Biology unit 1

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Lungs are supported and protected by the rib cage. The ribs can be moved by the muscles between them. This enables the lungs to be ventilated by a tidal stream of air, thereby ensuring that the air within them is constantly replenished.

The lungs are a pair of lobed structures made up of a seried of highly branched tubules, called bronchioles which end in tiny air sacs called alveoli.

Trachea is a flexible airway that is supported by rings of cartilage. Cartilage prevents the trachea collapsing as the air pressure inside falls when breathing in. Tracheal walls are made up of muscle, lined with ciliated epithelium and goblet cells. Goblet cells produce mucus that traps dirt particles and bacteria from the air breathed in. The cilia moves the mucus, laden with dirt and microorganisms upto the throat from where it passes down the oesophagus into the stomach.

Bronchi are 2 divisions of the trachea each leading to the lungs. They also produce mucus to trap dirt particles and have cilia that move the dirt laden mucus up towards the throat. Larger bronchi are supported by cartialage.

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Bronchioles walls made up of muscle lined with epithelial cells. Muscle allows them to constrict so they can control the flow of air in and out the alveoli.

Alveoli are minute air sacs at the end of the bronchioles. Contain some collagen and elastic fibres and are lined with epithelium. Elastic fibres allow alveoli to stretch as they fill with air when breathing in. Spring back during breathing out in order to expel carbon dioxide rich air. Alveolar membrane is the gas exchange surface.

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When air pressure of the atmosphere is greater than the air pressure inside the lungs, air is forced into the alveoli. This is called inspiration.(Inhalation)

When air pressure in the lungs is greater than that of the atmosphere, air is forced out of the lungs. This is called expiration.(exhalation)

Pressure changes within the lungs are brought about by the movement of two sets of muscles:

  • The diaphragm, which is a sheet of mucle that separates the thorax from the abdomen.
  • The intercostal muscles which lie between the ribs. There are two sets of intercostal muscles:
    • The internal intercostal muscles whose contraction leads to expiration.
    • The external intercostal muscles whose contractions leads to inspiration.



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Inspiration and expiration

Breathing in is an active process (uses energy) and occurs as:

  • The external intercostal mucles contract while the internal intercostal muscles relax.
  • The ribs are pulled upwards and outwards increasing the volume of the thorax.
  • The diaphragm muscles contract, causing it to flatten which also increases the volume of the thorax.
  • The increased volume of the thorax results in reduction of pressure in the lungs
  • Atmospheric pressure is now greater than pulmonary pressure and so air is forced into the lungs.

Breathing out is a largely passive process (it does not require as much energy) and occurs as:

  • The internal intercostal muscles contract while the external intercostal muscles realx.
  • The ribs move downwards and inwards decreasing the volume of the thorax.
  • The diaphragm mucles relax making it return to its upwardly domed position again decreasing the volume of the thorax.
  • The decreased volume of the thorax increases the pressure in the lungs.
  • The pulmonary pressure is now greater than that of the atmosphere, and so air is forced out of the lungs.
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Pulmonary ventilation is the total volume of air that is moved into the lungs during one minute. To calculate it we multiply together two factors:

  • Tidal volume which is the volume of air normally taken in at each breath when the body is at rest. This is usually around 0.5 dm3
  • ventilation (breating) rate = the number of breathes taken in one minure. This is normally 12 - 20 breaths in a healthy adult.

Plumonary ventilation is expressed as dm3 min-1

Pulmonary ventilation = tidal volume x ventilation rate

       (dm3 min-1)                (dm3)                (min-1)

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Exchange surfaces

They have large surface area to volume ration - to speed up the rate of exchange.

They are very thin - to keep the diffusion pathway short and so allow materials to cross rapidly.

They are partially permeable - to allow selected materials to diffuse easily.

There is a movement of he environmental medium - air - to maintain a diffusion gradient.

There is movement of the internal medium - blood - to make a diffusion gradient.

Diffusion is proportional to: surface area x difference in concentration

                                                    length of diffusion path

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Role of the alveoli in gas exchange

There are about 300 million alveoli in a human lung. Around each alveolus is a network of pulmonary capillaries so narrow that red blood cells are flattened against the thin capillary walls in order to squeeze through. These capillaries have walls that are only a single layer of cells thick. Diffusion of gases between the alveoli and the blood would be rapid because:

  • red blood cells are slowed as they pass through the pulmonary capillaries allowing more time for diffusion.
  • Distance between the alveolar air and red blood cells is reduced as red blood cells are flattened against the capillary walls.
  • The walls of both alveoli and capillaries are very thin so distance over which diffusion takes place is very short.
  • Alveoli and pulmonary capillaries have a large total surface area.
  • Breathing movements constantly ventilate the lungs, action of the heart constantly circulates blood around alveoli. Together they ensure a steep concentration gradient of the gases to be exchanged in maintained.
  • Blood flow through the pulmonary capillaries maintains a concentration gradient.
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Pulmonary TB

TB caused by 1 of 1 species of rod shaped bacteria: MYCOBACTERIUM TB or MYCOBACTERIUM BOVIS. Symptoms include a persistent cough, tiredness and loss of appetite that leads to weight loss. Fever and coughing up og blood may occur as the disease progresses.TB is spread through the air by droplets released into the air when infected individuals cough, sneeze, laugh or even talk.

