RESPIRATION

1
RESPIRATION

• Gas Exchange

2
PARTIAL PRESSURES

• In a mixture of gasses, the total pressure
  distributes among the constituents proportional
  to their percent of the total
• The concentration of a gas can therefore be
  expressed as its partial pressure

3
Partial Pressures in air

• Oxygen 21
• Nitrogen 79

• Po2 160 mm Hg
• PN2 600 mm Hg
• Total Pressure (at sea level) 760mm Hg

4
Effect of water vapor

• As fresh air enters the nose and mouth it is
  immediately mixed with water vapor
• Since the total pressure remains constant, the
  water vapor lowers the partial pressure of all
  other gases
• For this reason, the PO2 is lowered to about 149
  mmHg

5
DEAD SPACE VOLUME

• At the height of expiration, about 150ml of gas
  still occupies the respiratory tree
• This old gas is necessarily mixed with the
  incoming fresh air and further lowers the PO2 to
  about 100 mmHg

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GAS EXCHANGE ACROSS PULMONARY CAPILLARIES
Inspired Air PO2 160mmHg

• Both oxygen and carbon dioxide diffuse down their
  concentration (partial pressure) gradients

PCO2 0.03mmHg
LUNG PO2 100mmHg PCO2 40mmHg
PULMONARY CAPILLARIES
PO2 40mmHg PCO2 46mmHg
PO2 100mmHg PCO2 40mmHg
7
GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES

• Both oxygen and carbon dioxide diffuse down their
  concentration (partial pressure) gradients

TISSUE PO2 lt 40mmHg PCO2 gt 46mmHg
SYSTEMIC CAPILLARIES
PO2 40mmHg PCO2 46mmHg
PO2 100mmHg PCO2 40mmHg
8
Carbon dioxide/Bicarbonate Relationship
CO2 H2O lt---gt H2CO3 lt---gt H HCO3-
Carbon dioxide dissolved in water readily
combines with water to form carbonic acid. The
carbonic acid then dissociates into the hydrogen
ion and bicarbonate ion. The former reaction is
catalized by and enzyme called Carbonic Anhydrase
in many tissues.
9
GAS TRANSPORT IN BLOOD

• Oxygen physically dissolved 1.5
• Oxygen bound to hemoglobin 98.5
• Carbon dioxide physically dissolved 10
• Carbon dioxide bound to hemoglobin 30
• Carbon dioxide as bicarbonate 60

10
HEMOGLOBIN/OXYGEN DISSOCIATION
Resting PO2 Systemic
Normal PO2 Capillaries
Hemoglobin Saturation
PO2 of blood (mmHg)
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Agents which shift the Hb/O Dissociation curve
The Bohr Effect
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UNDERSTANDING THE HB/O DISSOCIATION CURVE

• The plateau Provides a margin of safety in the
  oxygen carrying capacity of the blood
• The steep portion Small changes in Oxygen levels
  can cause significant changes in binding. This
  promotes release to the tissues.

13
Agents which shift the Hb/O Dissociation curve
The Bohr Effect
14
Carbon dioxide/Bicarbonate Relationship
CO2 H2O lt---gt H2CO3 lt---gt H HCO3-
Carbon dioxide dissolved in water readily
combines with water to form carbonic acid. The
carbonic acid then dissociates into the hydrogen
ion and bicarbonate ion. The former reaction is
catalized by and enzyme called Carbonic Anhydrase
in many tissues.
15
Carbon Dioxide Transport in the Blood At the
tissues
Tissue Cell
Carbonic Anhydrase
Red Blood Cell
CO2 H2O ---gt H2CO3 ---gt H HCO3
Hb ---gtHbH
Hb ---gt HbCO2
HbO2 -----gt Hb O2
16
Carbon Dioxide Transport in the Blood At the
lungs
Alveolus
Carbonic Anhydrase
Red Blood Cell
CO2 H2O lt--- H2CO3 lt--- H HCO3-
Hb lt---HbH
Hb lt--- HbCO2
HbO2 lt--- Hb O2
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The Haldane Effect

• Removal of oxygen from hemoglobin increases
  hemoglobins affinity for carbon dioxide
• This allows carbon dioxide to ride on the empty
  hemoglobin

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RESPIRATORY CONTROL

• Pons Pneumotactic center
• Pons Apneustic center
• Medulla Dorsal respiratory group
• Medulla Ventral respiratory group

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Medulla Dorsal respiratory group

• Inspiratory neurons
• Pacemaker activity
• Expiration occurs when these cease firing

20
Medulla Ventral respiratory group

• Both inspiratory and expiratory neurons
• Inactive during normal quiet breathing
• Rev up inspiratory activity when demands for
  ventilation are high

21
Pons Pneumotactic center

• Fine tuning over medullary centers
• Switches off inspiration

22
Pons Apneustic center

• Fine tuning over medullary centers
• Blocks switching off of inspiritory neurons

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CARBON DIOXIDE CONTROLLS RESPIRATION

• High carbon dioxide generates acidity of blood in
  brain
• Acidity of blood in systemic circulation is
  prevented from directly influencing the brain due
  to the blood/brain barriers impermeability to H
  
• CO2 H2O lt---gt H2CO3 lt---gt H HCO3

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OXYGEN LEVELS MUST FALL DRASTICALLY TO AFFECT
BREATHING

• Receptors in carotid bodies
• Below 60 mmHg for oxygen partial pressure,
  breathing is stimulated
• This is a last-ditch, fail-safe mechanism only!