Section II Respiratory Gases Exchange

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Section II Respiratory Gases Exchange
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I Physical Principles of Gas Exchange
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• Partial pressure
• The pressure exerted by each type of gas in a
  mixture
• Diffusion of gases through liquids
• Concentration of a gas in a liquid is determined
  by its partial pressure and its solubility

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Partial Pressures of Gases

• Basic Composition of Air
• 79 Nitrogen
• 21 Oxygen
• 0 Carbon Dioxide

Pgas Pb x Fgas PN 760 x 0.79 600.4 mm
Hg P02 760 x 0.21 159.6 mm Hg
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Partial Pressure of Gases in Fluids
Each gas has a specific solubility O2 Solubility
coefficient 0.003 ml/100 ml Blood C02 0.06
ml/100 ml Blood (x 20 of 02)
Gases dissolve in fluids by moving down aPartial
Pressure gradient rather than a concentration
gradient
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Partial Pressure of Gases in Fluids
At equilibrium, if the gas phase has a PO2 100
mm Hg, the liquid phase also has a PO2
100 mm Hg
An easy way to talk about gases in fluids.
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• Transport of gases between the alveoli and
  (pulmonary) capillaries and eventually from the
  capillaries to the tissues
• diffusion dependent on perfusion and the partial
  pressure (pp) exerted by each gas
• gases diffuse from area of ? conc. (pp) to ?
  conc. (pp)

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Diffusion

• ?concentration ? ? ?pp of gas ? ?diffusion
• CO2 more soluble than O2, therefore it diffuses
  faster

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Diffusion Blood Transit time in the Alveolus
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II Gas exchange in the lung and in the tissue
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Oxygen and Carbon Dioxide Diffusion Gradients

• Oxygen
• Moves from alveoli into blood.
• Blood is almost completely saturated with oxygen
  when it leaves the capillary
• P02 in blood decreases because of mixing with
  deoxygenated blood
• Oxygen moves from tissue capillaries into the
  tissues

• Carbon dioxide
• Moves from tissues into tissue capillaries
• Moves from pulmonary capillaries into the alveoli

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Diffusion Gradients of Respiratory Gases at Sea
Level
NB. CO2 is 20x more soluble than O2 in blood gt
large amounts move into out of the blood down a
relatively small diffusion gradient.
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PO2 and PCO2 in Blood
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III. A-a gradient, the efficiency of the gas
exchange in alveoli
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In a healthy person, what would you expect the A
- a to be? No difference, greater than 0, or
less than 0
Normal A a, up to 10 mm Hg, varies with
age
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Factors contributing to A - a Gradient

• Blood Shunts
• Matching

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SIMPLE CONCEPT OF A SHUNT
No Gas Exchange SHUNT
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NEXT NEW CONCEPT
If the volumes used for exchange are aligned
We might consider the system to be ideally
matched
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Matching
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Matching ventilation perfusion
Ventilation and perfusion (blood flow) are both
better at the bottom (base) of the lung than that
at the top (apex). But the change in blood flow
is more steep than in ventilation. Therefore the
ventilation/perfusion ratio rises sharply from
the base to the apex.
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Matching ventilation perfusion (cont)
Result V/Q is greater or less than 0.8 in
different regions
If V/Q lt0.8 shunt like, If V/Q gt 0.8 little
benefit, Increases A - a gradient
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Severe Mismatch
Lung Disease with a Large A a gradient
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IV Factors Affecting the Gas Diffusion in the Lung
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• The Properties of the Gas
• Molecular weight. Diffusion rate is inversely
  proportional to the square root of the molecular
  weight
• Temperature
• Solubility in water
• Each gas has a specific solubility O2 Solubility
  coefficient 0.003 ml 02/100 ml Blood
• C02 0.06 ml/100 ml Blood (x 20 of 02)

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• 2. Partial Pressure of the Gases
• Alveoli ventilation
• Blood perfusion in the lung capillary
• Speed of the chemical reaction
• The slow speed of the chemical reaction HCO3-
  H ----- H2CO3 ---H2O CO2 reduces the CO2
  exchange in the lung.
• So, during the gas exchange in the external
  respiration, the exchange of CO2 is a little
  lower than that of O2.

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• 3. Properties of the Lung
• Area of the respiratory membrane
• Distance of the diffusion
• VA/Qc

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V Pulmonary Diffusion Capacity Concept The
ability of the respiratory membrane to exchange a
gas between the alveoli and the pulmonary blood
defined as the volume of a gas that diffuses
through the membrane each minute for a pressure
of 1 mmHg. DL V/(PA PC) V is a gas that
diffuses through the membrane each minute, PA is
the average partial pressure of a gas in the air
of alveoli, PC is the average partial pressure of
a gas in the blood of pulmonary capillary.
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• Factors Affecting the DL
• Body posture
• Body height and weight
• Exercise
• Pulmonary diseases

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VI Internal Respiration

• All cells require oxygen for metabolism
• All cells require means to remove carbon dioxide
• Gas exchange at cellular level

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• Concept Gas exchange between the capillary and
  the tissues throughout the body
• Process
• Factors affecting the internal respiration
• Distance between the cells and the capillary
• Rate of metabolic rate
• Speed of the blood flow in capillary

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EXTERNAL AND INTERNAL RESPIRATION
TISSUE CELL O2 FOOD
ATMOSPHERE
SYSTEMIC CIRCULATION
HEART
PULMONARY CIRULATION

LUNGS
CO2 H2O ATP