Gas Transport in the BloodGas Transport in the Blood

Dr Shihab Khogali

Ninewells Hospital & Medical School, University of Dundee

Understand the effect of partial pressure on O2 and CO2 carriage in the blood

Understand the means of O2 carriage in the blood

Understand the oxygen-haemoglobin dissociation curve and the significance of its sigmoid shape

Know the Bohr effect and its significance in O2 liberation at tissue level

Understand the means of CO2 carriage in the blood

Know the Haldane effect and its significance in the uptake of CO2 and CO2 generated H+ at tissue level; and CO2 liberation at the lungs

What is This LectureAbout?

See blackboard for detailed learning objectives

O2 Picked up by blood at the lungs must be transported to the tissues for cellular use

CO2 produced at tissues must be transported to the lungs for removal from the body

Pulmonarycirculation

Systemiccirculation

Alveoli

Atmospheric air

Oxygen Partial Pressures around the System

20

Atmosphere Tissues

10

Air

Gas PulmonaryCapillary

Diffusion

Arterial

PO

2 k

Pa

This means that if the partial pressure in the gas phase is increased the concentration of the gas in the liquid phase would increase proportionally

The partial pressure of a gas in solution is its partial pressure in the gas mixture with which it is in equilibrium

Henry’s Law

The amount of a given gas dissolve in a given type and volume of liquid (e.g. blood) at a constant temperature is: proportional to the partial pressure of the gas in equilibrium with the liquid

•Gaseous Phase

•Liquid Phase (gas in solution)

What is the Effect of Partial Pressure on Gas Solubility?

Dissolved Oxygen The O2 amount dissolved in blood is proportional to the partial

pressure (Henry’s Law)

3ml O2 per litre of blood at a PO2 of 13.3 kPa

Under Resting conditions (cardiac output 5L/min): 15 ml/min of O2 is taken to tissues as dissolved O2

Even at strenuous exercise (cardiac output of 30 L/min): 90 ml/min would be taken to tissues as dissolved O2

Resting O2 consumption of our body cells is about 250ml/min

O2 consumption may increase up to 25 folds during strenuous exercise

– Clearly, another mechanism is involved in O2 transport in the blood.

Oxygen Transport in the Blood

Most O2 in the blood is transported bound to haemoglobin in the red blood cells

Normal O2 concentration in the arterial blood is about 20 ml/100 ml (200 ml per litre) at a normal arterial PO2 of 13.3 kPa and a normal haemoglobin concentration of 15 grams/100 ml

Percentage of O2 carried bound to haemoglobin = 98.5%

Percentage of O2 carried in the dissolved form = 1.5% (3 ml per litre at a PO2 of 13.3 kPa )

O2 is present in the blood in two forms: (1) bound to haemoglobin (2) physically dissolved (very little O2)

Oxygen binding to haemoglobin

Haemoglobin can form a reversible combination with O2

Each Hb molecule contains 4 haem groups

Each haem group reversibly binds to one O2 molecule

Haemoglobin is considered fully saturated when all the Hb present is carrying its maximum O2 load

The PO2 is the primary factor which determine the

percent saturation of haemoglobin with O2

Oxygen Haemoglobin Dissociation Curve

O2 c

on

cen

trat

ion

ml/

100

ml

5.3 13.3

Blood PO2 (kPa)

% H

aem

og

lob

in S

atu

rati

on

8.0

% H

b s

atu

rati

on

0

100

O2 c

once

ntr

ati

on (

ml/1

00

ml)

0

20

PO2 (kPa)

Total O2

O2 combined with Hb

Dissolved O2

0 13

Oxygen Haemoglobin Dissociation Curve

Saturation

PO2 (kP)

0 13O2 c

once

ntr

ati

on (

ml/1

00

ml)

0

20

% H

b s

atu

ratio

n

0

100Hb =15

100

0

Hb =10

0

100Hb =20

Oxygen binding of haemoglobin

Binding of one O2 to Hb increases the affinity of Hb for O2

– co-operativity– Sigmoid

Flattens where all sites are becoming occupied

Significance of Sigmoid

Flat upper portions means that moderate fall in alveolar PO2

will not much affect oxygen loading

Steep lower part means that the peripheral tissues get a lot of oxygen for a small drop in capillary PO2 O

2 c

on

cen

trat

ion

ml/

100

ml

5.3 13.3

Blood PO2

(kPa)

% H

aem

og

lob

in S

atu

rati

on

8.0

Bohr Effect

% H

b s

atu

rati

on

PO2

PCO2

[H+]

Temperature

2,3-Biphosphoglycerate0

100

A shift of the curve to the right:- The Bohr Effect

Increased release of O2 by conditions at the tissues

Off-loading of O2 at Tissues

O2 c

onte

nt

(ml/1

0m

ls)

00

405.3

202.6

608.0

8010.6

10013.3

PO2 (mm Hg,

kP)

20

10

Arterial O2 Tension

Tissue O2 Tension

Curve in arterialconditions

Curve in tissueconditions

Additional O2 given up

Means of CO2 Transport in the Blood

Solution (10%)

As Bicarbonate (60%)

As Carbamino compounds (30%)

(1) CO2 in Solution

Henry’s Law

Carbon dioxide about 20 times more soluble than oxygen

About 10% of carried CO2 is in solution

(2) Bicarbonate: Most CO2 is transported in the blood as bicarbonate

Bicarbonate is formed in the blood by:-

CO2 + H2O H2CO3 H+ + HCO-3

CA

Carbonic Anhydrase

Occurs in red-blood cells

Bicarbonate Formation

Red blood cell

Capillary wall

CO2

H2O +H2CO3CA

HCO3

-H++

Cl-

Chloride shift

H+ + Hb HbH

(3) Carbamino Compounds

Carbamino compounds formed by combination of CO2 with terminal amine groups in blood proteins.

Especially globin of haemoglobin to give carbamino-haemoglobin

Rapid even without enzyme

Reduced Hb can bind more CO2 than HbO2

CO2 Dissociation Curve

5.3 6.6

CO

2 c

on

cen

trat

ion

(m

l/10

0ml)

45

55

CO2 partial pressure (kP)

PO2

5.3

13.3

a

v-

PO2

a = CO2 content in arterial bloodv- = CO2 content in mixed venous blood

The Haldane Effect

Removing O2 from Hb increases the ability of Hb to pick-up CO2 and

CO2 generated H+

The Boher effect and the haldane effect work in synchrony to facilitate:

O2 liberation and uptake of CO2 & CO2 generated H+ at tissues

Summary of CO2 Transport in the Blood