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Gas Exchange

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Plants require gas exchange to intake carbon dioxide and make glucose via photosynthesis, and to release oxygen as a byproduct. Animal whose cells respire aerobically require oxygen to produce energy for their metabolism, while releasing carbon dioxide which is the byproduct (yes, photosynthesis and respiration are converse processes; bear in mind plants do carry out both).


The factors which affect the diffusion rate of these gases through the organism are surface area, diffusion path, moist exchange surface and concentration gradients.


A large surface area with a short diffusion path length, moist exchange surface and steep concentration gradient of gases favours the fastest rate of diffusion. More space, quicker exchange, moist environment for gases dissolving better and a steeper gradient increase the rate of diffusion.


On the other hand, a small surface area that is dry, with a long diffusion path and a shallow concentration gradient results in little to no diffusion taking place.


Fick’s law describes the rate of diffusion in terms of these factors (except moisture), giving the following expression:



This expression gives an overall value to the interactions between these factors. Surface area and concentration difference must be high to favour diffusion, while thickness must be low. Plugging in any known values into the expression can give a value for the diffusion rate. This value can be compared to others to see how gas exchange differs between species, organs, etc.



Plants peak in their photosynthesis around midday, and cease to photosynthesise at night, based on the availability of light. Respiration occurs throughout as the energy-producing process, and occurs alone during the night without photosynthesis.


As such, the oxygen and carbon dioxide requirement of plants fluctuates throughout the day.



Based on Flick’s law, the rate of diffusion is optimised in plants by the large surface area of the leaves and thin width. This provides a short diffusion pathway, aided by the air pockets found in the leaf’s spongy mesophyll and the stomata opening when the demand for carbon dioxide increases i.e. during the day.




The mammalian 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, just off the alveolar ducts. 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. 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. 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. The interface created by the alveolar cells and the capillary cells is the respiratory surface.


Alveoli provide a large surface area for gas exchange, aided by a rich vascular supply i.e. the capillaries, and moisture which acts a surfactant to reduce surface tension. The good blood supply ensures the maintenance of a steep concentration gradient of oxygen (very high in air relative to very low in deoxygenated blood) and, conversely, of carbon dioxide (very low in air relative to very high in deoxygenated blood).


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.


Smoking damage

The chemicals contained in tobacco cigarette smoke (several thousand types) do damage to the cilia that line the respiratory tract and move dirt particles away from the lungs e.g. pollutants, pathogens, dust. These protrusions from the lungs, trachea and nose are sensitive to toxins and following long periods of sustained damage can cease to function. Cilia damage results in a failure to remove lung mucus and thicker mucus building up in the lungs. This causes an unproductive cough.


The buildup of debris in the respiratory tract renders someone more prone to bacterial infection such as chronic bronchitis. This manifests itself through inflammation of the airways and constitutes a kind of chronic obstructive pulmonary disease (COPD). Symptoms include coughing, increased mucus production, shortness of breath and wheezing. Chronic, as opposed to acute bronchitis lasts for more than 3 months.


Fibrosis is the accumulation of scar tissue as a result of damage or bacterial infection. It affects the aveoli primarily, as they are ever so small and fragile. So fibrosis is not a disease in its own right, but a result of others.


Scar tissue is fibrous connective tissue and prevents good lung function, therefore symptoms caused are coughing and shortness of breath. Damaged alveoli will not contribute to the diffusion of oxygen into red blood cells.


Emphysema is a nasty disease. It is caused by excessive smoking (or air pollutants) over a lifetime and results in a steep decline in lung function, to the point where very severe cases require an oxygen tank connected to the airways at all times.


Fibrosis occurs in the lungs as part of emphysema, which results in thicker alveolar walls which increase the diffusion pathway of oxygen and carbon dioxide, therefore decreasing the rate of diffusion. Another side effect is a loss of elasticity which makes breathing out more difficult.


Lung cancer is caused in the most part by prolonged smoking of tobacco or other products, and is a cancer that starts in the lungs. Symptoms include coughing, potentially coughing blood, persistent chest infections, chest pain, breathlessness and lack of energy. There are two main types of lung cancer, and depending on whether it has spread to the rest of the body and how soon it is identified, treatment includes surgery to remove the tumour, chemotherapy, radiotherapy and other emerging biological treatments.





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