Elastic properties of the lung

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Elastic properties of the lung

• The high compliance of the lung is related to a
  substance called surfactant.

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Respiratory Physiology
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Respiratory System

• What makes the air go in and out of the lungs?
• Gases move from regions of high pressure to
  regions of low pressure
• Boyles Law states that the pressure of a fixed
  mass of gas at constant temperature is inversely
  proportional to its volume

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Respiratory System
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Respiratory System

• When the volume of the lung increases the
  pressure w/in the lung decreases.
• Diaphragm contracts
• Chest expanses

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Respiratory System

• Actions of the respiratory muscles
• In humans
• Inspiration is a purely active process
• FRC- Functional Residual Capacity

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Respiratory System

• Actions of the respiratory muscles
• In horses
• Biphasic inspiration and expiration

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Respiratory System

• Pleural membranes
• Two pleural membranes
• Visceral (nearest the lung)
• Parietal (nearest the chest wall)
• Filled w/ a thin layer of pleural fluid
• Acts as a lubricant and as an adhesive

Pleural cavity
Diaphragm
Visceral pleura
Parietal pleura
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Respiratory System

• After alveoli ventilation
• the oxygen must diffuse from the alveoli into
  the pulmonary blood
• and diffuse the carbon dioxide in the opposite
  direction.
• All gases of concern in respiratory physiology
  are simple molecules that are free to move among
  one another.
• When air is passing through the nasal cavities
  and the upper airways, it is saturated with H2O.

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Respiratory System

• The alveolar air is quite different from
  inspired air because the CO2 is continuously
  expelled into the alveoli and O2 is taken from
  the alveoli.
• Mean values for alveolar air
• 13.6 O2,
• 5.3 CO2,
• 74.9 N, (nitrous oxide)
• 6.2 H2O.

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Respiratory System

• Diffusion the passive process whereby O2 passes
  from alveoli to capillary blood and CO2 passes
  in the reverse direction.
• For diffusion to occur,
• there must be a source
• of energy.
• This is provided by kinetic motion, the rapid
  and randomly striking of other molecules.

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Respiratory System

• Net diffuse of a gas in one direction occurs
  when
• a high concentration of gas is at one end of the
  chamber and a lower concentration at the other
  end.
• Diffusion is proportional to
• Partial pressure of each gas in the alveoli
  respiratory gas mixture.
• first in the alveolar membrane and then in the
  blood of the alveolar capillaries.
• pressure is determined by the gases concentration
  and solubility coefficient

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Respiratory System

• The alveolar-arterial pressure gradient is not
  constant.
• it decreases progressively as the blood passes
  through the alveolar capillary.
• it varies according to the stage of the
  respiratory cycle.
• Max. pressure gradients in the horse that can be
  encountered are 60 and 6 mmHg of O2 and CO2.

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Alveolar Diffusion

• At rest and moderate exercise levels, no
  measurable diffusion limitation is observed in
  the healthy horse.
• In contrast, during heavy but not necessarily
  maximal exercise, arterial hypoxemia and
  hemoglobin desaturation may occur in the horses.

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Alveolar Diffusion

• Hypoxemia is primarily related to one of the
  following factors
• 1. decrease in the partial pressure of O2 in the
  inspired air.
• 2. right-to-left vascular shunts.
• 3. ventilation / perfusion mismatching
• 4. diffusion impairment
• 5. alveolar hypoventilation and perfusion

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Alveolar Diffusion

• Factors that affect the rate of gas diffusion
  through the respiratory membrane
• pressure gradient of the gases between alveoli
  and capillary.
• physical properties of the gases
• diffusion coefficient of the gas
• surface area available
• thickness of the alveolar-capillary barrier

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Rate of Diffusion, V

• Area
• V Thickness x ?P x D
• Rate of diffusion of a gas is
• proportional to the area available for gas
  exchange. Area
• Inversely proportional to the thickness of the
  exchange surface Thickness
• Proportional to the diffusion constant D
• Greater when the pressure gradient between the
  gas w/in the alveoli and pulmonary arterial blood
  is greater ?P

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Pressure gradient

• Pressure difference
• Partial pressure (PP) is a measure of total of
  molecules of a particular gas striking a unit
  area of the alveolar surface of the membrane
  in unit time.
• and the pressure of the gas in blood represents
  the number of molecules that attempt to escape
  from the blood in the opposite direction.

