1. Lung function and Disease 3.1.4

3. Lung structure Lungs are the interface for the exchange of gases and their function is affected by both pathogens and lifestyle. All aerobic organisms require a constant supply of oxygen in order to release energy in the form of ATP during respiration. The waste gas product carbon dioxide also has to be removed because the build up can be harmful. We need lungs to enable oxygenation of our blood system in order to circulate to the relatively large volume of living cells. Mammals have to maintain a high body temperature and therefore have high metabolic and respiratory rates. Air enters via the trachea and then into the left and right bronchus (plural bronchi). The bronchi lead into a pair of lobed structures called the lungs. Lungs consist of a series of highly branched tubules called bronchioles which end up in tiny air sacs called alveoli.

5. Lung structure Why are a mammals lungs located within the body? Air is not dense enough to support and protect delicate structures They would cause loss of a great deal of water and dry out Because lungs are located internally, we need to have a means of moving the external medium (air) over the surface of our lungs. This movement is called ventilation.

6. Key structures Rib cage Rib cage is moved by muscles between ribs (intercostal muscles covered later) Causes ventilation by a tidal stream Means constant replenishment of air

7. Key structures Trachea Flexible and supported with cartilage Cartilage prevents trachea collapsing when air pressure falls Tracheal walls consist of muscle Lined with ciliated epithelium and goblet cells Goblet cells produce mucus to trap dirt particles and bacteria Cilia move mucus + dirt/bacteria up the throat into oesophagus to stomach

8. Key structures Bronchi Two divisions of the trachea – each leading to one lung Bronchi (plural) and left and right bronchus (singular) Similar cell structure as trachea – produce mucus and have cilia Larger bronchi have cartilage – cartilage reduced as bronchi get smaller

9. Key structures Bronchioles Series of branching subdivisions of the bronchi Walls made of muscle lined with epithelial cells Muscle permits constriction to control the flow of air in and out of the alveoli

10. Key structures Alveoli (plural); alveolus (singular) Alveoli are minute air sacs (about 100-300 μm) at the end of the bronchioles Contain collagen and elastic fibres Elastic fibres allow alveoli to stretch when breathing in When breath exits, elastic fibres spring back to aid expulsion of carbon dioxide Lined with squamous epithelium (‘scale’ = Latin – VERY thin) Alveoli membrane is the gas- exchange surface of the lungs Blood vessels line the alveoli to diffuse gases

11. Summary Questions 1)In addition to support and protection a) why are lungs of humans found in the rib cage? Prevention of loss of moisture / stop lungs drying out b) why is prevention of this very important? Moisture is vital for the efficient exchange of gases in the lungs 2) List key structures of air travelling from the nostril to the blood vessels surrounding the alveoli? Nostril → nasal cavity → trachea → bronchi → bronchioles → alveoli → blood vessels

12. Summary Questions 3) Explain how the various cells of the lung structure ultimately protect and aid the alveoli function correctly? Trachea/bronchi – cartilage (support); goblet cells (mucus); ciliated cells (expulsion of dirt/bacteria). Bronchioles – muscle (controls air flow). Alveoli – elastin (aids expulsion of air during breathing out); squamous cells (very thin to aid diffusion of gases)

13. Mechanisms of Breathing Remember in Biology we DO NOT call the process of intake of air ‘Respiration’ (that is the process of ENERGY production), we call this process Ventilation. We must remember the HIGH → LOW rule again for particle movement This time we are talking about air pressure.

14. Breathing in Breathing in INCREASES our lung VOLUME. This causes a DROP IN AIR PRESSURE – thus air moves from HIGH → LOW (from atmosphere to lungs).

15. Breathing out Breathing OUT – we are compressing the air (DECREASING the VOLUME) in our lungs causing HIGH pressure – thus air moves from HIGH → LOW (from the lungs to the atmosphere)

16. ACTIVE PROCESS – uses ENERY Largely PASSIVE PROCESS – requires little energy

17. What causes ventilation? Intercostal muscles Two sets – internal intercostal muscles and external intercostal muscles Internal intercostal muscles contract to lead to expiration (air OUT) External intercostal muscles contract to lead to inspiration (air IN) Note that when one set is CONTRACTION the other set is RELAXING – this is described as ELASTIC recoil Diaphragm A sheet of muscle between the thorax and abdomen The diaphragm is curved upward (domed position) when relaxed When diaphragm muscles contract, it flattens and moves down Causing an increase in thorax volume During NORMAL quiet breathing, the recoil of the elastic lungs is the MAIN cause of air being forced out (similar to balloons). Muscles become most important during strenuous condition such as exercise.

