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The Lungs
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What are the lungs?

You have to breathe if you want to live. It's quite a simple thing to understand, and most people do. People associate 'not-breathing' with 'death' - if you're looking to see if something is all right after a serious accident, one of the things you may check is their ability to breathe. So why is breathing - and therefore the lungs, with which we breathe - so important?

Well, along with the heart, the lungs are found in the thorax and are responsible for making sure every cell in the body is able to meet its energy needs. This means that it has to get oxygen (from the air) and nutrients or fuel (from food) to every cell in the body. The transport system is the circulatory system, and the 'delivery lorries' which carry the oxygen to every cell is the blood. The heart pumps blood to where it is needed, and the digestive system in the abdomen digests the food to obtain the nutrients. So all you need is to get oxygen from the air. Here's where the lungs get involved.

If you stop breathing for a minute - or if you hold your breath - you notice that your chest stops moving completely and it starts to become very difficult. This is because your lungs have stopped moving, and every part of your body is telling you that your lungs shouldn't have! It is essential that a steady supply of oxygen gets into the blood so that you can make use of the fuel which you get from your food.

The image on the right shows the route that air takes into the lungs. As you breathe in through your mouth or nose, air passes to the back of your throat where a pipe called the trachea takes a route down into the thorax. This pipe follows down to just below the height of the lungs, and then splits into the main bronchi - left and right. As you look at the person, their right is your left, so the names may appear unusual. The place where the trachea splits is called the carina.

If you try and walk through a wall, you simply won't be able to do it. However, if the wall were really thin with big holes in it, you probably would be able to fit through. It's much the same with the lungs. The lining or 'wall' of the trachea is thick, and so there's no way that oxygen could fight it's way through to the blood on the other side. However, oxygen can fit through the thinner walls which are found inside the lungs. So the air travels into the lungs and eventually reaches tiny sacs called alveoli which have thin walls to allow oxygen to travel through. More about that later.

As the air follows down through the bronchi, the bronchi begin to split. This gives the air several different options as to where it could go. Each of the places that the air could end up is given a different name - and they are referred to as lobes. The left side has two lobes - the upper lobe and lower lobe, separated by a kind of groove known as a fissure, in this case an oblique fissure.

The right side has two fissures - the upper lobe and middle lobe are separated by the horizontal fissure, and the middle lobe and lower lobe are separated by another oblique fissure. Each of these different lobes has a separate blood supply - which means that if something goes wrong with one of them, and surgery has to take place to remove one, then it can happen without too much disruption to the rest of the lungs. What a clever design!

What is the diaphragm?

As described briefly in the section on the thorax, the diaphragm is a muscle, made up of many fibres which form two domes. The right dome is slightly higher than the left because of the contents of the abdomen beneath: the liver, the largest solid organ in the body, is found just below. All the same, when the diaphragm contracts, it is still able to push down.

Air will flow from where there is high pressure to where there is low pressure, in an effort to try and balance it all out - if air moves towards an area of low pressure, the pressure in that area will increase as a result of the increased volume of air.

In this sense, the thorax is often compared to a large bell jar with an opening at the top to which the lungs are attached, and a domed bottom. When this domed bottom descends (like when the diaphragm contracts), the volume inside the belljar increases. This means the air particles are more spaced out, so the pressure decreases. Air from outside will enter through the opening at the top, heading towards this area of low pressure, and in doing so will fill the 'lungs'. When the domed bottom rises again, the pressure will increase, this time to a pressure which is higher than that of the outside; so the air will rush out of the lungs.

This is how the diaphragm works. Combined with contraction from intercostal muscles (which prevent the spaces between the ribs billowing in or out during breathing) and other muscles such as pectoralis major (which contracts to lift the rib cage), breathing is effectively achieved by increasing and decreasing the volume of the thoracic cavity (the space within the thorax).

What are the airways of the lungs?

The airways of the lungs are the route that air takes to get to the alveolar sacs where gas exchange can take place. They are like tunnels which run from the back of the throat and through the lungs, and form a maze of very tiny tubes for air to pass through.

In the same way that arteries have a thick lining close to the heart to prevent blood leaking, airways have a thick lining close to the mouth to prevent tearing. When air comes in, it isn't just oxygen that you find in it. There's also lots of other gases, and very small particles which you can't see, including dust and germs. These tiny particles come in at high speeds at the air rushes into the lungs, and this could tear the lining of the airways if it wasn't thick. So at the top of the lungs, the cells which line the airways make up a thick barrier to prevent injury.

