The lungs are the organs that we use to breathe, which is a mechanism for getting oxygen into the body and getting carbon dioxide out. The main function of the lungs is to make sure as much air as possible comes into contact with the blood in the veins in the lungs, so that oxygen passes in and carbon dioxide passes out efficiently.
Structure of the chest cavity
The chest cavity, or thorax, is bounded by the rib cage, and separated from the abdomen by the diaphragm, a flat sheet of tough connective tissue and muscle that helps us to breathe in and out. Within the chest cavity are the two lungs, the heart and the oesophagus together with various blood vessels and nerves.
The ribs form a protective cage around the lungs but this is moveable thanks to two sets of intercostal muscles, which lie between the ribs:
The external intercostal muscles contract to allow us to breathe in or inhale
The internal intercostal muscles contract when we want to breathe out or exhale more forcefully than normal. Quiet, gentle exhalation is largely a passive process – we just relax and it happens; it does not require energy.
There are two pleural membranes, one covering the lungs and one lining the ribs. Between the two membranes is the pleural space, filled with pleural fluid. This arrangement serves to attach the lungs to the ribs so that when the ribs move, the lungs do too. It’s a bit like pressing two wet pieces of glass together – they can slide sideways but it’s very hard to separate them.
If the pleural cavity is penetrated – by a knife or a broken rib, for example – air gets into the pleural cavity. In this case the lung shrinks back to its smallest size and breathing becomes very difficult because although the ribs can still move, they are not able to inflate the lungs. This situation is known as a pneumothorax, or collapsed lung.
What is inside the lungs?
The lungs themselves are spongy organs consisting largely of airways, air sacs and blood vessels. There is no muscle in lungs, so they cannot inflate by themselves. However, lungs are naturally elastic so that once inflated, they will tend to recoil and expel air. This minimises the effort needed to breathe out.
Air is delivered into the air sacs, called alveoli, via a network of tubes called the bronchial tree. Listed from largest to smallest there is the trachea, bronchi, bronchioles and finally the terminal bronchioles that link to the alveoli.
The trachea, bronchi and bronchioles are supported by rings of cartilage which prevent them from closing. The walls of the terminal bronchioles, however, have no cartilage and are largely made from smooth muscle. This difference is important in understanding asthma.
Breathing in and out
At sea level, there is about ten miles worth of atmosphere pushing down on us. This is quite a lot of pressure, and is known by scientists as ‘one atmosphere’.
To breathe in, we increase the volume in the thorax so that the pressure inside drops. This is done by contracting the external intercostal muscles and the diaphragm. The ribs are pulled up and out, while the diaphragm flattens. When the pressure in the chest becomes lower than the atmosphere, air goes into the lungs.
To breathe out we need to decrease the volume in the thorax and thereby increase the pressure. To do this we normally just relax. The weight of the rib cage and the natural elastic recoil of the lungs forces air out without much effort on our part. If we want to breathe out more forcefully, we can use our internal intercostal muscles.
Gas exchange at the alveoli
The fresh air that reaches the alveoli contains about 20% oxygen and 0.04% carbon dioxide; the rest is mainly nitrogen. The blood arriving at the lungs has lost some of its oxygen and has quite a lot of carbon dioxide, so an exchange takes place via the process of diffusion. The haemoglobin in the red cells picks up oxygen, becoming bright red in the process.
The air we exhale contains less oxygen and more carbon dioxide than atmospheric air, but the process is less efficient than people think. Exhaled air has about 16% oxygen and 4% carbon dioxide.
How do the lungs keep clean?
Household dust is quite an unpleasant mixture of human skin cells, bacteria, fungal spores, pollen, fibres from clothes and a variety of other substances, and we breathe it in with every breath.
The lungs are a splendid place for bacteria to live: warm, moist and lots of oxygen. So why don’t we succumb to lung infections and coughing fits most of the time?
The lungs have their own cleaning system to reduce debris and to avoid infection. The airways are lined with two types of cells: ciliated cells and mucus-producing cells. The cilia form a carpet of tiny hairs that move a continuous layer of mucus up and out of the lungs. Most of the dust particles become trapped in the mucus and moved to the oesophagus, where they are swallowed and destroyed by stomach acid.
Several different types of white cells also patrol the surface of the airways, mopping up foreign particles. All in all it’s a very efficient system, but smoking paralyses the hairs and other substances in tobacco irritate the airway lining so that they become inflamed, which means that they will become red, swollen and produce more mucus than usual.
What can go wrong?
Lung tissue is very delicate and problems with any of these mechanisms, damage by accidents, breathing in harmful substances, and attack by bacteria, viruses or fungi, can lead to lung problems. The next article in the guide gives an overview of what can cause coughing and breathlessness, two of the most common symptoms of lung problems, and then we go on to examine specific problems in greater detail.