M.TB is a resistant bacterium that can survive several weeks once the droplets have dried. Normally takes close contact with an infected person over a period of time rather than a casual meeting to transmit the bacteria. Spreaded more in crowded places or poorly ventilated conditions. TB can also be spread from cows to humans because M.Bovis also infects cattle. Milk may contain the bacterium.

  • People who are in close contact with infected people over long periods eg living and sleeping in overcrowded conditions.
  • Work or residue in long term care facilities where relatively large numbers of people live close together eg old peoples homes, care homes, hospitals or prisons.
  • Are from countries where TB is common.
  • Have reduced immunity:
    • Very young or very old                                 Malnourished
    • Aids                                                               Homeless
    • Diabetes, lung disease                                  Alcoholics/injecting drug users
    • undergoing treatment after transplant surgery
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Once M.TB has been inhaled by someone who is not immune to it:

The bacteria grow and divide within upper regions of the lungs where theres a high supply of oxygen.

The bodys immune system responds and white blood cells accumulate at the site of infection to ingest the bacteria.

Leads to inflammation and enlargement of the lymph nodes that drain that area of the lungs. Called primary infection.

Infection is usually controlled in a few weekes, however some is bacteria is usually remained. Years later these bacteria may re emerge to cause a 2nd infection. Called post primary TB.

Infection also arised in the upper regions of the lungs but is not easily controlled. Baceria destroo tissue of the lung and results in cavities and where the lung repairs itself scar tissue.

The sufferer coughs up damaged lung tissue containing bacteria along with blood.

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Pulmonary Fibrosis

Arises when scares form on the epithelium of the lungs causing them to become irreversibly thickened. To diffuse respiratory gases, linings of the alveloi need to be thin. In patiens with fibrosis, oxygen cant diffuse into the blood as efficiently because diffusion pathway has lengthened and volume of air that lungs can contain has been reduced.

Fibrosis reduces elasticity of the lungs. Fibrosis makes it difficult to breathe out, therefore ventilate the lungs. Effects of fibrosis on the lungs are:

Shortness of breath due to considerable volume of air space within the lungs being occupied by fibrous tissue. Means that less air and oxygen is being taken into the lungs each breath. Thickened epithelium of the alveoli means the diffusion pathway is increased so the diffusion of oxygen into blood is really slow. Makes is hard to maintain a diffusion gradient across the exchange surface.

Chronic dry cough occurs because fibrous tissue creates obstruction in the airways of the lungs. Bodys reflex reaction is to remove obstruction by coughing. SInce tissue is more or less immovable, nothing is expelled and the cough is described as dry.

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Chest pain are the consequence of pressure and hence damage from the mass of fibrous tissue in the lungs and further damage scarring due to coughing.

Weakness and fatigue results from the reduced intake of oxygen in the blood. Means that the release of energy by cellular respiration is reduced leading to tiredness.

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Example of a localised allergic reaction. Most common allergens that stimulate asthma are pollen, animal fur and faeces of the house dust mite. Can be triggered or made worse by exercise, cold air, infection, anxiety and stress. One or more of these allergens causes white blood cells on the linings of the bronchi and bronchioles to release chemical called histamine. It causes:

The lining of these airways to become 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 and so constricts the airways.

Overall theres a much greater resistance to the flow of air in/out of the alveoli. Makes it difficukt to ventilate the lungs and so maintain a diffusion gradient across the exchange surface.

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Symptoms of asthma are:

Difficulty in breathing due to constriction of the bronchi and bronchioles, their inflammed linings and the additional mucus and fluid within them.

Wheezing sound when breathing caused by air passing through the very constricted bronchi and bronchioles.

Tight feeling in the chest is the consequence of not being able to ventilate the lungs adequately because of the constricted brocnhi and bronchioles.

Coughing is the reflex response to the obstructed bronchi and bronchioles in an effort to clear them.

Genetics plays a huge part as for asthma runs through families.

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Disease develops over period of 20 years abd impossible to diagnose until the lungs have been irreversibly damaged. In emphysematous lungs the elastin has become permanently stretched and lungs are no longer able to force out all the air from the alveoli. Surface area of the alveoli is reduced and sometimes burst. Little exchange of gases can take place across the stretched and damaged air sacs.

Symptoms are:

Shortness of breath:results from difficukty in exhailing air due to loss of elasticity in the lungs, If lungs cannot empty then its difficult to inhale fresh air containing oxygen and so patient feels breathless. Smaller alveolar surface leads to reduced levels of oxygen in blood do patient tries to increase oxygen supply by breathing more rapidly.

Chronic cough consequence of lung damage and bodys effort to remove damaged tissue and mucus that cant be removed naturally due to cilia on the bronchi and bronchioles have destroyed.

Bluish skin due to low levels of oxygen in the blood as a result of poor gas diffusion in the lungs.

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To ventilate by inhalation   Asthma

To ventilate by exhalation  Fibrosis and emphysema

Provide large surface area  Emphysema

Provide short diffusion pathway  Fibrosis

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