater than Patm, Palv − Patm is positive and airflow is outward d. Intrapleural pressure (Pip) becomes more negative

c. Palv is greater than Patm, Palv − Patm is positive and airflow is outward