Bohr Effect
 Normal dissociation curve
 An increase or decrease in acidity,
temperature, or CO2 concentration causes a
shift in the curve
 This is due to an alteration in the molecular
structure of Hb
 Specifically, the Bohr effect is the presence
of H+ ions in contracting muscle unloads O2
from Hb
 Bohr effect is the reduced effectiveness of
hemoglobin to hold O2, especially in PO2
ranges of 20-50 mm Hg
 @PO2 in alveoli, Bohr effect in pulmonary
capillary blood is negligible
– Allows Hb to load completely with O2 as the
blood passes through the lungs, even during
maximal exercise
Red-Blood-Cell 2,3-DPG
 2,3-diphophoglycerate is produced within the RBC
during glycolysis (anaerobic)
 Binds loosely with subunits of Hb molecule
 Reduces the affinity for O2, shifting the curve
 Enhances the unloading of O2 in the tissue
 Unlike the response of H+ ions to unload O2
quickly, 2,3-DPG operates at a slower rate,
allowing adaptations to gradual changes in O2
availability
 If PO2 decreases, more O2 is released to the
tissues
 High levels of 2,3-DPG in RBCs for those who
live at high altitudes and those with
cardiopulmonary disorders
 Half-life is small, ~6 hours if return to low
altitudes
 Endurance training may increase 2,3-DPG after
maximal exercise of short duration, while
training has no benefit during prolonged, steadyrate exercise
 Females appear to have higher levels, may
compensate for lower Hb levels
Myoglobin
 Iron-protein found in skeletal and cardiac
tissue
 High concentration in red or oxidative fibers
 Similar to Hb in its ability to combine
reversibly with O2
 Each myoglobin molecule contains only one
iron atom, carries one O2 molecule
 Mb + O2  MbO2
Oxygen release at low pressures
 Myoglobin is an extra source of oxygen in muscle,
may facilitate the transfer of O2 to the
mitochondria
 Especially in the beginning of exercise and during
intense exercise, when there is a sharp drop in
cellular PO2
 Dissociation curve is rectangular hyperbola
 Myoglobin binds and retains oxygen at low
pressures much more readily than Hb
 Greatest release of oxygen occurs from
MbO2 when the tissue PO2 drops to 5 mm
HG or less
 Myoglobin has no Bohr effect
Training
 Slow twitch or slow oxidative fibers have
more myoglobin
 Myoglobin is some mammals is related to
activity level
 Results in humans are unclear
CO2 transport in blood
 Dissolved (~10%)
 20% CO2 bound to hemoglobin (carbaminohemoglobin)
 70% as bicarbonate
Regulation of blood pH
 Buffer system – seconds
– Phosphate buffer system
– Carbonic/carbonate system
– Blood proteins, esp. Hb
 Respiratory system – minutes
– Ventilation rate is controlled to keep sufficient CO2 in
blood to maintain pH
 Kidneys – days
– Excrete bicarbonate (HCO3-) at a rate that optimizes pH
Functions of the respiratory system
 Delivery of O2 to tissues
 Disposal of CO2 produced by the tissues
 Maintenance of a stable blood pH @7.4
Control of ventilation during exercise
 Humoral stimuli: changes in physical and
chemical properties in blood from normal
values at rest
 Neural stimuli: originates in the brain center
– Respiratory Center
– Mental conditions, e.g., emotions
– Inflation and deflation (stretch) of the lungs
– Muscle contraction and limb movement or
tension development