Healthy lungs are filled with air and are completely expanded in the thorax, so that there is absolutely no gap in the intrapleural space. There is just a thin layer of fluid. This sticks the visceral and parietal pleura tightly together, much like two microscope slides with a drop of water in between. When the thorax expands, the lungs must expand at the same time because the visceral and parietal pleura remain stuck to one another.
Atelectasis refers to the collapse of a lung, in which a gap opens in the pleural space. This gap may be in a small region, or all of the lung on one side can collapse. Most of you saw completely collapsed lung tissue when we dissected the pig heart.
With atelectasis, the lung affected will not inflate properly, since the inflation depends on the tight attachment of the visceral and parietal pleura.
Atelectasis may resolve on its own if the separation is not too large. But in more serious cases, a positive pressure ventilator might help. Also, a large syringe and needle might be used to withdraw gas from the space. If that doesn't work, a chest tube might be required. One end of the tube in surgically introduced into the space. The tube has a valve that allows gas to leave the intrapleural space, but not return.
One cause of atelectasis is a clogged bronchus. This might be due due to a tumor or to mucus. For example, mucus leading to atelectasis might occur following surgery.
In the region of the lung distal to the blockage, oxygen is continually removed from the alveoli by the blood flowing through the pulmonary capillaries. Red blood cells flowing to the pulmonary capillaries have hemoglobin which readily binds the oxygen. This lowers the pressure in the alveoli and this is what leads to the collapse.
Absorptive atelectasis is usually linked to obstructive atelectasis.
Atelectasis distal to a point of blockage is much more likely if the patient has been breathing supplemental oxygen. Air contains 79% nitrogen, which is inert and does not tend to move into or out of the alveoli. But if some of the nitrogen is replaced by oxygen due to the supplemental oxygen, more gas is removed from the alveoli and the pressures decreases more.
Pneumothorax refers to air or gas in the intrapleural space. One cause is a wound that punctures the chest wall, allowing air from outside the body to flow into the intrapleural space. As a result, the visceral and parietal pleural peel apart, and the lung on that side collapses. In first aid, this is called a "sucking chest wound", because inhalation causes air to flow through the wound into the intrapleural space. The first step in dealing with this is placing a piece of plastic over the hole to prevent air flowing into the intrapleural space.
A second possibility is that a serious lung infection, such as tuberculosis, has seriously damaged airways, allowing air to flow from a bronchus into the intrapleural space.
Another common cause of lung damage is emphysema, in which the tissue of the lung is destroyed. Atelectasis is especially likely if bullae are present. We will discuss bullae later.
Finally, there may be a spontaneous pneumothorax, in which there is no obvious cause. The lung simply pulls away from the chest wall. This can involve only a small area and might resolve spontaneously. Or it can be larger and require treatment. Spontaneous pneumothorax is several times more likely in men than women. A tall, thin young man is the most typical patient.
As you learned in the previous page, surfactant is secreted by type II alveolar cells and acts to reduce the surface tension in alveoli. This is essential for inhalation. Otherwise the lungs would be too difficult to inflate. By reducing the surface tension in alveoli, there is less tension in the walls of the alveoli and thus the lungs as a whole become more compliance and inflate easier. With less surfactant, there is more tension in the walls of the alveoli and thus there is a greater force tending to cause the lungs to collapse.
What is the composition of surfactant?
As the lungs develop in the fetus, surfactant is one of the last substances to be synthesized. This is why premature infants often show infant respiratory distress syndrome (IRDS). The baby struggles to breathe with tachypnea and cyanosis.
If necessary, the baby is put on a postive pressure ventilator. Then animal surfactant, extracted perhaps from cow lungs, can be infused into the airways. It works its way down to the alveoli and helps until the baby is synthesizing enough surfactant.
What is meant by tachypnea and cyanosis?