what causes the lungs to expand the lungs are only attached to the rest of the body at the root so in order for inhaling to work something has to make the lungs expand when the thorax expands and that isn't easy because much of the lung is made of elastic tissue which is constantly trying to recoil and shrink the lungs also water molecules inside the alveoli are constantly trying to make the lungs shrink so something has to be happening in there to keep the lungs expanded and then actually there are two things the transmural pressure gradient and compliance transmural pressure gradient the transmural pressure gradient is the difference between intra-alveolar and intrapleural pressure which is about minus four millimeters of mercury intra pleural pressure is always lower than intra-alveolar pressure this negative pressure difference is like a vacuum and it's required for ventilation to occur it arises from the opposite tensions created from elastic recoil in the lungs and the thoracic walls each side is pulling away from the other side not usually enough to separate them but just enough to create a little vacuum that keeps the lungs expanded the transmural pressure gradient pulls the lungs outwards and allows them to expand when the thoracic volume increases so if i illustrate the lungs just as one big alveolus surround them by the pleural cavity then the transmural pressure gradient is across this wall and it is -4 millimeters of mercury so the pressure is lower here inside the pleural cavity and it's higher inside the alveoli and that allows the lower pressure in the pleural cavity to pull outwards on the alveoli and keep them next to the thoracic wall if the lungs were not held against the inside of the thoracic wall this way they could not expand and no matter how hard the inspiratory muscles worked inhaling could not occur if this pressure gradient is disturbed for example by a pneumothorax which happens when there's a puncture in the thoracic wall then the lungs will collapse and they cannot be used for breathing until normal pressure gradients are restored by medical treatment usually a pneumothorax happens only on one side of the chest if there's only one puncture wound compliance during inhaling the lungs stretch allowing them to fill with air compliance refers to how easily the lungs expand during exhaling and you can also say that compliance describes how easily the lungs are kept in their expanded position against the inside of the thoracic wall the formula for compliance is c equals delta v divided by delta p where c is compliance delta v is the volume change and delta p is the pressure gradient so according to this formula compliance is the amount of air that can be inhaled for a given change in pressure the most important thing to know about compliance is that the lower the compliance the smaller the volume of air you can inhale at the normal pressure gradient so if you held the pressure gradient constant and you had low compliance you could inhale less air that means in order to compensate for low compliance the inspiratory muscles have to work harder to create a larger pressure gradient if you need to inhale the normal amount of air compliance is affected by several factors but the most important one is the surface tension of the fluid inside the alveoli the inner surface of the alveoli is covered with a thin film of fluid that's mostly water this keeps the alveolar cells from drying out and helps oxygen and carbon dioxide diffuse through the alveolar cells the water molecules in this layer of fluid are attracted to each other creating surface tension if there were no other forces involved the water molecules would clump together collapsing the alveoli so the surface tension of the water creates a force that pulls the walls of the alveoli inwards since the alveoli have to expand during inhaling the force of that surface tension pulls opposite to the force that expands the alveoli for inhaling surface tension opposes alveolar expansion and reduces compliance fortunately we have a substance called pulmonary surfactant that decreases surface tension in the alveoli pulmonary surfactant is a phospholipo protein complex produced by alveolar type ii cells surfactant molecules mix in with the water and they break up some of the hydrogen bonds reducing surface tension and increasing compliance surfactant production in fetal lungs is very low until about two weeks before birth so premature infants may not have enough surfactant to be able to expand their lungs and breathe after they're born they can be treated with artificial surfactant to reduce the surface tension in their alveoli and allow their lungs to expand so i'm going to review this concept of compliance and pulmonary surfactant and i will use again the concept of well this could be an alveolus or it could represent the entire lung so here's now alveolus and alveoli have a layer of fluid represent that by blue dots on the surface of the alveolar cells and that's necessary to keep them from drying out because if they were dry it would be difficult for carbon dioxide and oxygen to diffuse through them and interchange with molecules in the blood so again these blue dots represent molecules of water water unfortunately likes to get together with other water molecules and so if it were just water in there all of those water molecules are trying really hard to get in touch with all of the other water molecules so these arrows represent surface tension and if there were no other forces opposing that that alveolus would just collapse once they're collapsed they could only be opened by positive pressure from the outside so that would be a very serious situation and in this condition we would have low compliance but fortunately for us we have a molecule that's made by the alveolar type 2 cells and i'm going to go ahead and put in some water molecules so i will draw surfactant as red molecules and the effect of surfactant is to reduce the surface tension there's still some but notice that these yellow arrows are a lot smaller than they were before i added the surfactant and so pulmonary surfactant increases compliance by reducing surface tension you