Lungs

• A flexible airway that is supported by rings of cartilage
• Cartilage prevents airways from collapsing as air pressure is decreased when breathing in
• Walls are lined with ciliated epithelial cells and goblet cells
• Goblet cells produce mucus that traps particles from the air breathed in
• Cilia move this mucus up the throat so it can travel down the oesophagus and into the stomach

Bronchi

• Two divisions of the trachea, each leading to one lung
• Have goblet cells and cilia for the same purpose as the trachea does
• Has supportive cartilage which reduces in amount as the bronchi get smaller

Bronchioles

• Branching subdivisions of the bronchi
• Walls made of muscle lined with epithelial cells
• Muscles allows control of air entering alveoli

• Contain collagen and elastic fibres
• Elastic allows alveoli to snap back into shape, expelling carbon dioxide-rich air
• Alveolar membrane is the gas exchange surface

Characteristrics of alveoli:

• Very thin membrane for a short diffusion pathway
• Have a large surface area to volume ratio
• Partially permeable, to allow selected materials to diffuse easily

Surrounded by:

• A movement of the environmental medium (Air) to maintain a diffusion gradient
• A movement of internal medium (Blood) to maintain a diffusion gradient

• Each alveoli is surrounded by pulmonary capillaries, which are so thin that red blood cells are flattened against their walls.

Diffusion is therefore rapid because:

• Red blood cells are slowed as they pass through allowing more time for diffusion
• Distance between alveolar air and red blood cells is reduced as red blood cells are flattened against the walls
• Walls of alveoli and capillaries are very thin so gives a shorted diffusion pathway
• Both have a large surface area
• Lungs are constantly ventilated and blood is moved so a steep concentration gradient is maintained

Cause & symptoms

• Caused by mycobacterium tuberculosis or mycobacterium bovis
• Symptoms include a persistant cough, tiredness, and loss of appertite which leads to weight loss
• Fever and coughing up of blood can also occur as the disease develops

Trasmission

• Droplet trasmission by air
• Mycobacterium tuberculosis can survive several weeks on a surface once the droplets have dried
• Close extended contact with an infected person, living with lots of people, living in a country where TB is common or having reduced immunity can mean you're at greater risk of contracting TB
• Those with reduced immunity are: the elderly, the very young, those with AIDS, those with other medical conditions, those undegoing treatment with immunosuppressive drugs, the malnourished, alcholics or drug users and the homeless.

• Bacteria grow and divide in the upper regions of the lungs
• Immune system responds and white blood cells accumulate at the site of infection to ingest the bacteria
• Leads to inflammation and the enlargement of lymph nodes that drains the area. This is the primary infection.
• Bacteria remain after being controlled
• Bacteria may reemerge to cause a second infection. This is called the post-primary infection
• This infection arises in the upper parts of the lungs and the bacteria destroys the tissue of the lungs
• This results in cavaties and when the lung repairs itself, scar tissue
• Sufferers cough up damaged tissue containing bacteria and also blood. TB can spread to rest of body without treatment.

Cause

• Scar forms on epithelium of lungs making them become irreversibly thickened
• Oxygen cannot diffuse as effectively because of this this, as the diffusion pathway has been lengthened and the volume of air that the lungs can hold has been reduced
• It also reduces the elasticity of the lungs and therefore the lungs cannot expel air as effectively and so ventilation is more difficult.

Symptoms

• Shortness of breath - Reduced volume and elasticity makes maintaining a diffusion gradient more difficult
• Chronic, dry cough - fibrous tissue creates an obstruction that the body's reflex system tries to get rid of
• Pain in chest - the result of pressure and therefore damage of the mass of fibrous tissue
• Weakness & fatigue - reduced intake of oxygen leads to a decrease in energy released from cellular respiration which causes tiredness

Cause

• Allergens stimulate white blood cells on the linings of bronchioles and bronchi to release histamine
• Histamine makes airways inflamed
• Cells of epithelial lining secrete larger quantities of mucus than normal
• Fluid leaves the capillaries and enters airways
• Muscle surrounding the bronchioles contracts so narrows the airways

Greater resistance to air flow makes it more difficult to ventilate the lungs and maintain a diffusion gradient 

Symptoms:

Breathing difficulty, wheezing sound when breathing, tight feeling in chest, coughing

In emshysematous lungs, the protein elastin has become permanently stretched and the lungs are no longer able to expel all the air from the alveoli. Elastin normally allows the lungs to stretch when in inspiration and spring back when in expiration. the surface area of emphysematous alveoli is reduced and they sometimes burst so little gas exchange can occur.

Symptoms:

• Shortness of breath - results from the difficulty in exhaling due to loss of elasticity. The smaller alveolar surface area leads to reduced levels of oxygen in the blood and so the sufferer breathes in more to increase their oxygen supply.

• Chronic cough - Attempt to remove damaged tissue and mucus that cannot be removed normally due to cilia being destroyed

• Bluish skin colouration - due to low levels of oxygen in the blood

Inspiration

• External Intercostal muscles contract, internal intercostal muscles relax
• Ribs are pulled upward and outwards, increasing the volume of the thorax
• Diaphragm muscles contract causing it to flatten, also increasing the volume of the thorax
• Increased volume of thorax results in reduction of pressure in the lungs
• Atmospheric pressure is now greater than pulmonary pressure so air is forced into the lungs

Expiration

• Internal intercostal muscles contract, while external intercostal muscles relax
• Ribs move downwards and inwards, decreasing the volume of the thorax
• Diaphragm muscles relax, returning to its domed position, also decreasing the volume of the thorax
• Decreased volume of the thorax increases the pressure in the lungs
• Pulmonary pressure now greater than atmospheric pressure so air forced out of the lungs

During normal breathing, the elastic recoil of the lungs is the main cause of air being expelled