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Probably the most significant organs that divers need to understand.
Structure
 The left side of the lungs has upper and lower lobes and the right side of the lungs has upper, middle and lower lobes.
Outer surfaces of lungs covered by a very thin and slippery double membrane called Pleura. The space between inner chest and outer wall called Intra Pleural cavity or space.
The mediastinum is the name for the mass of tissues and organs separating the two lungs. It extends from the sternum, or breastbone, back to the vertebral column and is bounded laterally by the pericardium, the membrane enclosing the heart, and the mediastinal pleurae, membranes that are continuous with those lining the thoracic cage. The mediastinum comprises the heart and all other thoracic structures except the lungs. It is arbitrarily divided into the anterior, middle, posterior, and superior regions.
The air passes through the throat into the trachea or windpipe.
Breathing
 Inspiration (breathing in) The chest wall is pulled up and out by muscles between the ribs in the chest wall (external intercostal muscles) and the diaphragm moves down by tightening. This causes the size of the chest cavity to increase, lowering air pressure in the lungs and therefore air flows in.
Expiration (breathing out) This is normally passive and is due to elastic recoil of the lungs, chest wall, and diaphragm. this increases the pressure in the lungs forcing gas out. In a forced expiration e.g. when short of breath, coughing, clearing a regulator, etc. the abdominal muscles contract forcing the abdominal contents upwards against the diaphragm, pushing it upwards, at the same time muscles between the ribs (internal intercostal muscles) contract, pulling the chest wall in and down. this causes a much larger rise in pressure in the lungs and gas flows very rapidly out.
The mucus membranes in our mouth and nose warm and moisten the air, as well as trap particles of foreign matter.
Air enters the lungs from Larynx to Trachea to progressively narrowing tubes. The branches of the bronchi eventually narrow down to tubes of less than 1.02 mm (less than 0.04 in) in diameter. These tubes, called bronchioles, are covered by microscopic hairs called Cilia and divide into even narrower tubes, called alveolar ducts. Each alveolar duct ends in a grape like cluster of thin-walled sacs, called alveoli.
The Alveoli
Alveoli are small, thin air sacs that are arranged in clusters like bunches of balloons. (a single sac is called an alveolus). From 300 million to 400 million alveoli are contained in each lung. The air sacs of both lungs have a total surface area of about 93 sq m, nearly 50 times the total surface area of the skin.
 When you breathe in, by enlarging the chest cage, the "balloons" expand as air rushes in to fill the vacuum. When you breathe out, the "balloons" relax and air moves out of the lungs.
Tiny blood vessels (capillaries) surround each of the 300 million alveoli in the lungs. Oxygen moves across the walls of the air sacs, is picked up by the blood and carried to the rest of the body. Carbon dioxide or waste gas passes into the air sacs from the blood and is breathed out.
 The alveoli have very thin walls which are very close to the thin walls of the capillaries.
Contrary to the norm, here in the lungs the arterial blood has low oxygen and a high carbon dioxide content. Venous blood has a high oxygen and low carbon dioxide content. The gases equalise across these membranes.
This occurs due to difference in partial pressure of both O2 and CO2. This partial pressure causes a pressure gradient therefore gases diffuse across membrane. Similar pressure gradient occurs in blood tissue level where CO2 is released by tissues and O2 diffuses into tissues. The gas exchange occurs in the lungs by diffusion determined mainly by the solubility of the gases. CO2 diffuses about 20 times more rapidly than O2 because it is much more soluble.
The alveoli and bronchioles are coated on the inside with a detergent like substance called Surfactant, this reduces surface tension makes the lungs much easier to inflate and helps stop the collapse of air passages. Smoking destroys surfactant inhibiting reopening of the bronchioles.
Control of respiration
The Medulla is the Respiratory Control Centre located in the brain stem. This contains nerve cells called chemo receptors. These are sensitive to carbonic acid which is carbon dioxide in solution in the body. Carbonic acid is Hydrogen Ions. The respiration rate is thus controlled by carbon dioxide, which ensures that lungs remain functioning at a rate that suits the oxygen supply needed by the tissues.
The Respiratory Control Centre also regulates breathing in response to changes In blood CO2 level to a lesser degree by sensors in Aorta and Carotid Arteries which detect O2 levels. It responds to low oxygen levels and high carbon dioxide levels, but low oxygen alone will not stimulate breathing, high carbon dioxide must be present.
