Blood Gas Transport

Dr Taha Sadig Ahmed

Physiology Dept College of Medicine King Saud University

Riyadh

• Oxygen and Carbon Dioxide Transport• Objectives :• At the end of this lecture the student is expected to :• Understand the forms of oxygen transport in the blood,

the importance of each. • Differentiate between O2 capacity, O2 content and O2

saturation. • Describe (Oxygen- hemoglobin dissociation curve) • Define the P50 and its significance. • How DPG, temperature, H+ ions and PCO2 affect

affinity of O2 for Hemoglobin and the physiological importance of these effects.

• Describe the three forms of carbon dioxide that are transported in the blood, and the chloride shift.

• Transport of O2 and CO2 in body and body fluids

• O2 is mostly transported in the blood bound to hemoglobin

• If the PO2 increases Hb binds O2 • If PO2 decreases Hb releases O2 • O2 binds to the heme group on hemoglobin, with 4 oxygens /per Hb molecule

• Terminology (1) • O2 content: amount of O2 ( in ml ) carried by 100 ml blood

( arterial or venous ).• When blood is 100 % saturated wit oxygen , as in the pulmonary

capillary, each ml of Hb carries 1.34 ml of oxygen ; therefore oxygen content would be 15 gm Hb x 1.34 = 20 ml oxygen carried by 100 ml blood ( because Hb concentration = 15 gm per 100 ml blood ).

• But when blood is only 75% saturated ( as in pulmonary artery) , each 100 ml blood contains 19.4 ml oxygen ( per 100 ml blood) .

• Amount of oxygen released from Hb in the tissues = 5 ml oxygen from each 100 ml blood .

• There fore, oxygen content of venous blood =( 19.4 – 5 ) = 14.4 ml oxygen per 100 ml blood

• During strenuous exercise , the oxygen uptake by the tissues increases 3 folds to become 15 ml O2/100 ml blood ; so the oxygen content of venous blood during strenuous exercise = ( 19.4 – 15 ) = 4.4 ml oxygen per 100 ml blood

• At rest the tissues consume 250ml O2 and produce 200 ml CO2 .

The Oxyhemoglobin Dissocaiation Curve

75% saturation , 15 ml content ( in pulmonary artery i.e,. venous blood)

100% saturation , 20 ml content ( in rterial blood )

• (A) at rest , • amount of O2 given up ( oxygen uptake , unloaded ) at the tissues

= 5 mlO2/100 ml blood• Therefore , what remains in venous blood = (20-5) = 15 ml O2 / 100 ml blood

• (B) During severe /strenuous exercise ,• During strenuous ( severe) exercise the oxygen uptake ( extraction

) by the tissue increases 3 folds ( from 5 ml to 15 ml ) so 15 ml O2 is extracted by the tissues from each 100 ml of blood

• Therefore , what remains in is venous blood will contain ( 20 -15 ml ) =

5ml O2 /100 ml blood .

• At rest, tissues consume 250 ml O2 /min and produce 200 ml CO2/min .

• Oxygen Transport in Blood• 3% dissolved in plasma • 97% bound to hemoglobin (oxyhemoglobin) • Higher PO2 results in greater Hb saturation. • The relation between PO2 and Hb-O2 is described by

Oxyhemoglobin Saturation Curve , which is not linear ( straight line ) but Sigmoid

• ( s-shaped )• It is not linear but sigmoid (S- shaped) in shape (S- shaped) • Normal P50 = 26.5 mmHg

Oxyhemoglobin Dissociation Curve Right and Left Oxyhemoglobin Dissociation Curve Right and Left ShiftsShifts

Right shift means Right shift means the oxygen is unloaded from Hb the oxygen is unloaded from Hb ( Hb giving away O2 ) to the tissues from Hb, ( Hb giving away O2 ) to the tissues from Hb,

Left shift means Left shift means loading or attachment of oxygen loading or attachment of oxygen to Hb . to Hb .

The P 50 and curve shifts The P 50 and curve shifts P50:The arterial PO2 at which 50% of the Hb is saturated with O2, normally P50= 26.5 The position of the dissociation curve can be determined by measuring the P50 Decreased P50 means increased affinity of Hb to O2 or shift of the curve to left Increased P50 means decreased affinity of Hb to O2 or shift of the curve to right.

Factors that shift the curve to the right are Factors that shift the curve to the right are

(1) increased blood H+ (1) increased blood H+ ( Arterial blood pH , PHa) ( Arterial blood pH , PHa)

(2) increased PCO2 (2) increased PCO2 ( arterial blood pCO2 , PaCO2( arterial blood pCO2 , PaCO2) { ) { which will also lead to (1) }which will also lead to (1) }

(3) increased temperature , (3) increased temperature ,

(4) increased blood 2,3 DPG (4) increased blood 2,3 DPG

2,3-DPG is an organic phosphate present in human 2,3-DPG is an organic phosphate present in human RBCs . It binds to deoxyhaemoglobin RBCs . It binds to deoxyhaemoglobin thus lowering thus lowering Hb affinity for oxygen Hb affinity for oxygen facilitates O2 removal from Hb facilitates O2 removal from Hb facilitating O2 deivery to the tissues .. facilitating O2 deivery to the tissues ..

