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Lungs, Respiratory Tract and Breathing

Where are the lungs found?

The lungs are found in the chest on the right and left side. At the front they extend from just above the clavicle (collarbone) at the top of the chest to about the sixth rib down. At the back of the chest the lungs finish around the tenth rib. The pleura (the protective membranes which cover the lung) continue down to the twelfth rib. From front to back the lungs fill the rib cage, but are separated by the heart, which lies in between them.

Air that we breathe enters the nose, flows through the pharynx (throat) and larynx (voice box) and enters the trachea (windpipe). The trachea eventually divides into two parts called bronchi. The right main bronchus (bronchus is the word for one bronchi) supplies the right lung; the left main bronchus supplies the left lung. These bronchi then go on to divide into smaller bronchi. Eventually the bronchi become known as bronchioles – the smallest air tubes in the lungs. This system of air tubes can be thought of as an upside down tree, with the trachea being the main trunk and the bronchi and bronchioles being the branches. The medical term for all the air tubes is ‘the respiratory tract’.

At the end of the smallest bronchioles are alveoli. Alveoli are tiny sacs that are lined by a very thin layer of cells. They also have an excellent blood supply. The tiny alveoli are the place where oxygen enters the blood and where carbon dioxide (CO2) leaves the blood.

The lungs are divided into different parts by what are known as fissures. Fissures are separations of the tissue of the lung that divide the lung into lobes. The right lung has three lobes called upper, middle and lower lobes. The left lung only has two lobes, the upper and lower.

What do the lungs do?

The lungs’ main function is to help oxygen from the air we breathe enter red blood cells. Red blood cells then carry oxygen around the body to be used in the cells found in our organs and tissues. The lungs also help the body to get rid of CO2 gas when we breathe out. There are a number of other jobs carried out by the lungs that include:

  • Changing the pH of blood (whether the blood is more acid or alkali) by increasing or decreasing the amount of CO2 in the body.
  • Filtering out small blood clots formed in veins.
  • Filtering out small gas bubbles that may occur in the bloodstream.
  • Converting a chemical in the blood called angiotensin I to angiotensin II. These chemicals are important in the control of blood pressure.

How do the lungs and breathing work?

Breathing in is called inhalation. For air to flow into the lungs there must be a difference in the air pressure in the lungs and the pressure outside. Air is made up of tiny particles including oxygen. If these particles are held together, in a bottle for example, they push on the sides of the bottle. This ‘push’ is what is known as pressure. If the size of the bottle and the amount of air in it stays the same, the pressure in the bottle will stay the same. But the pressure in the bottle can change. If the size of the bottle increases without allowing more air in, the pressure in the bottle goes down. This is because there are fewer particles inside than outside. If you then removed the lid, air would flow into the bottle. This would make the pressure on the inside the same as the outside.

Getting fresh air into the lungs works on a similar principle. For inhalation to happen, the lungs must get bigger. This lowers the pressure inside the lungs in comparison to the outside. Air rushes into the lungs to make the pressure equal – a breath in.

The size of your lungs varies according to what you are doing. The body has a special set of muscles that help to make the lungs increase in size. The most important muscle of inhalation is the diaphragm. Found beneath the lungs, the diaphragm is a dome-shaped muscle. When this muscle contracts (gets tighter), it flattens and the lungs increase in size. During exercise the diaphragm flattens more than when you are resting. This causes the lungs to expand more, causing more air to flow in.

Exhalation is the process of breathing out. Essentially this is the opposite of inhalation, except that it is usually a passive process. This means that muscle contractions are not generally required. Exhalation also relies on the difference in pressure between the inside and outside of the lungs. However, in this case, the pressure on the inside is greater than on the outside. This causes air to flow out of the lungs.

The lungs receive deoxygenated blood (blood that has lost its oxygen) from the heart through blood vessels called the pulmonary arteries. The deoxygenated blood is then sent to the alveoli. Here, oxygen that has been funnelled through the bronchi and bronchioles can pass across the thin membranes found in the alveoli. A chemical substance within red blood cells haemoglobin called has a great attraction to oxygen. Haemoglobin binds oxygen tightly within red blood cells, allowing oxygen to be carried in the bloodstream. At the same time as oxygen is going into the bloodstream, CO2 is coming out. CO2 moves out of the blood and enters the alveoli. This allows the CO2 gas to be exhaled.

Once the blood passing through the lungs picks up oxygen it is known as oxygenated blood. This blood returns to the heart in the pulmonary veins. Once in the heart the oxygenated blood is then pumped around the body. The oxygen carried by the red blood cells can then be used in the body’s cells.

The basic rhythm of breathing is controlled by the brain. Part of the brain called the brain stem has a special area dedicated to maintaining your breathing pattern. Nerves originating in this area generate electrical impulses. These impulses control the contractions of your diaphragm and the other muscles of breathing. This is all done without thinking. However, other parts of the brain can temporarily overrule the brain stem. This is how we are able consciously to hold our breath or change our pattern of breathing.

While the brain controls the basic rhythm, it also receives information from sensors in the body. These sensors are nerve cells and provide information that influences the rate and depth of breathing. The main sensors monitor levels of CO2 in the blood. When the levels of CO2 rises, the sensors send electrical impulses to the brain. These impulses cause the brain to send more electrical signals to the muscles of breathing. Breathing gets deeper and faster and more CO2 is exhaled. The blood level of CO2 then decreases and the sensors stop sending signals to the brain.

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