Breathing
As most divers know we breathe the following mixtures:
| Component |
Inhaled air |
Exhaled Air |
Difference |
| Nitrogen |
78% |
78% |
|
| Oxygen |
21% |
17% |
4% |
| Carbon Dioxide |
0.033 |
0.4% |
0.4% |
| Other |
1% |
1% |
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We do not consume all the oxygen in the air we breathe and this explains why mouth-to-mouth resuscitation works. The air we breathe out contains besides about 16% oxygen which is just enough to sustain life. Rebreathers work by circulating and reusing the air, however they must replace the Oxygen used and remove (scrub) the Carbon dioxide.
Important Terminology
| Term |
Capacity |
Description |
| Total Lung Capacity (TLC) |
5-6 litres |
Total volume of air that can be held by both lungs |
Vital Capacity
(VC) |
4-5 litres |
Maximum volume that can be breathed out after maximum inspiration |
| Residual Volume (RV) |
1.5 litres |
Amount of gas left in lungs after maximum expiration. RV is about 25% of TLC |
Tidal Volume
(TV) |
0.5 litres (approx.) |
Volume of gas moved in and out during normal breathing resting cycle |
| Dead Air Space |
150ml |
The air that is not inhaled or exhaled in one breathe. |
Respiratory Minute Volume
(RMV) |
25 litres |
Total amount of gas moved in and out of lungs during one minute (approximately 6 litres per minute at rest and 100 litres per minute with strenuous exercise). |
| Surface Air Consumption (SAC) |
25 Litres |
A typical breathing rate per minute for a diver. |
Barotrauma of the lungs
Lung squeeze or over-expansion of the lungs is very dangerous, it's probably the number 1 rule of diving - never allow a pressure difference to occur in the lungs, especially on the ascent That is NEVER HOLD YOUR BREATH whilst underwater.
Lung Squeeze - The average lungs are designed to go from about 6 litres when full to a minimum of 1.5 litres (Residual Volume) when you breath out. If they shrink beyond the Residual volume, serious problems will occur.
Free divers may take a breathe on the surface and then descend, as you would expect they are subject to Boyle's law and the lungs will compress according to the depth.
Over expansion of the Lung - As most divers know if you took a balloon down to 30 metres, filled and sealed it with 1 litre of air (at 4 BAR) then ascend, the balloon would grow in size as the ambient water pressure lessoned and the air inside the balloon expanded. At the surface it will be 4 times the size that it was at 30 meters. The biggest size change would be in the last 10m where it will have doubled in size.
The same will happen to our lungs if we seal them by holding our breath whilst ascending. A normal person has very little tolerance in terms of lung expansion so physical damage can easily occur in just a few metres of depth, particularly near the surface.
Skip breathers are prone to these risks as they habitually hold their breath and may do for anything up to a minute. Furthermore they may take a huge breath before doing so. If they ascend during this time, either deliberately or as a result of an up current, they may suffer physical lung damage. Unlike the ears, divers cannot feel the pressure in their lungs at this level and there is no pain to act as a warning.
The unfortunate sequence of events are as follows:
- The lungs over inflate.
- Each alveolus over inflates.
- The increasing air pressure breaks the thin walls of the alveoli and is forced directly into the blood capillaries.
- This causes bubbles in the blood stream (Pulmonary veins) which carries them to the heart
- The bubbles then move out of the heart to the body.
- The bubbles travel until they can go no further because the blood vessels are getting smaller and smaller. The bubbles are then trapped and clots may form around them, downstream tissues will be starved of oxygen and may die.
- Where this blockage occurs will determine the seriousness of the event. For example if this happens in the brain or the nervous system it could be extremely serious, and could end up with a stroke, paralysis or worse. If this happens in the skin the effects may be minor.
- If the excess pressure increase further, then the lungs may rupture.
- This can lead to haemorrhage which could flood the lungs and make breathing more difficult.
- If sufficient air gets trapped in the pleural cavity, the lung will collapse.
- Air may escape into the surrounding tissues causing crepitus (cracking under the skin).
Treatment
First aid for a diver with lung injuries is similar to that of DCS, i.e., 100% oxygen, make the casualty safe, reassure and get medical aid fast. See DCS for more information
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