A) Transport of Oxygen
in the blood
·
97% of oxygen carried with Hb from lungs to
tissues
·
remaining 3% dissolved in plasma
·
oxygen reversibly combines with Hb maximum
amount of O2 that can combine with the Hb of blood:
i) 15 gms Hb per 100 ml blood
ii) 1 gm Hb combines with 1.34 ml O2
iii) 100 ml blood
combines with 20 ml O2 [100% saturated]
·
amount of O2 released from Hb in the
tissues:
i) Normal arterial blood: 100 ml blood
combines with 19.4 ml O2 [97% sat; PO2 95]
ii) Venous blood: 100 ml blood combines with
14.4 ml O2 [75% sat; PO2 40 mm Hg]
iii) Thus, 5ml of O2 is transported by each
100 ml blood through the tissues per cycle
·
transport of O2 during exercise:
i) Exercise —> increased cellular O2
utilization -> decreased
interstitual PO2 [15mmHg]
ii) Venous blood: 100 ml blood combines with
4.4 ml O2 [20% sat; PO2 18 mmHg]
iii) Thus, 15ml of O2 is transported by each
100 ml blood through the tissues per cycle
iv) Therefore, increased cellular O2
utilization -> increase rate of O2 release from Hb
·
utilization coefficient
i) utilization coefficient = fraction O2
released from blood as passes via tissue capillaries
ii) normally 0.25 [25%]
iii) strenuous exercise:- 0.75 - 0.85
·
Hb helps maintain a constant PO2 in tissue
fluids (oxygen buffer function of Hb) despite exercise or changes in
atmospheric changes in PO2
·
Effect of blood flow on metabolic use of oxygen
i) total amount of O2 available each minute
for use in any given tissue is determined by:
a) quantity of O2 transported in
each 100 ml blood
b) rate of blood flow
ii) if rate of blood flow approaches zero,
amount of O2 available also approaches zero
·
Transport of Oxygen in dissolved state
i) Normal arterial blood: 100 ml blood has
dissolved 0.29 ml O2 [PO2 95 mmHg]
ii) Venous blood: 100 ml blood has dissolved
0.12 ml O2 [PO2 40 mm Hg]
iii) Thus, 0.17ml of O2 is
transported by each 100 ml blood through the tissues per cycle in the dissolved state
Bohr Effect: increase in CO2
in blood will cause O2 to be displaced from the Hb thereby promoting
O2 release in tissues [ie oxygen dissociation curve shifts to the
right]; reverse effect occurs in the lungs
B) Transport of Carbon
dioxide in the blood
·
Normally 4 ml of CO2 is transported
from the tissues to the lungs in each 100 ml blood
·
Gaseous CO2 (generally not
bicarbonate) diffuses out of the cell
·
Chemical forms in which CO2 is
transported:
1) Dissolved state [7%]
i) arterial blood PCO2= 40 mmHg;
2.4 ml CO2 in 100 ml blood
ii) venous blood PCO2=45 mm Hg;
2.7 ml CO2 in 100 ml blood
iii) therefore, 1.3 ml is transported as
dissolved CO2 by each 100 ml blood
2) Bicarbonate [70 %]
i) reaction of CO2 with water in
rbc—> carbonic acid
ii) carbonic anhydrase catalyzes the
reaction of CO2 & H2O 5000 X
iii) carbonic acid —> H+ &
HCO3-
iv) H+
combines with Hb (Hb is a powerful acid-base buffer)
v) HCO3- diffuse into
plasma; Cl- diffuses into rbc [chloride shift]
vi) administration of an carbonic anhydrase
inhibitor —> reduced CO2 transport —> elevated tissue PCO2
3) Carbaminohemoglobin [23%]
i) CO2 combines reversibly with
Hb (and to a much lesser extent other plasma
proteins)
·
Haldane
effect
i) is the effect of the oxygen-hemoglobin
reaction on CO2 transport
ii) binding of O2 with Hb tends to
displace CO2 from the blood
iii) Tissues: have increased CO2
uptake due to O2 removal from Hb
iv) Lungs: have increased release of CO2
because of O2 pickup by Hb
v) Due to increased acidity of Hb when
combined with O2
vi) approximately doubles
the amount of CO2 picked up in the tissues and released in the lungs
·
the formation of carbonic acid decreases the pH
in venous blood [effect is attenuated by buffers]
K. C. Potger
Copyright © 2001