AQA AS Biology Chapter 4.5 - LUNG DISEASES

Revision notes on chapter 4.5 of the AQA AS Biology textbook concerning lung diseases:

Pulmonary Fibrosis

Asthma

Emphysema

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Chapter 4.5 ­ Lung Diseases ­ Notes
Pulmonary Fibrosis:
Arises when scars form on the epithelium of the lungs, causing them to thicken
irreversibly. This means that oxygen cannot diffuse into the blood as efficiently as the
diffusion pathway has been lengthened. Fibrosis also reduces the elasticity of the
lungs. Effects and explanation of their causes:
Shortness of breath: due to a considerable volume of the lung's air space
being occupied by fibrous tissue. This means less air is taken into the lungs
with each breath. The diffusion of oxygen into the blood is very slow due to
the increase in the length of the diffusion pathway. Due to compromised
elasticity, it is very difficult to ventilate the lungs. This makes it difficult to
maintain a diffusion gradient across the exchange surface.
Chronic, dry cough: due to the fibrous tissue, there are obstructions in the
lung's airways. The body's reflex action is to cough, but the tissue is
immovable, so nothing is expelled, which is why it is a dry cough.
Chest pain: consequence of pressure and damage from the mass of fibrous
tissue in the lungs, and further damage and scarring due to coughing.
Weakness/fatigue: due to reduced intake of oxygen into the blood. The
release of energy by cellular respiration is reduced which causes fatigue.
Asthma:
A localised allergic reaction, which affects 10% of the world's population and is
caused by many common allergens, such as pollen and animal fur. It can be worsened
by air pollutants, exercise, cold air, infection, anxiety and stress. The allergens cause
white blood cells on the linings of the bronchi and bronchioles to release a chemical
called histamine, which has the following effects:
- The lining of the airways becomes inflamed
- The cells of the epithelial lining secrete larger quantities of mucus than normal
- Fluid leaves the capillaries and enters the airways
- The muscle surrounding the bronchioles contracts, constricting the airways
This means that there is a greater resistance to the flow of air in and out of the
alveoli, which makes it difficult to ventilate the lungs, and so maintain a diffusion
gradient across the exchange surface. There are many symptoms of asthma.
Breathing difficulties occur due to the constriction of the bronchi and bronchioles,
their inflamed linings and the additional mucus and fluid within them. Wheezing is
caused by the air passing through the very constricted bronchi and bronchioles. Tight
feelings in the chest are the consequence of not being able to ventilate the lungs
adequately due to the constrictions. Coughing is a reflex response to try to clear the
obstructed bronchi. Asthma is a genetic disease. There has been a distinct increase in
the number of cases, which could be due to air pollution and an increase in the
variety of chemicals used in foods and manufactured products.
Emphysema:
The disease develops over a period of 20 or so years, so is virtually impossible to
diagnose until there is irreversible damage. In emphysematous lungs, the elastin is
permanently stretched, so air cannot be forced out of the alveoli. The surface area of
the alveoli is reduced, and this can result in them bursting. Shortness of breath
results from difficulty in exhaling air due to loss of elasticity. Because it is difficult to
empty the lungs of air, it is also difficult for fresh air to be taken in, so the sufferer

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The smaller alveolar surface area leads to reduced oxygen levels in
the blood, so the patient breathes more rapidly. Chronic coughing is due to the lung
damage and the body's efforts to remove immovable damaged tissue and mucus.
This cannot be removed naturally as the cilia on the bronchioles have been destroyed.
Bluish skin colouration is due to low oxygen levels in the blood due to poor gas
diffusion in the lungs. Giving up smoking can seriously reduce the rate of further
deterioration.…read more

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