2,3 DPG increases in the RBCs in anemia and 2,3 DPG increases in the RBCs in anemia and hypoxemia, and thus serves as an important adaptive hypoxemia, and thus serves as an important adaptive response in maintaining tissue oxygenation response in maintaining tissue oxygenation

Fetal HbFetal Hb: has a P50 of 20 mmHg in comparison to 27 : has a P50 of 20 mmHg in comparison to 27 mmHg of adult Hb. mmHg of adult Hb.

Factors that shift the curve to the left are Factors that shift the curve to the left are

(1) decreased blood H+(1) decreased blood H+

(2) decreased increased PCO2(2) decreased increased PCO2

(3) decreased temperature , (3) decreased temperature ,

(4) decreased blood 2,3 DPG (4) decreased blood 2,3 DPG

of the dissociation curve during exercise of the dissociation curve during exercise

Exercise increases Temp, H+, 2,3 DPG and shift Exercise increases Temp, H+, 2,3 DPG and shift the curve to right (Rt) . the curve to right (Rt) .

Transport of Oxygen in the dissolved stateTransport of Oxygen in the dissolved state

Only 3% of O2 is transported in the dissolved state, Only 3% of O2 is transported in the dissolved state,

at normal arterial PO2 of 95 mmHg , about 0.29 ml of at normal arterial PO2 of 95 mmHg , about 0.29 ml of oxygen is dissolved in each 100ml of blood. oxygen is dissolved in each 100ml of blood.

When the PO2 of the blood falls to 40 mmHg in tissue When the PO2 of the blood falls to 40 mmHg in tissue capillaries, only 0.12 of oxygen remains dissolved. capillaries, only 0.12 of oxygen remains dissolved.

i.e 0.17 ml of oxygen is normally transported in the i.e 0.17 ml of oxygen is normally transported in the dissolved state to the tissues per each 100 ml of blood dissolved state to the tissues per each 100 ml of blood

Combination of Hb with CO Combination of Hb with CO displacement of displacement of oxygenoxygen

CO combines with Hb at the same point on the Hb CO combines with Hb at the same point on the Hb molecule as does oxygen, molecule as does oxygen,

it binds with Hb about 250 times as much as O2 (affinity it binds with Hb about 250 times as much as O2 (affinity of Hb to CO is very high (250 times) that to O2.of Hb to CO is very high (250 times) that to O2.

It causes Lt shift of the O2-Hb curvevIt causes Lt shift of the O2-Hb curvev

Transport of Carbon Dioxide in BloodTransport of Carbon Dioxide in Blood

carbon dioxide is transported in three forms. carbon dioxide is transported in three forms.

(2) As bicarbonate ions (2) As bicarbonate ions 70 % 70 %

(3) As Carbaminohemoglobin ( combined with Hb) (3) As Carbaminohemoglobin ( combined with Hb) 23 23%.%.

(3) (3) Dissolved in plasdma Dissolved in plasdma 7% 7%

Each 100 ml of blood carry 4 ml of CO2Each 100 ml of blood carry 4 ml of CO2

Transport of Carbon Dioxide in BloodTransport of Carbon Dioxide in Blood

CO2 is transported in blood in three forms :CO2 is transported in blood in three forms :

(1) As bicarbonate ion (1) As bicarbonate ion 70 % of the total blood CO2 70 % of the total blood CO2

(2) As carbamino hemoglobin ( combined to Hb ) (2) As carbamino hemoglobin ( combined to Hb ) 23 % 23 %

(3) Dissolved (3) Dissolved 7 % of the total blood CO2 7 % of the total blood CO2

Each 100 ml of blood carries 4 ml of CO2 from the tissues . Each 100 ml of blood carries 4 ml of CO2 from the tissues .

Formation of Bicarbonate and the Formation of Bicarbonate and the

Chloride ShiftChloride Shift

in the tissues in the tissues

in the pulmonary capillin the pulmonary capillariesaries

  

Q: What is the chloride shift ? It is diffusion of chloride into the red blood cell when HCO3- leaves it. Exchange of bicarbonate ion (HCO3

-) and chloride ion (Cl-) across the membrane of red blood cells in order to main electrical neutrality

• Haldane Effect • Removal of oxygen from Hb increases binding of carbon

dioxide to Hb .• In addition to enhancing removal of carbon dioxide

from oxygen-consuming tissues, the Haldane effect promotes dissociation of carbon dioxide from hemoglobin in the presence of oxygen .

• In the oxygen-rich capillaries of the lung, this property causes the displacement of carbon dioxide to plasma as low-oxygen blood enters the alveolus and is vital for alveolar gas exchange

• Bohr Effect• Increased carbon dioxide decreased Hb affinity for

O2 release of O2 fro Hb .• Conversely , a decrease in CO2 increased pH

resulting in Hb binding more & more O2 .

• Respiratory Exchange Ratio (Respiratory Quotient , RQ )

• R = Rate of carbon dioxide output Rate of oxygen uptake

• Normally it is 4/5= 82% • When Carbohydrate diet is used R ( RQ) = 1 • When fats only is used R ( RQ) =0.7 • A person on normal ( mixed) diet RQ =0.82