The Respiratory System

  • Created by: rosieevie
  • Created on: 20-01-17 17:11


  • Conducting zone - nose, pharynx, larynx, trachea, bronchi, bronchioles, terminal bronchioles
    • Filter, warm and moisten air to conduct it into lungs
  • Respiratory zone - O2 and CO2 exchange site within blood 
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Structure of Respiratory System

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Trachea, Bronchi and Bronchioles


  • Cartilagenous rings - hold trachea open + flexible
  • Pseudostratified (looks like layers but isn't) ciliated columar epithelium 
    • Cilia beat towards mouth = mucus swallowed (paralysed by nicotine)
  • Goblet cells - create mucus in lumen to trap dust etc


  • Conducting surface and branch repeatedly
  • Large in diameter


  • Smooth muscle and airflow holds airways open
  • No fibrocartilengous layer
  • Ciliated epithelial cells
  • Act as gas exchange site
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Thoracic Cavity

  • Consists of lungs and diaphragm
  • Lungs attached to wall due to interpleural pressure (~4mmHg) = vaccum - lung can move but remain attached
  • Drives air movement by volume changes = pressure changes
  • Intrapleural pressure always negative - lungs will collaspe otherwise
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Lung Compliance

  • Relating to lung elasticity
  • Decrease - sitffer lung = harder to infalte
  • Increase - floppy lung = tissue collapse (especially on exhalation) - emphasema

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  • Air-filled sacs - expand during inhalation
  • Covered with extensive vasculature - gasous exchange
  • 3 cell types:
    • Type I cell - simple squamous - flat and narrow = thin diffusion pathway
    • Type II cell - produce surfactant = detergent which decreases surface tension
    • Alveolar macrophages - white blood cell = defence (purify air/immune response)
  • Alveolar surface tension - keeps alveolar open and prevents collapse into bigger structures due to pressure difference (surfactanct reduces and equalises pressure)


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Cells Lining Airways

Stratified Squamous Epithelial Cells

  • Several layers thick = protection
  • Withstand abrasion

Ciliated Columnar Epithelial Cells

  • Cilia - beat towards mouth so mucus is swallowed
  • Paralysed by nictotine

Goblet Cells:

  • Secrete mucus - traps dust, bacteria

Alveolar Cells

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Inspiration and Expriation

  • Expiration - passive stage (2-3s)
    • Diaphragm relaxed (dome) 
    • External intercostal muscles relaxed
    • Rib cage in and down
    • Decrease in volume - increase in alveolar pressure
    • Intrapleural pressure increases
  • Inspiration - active stage (1-1.5s)
    • Diphragm contracts - downward movement
    • External intercostal muscles contract 
    • Rib cage up and out
    • Increases volume - decreases pressure 
    • Intreapleural pressure decreases
  • Quiet breathing - exhalation is passive
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  • Thick pipe - faster airflow
  • Increase in diameter increases flow
  • Airway diameter regulated by elastic tissues

∝ r4

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Spirometry Trace Labelled


  • Volume - amount of space take up by an object
  • Capacity - measure of object's ability to hold a substance
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Spirometry Trace Definitions

Residual volume - air remaining in lungs after fully exhaling (keeps lungs open)

Resting tidal volume - air entering lungs at rest (~500ml)

Expiratory reserve volume - maximal volume of air expelled after exhaling

Inspiratory reserve volume - maximal volume of air inhaled during inhalation

Total lung capacity - predicted vital capacity + residual volume

Vital capacity - greatest expelled air volume after taking the deepest possible breath

Functional residual capacity - air remaining in lungs after passive expiration

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Measuring Residual Volume

Amount of air left in the lungs after full exhaling (keeps lungs open)

Cannot be measured using conventional spirometry but can be measured by:

  • A gas dilution test. A person breathes from a container containing a documented amount of a gas (either 100% oxygen or a certain amount of helium in air). The test measures how the concentration of the gases in the container changes.
  • Body plethysmography. This test measures the total amount of air the lungs can hold (total lung volume). For this test, a person sits inside an airtight booth called a plethysmograph and breathes through a mouthpiece while pressure and air flow measurements are collected.
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Blood Gaseous Exhange

Oxygen diffuses into blood along partial pressure gradient - changes caused by breathing and circulation.

Warm, humid air in lungs means lower pp in alveoli than atmospheric air

Dalton's Law - in a mixture of gases (air) the total pressure is sum of gas partial pressures

Partial pressure - the pressure a gas would exert if only gas present

At sea level (mostly N2 and O2) - Ptotal = 760mmHg

  • N2 = 760mHg x 0.78 = 560mmHg
  • O2 = 760mmHg x 0.21 = 160mmHg

Changing altitude = change oxygen's partial pressure

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Ventilation and Perfusion

Ventiliation (alveoli) matched to perfusion (pulmonary capillaries) = efficient process

Ventilation = air into alveolus

Perfusion = blood moving past alveoli

Decrease in ventilation (alveolar blockage) -> increase PCO2 and decreases PO2 = blood not oxygenated -> tissue around damaged alvoli constricts arteries, diverting blood to other alveoli