18. YOUR TASK – in pairs, give summaries of key structural involvement in: 1) Inspiration 2) Expiration

19. Inspiration External intercostal muscles contract Internal intercostal muscles relax Ribs are pulled upward & outward – increasing thorax volume Diaphragm muscles contract, causing it to flatten – also increases volume Increase of thorax volume causes reduction of lung pressure Air moves HIGH → LOW (from atmosphere to Lungs)

20. Expiration Internal intercostal muscles contract External intercostal muscles relax Ribs are pulled downward & inward – decreasing thorax volume Diaphragm muscles relax, return to domed shape – also decreasing volume Decrease of thorax volume causes increase of lung pressure Air moves HIGH → LOW (from Lungs to atmosphere)

21. What is pulmonary ventilation? The total volume of air that is moved into the lungs during one minute To calculate this we need multiply two factors: 1) TIDAL VOLUME = volume of air normally taken in at EACH breath when body is at REST (usually 0.5dm 3 ) 2) VENTILATION (Breathing) RATE = the number of breaths taken in one minute. (usually 12-20 breaths) Pulmonary ventilation = tidal volume x ventilation rate (dm 3 min -1 ) (dm 3 ) (min -1 )

22. Summary questions answers

23. Summary Questions Answers

25. Airway Number Approx. Diameter Cartilage (and shape) Goblet Cellsproducing mucus Smooth Muscle Elastic Fibres Cilia Type of Epithelium Site of gaseous exchange Trachea 1.8cmFew ciliated columnar No Bronchus (Plural bronchi) 1.2cmFew ciliated columnar No Bronchiole300,000 0.5m m LessFew Ciliated Cuboidal No Alveoli 3 x 10 9 100 - 300 um No

26. Airway Number Approx. Diameter Cartilage (and shape) Goblet Cellsproducing mucus Smooth Muscle Elastic Fibres Cilia Type of Epithelium Site of gaseous exchange Trachea 11.8cm Yes C- shaped rings Yes Few Yes Move mucus ciliated columnar No Bronchus (Plural bronchi) 21.2cm Yes in larger bronchi Yes Few Yes Move mucus ciliated columnar No Bronchiole300,0000.5mmNo LessFew Yes (surface area) Ciliated Cuboidal No Alveoli 3 x 10 9 100 - 300 um No YESNo Simple squamous Yes

27. Tissue activityInhalationForced Exhalation External intercostal muscle Relax Internal intercostal muscle Contract DiaphragmContracts (flattens) Elastin fibres in alveoli Expand Rib movementInward & Downward Pressure changes (relative to each other) Volume of LungsDecreased Pressure in lungs/alveoli Low Pressure of environment Air movementLungs → Environment

28. Tissue activityInhalationForced Exhalation External intercostal muscle ContractRelax Internal intercostal muscle RelaxContract DiaphragmContracts (flattens)Relaxes (domed shape) Elastin fibres in alveoli ExpandSpring back Rib movementUpward & OutwardInward & Downward Pressure changes (relative to each other) Volume of LungsIncreasedDecreased Pressure in lungs/alveoli LowHigh Pressure of environment HighLow Air movementEnvironment → LungsLungs → Environment

29. Exchange of gases in the lungs Ventilation of gases within lungs is essential to provide a constant supply of oxygen to create a diffusion gradient within the alveolar surface.

30. CharacteristicBenefit to gas exchange Large surface area to volume ratio Squamous epithelial cells Partially permeable Movement of environmental medium (air) Movement of internal medium (blood) Moisture

31. CharacteristicBenefit to gas exchange Large surface area to volume ratio Speeds up gas exchange Squamous epithelial cells Very thin to keep diffusion distance small Partially permeableSelective materials permitted across Movement of environmental medium (air) Maintain diffusion gradient (O 2 in & CO 2 out) Movement of internal medium (blood) Maintain diffusion gradient (O 2 in & CO 2 out) MoistureAids optimal gas diffusion

32. As also covered in Cell Membranes – the following relationship between key factors is described as Fick’s Law. This relationship also applies to gas exchange in the lungs RD ≈ SA x CG DD Rate of Diffusion is proportional to: Surface Area of exchange x Concentration Gradient Diffusion Distance

33. Role of the alveoli in gas exchange We have about 300 million alveoli in EACH human lung. Their total surface area is 70m 2 (half a tennis court) Each alveoli is lined with squamous epithelial cells (0.05μm to 0.3μm thick) AROUND each alveoli is a network of pulmonary capillaries Very narrow (7-10μm thick) so that red blood cells are flattened against the walls Also have a single very thin wall (0.04μm – 0.2μm)

35. Summary Questions 1)How does each of the following features contribute to gas exchange efficiency? a)The wall of each alveolus is not more than 0.3μm thick? Diffusion distance small – thus rapid movement b) There are 300 million alveoli in each lung? Collectively, very large surface area c) The surfaces of the alveoli are moist? Permits optimal gas exchange d)Each alveolus is covered by a dense network of pulmonary blood capillaries? Collectively, provide a VERY large surface interface with blood e) Each pulmonary capillary is very narrow? Enables slowing down of blood (thus diffusion occurs faster) and blood cells pushed against capillary way to make diffusion distance smaller also.

36. Summary Questions If the number of alveoli in each lung was increased to 600 million and the pulmonary ventilation was doubled, how many times greater would the rate of diffusion be? Show your working with arbitrary values. E.g. say CG = 3 and DD = 10 (arbitrary values = made up) RD = SA xCG/DD RD (300) = 300x3 / 10 = 900 / 10TWICE! = 90 RD (600)= 600x3 / 10 = 1800 /10 = 180