Lung Function and Disease

Lung structure

Nose

o Air is filtered in nostrils with small hairs

o Air is moistened and warmed by nasal cavities

o Mucus traps foreign particles while cilia propels particles towards the throat

Air passes into the pharynx → larynx → trachea

o The epiglottis is found within the larynx

o Breathing: epiglottis projects upwards → larynx is open

o Swallowing: larynx pulled up / epiglottis flaps back and blocks larynx / prevents food from entering airway

Trachea

o Contains C-shaped cartilage rings / prevents collapse of tube

o Divides into 2 tubes with smaller diameter called bronchi

o Bronchus is supported with ciliated epithelia to prevent microorganisms

o Right bronchus is bigger than the left one → common site for inhaled foreign bodies

Bronchi further divide into bronchioles

o Their diameter can be controlled by smooth muscles

o Form alveoli (100µm in diameter)

Fick's law

Rate of diffusion is proportional to (surface area x conc. difference) / distance

Applies to exchange of food, waste, gases, and heat with surroundings

Large organisms

o Have a small surface area : volume ratio

o Decreases the rate of diffusion

o Large animals lose less heat than small animals

o Don't require a high metabolism to maintain body temperature

o Feed only once

Small organisms

o Lose heat very readily

o Need a high metabolism to maintain body temp

o Must feed continuously

 

Efficient gas exchange requires:

o Large surface area

o Large concentration gradient (low O2 on one side, high O2 on the other site of the membrane)

o Short diffusion pathway (thickness of membrane molecules must travel to diffuse across)

Alveolar Gas Exchange

- Greater partial pressure of O2 in alveolar air / more O2 dissolves in blood (Henry's Law)

- Two types of alveoli cells

Type I cells

o Composed of endothelium - layer of two thin cells

o This allows diffusion of gases (short diffusion pathway) down their conc. gradients

o O2diffuses from air to blood; CO2diffuses from blood to air

Type II cells

o Secrete surfactant that keep alveoli constantly moist

o Allows oxygen to dissolveand to diffuse through the cells into the blood

o In the blood, it is taken up by haemoglobin

- Alveoli contain phagocytes to kill bacteria that have not been trapped by mucus

Ventilation

o Flow of air in and out of alveoli

o Maintains large concentration gradient

Ventilation

- Tidal volume, VT, volume of air inhaled and exhaled in a normal single breath (≈0.5 L)

- Functional residual capacity, FRC, volume remaining in lungs after exhalation of tidal volume (≈2.5 L)

- Expiratory reserve volume, ER, volume of a maximal exhalation (≈1.5 L)

-  Residual volume, RV, volume remaining in lung after maximal exhalation (≈1L)

- Inspiratory reserve volume, IR, additional volume that can be inhaled after inhalation of tidal volume

- Vital capacity, VC, maximum volume of exhalation after lungs are maximally filled

o Best clinical indicator of breathing

- Minute ventilation is the overall flow of air into lungs (analogous to cardiac output)

o Minute Ventilation = Tidal Volume x Respiratory Rate

o (0.5 litre/breath * 10 breaths/min = 5 litres per minute)

- "Dead space" - not all O2 available in air is available to alveoli

o Fresh air mixes with exhaled air during inspiration

o Alveolar ventilation takes dead space into account

o Alveolar ventilation = (Tidal Volume - Dead Space) x Respiratory Rate

o (350 ml x 10 breaths per minute = 3500 ml or 3.5 litres)

Measurements of Ventilation

- A spirometer is used to measure expired breath

 - Restrictive disorders, such as pulmonary fibrosis, reduce compliance and vital capacity

Four measures are called respiratory volumes

o Tidal volume

o Inspiratory reserve volume

o Expiratory reserve volume

o Residual volume

Others, called respiratory capacities, are calculated by adding 2 or more of the respiratory volumes

 

Alveolar Gas Exchange

The greater the partial pressure of O2 in alveolar air the more O2 will dissolves in blood (Henry's Law)

It seems that Henry was a master of stating the obvious! Let's try to further understand the process of how oxygen moves across the alveolus and into the blood.

The aleveoli - the site of gas exchange in the lungs is composed of epithelial cells. This has evolved to allow efficient diffusion of gases (large surface area short diffusion pathway) down their concentration gradients. O2 therefore diffuses from air to blood where it then associates with haemoglobin and CO2 diffuses from blood to the air in the alveolus.

A protein called surfactant is produced by the alveoli, which prevents the alveolar surfaces from sticking together when they deflate. The alveoli also contain phagocytes to kill bacteria that have not been trapped by mucus which may later cause disease.

Lung Disease

There are many different conditions which can impair the function of the lungs. Obviously with such an important role, any harm done to the lung tissue can have major effects on a person's health and fitness. With such a clinical focus, this is a key area of 'How Science Works' and you should be able to analyse data for correlations and causal relationships between human activity/behaviour and the onset of disease.

