RESPIRATION PHYSIOLOGY: BLOOD GAS TRANSPORT
Oxygen Transport

OXYGEN TRANSPORT

A. Dissolved O2 (physical solution)

1. amount directly proportional to PO2
2. solubility coefficient 3 ml O2 / liter plasma / 100 mmHg PO2
3. generally insignificant at normal PO2
4. can become significant in hyperbaric conditions (very high P-O2)

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(Notes =>)

B. Bound to Hemoglobin

1. Reaction

Hb   +   4O2   =  Hb(O2)4
1 gm     1.34 ml

Hb        = Reduced hemoglobin (deoxygenated, blue or cyan)
Hb-O2   = Oxygenated hemoglobin, oxyhemoglobin (red)

Note: Cyanosis (bluish color of skin or mucosa) occurs when blood concentration of reduced Hb > 60-80 gm/liter (6-8 gm/dl)
(see figure =>)

Note affinity for carbon monoxide (200x O2 affinity)
(see figure =>)

(Notes =>)

2. Binding Capacity

1 gm of normal Hb can bind a maximum of 1.34 ml O2
(100% saturation)

3. Shape of Hb-O2 dissociation (or saturation) curve

a. sigmoid, due to the fact that each oxygen that binds to Hb increases the attraction of Hb for the next oxygen (up to full saturation)
b. curve flattens at higher PO2 since relatively few binding sites remain
c. shape determines

1) alveolar PO2 necessary to saturate Hb
2) tissue PO2 necessary to release O2 from Hb

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Note points

PO2 =   40 mmHg ⇒ Saturation = 75%     (Pv-O2 for a normal person at rest)
PO2 = 100 mmHg ⇒ Saturation = 97.5%  (Pa-O2 for a normal person at rest and in exercise)
P50  =   26 mmHg ⇒ Saturation = 50%    (for normal Hb)

4. Utility of sigmoid shape

a. blood almost completely saturated at normal alveolar P-O2
b. saturation in lungs not strongly dependent on P-O2 (in normal range)
c. oxygen can unload in tissue at reasonably high P-O2

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5. Influences on the Dissociation Curve  (see figure =>)

Note:  Curve is not constant but can shift (increase/decrease Hb-O2 affinity) with changes in physiological conditions

a. pH: when H+ combines with Hb, Hb-O2 attraction is reduced (noncompetitive inhibition)

H+ ↑⇒ Right shift of dissociation curve

b. PCO2: when CO2 combines with Hb, Hb-O2 attraction is reduced (noncompetitive inhibition)

PCO2↑⇒Right shift of dissociation curve

Note: O2 dissociation curve shift due to CO2 and reduced pH is known as the Bohr effect

 

c. Blood temperature: increased temperature reduces Hb-O2 attraction

Temperature ↑⇒ Right shift of dissociation curve

Note: a, b, and c above aid in releasing O2 from Hb in rapidly metabolizing tissue; also aid in uptake of O2 by Hb in the lungs  (see figure =>)

d. Diphosphoglycerate (DPG): When present, binds to Hb, interfering with O2 uptake

DPG ↑⇒ Right shift of dissociation curve

DPG produced in red cell

In response to chronic hypoxia, DPG production increases; aids in release of O2 from Hb
Note: Loss of DPG during blood storage interferes with O2 delivery to tissue

e. Molecular species of Hb: fetal Hb binds O2 more tightly then adult Hb (less sensitive to DPG)

Note: Enables fetal Hb to better compete with maternal Hb for O2 in the placenta

C. Normal Hb Concentration in Blood

Male               160 gm Hb / liter blood (16 gm/dl)
Female             140 gm Hb / liter blood (14 gm/dl)
"Physiological"    150 gm Hb / liter blood (15 gm/dl)

Also can use Hematocrit as index of hemoglobin concentration

Hematocrit = Red cell volume / total volume of blood sample
Measured by spinning a sample of blood; red cells pack bottom of tube
Normal values:  Young men 48%, young women 42%, average 45%

D. Total Blood Oxygen Content

1. Sum of dissolved (generally minor) and bound to Hb (most)

CO2    = Hb-O2 + dissolved O2 (units = ml O2 / liter blood )

= 1.34 x CHb x %Saturation + 3 x ( P-O2 / 100 )

2. Systemic arterial blood (normal)

CHb    = 150 gm/l

Pa-O2    = 100 mmHg ( equilibrate with alveolar PO2 )

% Sat    = 97.5%

so

Ca-O2 = 1.34 x 150 x 0.975 + 3 x (100/100)

= 196 ml /l + 3 ml/l

≈ 200 ml O2 / liter blood (almost completely saturated) (or 20 ml O2 / dl blood)

2. Uptake in systemic capillaries  (see figure ==>)

Determined from Uptake:  VO2 = CO x ( Ca-O2 - Cv-O2 )

Note:  CO = Cardiac Output -- amount of blood pumped out of each side of the heart each minute;  normal value at rest is 5 liters/minute

At rest and with normal cardiac output:  Uptake @ 50 ml O2 / liter blood

3. Systemic venous blood (normal, at rest)

Cv-O2 = Ca-O2 - Capillary Uptake

          = 150 ml O2 / liter blood

so

%Sat = 150/200 = 75%

Pv-O2 = 40 mmHg (from the dissociation curve)

Note O2 reserve in blood

Note effect of increased O2 uptake, as in exercise

Note effect of anemia (low blood Hb)

Note effect of polycythemia (high blood Hb)
Note: polycythemia vera (disease) vs. polycythemia as response to hypoxia (normal response)

RsBldgOxgn.htm -- B1K30

© AC Brown 2011