Download - Co2 transport in blood
TRANSPORT OF TRANSPORT OF CARBONDIOXIDE IN BLOOD CARBONDIOXIDE IN BLOOD
CO2 is a by product of aerobic metabolism
in mitochondria.
CO2 readily hydrate to form carbonic acid,
so it can be a source of significant
acidosis if allowed to accumulate.
Thus it is very important to eliminated it
from the body.
There is continuous gradient for CO 2
tension from mitochondria to cytoplasm,
ECF, venous blood, and alveoli where the
CO2 is finally eliminated.
In arterial blood the vol of CO 2 is 48 ml%
and PCO2 is 40 mmHg
In venous blood, the vol of CO 2 is 52ml%
and PCO2 is 46 mmHg.
Each 100 ml of venous blood release 4 ml
CO2 while passing through lungs.
CO is 5 L/min CO2 eliminated from body
is
4 x 5000 = 200ml/min 100
EXCHANGE OF CO2 BETWEEN BLOOD AND
TISSUE
1. Diffusion of CO2 from tissue into blood :
PCO2 is high in cells (46
mmHg due to metabolic
activity. PCO2 is arterial
blood is 40mmHg
Pressure gradient of 6
mmHg is responsible for
diffusion of CO2 from
tissue to blood.
2. Diffusion of CO2 from blood to Alveoli :
PaCO2 in alveoli is 40 mmHg PCO 2 in blood
is 46 mmHg.
The pressure gradient of 6 mmHg is
responsible for diffusion of CO 2 from blood
into alveoli.
3. Diffusion of CO2 for alveoli to atmosphere :
In atmosphere PCO2 0.3 mmHg whereas in
alveoli it is 40 mmHg. So, CO 2 leaves
alveoli easily.
TRANSPORT OF CARBON DIOXIDE
CO2 is transported in blood :
i. Dissolved form (7%)
ii. Carbonic acid
iii.As bicarbonate (63%)
iv. As carbamino compound (30%)
i. Dissolved form :
CO2 diffuses into blood and dissolves in
plasma forming simple solution
7% of total CO2 is transported as dissolved
state in blood.
ii. Carbonic acid :
Part of dissolved CO 2 in plasma, combines
with water to form carbonic acid.
This reaction is very slow and is negligible.
iii.As Bicarbonates :
63% of CO2 is transported as bicarbonate.
From plasma, CO 2 enter RBC. Inside RBC, CO 2
combines with water to form carbonic acid. Reaction in RBC is very rapid due to enzyme carbonic anhydrase.
Carbonic acid is very unstable. Almost all
carbonic acid formed in RBC dissociate
into bicarbonate and H + ion.
se conc of bicarbonate inside RBC
causes diffusion of bicarbonate ion into
plasma.
Chloride shift :
NaCl in plasma dissociate into Na + and cl
ions. When HCO3 move out of RBC into
plasma, to maintain electrolyte balance Cl
move into RBC.
This is called chloride shift as hamburger phenomenon.
H+ ion are buffered by Hb inside cell.
HCO3 combines with Na+ ion in plasma to
form NaHco3 and this form is transported in
blood.
Reverse chloride shift :
Bicarbonate has to be reverted back into
CO2, which has to be expelled out.
When blood reaches alveoli, NaHCO 3 in
plasma dissociate into Na + and HCO3. HCO3
moves into RBC and Cl moves out of RBC
into plasma. This is called reverse chloride
shift.
At same time, O2 enters RBC and displace
H+ from Hb. H+ then combines with HCO 3 to
form carbonic acid, which dissociates into
H2O and CO2. CO2 is expelled out.
iv. As Carbamino Compound :
30% of CO2 is transported as carbamino
compound.
CO2 combines with Hb to form carbamino
hemoglobin.
CO2 combines with plasma protein to form
carbamino protein.
Carbamino hemoglobin and carbamino
protein are together called carbamino
compound.
parameter Arterial blood Venous blood
Pco2 40 mm Hg 45 mm Hg
Dissolved CO2 27ml/l 29ml/l
Total CO2 content
490ml/l 530ml/l
Blood volume 1.25 l 3.75l
Volume of CO2 613ml 1988ml
CO2 Dissociation Curve :
Amount of CO2
combining with blood
depends upon the PCO 2.
The relationship
between PCO2 and
quantity of CO2
combined with blood is
demonstrated by a
curve called CO2
dissociation curve.
CO2 dissociation curve
shows CO2 content in
blood is 48 ml% when
PCO2 is 40mmHg.
CO2 content is 52 ml% when PCO 2 is 46
mmHg. It becomes 70ml% when PCO 2 is 100
mmHg.
Combination of more amount of O 2 with Hb
displace CO2 from Hb. This effect is called
Haldane’s effect.
So, excess of oxygen content in blood
causes shift of CO2 dissociation curve to
right.
Cause for Haldane’s effect :
Due to combination with O 2, Hb becomes
strongly acidic. Highly acidic Hb has less
tendency to combine with CO 2. So, CO2 is
displaced from blood.
Significance of Haldane’s effect :
Haldane’s effect is essential for the
release of CO2 from blood into
alveoli.
It is essential for uptake of O 2 by blood.
COCO 22 on O on O 22 Saturation of Hb Saturation of HbCO2 unloads O2 from Hb (Bohr Effect)
At any PO2 less O2 bound
HYPERCAPNIA
Hypercapnia is defined as an arterial
PCO 2 above 46 mmHg that does not
represent compensation for a
metabolic alkalosis.
PHYSIOLOGICAL EFFECTS OF HYPERCARBIA
1) On CVS :
Causes direct depression of both cardiac muscle
and vascular smooth muscle, but at same time it
cause reflex stimulation of sympathoadrenal
system.
Moderate to severe hypercapnia results in
increase HR and myocardial contractility with
consequent stroke vol and cardiac output, while
systemic vascular resistance is reduced.
2) Respiratory system :
The max stimulatory respiratory effect is by
PaCO2 of about 100mmHg . Of 5 mmHg PaCO 2 can
result in two fold in minute ventilation.
With higher PaCO 2 stimulation is reduced and at
extremely high level respiration is depressed and
later ceases. Hypercapnia causes broncho dilation in both
healthy persons and patients with lung diseases.
3) CNS :
Cerebral blood flow α PaCO 2 between 20 –
80 mmHg. Blood flow changes 1-
2ml/100g/min per mmHg change in PaCO 2
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