Further down, however, the lining gets thinner so that gas exchange can take place. Oxygen can't get through thick walls, but it can get through thin walls. So, from the respiratory bronchioles onwards, the lining of the airways is thin enough for gas exchange to take place.

The airways are named depending mainly on their structure, but it is often difficult to see exactly what they're made of unless you have a very powerful microscope. Fortunately, the structure changes at particular points, which makes it easy to identify. As you go down the main tube leading to the lungs - the trachea - the airway splits into two at a division called the carina. Each of the tubes which leads from this is called a main bronchus (that is, you have 1 bronchus, many bronchi), and each of these two tubes splits into two; and each of these four tubes splits into two .... and so on and so on. After the tubes have split once, then, they are called main bronchi. After they have split three times, they are referred to as the large bronchi, and after 10 times it is the smaller bronchi. After 15 you have the bronchioles, and the respiratory bronchioles, at which gas exchange can take place, occur after the 18th divison. The alveolar ducts occur at the 21st division, and the 23rd division gives you your alveoli. In theory this would give you over 8 million alveoli; in fact there are in the region of 300 million alveoli!!

What are alveoli?

Alveoli or alveolar sacs are the sacs which you're left with at the end of the airways, and are probably the most important part of the lungs. They are where deoxygenated blood loses its carbon dioxide and receives the oxygen it needs to become oxygenated. Blood from the right side of the heart is deoxygenated because it arrived at the heart from the rest of the body, and while it was in the body it delivered its oxygen. So, when it is pumped out of the heart, it is pumped into the lungs. Here it travels initially through something called the pulmonary artery (pulmonary meaning something to do with the lungs, and artery meaning a blood vessel which is leading away from the heart), which divides into smaller vessels - and eventually tiny capillaries.

So blood cells travel in single file along these tiny blood vessels. These capillaries are wrapped around the alveolar sacs, with only a thin cell border between the blood and the sacs; so when the blood, which is severely lacking in oxygen, arrives in these capillaries, oxygen diffuses across a large concentration gradient into the blood cells; that is, because there is loads more oxygen in the air-filled alveoli, it effectively wants to find somewhere which has less oxygen, to 'share the wealth', and therefore moves into the blood. The blood cells, now rich in oxygen, leave fully oxygenated to return to the heart in the pulmonary vein.

What is gas exchange?

Gas exchange is a process where gases are exchanged (no surprises there!) and, in case it wasn't quite obvious, the exchange is between carbon dioxide and oxygen. Oxygen comes from its rich supply in the air contained within the alveoli, and passively diffuses into the capillaries so that it can be dissolved in the blood and taken up by the red blood cells. Carbon dioxide, meanwhile, diffuses out of the blood and into the alveoli.

Proper consideration of gas exchange introduces two very important terms: ventilation and perfusion. I was always quite puzzled by these things, but they're actually quite simple, and they help us to understand two key aspects of the whole gas exchange process - two key things which are necessary for it to take place properly.

Ventilation is the access of air to the lung, and particularly the alveoli. If someone has good ventilation, it means they're getting lots of air in, and they're able to get it out again. There's a good turn around of air! If someone has poor ventilation it probably means there's something getting in the way of air coming in, like a peanut which has been swallowed, or a lot of mucus or pus which is lining the airways.

Perfusion is the access of blood to the lung capillaries. If you can't get blood to the capillaries, there's no way for the oxygen to get into the blood, nor for the carbon dioxide to escape. If alveoli were like a corner shop, you can't pick up your shopping unless you actually go there. It's all very well driving somewhere in the region, but unless you actually get to the corner shop itself, you can't pick up the goods. In the same way, blood can't pick up oxygen unless it gets to the alveoli. Low perfusion could be due to a pulmonary embolism blocking one of the vessels.

Ventilation is given the letter "V", which of course makes sense. Unfortunately somebody decided to give perfusion the letter "Q".

The whole "ventilation-perfusion" situation enables doctors to use something called a ventilation-perfusion scan, which takes two pictures of the lungs - one which looks at whether or not there's air getting to the whole of the lungs, and whether or not there's blood getting to the whole of the lungs. They're useful when you're trying to work out why someone might be breathless, as they can show what's causing the reduced gas exchange.

Gas exchange is therefore quite a simple concept, but importantly it relates to ventilation and perfusion, and it's important to know which of these two (or both!) is responsbile if gas exchange is being impaired.

Further Reading