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 centerPons: Apneustic centerMedulla: Dorsal respiratory groupMedulla: 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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