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A. How do the lungs really Inspire and Expire?
How does air go in and out of the lungs? There are many ways to tell this story but I have chosen to start with a flexible bottle … like a balloon.
In the diagram below, you see, left, a sketch of a normal open bottle. For the fun, I have rendered the neck of the bottle into something that is similar to the respiratory upper ways.
As this bottle is open, the air inside is at equilibrium (= same pressure) with the air outside and air will not flow in or out of the bottle.
To get air to flow out of the bottle, the size of the bottle has to decrease (right sketch). This will increase the pressure inside and therefore push the air out. Simple!
If the pressure inside the lungs is lower than outside, then air will flow into the lungs (inspiration). If the pressure inside is higher, air will flow out of the lungs (expiration).
What is the pressure actually outside? In reality, this pressure is the atmospheric pressure; the column of air that weighs down upon us.
This pressure amounts, at sea level, to about 760 mmHg. This varies a bit according to the weather etc.
So, the pressure outside is (about) 760 mmHg. The pressure inside the lungs, normal size, is obviously the same (at least, when your mouth and nose are open!).
If the lungs increase in size (larger volume), then the pressure will (slightly) decrease, and this will create an inflow of air. The opposite occurs when the lung volume is reduced; then the pressure is increased and air is squeezed out.
8.
The principle is similar to that used in bellows.
If the surface of the lung where simply fixed to the inside of the chest wall, then the pushing and the pulling, the constant stretching and compressing by the ribs will bruise and damage the lung surface.
Therefore, a special system has been built between the lungs and the rib cage to protect the delicate lungs and, at the same time, to “fasten” or to fix the lungs to the rib cage.
This special system consists of:
- two pleura’s
- the pleural space
- the pleural fluid
- the pleural pressure.
The lungs are covered with a special membrane called a ‘pleura’. Because it is fixed to the lungs, this membrane is called the visceral pleura (viscera = internal organ).
Between the two pleurae’s, there is a fluid (= pleural fluid) to lubricate the two surfaces. This lubrication is necessary because when one breathes, as the lungs expands or deflate, the two pleura will rub against each other. Try this at home!
This makes that the two pleural spaces are NOT connected to each other. In fact, this divides the chest into a right lung cavity and a left lung cavity.
Try this at home!: Take two hard but flat surfaces. Two glass plates are ideal but you can also try this with two flat plastic surfaces. Place them on top of each other and rub one on top of the other. Notice how rough this actually is. Now place some water between the two plates and rub again: Aaah! This is much smoother!
If the pleural space is dry, as in pleuritis (= infection of the pleura), then the two pleurae will rub against each other without the lubrication of the pleural fluid and this is painful!
These are like a fold or a root, one for the right and one for the left lung where the blood can go in and out and where the left and right bronchia connect to the lungs.
Note that the artery here is colored blue and the veins red! Why did I do that?
In the second diagram, we see the right hilum with the entry and exit sites for the right bronchus, the right pulmonary artery, the two right pulmonary veins and the lymph vessels (not shown) and the nerves (also not shown) and the bronchial circulation (also not shown). The left hilum looks very much the same.
Remember that there is a third ‘space’ inside the chest; the mediastinum. This space, located between the two lungs, contains the heart, all the major blood vessels, the trachea and the esophagus. It is the central ‘shaft’ inside the chest.
Why did I do that? Because the arterial blood from the right heart is oxygen-poor while the venous blood, having been oxygenated in the lungs, is now oxygen-rich.
But if you suck the air out of the balloon, then the balloon will shrink and the walls of the balloon will “stick to each other”.
Here are a few examples:
- sucking a fluid through a drinking straw
- drawing a fluid into a syringe
- inspiration!
The lungs themselves are very elastic. If you would take them out of the chest, they would collapse into a small ball.
The ribs, if they were not strong, would collapse under this negative pressure like the balloon when you suck the air out.
But because the ribs and its cage are strong, the visceral pleura, with the lungs attached to it, is now pulled towards the parietal pleura as shown in the diagram.
Therefore, this negative pleural pressure will make the visceral pleura ‘stick’ to the parietal pleura.
This sliding is made easier because of the pleural fluid to lubricate this friction.
This is exactly what constantly happens during our inspiration and expiration.
But there is a risk!! The risk is what happens if you get a hole to the pleural space.
If for some reason there is a hole in the pleura, then air will flow into the pleural space, the pleural pressure will immediately become equal to the outside pressure and, pfhee…, the lungs will collapse!
This condition is called a pneumothorax (pneumo = air in the chest (=thorax)).
A hole through the parietal pleura is most often caused by a trauma, a knife, a bullet, a car accident etc. Something sharp has punched a hole through the rib cage.
A hole through the visceral pleura is often caused by the bursting of one or more alveoli to the pleural space. Often because of wear and tear, too much coughing, smoking etc.
In most cases, the pneumothorax is one sided; either the left or the right lung collapses.
With one lung collapsed however, you don’t die, as the other normal lung is enough to keep you alive! But a collapsed lung is painful and you need help.
The three curves drawn above shows the changes in volumes and pressures during the breathing of the lungs.
When the chest expands (by flattening the diaphragm and raising the ribs), then the volume in the chest increases.
This increase in chest volume will, by itself, lower the pleural pressure.
This decrease in pleural pressure will “stretch” the lungs towards the chest wall thereby increasing the lung volume.
Since there is an opening between the lungs and the outside world (through the respiratory airways), air will then flow into the lungs: inspiration!
The same chain of events occurs during expiration, only in the opposite direction! (decrease in chest volume > decrease in pleural pressure > decrease in lung volume > expiration).