3.3.2: Gas exchange (The human gas exchange system)

Okay we are *almost* done with gas exchange.

Mammals must absorb/remove a large volume of oxygen/carbon dioxide, respectively, because they are relatively large organisms with a large volume of living cells and they have to maintain a high body temperature which relates to them having high metabolic and respiratory rates.

First off, we need to know a bit about how much air is taken in/out of the lungs in a given time. This is the pulmonary ventilation rate and can be calculated using the equation…

pulmonary ventilation rate = tidal volume x breathing rate

Tidal volume is the volume of air normally taken in at each breath when the body is at rest

Breathing/ventilation rate is the number of breaths taken in one minute

The lungs are the site of gas exchange in humans. They are situated inside the body as air is not dense enough to support/protect them (they are very delicate) and also the body would lose a large amount of water/dry out. The structure of the lungs is as follows:

• The trachea: flexible airway supported by cartilage rings (similar to insect tracheae). The cartilage prevents collapse when pressure decreases when breathing in. Walls are made of muscle lined with goblet cells and ciliated epithelium. Produce mucus to trap dirt
• Bronchi: two divisions of the trachea. Produce mucus to trap dirt but do not contain cartilage all the way along them. Are also ciliated to move dirt particles to the throat
• Bronchioles: series of branching divisions of the bronchi. Walls made of muscle  which can constrict to control how of air in/out of alveoli
• Alveoli: air sacs at the end of bronchioles. Between them is are some collagen and elastic fibres. Elastic fires allow them to stretch and recoil during inhalation/exhalation respectively to expel co2 rich air. This is the gas exchange surface. Lined with epithelium. Around each alveolus is a network of capillaries. Red blood cells are slowed and flattened against the capillary walls, increasing the time for diffusion and decreasing the diffusion distance.

Yeah, that makes sense, but how does the air actually get in and out of the lungs?

To maintain a diffusion gradient, air is constantly moved in and out of the lungs (ventilation). When air pressure in the lungs exceeds atmospheric air pressure air is forced out of the lungs (expiration/exhalation). When air pressure of the atmosphere is greater than air pressure inside the lungs, air is forced into the lungs. These pressure changes are as a result of certain muscles. Inspiration:

• Diaphragm contracts, increases the thorax volume
• External intercostal muscles contract
• Internal intercostal muscles relax
• Ribs are pulled up and out
• Increased thorax volume decreases lung pressure

Expiration:

• Diaphragm relaxes, decreasing the thorax volume
• External intercostal muscles relax
• Internal intercostal muscles contract
• Ribs move inward and down
• Decreased thorax volume increases lung pressure

NOTE: I find it SO hard to remember which of the intercostal muscles relax/contract for inhalation/exhalation - if anyone has a way of remembering it please let me know (in the comments??)!!

NOTE 2.0: During normal breathing it is the recoil of alveolar elastic tissue which mainly forces the air out. Strenuous exercise causes various muscles to play a part so gases are exchanged faster = more oxygen in = more respiration = more ATP = less anaerobic respiration/reduced oxygen debt

Okay so one last bit. The spec says we should be able to interpret information relating to the effects of lung disease on gas exchange etc. Here we go…

Specific risk factors increase the risk of lung disease (COPD). These include:

• Smoking
• Air pollution - pollutant particles and gases
• Genetic make up - people may be genetically more/less likely to obtain lung disease (explains why some life long smokers never get lung disease)
• Infections - if you frequently get chest infections you’re more likely to have a higher chance of obtaining lung disease
• Occupation - individuals working with harmful chemical/dusts/gases may have an increased risk of obtaining lung disease

Finally, don’t forget that correlation does not mean cause!!