Chapter 17 Mechanics of Breathing

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ChipzThatLeo
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270251
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Chapter 17 Mechanics of Breathing
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2014-04-11 22:51:03
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Breathing
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Mechanics of Breathing
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  1. The four primary functions of the respiratory system are:
    • 1. Exchange of gases between the atmosphere and the blood.
    • 2. Homeostatic regulation of body pH
    • 3. Protection from inhaled pathogens and irritating substances
    • 4. Vocalization
  2. What is respiration?
    • External respiration is the sum of four processes

    • 1. Ventilation (gas exchange) involves
    • inspiration (inhalation) to bring air into lungs and expiration (exhalation) to carry air out of lungs

    Exchange of O2and CO2 between lungs and blood

    3. Transport of O2 and CO2 by blood

    4. Exchange of gases between blood and cells
  3. Anatomical Structures
    of Respiration?
    • • Respiratory system consists of two
    • parts:
    • 1. Upper respiratory tract (mouth,
    • nasal cavity, pharynx, larynx
    • and vocal cords

    • 2. Lower respiratory tract (trachea, two
    • bronchi, branches, and lung tissue)
  4. Structures in the Thorax
    • • Lower respiratory system is in the thorax
    • • Thorax is bounded by muscle and bone called thoracic cage
    • • Diaphragm muscle forms floor
    • • Ribs and intercostal muscles form sides and top
  5. Construction of the
    Lungs
    • • Lungs are mostly air
    • • Support for lungs:
    • – two pleuralmembranes
    • surround a lung
    • – Fluid fills pleural cavity between membranes
    • • Connection with the atmosphere is by
    • way of a conducting system
  6. Conducting system
    • • Trachea and bronchi have cartilage skeleton to prevent collapse
    • • Bronchi branch into bronchioles
    • • Bronchioles are collapsible tubes of
    • smooth muscle lined with epithelia
    • • Bronchioles terminate in alveoli
    • • Alveoli form a surface
    • for gas exchange
  7. Construction of the Alveoli
    • • An alveolus is a single layer of epithelium
    • • Type 1 cell: most alveolar cells facilitate diffusion of gases between lung tissue and
    • atmosphere
    • • Type II cell: secretes surfactant
  8. Alveolar‐capillary Junction
    • • Fast diffusion between
    • alveolus and capillary
    • depends on short
    • distance
    • • Alveolar epithelial cells
    • and pulmonary
    • capillary endothelial
    • cells are thin to reduce
    • distance
    • • Gases must diffuse in
    • plasma to reach red
    • blood cells (RBCs).
  9. Gas Laws
    Gas Pressure Drives Flow

    • • Air is a mixture of gases
    • • Each gas has a partial
    • pressure based on %
    • composition
    • • PO2 = 160 mm Hg
    • • PCO2 = 0.25 mm Hg
    • • Partial pressure
    • gradient (ΔP) and
    • resistance (R)
    • determine flow (F) of
    • individual gases
    • according to
    • F  ΔP/R
    • TOTAL PRESSURE 760 mm Hg
    • • R is inversely related to the fourth
    • power of the radius of a bronchiole
  10. Ventilation
    • Ventilation (Breathing) exchanges air between the environment and
    • interior air spaces of the lungs.
    • • The exchange occurs in a respiratory cycle
    • – Inspiration
    • – Expiration
    • • Result of ventilation is Bulk flow based on physical principles:
    • 1. Flow is from regions of high pressure to low pressure
    • 2. Muscular pump creates pressure gradient
    • 3. Resistance to flow is influenced by diameter of the tubes
    • carrying air flow.
  11. What governs bulk flow of air?
    • Air moves from a region of high
    • pressure to a region of low pressure
    • • Lungs expand during inspiration
    • • Lungs return to resting volume during
    • exhalation
  12. Gas Laws
    • • Flow of air is governed by
    • Boyle’s law:
    • – if volume increases,
    • pressure decreases
    • – if volume decreases,
    • pressure increases
    • • Inspiration:
    • – Lung volume increased
    • – Lung pressure must
    • decrease
    • – Flow is inward (high
    • pressure outside to low
    • pressure inside lung)

