External Respiration – Specifics• Partial pressures in alveoli different from

atmospheric partial pressures

Gas Partial Pressure

Surrounding Air, mm Hg

Alveolus, mm Hg

PO2 159 104

PCO2 0.3 40

PH2O 3.5 47

External Respiration: Reasons for Difference

• humidification of inhaled air • gas movements

– increases alveolar PCO2

– decreases alveolar PO2

• mixing of old and new air (not all alveolar air is exchanged with each breath)

External Respiration:Gas Movements

• O2 loading (into blood)

• CO2 unloading (out of blood)

Ventilation-Perfusion Coupling• circulatory system works in coordination

with respiratory system to maximize effectiveness of exchange

• PO2 in bronchioles affects arteriolar diameter – more blood goes to areas with higher oxygen levels– decreased airflow decreased PO2 in airway

vasoconstriction of pulmonary arterioles – increased airflow increased PO2 in airway

vasodilation of pulmonary arterioles

Ventilation-Perfusion Coupling• PCO2 in bronchioles affects bronchiolar

diameter – areas that are stagnating build up CO2 increase flow through those areas– decreased airflow increased PCO2 in airway

bronchodilation– increased airflow decreased PCO2 in airway

bronchoconstriction

Internal Respiration: Gas Movements

• O2 enters tissues from blood

• CO2 leaves tissues to enter blood

Factors Affecting Internal Respiration

• surface area for exchange – size of capillary bed[s] serving tissue– varies from tissue to tissue

• partial pressure gradient between blood and tissue– ventilation keeps arterial PO2 high, PCO2 low

• rate of blood flow– varies with needs of tissue– increases during activity– decreases when tissue is inactive

Metabolism Review• Aerobic: Glucose + 6 O2 --> 6 CO2 + 6 H2O +

ATP

• Anaerobic: Glucose --> lactic acid + ATP

• By-products of glucose catabolism include:– BPG (bis-phosphoglycerate; intermediate of glycolysis)– heat (reactions are inefficient)– H+ (lowers pH), from:

• CO2 combines with H2O to produce carbonic acid or• lactic acid