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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