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Gas exchange in mammals and plants

In complex, multicellular organisms cells organise themselves in such a way that enables a greater structure and function. Cells differentiate into specific structures and functions, and organise themselves as such into tissues. The different tissues can then form organs with yet a higher level of structure and function, and organs can work together in certain broad areas of the organism’s structure and function by taking part in organ systems.

Isn’t that grand? Let me tell you a little secret. For anyone who’s seen the film Life starring our first Martian life form called Calvin, there is way that cells can associate the way Calvin does. In this case, cells aren’t at all differentiated to do different things in tissues, organs, etc. but instead maintain their single cell status among equal single cells. They associate at a higher level to produce certain greater effects, and can even look as if the structure were multicellular or complex, but it isn’t really, or at least it isn’t in the same way it is for true multicellular organisms.

This is the case for algae as well as for fungi and creepy Martian creatures.

Gas exchange in mammals

The mammalian (hence human) gas exchange system is made of the trachea, from which the bronchi branch off, followed by the bronchioles into the lungs, and finally the alveoli, which are the functional unit of the lungs. Of course this is nonsense without an image:

Air enters the lungs via the trachea, bronchi and bronchioles into the tiny air sacs – the alveoli.

Multiple tissues including smooth muscle, ciliated epithelial tissue, elastic fibres and cartilage make up the respiratory organs. Ciliated epithelial tissue is found in the airways. It consists of epithelium with membrane protrusions, hair-like, that move in order to clear out debris from the lungs. Cartilage and elastic fibers all work in unison to provide structural stability to the trachea and lungs during ventilation. They enable the big range of motion during inhaling and exhaling without collapsing the lungs. The ability to expand the tissue with air is ensured by these types of tissue.

Smooth muscle provides constriction which helps to limit the amount of air with toxins or debris that is inhaled, as the case may be.

Surfactant molecules are secreted in the lungs to prevent friction, surface tension and the lungs collapsing.

The epihelium of the alveoli is extremely thin (just one-cell wide, in fact) to allow fast diffusion of oxygen into the red blood cells, and of carbon dioxide out of them. This tissue is called squamous epithelial. The capillaries surrounding alveoli are so narrow, that the red blood cells have to be squished in order to pass through. This shortens the diffusion pathway, which in turn increases the rate of diffusion.

What allows diffusion to take place, of course, is the concentration gradient formed between the air in the alveoli and the red blood cells. Red blood cells deprived of oxygen and loaded with carbon dioxide (the blue/purple ones) will release carbon dioxide into the fresh air, then take up oxygen from it afterwards.

Since lungs aren’t made of muscle, how is their movement brought about in ventilation (breathing)? Intercostal (between-ribs) muscles and the diaphragm are responsible. Their contraction is caused by nerve signals from the respiratory centre in the medulla (in the brain). This results in the intercostal muscles pulling the ribs up, while the diaphragm is flat, and the abdominal organs are pushed downwards. The thorax (chest cavity) increases in volume, so lowers its pressure below that of the atmosphere, resulting in air being drawn into the lungs. Exhaling, on the other hand, does not require muscular activity. Elastic recoil of the muscles, as well as the weight of the ribcage and abdominal organs, result in the pressure inside the lungs increasing, therefore pushing the air back outside.

In medicine, it’s important to have a calculation for the amount of air a person takes in or out over time. Various measures can be taken from a patient by using a spirometer that they breathe into.

Pulmonary ventilation = Tidal volume x Ventilation rate

Tidal volume is the volume of air inhaled or exhaled in one breath.
Ventilation rate is the number of breaths taken in one minute.

So, if someone was breathing 20 times a minute, a volume of 300 cm3 per breath, then what is their pulmonary ventilation?

By the above formula, 300 cm3 x 20 = 6000 cm3. Simplified, this is 6 litres per minute.

Vital capacity is the amount of air that can be expelled after a maximum inhalation. The volume left over after exhalation is the residual volume which is around a litre in humans.

Peak expiratory flow rate (PEFR) is the top rate at which exhaling can occur. It indicates how easily exhaling is done, and whether there are obstructions in the way.

Forced expiratory volume in the first second of expiration (exhaling) is termed FEV1. The ratio of FEV1 to the vital capacity shows how much of the full forced expired volume is expired in the first second. Levels of below 80% indicate obstruction.

In the case of emergency where someone has stopped breathing i.e. respiratory arrest, breathing can be carried out for them manually via mouth-to-mouth – expired air resuscitation. It involves using your exhaling as their inhaling. There is enough oxygen in the breath to sustain the victim. Breathing can be done through the mouth or nose.

Electronic ventilation equipment can also be used in certain circumstances such as intensive care units and at home by patients with sleep apnoea or chronic obstructive pulmonary disease. The rate of breathing is adjusted for adults, children and babies. Breathing rate decreases with age, while mouth-to-mouth is easier on children if it is given over both the mouth and the nose, to create a good seal over the smaller mouth.

Gas exchange in plants

Like insects, plants must meet the opposing demands of water retention and gas exchange. The site of photosynthesis in plants, as well as the gas exchange site, is the leaf. This is what a section through a leaf looks like:

1. The mesophyll cells are surrounded by quite a lot of empty space for air to mingle around, providing plenty of surface area for gas exchange by diffusion.
2. Air with its carbon dioxide (necessary for photosynthesis) enters the leaf through the stomata.

Stomata are holes on the leaf surface, made by the guard cells. They can open and close (using ATP) depending on environmental factors such as humidity, temperature and wind.

This controls the amount of water loss. Oxygen, the byproduct of photosynthesis, also leaves the leaf through the stomata.

In addition to the stomata, there are special areas on the woody tissue of flowering plants that aids in gas exchange, called lenticels





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