respiration gas exchange

RESPIRATION

Gas Exchange

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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


Partial Pressures in air

❚ Oxygen = 21%

❚ Nitrogen = 79%

❚ Po2 = 160 mm Hg

❚ PN2 = 600 mm Hg

❚ Total Pressure (at sea level) = 760mm Hg


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


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


GAS EXCHANGE ACROSS PULMONARY CAPILLARIES

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

Inspired Air PO2 = 160mmHg

PCO2 = 0.03mmHg

LUNG PO2 = 100mmHgPCO2 = 40mmHg

PO2 = 40mmHgPCO2 = 46mmHg

PULMONARY CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg


GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES

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

TISSUE PO2 < 40mmHgPCO2 > 46mmHg

PO2 = 40mmHgPCO2 = 46mmHg

SYSTEMIC CAPILLARIESPO2 = 100mmHgPCO2 = 40mmHg


Carbon dioxide/Bicarbonate Relationship

CO2 + H2O <---> H2CO3 <---> 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.


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%


HEMOGLOBIN/OXYGEN DISSOCIATION

PO2 of blood (mmHg)

% HemoglobinSaturation

Resting PO2

SystemicNormal PO2

Capillaries


Agents which shift the Hb/O Dissociation curve: The Bohr Effect


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.


Agents which shift the Hb/O Dissociation curve: The Bohr Effect


Carbon dioxide/Bicarbonate Relationship

CO2 + H2O <---> H2CO3 <---> 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.


Carbon Dioxide Transport in the Blood: At the tissues

Tissue Cell

Red Blood Cell

CO2 + H2O ---> H2CO3 ---> H+ + HCO3

Carbonic Anhydrase

+ Hb --->HbH+ Hb ---> HbCO2

HbO2 -----> Hb + O2


Carbon Dioxide Transport in the Blood: At the lungs

Alveolus

Red Blood Cell

CO2 + H2O <--- H2CO3 <--- H+ + HCO3-

Carbonic Anhydrase

+ Hb <---HbH+ Hb <--- HbCO2

HbO2 <--- Hb + O2


The Haldane Effect

❚ Removal of oxygen from hemoglobin increases hemoglobin’s affinity for carbon dioxide

❚ This allows carbon dioxide to “ride” on the empty hemoglobin


RESPIRATORY CONTROL

❚ Pons: Pneumotactic center❚ Pons: Apneustic center❚ Medulla: Dorsal respiratory group❚ Medulla: Ventral respiratory group


Medulla: Dorsal respiratory group

❚ Inspiratory neurons

❚ Pacemaker activity

❚ Expiration occurs when these cease firing


Medulla: Ventral respiratory group

❚ Both inspiratory and expiratory neurons

❚ Inactive during normal quiet breathing

❚ Rev up inspiratory activity when demands for ventilation are high


Pons: Pneumotactic center

❚ Fine tuning over medullary centers

❚ Switches off inspiration


Pons: Apneustic center

❚ Fine tuning over medullary centers

❚ Blocks switching off of inspiritory neurons


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 barrier’s impermeability to H+

❚ CO2 + H2O <---> H2CO3 <---> H+ + HCO3


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!


respiration gas exchange

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