ANSC 222 unit 2 (respiratory gas exchange)

CO2

more soluble than oxygen

PO2

• Partial pressure of dissolved O2 in air or in plasma (not hemoglobin)

• This force pushes O2 from alveoli into blood and from blood to tissues

• Measured in mmHg

Hb saturation

• The fraction of binding sites occupied by O2 molecules

• This force pushes O2 from alveoli into blood, blood into tissues

• measured as %

O2 content

• the actual amount of O2 carried (Bound+dissolved)

• depends on the Hb concentration, saturation, and PO2

• measured% (aka 20%=20mL O2 gas per 100 mL blood)

• ~20 mL O2 gas is ~.03 mL in blood

Each gas diffuses down it’s own pressure gradient

PO2~60 mmHg

inspired air= 160→ alveolar gas = 100 → arterial blood = 95 → tissue cells = <40 → venous blood = <40

PCO2~ 60 mmHg

inspired air= .25→ alveolar gas = 40 → arterial blood = 40 → tissue cells = >46 → venous blood = >46

Gas transport in blood

Oxygen: poorly soluble in water

• 97% is carried by RBCs

• 3% dissolved in plasma

Carbon dioxide: highly soluble in water

• 23% carried by RBC’s

• 77% is dissolved in the plasma (carbonic acid, bicarbonate, and hydrogen ions)

CO2

• far more soluble than O2 ~20x

• a 6 mmHg CO2 gradient moves roughly as much as gas a 6o mmHg O2 gradient

Dissolved O2

• Per 100 mL blood plasma -.3mL

• At PO2 100 mmHg, that supplies only ~15 of the 250 mL/min we need; hence, hemoglobin

Red blood cells

• iron in heme binds O2: 4 heme molecules per hemoglobin= 4 O2 molecules per hemoglobin

• 2 beta+ 4 heme + 2 alpha

• each rbc has ~250 million hemoglobin molecules

• ~1 billion O2 molecules per RBC

Hemoglobin affinity for O2

• When Hb does not have any bound O2, it is termed deoxyhemoglobin

• when hemoglobin binds O2, it is termed oxyhemoglobin

• Hemoglobin allows blood to transport

• 30-100x more O2 than dissolved in plasma

Factors affecting the ability of hemoglobin to bind O2

1. Cooperativity

• Heme-heme interaction: as each heme group accepts oxygen, it becomes progressively easier for the next heme group in the molecule to pick up oxygen

Factors affecting the ability of hemoglobin to bind O2

2. Partial pressure of O2

• gas diffuses from high to low pressure areas

• the pressure difference causing O2 to diffuse from the alveoli into the blood is 64 mmHg (104→40)

• The final pressure difference causing O2 to diffuse into the tissues is 55 mmHg (95→40)

Factors affecting the ability of hemoglobin to bind O2

3. CO2 and pH

• CO2 makes the blood more acidic

• Low pH decreases Hb affinity for O2

• helps O2 unloading into metabolically active tissues (CO2- rich)

• also helps O2 uptake in the lungs (as CO2 is released)

• carbonic anhydrase - catalyzes this conversion in RBC‘s

Factors affecting the ability of hemoglobin to bind O2

4. Temperature

• increased temperature facilitates the release of O2

CO2 transported as

1. CO2= 7%

2. Hgb x CO2= 23%

3. HCO3= 70%

CO2 and pH

• CO2 in plasma reacts with H2O to form carbonic acid, which lowers the pH

• H2CO3 breaks down into hydrogen ions and bicarbonate

• Hydrogen ions bind to hemoglobin, changing the shape of the globin proteins and lowering the affinity for O2

Bohr effect

Hemoglobins oxygen affinity is in inversly related both to the concentration of carbon dioxide and blood acidity

Hemoglobin affinity for CO2

• Hemoglobin binds CO2; it is termed carbaminohemoglobin (CO2Hgb)

• Hemoglobin can bind four four CO2

Haldane effect

Hemoglobin’s affinity for CO2 is inversely related to O2 levels; in the presence of O2, CO2 dissociates from hemoglobin

Deoxyhemoglobin

Can carry increased amounts of CO2, whereas oxyhemoglobin has decreased affinity for CO2

Hemoglobin affinity for CO2

Partial pressure of CO2 (PCO2): gas diffuses from high to low-pressure areas

• CO2 can diffuse about 20n times as rapidly as oxygen

Pressure diffeence

Diffuse out of tissues → 5 mmHg (45→ 40)

Diffuse out of blood into alveoli → 5 mmHg (45→40)

Oxygen-hemoglobin dissociation curve

1. Blood pH

2. CO2

3. Temperature