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Pressure gradient

• When PP of gas in the alveoli is greater than
  pressure in the blood, net diffusion (O2) from
  alveoli into blood occurs.
• When pressure of the gas in the blood is greater
  than the PP in the alveoli, net diffusion (CO2)
  from the blood into the alveoli occurs.

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Diffusion coefficient

• Diffusion coefficient
• Transfer of each gas through the respiratory
  membrane depends on its diffusivity (D).
• D Solubility Molecular wt.
• Proportional to the solubility of the gas.
• And is inversely influenced by its molecular
  weight.
• Diffusivity of CO2 is 20xs that for oxygen.
• This is because CO2 is 24xs more soluble than
  O2.
• But CO2 diffuses through the membrane 20 slower
  than O2.
• Because of its higher molecular weight
• O2 in turn diffuses about twice as fast as N.

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Alveolar-capillary surface

• The horse appears to have an appropriately
  enlarged alveolar-capillary surface area.
• During exercise this area is enlarged by the
  recruitment of unperfused vessels induced by the
  increase in pulmonary pressure.

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Surface Area

• Surface area of respiratory membrane
• Can be decreased by
• removal of a lung
• emphysema abnormal accumulation of air in
  tissues and organs.
• When total surface area is decreased to about
  1/3 to 1/4 normal,
• exchange of gases through the membrane is impeded
  to a significant degree.

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Respiratory Membrane

• Different layers of the respiratory membrane
• 1. A layer of fluid lining the alveolus and
  containing surfactant that reduces the surface
  tension of the alveolar fluid.
• 2. The alveolar epithelium composed of thin
  epithelial cell.

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Respiratory Membrane

• 3. An epithelial basement membrane
• 4. A thin interstitial space between the
  alveolar epithelium and the capillary membrane.
• 5. A capillary basement membrane that in many
  places fuses with the epithelial basement
  membrane.
• 6. The capillary endothelial membrane.

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Respiratory Membrane
Epithelial B.M
Capillary B.M
Interstitial space
Capillary Epithelium
Alveolar Epithelium
O2
RBC
CO2
Alveolus
Capillary
Cross section of respiratory membrane.
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Thickness of respiratory membrane

• Causes of thickness of respiratory membrane
• Edema fluid in the interstitial space and
  alveoli
• Pulmonary disease may cause fibrosis of the
  lungs.
• Any factor increasing the thickness of the
  membrane 2 to 3 times can interfere
  significantly w/ normal gas exchange.

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Gas Exchange

• Compared with other mammals of similar size,
  horses achieve a higher VO2max per kg of body wt.
  by building and maintaining more of the
  following structures in the O2 transport chain
• Heart size
• Hemoglobin
• Peripheral capillary bed
• Larger skeletal muscle mass that contains a
  higher density of mitochondria.

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Blood Gas Transport

• Once O2 passes through the alveolar-capillary
  barrier, it either
• dissolves in the plasma or
• combines with hemoglobin.
• Each molecule of hemoglobin can bind up to 4
  molecules of O2, forming the oxyhemoglobin
  complex.
• Contributes to the maintenance of an adequate
  pressure gradient during alveolar-capillary
  diffusion

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Blood Gas Transport

• Oxygen content of blood is mainly determined by
• The hemoglobin concentration
• Its saturation with O2
• When RBC are reduced (as in anemia) the O2
  content is reduced.
• When PCV is increased, the O2 content increases
  even if arterial O2 partial pressure is reduced.

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Blood Gas Transport

• The increase in PCV and the consequent increase
  in hemoglobin provides almost 50 to 60 more
  binding sites for O2 during exercise.
• However, too great an increase in PCV may be
  disadvantageous.
• Why

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Tissue Diffusion

• Once released into the tissue, O2 will be
  bound to myoglobin.
• the main function of which is the transfer of
  O2 w/in the muscle cells.