• The gaseous exchange system links the circulatory system with the atmosphere – It is adapted to
• • Clean and warm the air that enters
  • Maximize the surface area for diffusion
  • Minimise the diffusion distance for oxygen and carbon dioxide
• Goblet cells are cells of the ciliated epithelium that produces mucus
• Upper part of the goblet cells is swollen with droplets containing mucin which has been secreted by the cell
• The rest of the cell which is contains the nucleus is quite slender-like the stem of a goblet
• Mucus is also made in the glands beneath the epithelium
• It is a slimy solution of mucin which is composed of glycoproteins with many carbohydrate chains to make them sticky and trap particles in inhaled air
• Some chemicals such as SO₂ and NO₂ dissolve in mucus to form acid solution which can irritate the airways
• Between goblet cells are the ciliated cells
• Continual beating of their cilia carries the carpet of mucus upwards towards the larynx
• Each cilium has a tiny hook at the end to sweep mucus which moves at a speed of 1 cm per minute
• When mucus reaches the top of the trachea it is usually swallowed so that pathogens are destroyed by acid in the stomach
• Phagocytic white blood cells called macrophages patrol the surfaces of the airways scavenging small particles such as bacteria and fine dust particles
• During an infection, they are joined by other phagocytic cells which leave the capillaries to help remove pathogens

Pathway taken by an oxygen molecule as it passes from the atmosphere to the blood in the lungs

Mouth/nostrils, nasal passage, larynx, trachea, bronchus, terminal and respiratory bronchioles, alveolar duct, alveolus, epithelium, connective tissues, endothelium of capillary, plasma, red blood cells  Trachea

• It leads from the throat to the lungs
• At the base of the trachea are two bronchi which subdivide and branch extensively forming a bronchial tree in each lung
• Ciliated epithelium and goblet cells present and also smooth muscles are present
• Connective tissue with elastic fibres and collagen are present
• Has cartilage which keeps it open and lowers its resistance
• Cartilage also prevent them from collapsing or bursting as the air pressure changes during breathing
• There is a regular arrangement of C-shaped cartilage rings in the trachea

Smooth muscles

• Bronchioles are surrounded by smooth muscles
• Can contract and relax to adjust the diameter of their tiny airways
• During exercise they relax to allow a greater flow of air to the alveoli
• Absence of cartilage makes the adjustments possible

Elastic fibre

• Allows bronchioles/alveoli to inflate/expand/stretch
• To hold inspired air to maximum volume/maximum surface area
• Recoil during expiration/expulsion of air – Prevent alveoli bursting

Cartilage

• Keeps the airways open and air resistance low
• Prevent them from collapsing or bursting as the air pressure changes during breathing
• In the trachea there is a regular arrangement of C-shaped rings of cartilage, in the bronchioles there are irregular blocks of cartilage

Goblet cells

• Secrete mucus
• Solution of mucin/glycoproteins with many chains of carbohydrates
• To trap small particles e.g. bacteria/spores/dust
• Moved by cilia upwards – Protect alveoli

Bronchioles

• Found at the base of the trachea
• Have irregular blocks of cartilage
• Surrounded by smooth muscles which can contract or relax to adjust the diameter of these tiny airways
• During exercise they relax to allow a greater flow of air to the alveoli
• Absence of cartilage makes the adjustments possible
• Ciliated epithelium and goblet cells are present and also smooth muscles are present – Connective tissue with elastic fibres and collagen present

Alveolus

• Found at the end of the pathway between the atmosphere and the blood stream
• Have very tiny epithelial lining surrounded by many blood capillaries carrying oxygenated blood
• The short distance between air and blood means that oxygen and carbon dioxide can be exchanged efficiently by diffusion
• Alveolar walls contain elastic fibres which stretch during breathing in and recoil during expiration to help force air out
• This elasticity allows the alveoli to expand according to the volume of air breathed in
• When fully expanded during exercise, the surface area available for diffusion increases and the air is expelled efficiently when the elastic fibres recoil
• Lining each alveoli is moist squamous epithelium which consists of very thin flattened cells, reducing the distance over which diffusion must occur
• Collagen and elastic fibres are also present
• Special cells in the alveolar walls secrete a detergent-like chemical on the inside lining of the alveolus
• This is called surfactant
• It lowers the surface tension of the fluid layer lining the alveolus and thereby reducing the amount of effort needed to breathe in and inflate the lungs
• Surfactant also speeds up the transport of oxygen and carbon dioxide between the air and the liquid lining the alveolus and helps to kill any bacteria which reach the alveoli
• Surfactant is constantly secreted and reabsorbed in a health lung
• No cilia present
• No goblet cells, squamous epithelium thin flattened cells with liquid surfactant on inner surface and blood capillaries on outer surface

Adaptations of the alveolus for gaseous exchange

• Large surface area
• Thin epithelium therefore gases have a short distance to travel
• Surfactant is present
• A steep gradient is maintained by ventilation, a good blood supply and the presence of an oxygen carrying haemoglobin

Describe the functions of goblet cells, smooth muscle and elastic fibres in the gaseous exchange system.         

