M5(2) Hb Dissociation Curve and Carbon Dioxide Transportation

oxyhemoglobin dissociation curve

describes the binding or dissociation of oxygen (reflected in SaO2%) due to PO₂

high PO2

represents the loading of oxygen from lungs onto hemoglobin

• high pressure = high O2 saturation

low PO2

represents unloading of oxygen from hemoglobin into tissues

• low pressure = low O2 saturation

hemoglobin

easier to load/unload oxygen depending on PO₂

right shift of dissociation curve

AKA decreased affinity

• maximal PO2 that can be achieved is lower

  • lower O2 saturation

    • more O2 offloading

temperature and pH

high temperature and low pH will shift the curve to the right (dec affinity)

key points of dissociation curve

1. Hb does not fully saturate

2. Plateau at pulmonary capillaries

3. Steep slope at systemic capillaries

4. Hb saturation stays high until PO₂ ~60 mmHg

Hb does not fully saturate

even at very high PO₂ (like in alveoli ~100 mmHg), Hb reaches ~97–98% saturation, but not 100%.

• demonstrates that Hb is not bound too tightly to oxygen, optimized for both loading in lungs and unloading in tissues

loading portion of curve

at PO₂ ~80–100 mmHg (lungs / pulmonary capillaries), the curve is flat.

• SaO₂ stays high even with large changes in PO₂, allowing oxygen loading to occur

unloading portion of curve

around PO₂ ~20–40 mmHg (tissues / systemic capillaries), the curve is steep.

• SaO₂ stays high even with large changes in PO₂, allowing oxygen loading to occur

Hb saturation stays high

Hb saturation stays high until PO₂ ~60 mmHg

above ~60 mmHg, Hb remains >90% saturated.

• this is why people can tolerate moderate drops in arterial PO₂ without severe hypoxia.

• below 60 mmHg → curve drops rapidly → small decreases in PO₂ = big loss in saturation (danger zone)

supplemental O2 at sea level

at sea level and lower altitudes, alveolar PO₂ is ~100 mmHg

• on the curve, Hb is already 97–98% saturated at this point

• very little or no Hb loading occurs

Bohr effect

refers to the effect of CO₂ and H+ on O₂ affinity

• shifting of the oxyhemoglobin dissociation curve in response to changes in CO₂ and pH

inc in CO2

Increase in CO₂ or H⁺ (lower pH, more acidic):
→ Curve shifts right
→ Hemoglobin’s affinity for O₂ decreases
→ More O₂ is unloaded to tissues

dec in CO2

Decrease in CO₂ or H⁺ (higher pH, less acidic):
→ Curve shifts left
→ Hemoglobin’s affinity for O₂ increases
→ Hemoglobin holds onto O₂ more tightly

2,3 DPG

2,3 diphosphoglycerate, inc during exercise

• produced in RBCs during glycolysis

• binds deoxyhemoglobin → decreases O₂ affinity

Bohr effect and exercise

Bohr effect predominates during intense exercise

Hb Bohr effect

Minimal effect in pulmonary capillaries (lungs)

Bohr effect does not affect plateau region, Hb is nearly fully saturated

• small changes in CO₂ and pH don’t significantly alter Hb saturation

• efficient oxygen loading

Hb Bohr effect

Large effect in systemic capillaries (active tissue)

• Bohr effect shifts the curve to the right:

  • Hb affinity for O₂ decreases.

  • O₂ offloading to tissues increases exactly where it is needed most

P50 increase

P₅₀ (PO₂ at which Hb is 50% saturated)

• occurs at higher partial pressure, promoting O₂ offloading

a-vO2 difference

the difference in oxygen content between arterial blood (O₂-rich) and venous blood (O₂-depleted)

• difference in how much O₂ tissues are using

• at rest: ~4-5 mL O₂ / dL blood

• during exercise: 3x resting level

O2 release

O₂ release from hemoglobin depends on PO₂ gradients:

• even without increasing blood flow, O₂ moves from blood → tissues along a gradient.

• local tissue PO₂ ↓ → more O₂ released from hemoglobin

O2 release during exercise

during exercise, PO₂ drops drastically

• muscle PO₂ drops further due to high consumption

myoglobin

found in skeletal and cardiac muscle fibers

• acts as intramuscular O₂ storage

• globular protein w/ one heme group containing iron → can bind one O₂ molecule

myoglobin O2 affinity

high affinity for O₂, even at low PO₂ → holds onto oxygen tightly

• good for short-term O₂ storage in muscle

• ensures O₂ only released when mitochondria need it most

myoglobin and O2 delivery

facilitates O₂ transfer to mitochondria

• myoglobin stores O₂ in muscle fibers and delivers it when PO₂ is very low

• important at the start of exercise and during high-intensity activity, when O₂ demand exceeds supply from blood

myoglobin Bohr effect

myoglobin’s O₂-binding is not influenced by acidity (pH), CO₂, or temperature

CO2 transport in blood

CO₂ is transported in blood in 3 ways

• dissolved in plasma

• bound to hemoglobin

• as plasma bicarbonate

dissolved in plasma

CO₂ is dissolved in plasma (5%)

• small fraction because CO₂ is more soluble than O₂, but still limited

• follows Henry’s Law

bound to Hb

CO₂ binds directly to amino groups on hemoglobin

• helps transport CO₂ w/o changing plasma pH too much

carbonic anhydrase

catalyzes the reversible reaction b/w carbon dioxide and water to form carbonic acid

• carbonic acid spontaneously dissociates into bicarbonate

bicarbonate

CO₂ diffuses into RBCs and is converted into HCO₃^- via carbonic anhydrase

• transported via plasma in blood

carbonic anhydrase at tissue

CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃ (bicarbonate)

carbonic anhydrase at lungs

H⁺ + HCO₃ (bicarbonate) → H₂CO₃ → CO₂ + H₂O