Gaseous Exchange

Gaseous Exchange in humans

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quantity of materials needed are proportional to the volume of the organism

diffusion of substances is proportional to surface are

large organism = small SA:V ratio small organisms = large SA:V ratio

unicellular organism = very large SA:V ratio

the distance materials have to travel is very short so materials are obstained through diffusion

large organisms cannot rely on diffusion alone to provide materials-need special gaseous exchange surfaces and transport systems e.g. gills in fish, lungs in humans

humans = small SA:V and high metabolism so need specialised gaseous exchange surfaces and transport systems

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Surface Area to Volume ratio

exchange surfaces: alveoli, blood vessel walls, villi, microvilli - need large SA:V to be effective


flattened body (flatworms)

central region filled with gut (not metabolising cells)

specialised exchange surfaces with large SA:V for diffusion and transport system

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human gaseous exchange system links circulatory system with atmosphere

adapted to:

provide clean, warm air (moist, hairy nose)

large SA for gaseous exchange

extensive and close blood supply so that diffusion of gases is over a short distance and concentration gradient maintained with constant blood flow

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General Principles for Efficient Gas Exchange 1


actual repiratory surface on alveoli inside lungs

average adult has approx. 600million alveoli=total SA approx. 100m2 (alveoli folded to increase SA)

bronchioles highly branched giving large number pathways for air to move in and out of lungs


without this, nothing would diffuse

blood carries oxgenated blood away from alveoli and deoxygenated to alveoli

ventilation brings oxygen into to alveoli and takes carbon dioxide away

capillaries surrounding alveoli are narrow which slows down blood flow allowing time for gaseous exchange

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General Principles for Efficient Gas Exchange 2


walls of alveoli and capillaries are composed of a simple layer of flattened epithelial cells (2 thin cells only 0.1-0.5um) allow rapid diffusion


water diffuses rom alveoli cells into alveoli so they are constantly moist

oxygen dissolves into this water before diffuses through these cells into the blood where it's taken up by haemoglobin in the RBC


where appropriate, good blood supply to carry oxygen away to the tissues and main a steep concentration gradient

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site of gaseous exchange

tiny hollow saces of thin, flattened sqamous epithelial cells (short distance to diffuse)

surface is 0.2um thick

elastic fibres to allow expansion when breathing in and recoil when breathing out

water lining alveoli allow diffusion of gases

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when a person breathes out, the alveoli deflate and inner surface can touch each other

surface tension of the water on the alveoli could make them stick together and therefore not inflate again

is like a detergent secreted by some cells which reduces surface tension of the water

stops alveoli from collapsing

has antibacterial effect


often struggle to breath (respiratory distress syndrome: RDS)

artificial surfactant may be used to treat premature babies before their own lungs secrete surfactant

artificial surfactant decreases amount of time infants spend on the ventilation

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inspiratory resever volume-volume of extra air breathed in

tidal volume-volume of air breathed in/out at rest

residual volume-volume or air remaining to prevent lungs from collapsing

expiratory reserve volume-volume of air one can breath in or out in one breath

vital capacity-maximum volume of air one can breath in or out in one breath

total lung capacity-total volume of air in the lungs

limewater used to absorb the carbon dioxide breathed out to prevent you breathing it back in

nose clip to prevent oxygen being lost by your nose so it doesn't affect results

trace gradually falls because the volume of air in the drum will gradually decrease as it is being breathed out as carbon dioxide and absorbed into the limewater

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Chronic Bronchitis

tar promotes mucus production (goblet and mucus glands)

cilia destroyed

persistent cough develops to remove mucus

bacteria, viruses and dust block the bronchioles-bronchioles damaged

smooth muscle and epithelial membranes develop scar tissues which narrow airways

lungs susceptible to pneumonia

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many infections e.g. industrial dust, smoking, lead to migration of phagocytes from blood to respiratory tract

they release enzyme elastase (protease) to clear a pathway for phagocytes to clear irritant in alveoli

elastase destroys elastin in alveoli-unable to expand a recoil and alveoli sometimes bursts and large spaces appear

very little air exchanged with each breath so breathing rate increases due to lack of oxygen

blood pressure increases due to resistance of blood vessels in the lungs to blood

right side of hreat increases

irreversible-oxygen mask required

emphysema and chronic bronchitis known as chronic pulmonary obstructive disease (CPOD)

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Lung Cancer

tar in tobacco contains carcinogens

carcinogens react with DNA of lung epithelial cells causing mutation of cells

a malignant tumour is formed

cells break off, enter lymph system and carried to other organs (metastasis) to form secondary tumours

cannot be detected until tumours are 1cm2 causing person to cough blood and the tumour surgically removed

secondary tumours treated with radiation of chemotherapy

cigarette smoking linked to mouth, oesophagus, larynx, bladder, pancreas, kidney and cervical cancers

lung cancer diagnosed by bronchioscope (endoscope views epithelial lining), chest x-ray or CT scan

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Epidemiological Data - Smoking and Lung diseases

50% smokers die of smoking-related disorders

98% people with emphysema are smokers and 20% smokers have emphysema

90% deaths from COPD are smokers-COPD rare in non-smokers

pneumonia and influenza 2x as high in smokers and lung cancer 18x more likely in smokers than non-smokers

25% smokers die of lung cancer

risks increase if: inhale, start young, high tar, high number, smoke a long time

risks decrease if: stop smoking

takes 10 years to return to non-smoker risk

as smoking in developed world decreases, smoking related diseases are decreasing

increasing in developing world

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Epidemiological Data - conclusions and evidence


it is an associative link

experiments on animals show a direct causal link between smoking and lung cancer


carcinogens and co-carcinogens identified in tar

animals exposed to cigarettes developed tumours similar to those in humans-carcinogens from tar painted onto skin of mice; cancerous growths developed

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normal airway = some mucus, wide and open, not inflamed

asthmatic airway = membranes inflamed (histamine released), smooth muscle contracts and narrows lumen

asthmatic airway during attack = excess mucus produced blocks airways and creates wheezing sound


allergy-esp. dust mites (protein found in faeces), pollen, pet hair

atmospheric pollution and smoking

exercise induced asthma


often develops after viral infection of lungs

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steroids (e.g. beclmethasone):

  • most common as they mimic action of hormones
  • taken through inhaler
  • slow to take effect but long lasting
  • taken regularly to control asthma
  • reduce inflammation
  • used to stop an already happening attack

beta-agonists (e.g. salbutamol):

  • via inhaler
  • immediate relief
  • relax smooth muscle
  • keep airways open (bronchodilators)
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Smoking and Disease

Carbon Monoxide

  • combines readily with haemoglobin (more so than oxygen)
  • causes reduction of oxygen in blood so physical activity harder to partake in
  • hardening of arteries esp. coronary arteries supplying heart muscle


  • makes tobacco addictive-stimulates release of adrenaline
  • causes raised blood pressure and increased heart rate
  • can lead to atherosclerosis-build up of fatty acids in arteries so increases blood pressure
  • coronary heart disease and strokes


  • goblet cells in lungs to over-produce mucus
  • ciliated cells destroyed which waft mucus and dirt
  • build up of mucus in bronchioles and bacteria and viruses multiply in mucus
  • results in chronic bronchitis
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Ventilation of the Lungs


intercostal muscles contracts -> relaxes

ribs up an out -> down and in

diaphragm contracts and flattens -> relaxes

volume of thorax increases -> decreases

pressure in thorax decreases -> increases

outside air (atmospheric) pressure greater thus air moves in -> less thus air moves out

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