Goblet cells

• secrets mucus
• solution of mucin/glycoproteins;
• with many carbohydrate chains;
• to glycosylation/adding sugars to proteins;
• to trap particles/bacteria/dust/spores;
• moved by cilia;
• protects alveoli Smooth muscles
• contract/relax;
• adjust diameter of bronchioles R bronchi, trachea     [2]

Elastic fibres

• stretch during breathing in;
• Recoil during expiration;
• Allows alveoli to expand (according to the volume of air breathed in) [3 max 2]

Elastin 

• forms fibres;
• allows bronchioles/alveoli to inflate/stretch/expand;
• to hold inspired air/to maximum volume/to maximum surface area;
• recoil during expiration;
• prevent alveoli bursting;

Chronic Bronchitis

• Is caused mainly by smoking
• Tar in tobacco smoke inhibits the clearing action of the lungs
• It stimulates the goblet cells in the epithelium of the airways to secrete more mucus and as so inhibits the sweeping action of the cilia
• As a result mucus accumulates in the bronchioles and the smallest of these are obstructed
• As mucus is not moved or at best only moved slowly, dirt bacteria and viruses collect and cause smoker‘s cough
• The changes to the linings of the airways can be summarized as below
  • Mucus glands in the trachea and bronchi enlarge
  • Mucus glands and goblet cells secrete much more mucus
  • Cilia are destroyed
  • Epithelia are replaced by scar tissue
  • Smooth muscle become thicker
  • Airways are blocked by mucus
• These changes constitute chronic bronchitis
• Sufferers have a severe cough producing large quantities of phlegm which is a mixture of mucus and bacteria and some white cells
• Bronchitis usually contributes to the development of emphysema

Emphysema

• Is a condition in which bronchioles collapse, leaving large spaces where surface area for gaseous exchange used to be
• This is because the lungs are constantly infected, causing phagocytes to line the airways and to reach them they release a protein digesting enzyme elastace
• It destroys the elastin in the walls of the alveoli allowing phagocytes to enter and remove bacteria
• The initial inflammation is caused by infections which in turn is caused by the accumulation of mucus as a result of tar in the lungs
• As the protective system is compromised by bronchitis, infections become more common
• This leads to inflammation in the lungs: more macrophages line the lungs and phagocytes join to form the blood
• These phagocytes release a protein digesting enzyme elastace to reach the surface of the alveolus
• Elastace destroys elastin in the connective tissue so that the alveoli do not stretch and recoil when breathing in and out
• Because of this, the bronchioles collapse during exhalation trapping air in the alveoli, which often bursts
• Large spaces appear where they have burst and this reduces the surface area for gaseous exchange
• Cells in the alveoli release an elastace inhibitor (α – antitrypsin) but some smokers produce less of this than non-smokers so there is little to reduce the effects of the phagocytes
• Smokers are susceptible to developing emphysema
• The loss of elastin makes it difficult to move air out of the lungs
• The air remains in the lungs and is not refreshed during ventilation
• Together with the reduced surface area for gaseous exchange, this means that many people with emphysema do not oxygenate their blood very well and have a rapid breathing rate and the blood pressure also increases as blood vessels in the lungs become more resistant to the flow of blood
• As lung function deteriorates, wheezing occurs and breathlessness becomes progressively worse
• It may become so bad in some people that they cannot get out of bed

Describe the changes that occur in the lungs of people with bronchitis and emphysema Bronchitis

• Enlargement of mucus glands in the airways (trachea and bronchi)
• Increased secretion of mucus
• Narrowing and obstruction of airways
• Thickening of smooth muscles
• Inflammation
• Epithelia replaced by scar tissue
• Destruction of cilia
• Blockage of airways or passages by mucus –  Coughing up of phlegm

Emphysema

• Digestion by phagocytes of pathways through alveolar walls
• Loss of elastin due to digestion by elastace/phagocytes
• Overextension and bursting of alveoli to form large air spaces
• Decrease in surface area for gaseous exchange
• Lack of recoiling of air spaces when breathing out
• Decrease in volume of air forced out from the lungs – Shortness of breathe

Describe the changes that occur as emphysema develops in a smoker’s lungs

• inflammation;
• phagocytes move from blood to airways;
• release enzyme/elastace, to digest elastin/connective tissue;
• alveoli do not stretch and recoil/loose elasticity;
• alveoli burst;
• alveoli do no deflate to help force air out;
• elastace inhibitor in lung inactivated by smoke;
• decrease in surface area for gas exchange;
• lung air spaces/description of lung;

Describe the events in the development of lung cancer

• carcinogens cause mutations;
• g. UV light/tar/X-rays/asbestos;
• oncogenes transformed by carcinogens;
• cancerous cells do not respond to signals from other cells;
• uncontrollable cell division/mitosis;
• cancerous cells not removed by immune systems;
• absorption of nutrients from other surrounding cells;
• tumour gets bigger; – begnin tumours;
• malignant tumours;
• tumour supplied with blood and lymph vessels;
• metastasis;
• blockage of intestines/lungs/blood vessels
• secondary growth;

Describe how emphysema and chronic bronchitis affects the gaseous exchange system

Emphysema

• digestion/destruction of elastin
• by phagocytes/elastace
• bursting of alveolus
• decrease in surface area for gaseous exchange
• lack of air spaces during breathing out

Chronic bronchitis

• enlargement of mucus glands in the trachea and bronchi;
• increased mucus secretion;
• destruction of cilia;
• epithelia replaced by scar tissue;
• thickening of smooth muscle;
• blockage of airways or passages by mucus