Asthma

At least one in ten people suffer from asthma at some point during their lives with the majority of cases presenting in childhood. The condition is caused by inflammation of the bronchioles. In an asthma attack:

1. The smooth muscle in the bronchiole wall contracts which narrows the lumen
2. The epithelial cells lining the bronchiole secrete more mucus than normal which obstructs the movement of air through the respiratory system
3. Breathing rate increases but the tidal volume is reduced
4. Gas exchange in the alveoli is reduced

Asthma attacks can be triggered by many different stimuli. Common triggers include:

- Some diseases e.g. the common cold and flu

- Exposure to air pollution or dust

- Exposure to known allergens such as pollen, animal fur or certain foods

- Exercise - especially in cold air

- Psychological factors such as stress

There is no 'cure' for asthma but the condition can be treated and managed through the use of inhalers which administer drugs to the respiratory system. Normally an asthma sufferer would have two inhalers to be used in different ways. They will have one 'Preventer' which when used release anti-inflammatory drugs such as steroids to reduce the underlying inflammation and hopefully reduce the likelihood that a person will have an asthma attack. They will also be given an inhaler which should be used to relieve symptoms during an attack. This contains substances which dilate the bronchioles which should make breathing easier.

Pulmonary Fibrosis

Fibrosis is the scarring of body tissue in this case - the lungs. The scarring causes a loss in elasticity of the tissue between the alveoli and contorts the bronchioles and alveoli. These pathological effects reduce lung capacity. The condition is highly linked to occupational hazards such as working with substances such as asbestos, coal dust and metal dust. Widespread fibrosis caused by inhalation of harmful substances is called emphysema. Infectious diseases such as tubercolosis can also leave small regions of the lungs with scarring. Fibrosis therefore refers to the consequence of diseases which produce lung damage.

A patient suffering from pulmonary fibrosis would have shortness of breath and/or a cough. At present there is no treatment for this condition and a lung transplant is the only treatment option which will improve long - term survival.

Pulmonary Tuberculosis

This condition is caused by rod-shaped bacteria: Mycobacterium tuberculosis OR Mycobacterium bovis.

Symptoms:

- persistent cough

- tiredness

 - loss of appetite and weight loss

- fever

- coughing up blood

Transmission:

This disease is spread through the air in droplets released when an infected person coughs or sneezes. Coughs and sneezes really do spread diseases! Unusually the bacterium causing TB can survive for a long period of time even in dried droplets. This means that close contact with an infected person over a period of time can lead to transmission of the disease.

It is especially common in communities where living space is relatively small or crowded working environments. There are some countries in which TB is particularly prevalent and anyone moving to Britain from these countries will need to be tested for the bacteria before being allowed to enter the country without first being treated for TB.

People most at risk of contracting TB are those who:

- are in close contact with infective individuals

- live or work in care facilities

- are from countries in which TB is prevalent

- have reduced immunity (the very young or very old, those with AIDS, people taking immunosuppresants, malnourished individuals, alcoholics, homeless people)

Once inside a person's respiratory system there is a plentiful supply of oxygen allowing rapid growth and division of the bacteria. At this early stage the person often develops pneumonia. The white blood cells of the immune system respond rapidly to the infection to try and prevent the bacteria from spreading. This process results in the formation of scar tissue which contains the infection in an inactive state.

If the body's immune system becomes weakened the TB bacteria can break through the scar tissue resulting in the return of pneumonia and the spreading of the bacteria to other parts of the body (the kidneys, bone and linings of the brain and spinal cord are the most common sites affected).

Symptoms and Treatment:

There is a relatively long time between the time of infection and the onset of symptoms. A patient will present with tiredness, weight loss, fever, coughing, chest pain and shortness of breath (this is due to the presence of scar tissue in the lungs).

Inactive TB may be treated with an antibiotic and active TB will usually require several antibiotics to combat the infection.

Emphysema

Emphysema can be an inherited condition but most cases arise as a result of smoking. The toxins passed into the lungs when smoking trigger an immune response which ultimately leads to the destruction of the lung tissue.

Your lungs contain white blood cells which 'patrol' the lungs phagocytosing any harmful pathogens or particles which are inhaled. These phagocytes release enzymes which catalyse the breakdown of proteins found in the connective tissue between the alveoli and bronchioles. This makes it easier for them to move around the lung tissue to engulf and kill the invading pathogen or particle. Elastin is one of these proteins which phagocytes destroy. The elastin fibres act as an elastic band would, snapping back into shape after they have been stretched. This allows the stretch and recoil of the alveoli (the site of gas exchange).

Excessive destruction of elastin is normally prevented by the production of a substance called α1-antitrypsin which acts to prevent the action of elastase (the enzyme which catalyses the breakdown of elastin). The smoke from cigarettes contains several chemicals which stop lung cells from producing α1-antitrypsin. This means, that the destruction of elastin will increase, damaging the elastic tissue of the lungs making it harder for a person to exhale. Other proteins are also destroyed by the enzymes secreted by the pathogens meaning that the alveoli walls can be damaged and the surface area available for gas exchange is reduced.

A reduced area for gas exchange means that a person with emphysema's blood will contain a reduced concentration of oxygen. This will limit the amount and rate of aerobic respiration achievable by their cells making any activity a great effort.

Organisms Exchange Material With Their Environment

What is Surface Area and Volume Activity

Gas Exchange - Fish and Humans

Adaptations Allowing Animals to Meet their Oxygen Demands

Anatomy and Physiology of the Respiratory System

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Lung Function and Disease Worksheet