V/Q matching - ratio used to assess efficiency and adaquacy of oxygen from air reaching capillaries

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Diffusion Across a Membrane (Fick's Law)

Vgas = (AD/T) x (P1 - P2)

  • Vgas = diffusion rate
  • A = Area of pathway (directly proportional - large alveolus SA)
  • D = gas solubility (directly proportional)
    • Limiting factor - reduced by haemoglobin and carbonic anhydrase
  • T = pathway thickness (inversly proportional - single squaemous epithelial layer)
  • P1 and P2 = pressure differential (directly proportional - high diffusion gradient maintainined by ventilation and circulation)
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Red Blood Cells

  • Biconcave cells
  • Flexible membranes - change shape to pass through narrow capillaries
  • Mammalian blood cells - no nucleus (short lifespan - unable to repair and O2 is toxic)
  • Bird and reptile blood cells have nuclei
  • Haemoglobin - 4 globin proteins w/ 4 haem groups (1 Fe2+ ion = binds to O2)
  • 1 RBC can carry 4 O2 molecules
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Oxygen-Haemoglobin Dissociation Curve


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Oxygen-Haemoglobin Dissociation Curve 2

  • S-shaped curve
  • Mechanism of O2 unloading
  • Unloading cooperatively - made easier by haem shape change
  • Right shift (exercise) - lower stauration at same pO2 = more O2 delivered
    • High CO2
    • Low pH (high H+ conc)
    • High tempreature
    • High BPG (by-product of glycolysis)
  • Left shift (cold) - higher saturation at same pO2 = less O2 delivered
    • High pH
    • Low temperature
  • Blood leaving lungs and entering systemic arteries (arterial blood) - PO2 95mmHg (97%)
  • Venous blood returning from peripheral tissues (resting tissue) - PO2 400mmHg (75%)
  • Active tissue, heavy exercise (35% saturation) - as low as PO2 15mmHg
  • Utilization coefficient - % oxygen haemoglobin gives up to tissues = 25% (exercise <75%)
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  • Found in skeletal muscle
  • O2 storage/transfer molecule
  • 1 subunit - monomeric protein w/ haem ring
  • Carries 1 O2 mol - not cooperative
  • Dissociation at low pO2
  • 20x more myoglobin in whales = able to store O2 at low depths
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Carbon Dioxide Transport

  • Binds to globin protein of haemoglobin = carbamminoHb (20%
  • Dissolves in blood plasma (10%)
  • HCO3- in blood plasma (70%)

Carbon dioxide conversion to HCO3-:

RBC facilitate conversion using carbonic anhydrase enzyme:

RBC O2 carrying role not independent of CO2 carrying role = related reactions - removing H+ ions from O2 reaction drives CO2 reaction

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Control of Respiratory Activity

Mediated by nervous system - mediators in the brain stem:

  • Dorsal respiratory group - inspiraton
  • Ventral respiratory group - expiration and inspiraton
  • Pneumotaxic center - rate and depth of breathing (smooth it out)
  • Apneustic centre - inhibits pneumotaxic centre (controls intensity of breathing)
  • Pre-Botzigner complex - controls respiratory rhythm
  • Nucleus tractus solitarius - pacemaker

Send outputs to respiratory muscles via respiratory motor pathways

Expiration usually passive process - no need for NS input

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Modifying Respiration

  • -Higher centers in brian (voluntary) - overidden by simpler centres
  • Medullary chemoreceptors - detect changes in pH (CO2 concentration)
  • Carotid/aortic body chemoreceptors - detect decreases in CO2 conc
  • Hering-Breuer reflex - stretch response in lungs - decrease respiration to prepare for changes in activity - prevents overstretching and damaging muscles
  • Proprioceptors - muscles and joints - increase respiration
  • Receptors for touch, temperature and pain stimuli
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Medullary and Peripheral Chemoreceptors

Medullary Chemoreceptors

  • H+ ion concentration (indirectly CO2 levels - cannot cross blood vessel walls)
  • In cerebrospinal fluid
  • Signals to respiratory control centre - increases respiration
  • 5mmHg CO2 increase = x2 respiratory rate
  • Chronic exposure to high CO2 levels desensitises them

Peripheral Chemoreceptors

  • Detect pO2 
  • Carotid areas of neck and aortic bodies (arterial blood)
  • pO2 drop below 70mmHg to activate (usually 100mmHg)

CO2 prime driver of respiration - 5mmHg increase doubles respiration rate (increase pO2 by 30mmHg before O2 drives)

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Real-life Respiratory Situations

Hyperventilating before diving:

  • Decrease pCO2 = reduced respiratory urge
  • Decreases pO2 = blackout
  • Blackout = death

Giving a paitent with COPD O2:

  • Paitent has long term pCO2 increase
  • Receptors desensitised
  • Low pO2 drive
  • Giving O2 increases pO2 = lost respiratory drive
  • Death - paitent stops breathing

Holding breath until dying:

  • Not possible
  • Would lose conciousness 
  • As soon as this occurs breathing would start up again
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