    • • Expiration:
    • – Lung volume decreased
    • – Lung pressure must
    • increase
    • – Flow is outward (high
    • pressure inside to low
    • pressure outside lung)
  13. How does inspiration move air into alveoli?
    • • Skeletal muscle
    • contract to expand
    • thoracic cage
    • 1. Diaphragm moves
    • down
    • 2. Contraction of
    • intercostal and
    • scalene muscles pull
    • ribs up and out
    • • When lungs expand
    • pressure is reduced in
    • alveoli. Flow follows.
  14. Inspiration Depends on
    Compliance of Lung
    • • Compliance is ability of lung to be stretched
    • • Low compliant (stiff) lung tissue requires
    • more force to expand
    • – diaphragm, intercostal muscles, and
    • scalene muscles to work harder
    • • Ideal lung tissue has high compliance
  15. Role of Surfactant in Achieving High Compliance
    • • Surfactant secretion is necessary to keep compliance high
    • • Surfactant is secreted by Type II cells of alveolar epithelium
    • • Surfactant synthesis begins about 25th week of fetal development
    • • Earlier than 25 weeks, premature survival can be improved by
    • treatments with surfactant.
  16. Role of Bronchoconstriction in Inspiration
    • • Bronchoconstriction is the name given to decreased diameter of
    • bronchioles
    • – Bronchioles have smooth muscle like arterioles
    • – Constriction of smooth muscle reduces bronchiole diameter
    • – Constriction causes increase in resistance to flow during
    • inspiration
    • • Mucus can accumulate in bronchi; this blocks flow
    • • Bronchoconstriction can be the result of immune response
    • – Bronchioles can collapse causing large increase in resistance
    • • Trachea and bronchi do not contribute to bronchoconstriction
    • – fixed by cartilage rings that reinforce walls
    • • Alveoli do not contribute to bronchoconstriction
    • – Alveoli have no muscle layer
  17. How does expiration move air out of alveoli?
    • • Lungs are elastic
    • – Lungs are stretched by
    • volume increase during
    • inspiration
    • – Lungs recoil (elastic
    • recoil) when muscles
    • relax
    • • Hence, return of thorax and
    • diaphragm to resting state is
    • passive
    • • Forced exhalation requires
    • contraction of abdominal
    • muscles
  18. What if elasticity of lung is reduced ?
    • • Elastic fibers in connective tissue accounts for elasticity
    • • Smoking stimulates breakdown of elastic fibers
    • • An emphysema patient must force air out of lungs
  19. What is a collapsed lung?
    • Lungs are pulled to the margin of the ribs by a force
    • • If pleural sac is opened:
    • – Force is relieved
    • – Air flows in
    • – Air displaces lung inward
    • • collapsed lung is pneumothorax
  20. How is ventilation measured?
    • Ventilation (breathing) is a
    • cycle of inhalation and
    • exhalation
    • • A spirometer measures
    • volumes of air moved in
    • and out of lungs
    • • Tidal volume is the air
    • moved during normal
    • breathing
    • • Vital capacity is the
    • volume moved by a
    • forced inhalation followed
    • by a forced exhalation
  21. How is vital capacity
    related to lung
    function?
    • Deepest breath
    • measures inspiratory
    • reserve volume (IRV)
    • • IRV depends on
    • mechanics of
    • inspiration
    • • Forced exhalation
    • measures expiratory
    • reserve volume (ERV)
    • • ERV depends on
    • mechanics of
    • expiration
  22. Efficiency of Breathing
    • • Alveolar ventilation
    • measures efficiency
    • • Only a fraction of
    • tidal volume is
    • fresh air
    • – Dead space has
    • stale air
    • – Dead space is
    • constant
    • • Deep breathing
    • increases the
    • fraction of fresh air
    • in each breath
  23. Consistency in alveolar ventilation
    • • Normal quiet breathing
    • delivers constant gas
    • composition
    • – PO2 = 100 mm Hg
    • – PCO2 = 40 mm Hg
    • – PO2 and PCO2 change little
    • between breaths
    • • Hyperventilation does
    • improve alveolar ventilation
    • (efficiency)
    • – PO2 increases
    • – PCO2 decreases

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