The respiratory system

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The respiratory system

• HB100 Sport and exercise physiology

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Gas transport mechanisms
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The respiratory system function

• Why breathe?
• O2 transport
• Acid/base control
• Goal maintain arterial PO2, PCO2 and pH
• Why have lungs?
• Comparative physiology
• Lecture topics
• Lung structure and lung volumes
• Pulmonary gas exchange
• O2 carriage in the blood
• Control of ventilation

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Respiratory tract structure
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Alveolar ventilation

• Branching of respiratory tree ends in alveoli
• gas exchange occurs between alveoli and pulmonary
  circulation
• Surface area 60 - 80 m2
• Barrier only 2 cells thick
• Maximises transport of gases to and from blood

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Lung volumes II

• Dead space Physiological and anatomical
• air that does not participate in gas exchange
• anatomical dead space air in airways
• Physiological dead space alveolar air that does
  not transfer gases to capillaries -
  ventilationperfusion
• Dead space volume 150 - 200 mL (30 resting
  tidal volume)
• Dynamic lung volumes
• Forced expiratory volume (FEV1.0)
• Maximal voluntary ventilation (MVV)
• Ventilation in maximal exercise less than MVV...
• Static and dynamic lung volumes do not directly
  reflect physical fitness - dynamic lung
  performance may be used as a screening tool for
  respiratory disease

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Partial Pressures

• Gas exchange depends on partial pressure
• Ambient air contains 20.93 O2, 0.03 CO2, 79.04
  N2 and water vapour. Because the total ambient
  pressure is 760 mmHg, each gas exerts its own
  partial pressure.
• Partial pressure concentration x total pressure
• Ambient PO2 159 mmHg at sea level
• Ambient PO2 25 mmHg on summit of Everest
• Gas transferred between air and fluid depends on
  pressure difference and solubility (Henrys Law)
• alveolar PO2 105 mmHg, mixed venous PO2 40
  mmHg
• CO2 25 times more soluble than O2

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O2 Transport haemoglobin

• Oxygen is carried in two ways
• in solution - dissolved oxygen
• Bound to haemoglobin
• If only the blood plasma were used for oxygen
  transport, blood flow would need to be 80 L.min-1
  at rest, more than twice the highest value ever
  recorded!
• Haemoglobin (Hb), an iron containing globular
  protein, increases the carrying capacity of the
  blood by 65-70 times
• 1 L of arterial blood at sea level contains 197
  mL O2 bound to Hb (150 g Hb, 1.34 mLO2.g Hb-1)
  and 3 mL dissolved O2 (200 mLO2.L-1 blood)
• Each Hb molecule can bind to 4 O2 molecules, and
  this depends largely on the PO2 of the solution

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The lung as a limiting factor in exercise

• Mechanical airway limitation - alveolar
  ventilation cannot increase with increase in
  exercise intensity
• MVV usually higher than VE at VO2max
• Athletes MVV may be close to or equal to VE at
  VO2max
• Alveolar/capillary O2 disequilibrium arterial
  hypoxemia
• SaO2 gt 93 at VO2max in normal people
• elite athletes may experience arterial
  desaturation in maximal exercise
• Respiratory muscle fatigue
• unlikely during short term exercise diaphragm
  muscle similar to cardiac muscle in oxidative
  capability
• Respiratory muscle steal O2 cost of breathing
• Dyspnea - may cause volitional exercise
  termination
• Well motivated subjects?

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Lung training?

• The lung appears to be overbuilt in the untrained
  human, the muscular and cardiovascular systems
  limit aerobic performance
• However, the lung is also the least trainable
  component of the gas exchange mechanisms
• Cardiovascular and muscular systems are highly
  trainable
• Elite endurance athletes may be limited by lung
  function, as are those with respiratory disease
• Race horses ARE limited by lung function -
  locomotory demands dictate the ventilatory
  response 11 entrainment

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Conclusions

• The respiratory system is structured to maximise
  gas exchange from the atmosphere to the blood
• Gas exchange depends upon differences in partial
  pressures between the alveoli, blood, and muscle
• Ventilation must be controlled to efficiently
  maintain oxygen delivery and arterial blood gas
  status
• VE closely related to VCO2 until very high
  exercise intensities
• Lung appears to be overbuilt in relation to
  exercise demands in untrained humans, but may
  represent a limiting factor for exercise in
  athletes, athletic species and those with
  respiratory disease

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Further reading

• Relevant chapters can be found in all good
  physiology textbooks (esp. McArdle et al. and
  Willmore and Costill)
• Names to look out for
• Jerry Dempsey
• John West (not the Fishmonger!)
• Brian Whipp (ventilatory control)
• Paper Dempsey, J.A., and P.D. Wagner.
  Exercise-induced arterial hypoxemia (Invited
  Brief Review) J. Appl. Physiol. 87 1997-2006,
  1999.