RESPIRATION Gas Exchange

PARTIAL PRESSURES zIn a mixture of gasses, the total pressure distributes among the constituents proportional to their percent of the total zThe concentration of a gas can therefore be expressed as its partial pressure

Partial Pressures in air zOxygen = 21% zNitrogen = 79% z Po 2 = 160 mm Hg z P N 2 = 600 mm Hg z Total Pressure (at sea level) = 760mm Hg

Effect of water vapor zAs fresh air enters the nose and mouth it is immediately mixed with water vapor zSince the total pressure remains constant, the water vapor lowers the partial pressure of all other gases zFor this reason, the P O 2 is lowered to about 149 mmHg

DEAD SPACE VOLUME zAt the height of expiration, about 150ml of gas still occupies the respiratory tree zThis “old gas” is necessarily mixed with the incoming fresh air and further lowers the P O 2 to about 100 mmHg

GAS EXCHANGE ACROSS PULMONARY CAPILLARIES zBoth oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients Inspired Air P O 2 = 160mmHg P CO 2 = 0.03mmHg LUNG P O 2 = 100mmHg P CO 2 = 40mmHg P O 2 = 40mmHg P CO 2 = 46mmHg PULMONARY CAPILLARIES P O 2 = 100mmHg P CO 2 = 40mmHg

GAS EXCHANGE ACROSS SYSTEMIC CAPILLARIES zBoth oxygen and carbon dioxide diffuse down their concentration (partial pressure) gradients TISSUE P O 2 < 40mmHg P CO 2 > 46mmHg P O 2 = 40mmHg P CO 2 = 46mmHg SYSTEMIC CAPILLARIES P O 2 = 100mmHg P CO 2 = 40mmHg

Carbon dioxide/Bicarbonate Relationship CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - 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 zOxygen physically dissolved = 1.5% zOxygen bound to hemoglobin = 98.5% zCarbon dioxide physically dissolved = 10% zCarbon dioxide bound to hemoglobin = 30% zCarbon dioxide as bicarbonate = 60%

HEMOGLOBIN/OXYGEN DISSOCIATION P O 2 of blood (mmHg) % Hemoglobin Saturation Resting P O 2 Systemic Normal P O 2 Capillaries

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

UNDERSTANDING THE HB/O DISSOCIATION CURVE zThe plateau: Provides a margin of safety in the oxygen carrying capacity of the blood zThe 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 CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - 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 CO 2 + H 2 O ---> H 2 CO 3 ---> H + + HCO 3 Carbonic Anhydrase + Hb --->HbH + Hb ---> HbCO 2 HbO > Hb + O 2

Carbon Dioxide Transport in the Blood: At the lungs Alveolus Red Blood Cell CO 2 + H 2 O <--- H 2 CO 3 <--- H + + HCO 3 - Carbonic Anhydrase + Hb <---HbH + Hb <--- HbCO 2 HbO 2 <--- Hb + O 2

The Haldane Effect zRemoval of oxygen from hemoglobin increases hemoglobin’s affinity for carbon dioxide zThis allows carbon dioxide to “ride” on the empty hemoglobin

RESPIRATORY CONTROL zPons: Pneumotactic center zPons: Apneustic center zMedulla: Dorsal respiratory group zMedulla: Ventral respiratory group

Medulla: Dorsal respiratory group zInspiratory neurons zPacemaker activity zExpiration occurs when these cease firing

Medulla: Ventral respiratory group zBoth inspiratory and expiratory neurons zInactive during normal quiet breathing zRev up inspiratory activity when demands for ventilation are high

Pons: Pneumotactic center zFine tuning over medullary centers zSwitches off inspiration

Pons: Apneustic center zFine tuning over medullary centers zBlocks switching off of inspiritory neurons

CARBON DIOXIDE CONTROLLS RESPIRATION zHigh carbon dioxide generates acidity of blood in brain zAcidity of blood in systemic circulation is prevented from directly influencing the brain due to the blood/brain barrier’s impermeability to H + zCO 2 + H 2 O H 2 CO 3 H + + HCO 3

OXYGEN LEVELS MUST FALL DRASTICALLY TO AFFECT BREATHING zReceptors in carotid bodies zBelow 60 mmHg for oxygen partial pressure, breathing is stimulated zThis is a last-ditch, fail-safe mechanism only!