Lecture 29 : Blood Gas Transport

Question 1

How does O2 transport occur?

Answer 1

1. Dissolved in plasma O2 → 3 mL, 1.5%
2. Bound to haemoglobin (Hb) → 197 mL, 98.5%
   Total: 200 mL, 100%

Question 2

What is the O2 carrying capacity of blood?

Answer 2

~200 mL per L of blood combined with cardiac output (5 L/min) → 1000 mL/min

Question 3

What O2 is responsible for the PO2 in blood?

Answer 3

O2 dissolved in plasma
- O2 bound to Hb is no longer a gas

Question 4

What is the concentration of Hb in blood?

Answer 4

~150g/L

Question 5

How much O2 combines with each gram of Hb?

Answer 5

If each gram of Hb is completely saturated with O2 then each gram of Hb combines with 1.34 mL O2

Question 6

What is the maximum amount of O2 combined with Hb (Hb-O2 carrying capacity) in one litre of blood?

Answer 6

1.34 x Hb conc. (150 g/L)
= 1.34 x 150 mL O2/L
= 201 mL O2/L of blood

Question 7

Describe the characteristics of haemoglobin:

Answer 7

• Protein complex
• 4 subunits (tetramer)
• Subunits known as globin
• 1 heme group (white) attached to each globin subunit
• 1 Hb protein complex has 4 heme groups & 4 globin

Question 8

What causes the red appearance of haemogoblin?

Answer 8

The iron atom of heme

Question 9

What occurs as O2 binds?

Answer 9

Heme groups become more exposed increasing O2 binding

Question 10

What is cooperative O2 binding?

Answer 10

Binding of O2 induces conformational change in globin, supports further oxygen binding

Question 11

How is O2 saturation of arterial and venous blood calculated?

Answer 11

amount of O2 bound to Hb/ maximal capacity of Hb to bind O2 x 100%

Question 12

What is the percent of O2-Hb saturation of arterial blood?

Answer 12

197/201 x 100% = 98%

Question 13

What is the percent of O2-Hb saturation of venous blood?

Answer 13

150/201 x 100% = 75%

Question 14

At what systemic arterial PO2 is haemoglobin 98% saturated?

Answer 14

100mmHg

Question 15

At what systemic venous PO2 is haemoglobin 75% saturated?

Answer 15

40mmHg

Question 16

What determines the amount of O2 bound to Hb?

Answer 16

The PO2 in blood

Question 17

What is the atrial-venous difference in O2 content at rest?

Answer 17

The difference is the amount of O2 extracted by the tissue bed
CAO2 = 200mL/L
CVO2 = 150mL/L
= 50mL/L

Question 18

What is anaemia?

Answer 18

Condition where Hb concentration is less than normal and therefore there is a decreased capacity to carry oxygen

Question 19

What causes anaemia?

Answer 19

1. Low number of red blood cells (reduced Hb)
2. Iron deficiency

Question 20

Can an anaemic patient have 100% saturation

Answer 20

Yes

Question 20

What are the advantages of the steepness in a O2- haemoglobin dissociation curve?

Answer 20

• Large quantities of O2 can be off-loaded from Hb with only a small decrease in Po2
• At 60 mmHg Po2 → 90% of total Hb is combined with O2

Question 20

Describe the characteristics of the plateau in a O2-Hb dissociation curve:

Answer 20

• Po2 between 60 and 120 mmHg
• Increase in Po2 in this range causes only a modest increase in the Hb saturation % of O2

Question 20

Describe the characteristics of the steep slope in a O2-Hb dissociation curve:

Answer 20

• The curve is sigmoidal (s-shaped)
• Has a steep slope between PO2 of 10 – 60 mmHg
• Favours oxygen offloading at PO2 ~ 40 mmHg (interstitial tissue)
• Makes O2 readily available in tissue

Question 21

What are the advantages of the plateau in a O2-Hb dissociation curve?

Answer 21

• Permits a good saturation with O2, even if alveolar Po2 and thus arterial Po2 falls to 60mmHg ( ~ 90% saturation)
• Maintains O2 saturation at high altitude or has a lung disease

Question 22

How is the affinity of Hb for O2 assessed?

Answer 22

By the PO2 at which haemoglobin is 50% saturated with O2 - P50

Question 23

What is the P50 for arterial blood?

Answer 23

~25mmHg

Question 24

What would a decreased affinity of Hb for O2 causes?

Answer 24

1. Increase in P50 2. Rightwards shift of the Hb-O2 dissociation curve -> facilitates release of O2 from Hb - Lower O2 attraction - Occurs in tissue (high CO2)

Question 24

What would an increased affinity of Hb for O2 causes?

Answer 24

1. Reduction in P50 2. Leftwards shift of the Hb-O2 dissociation curve -> facilitates loading of O2 on to Hb - Higher O2 attraction - Occurs in lungs (low CO2)

Question 24

What is the Bohr effect?

Answer 24

O2 affinity of haemoglobin is dependent on CO2 , H+, concentrations and temperature

Question 24

What the effect on Hb-O of increasing PCO2 /H+ and temperature?

Answer 24

1. Increase in P50 2. Rightwards shift of the Hb-O2 dissociation curve -> reduces affinity of Hb for O2 and facilitates release of O2 from Hb in the tissues

Question 25

How is CO2 transported in blood?

Answer 25

1. Dissolved in plasma and in the cytoplasm of the erythrocytes (~10%) 2. Bound reversibly to hemoglobin in the erythrocytes forming carbamino haemoglobin, HbCO2 (~30%) 3. In the form of the bicarbonate ion, HCO3- (~60%)

Question 26

What CO2 is responsible for the PCO2 in blood?

Answer 26

CO2 dissolved in plasma - CO2 bound to Hb is no longer a gas

Question 27

When CO2 is bound to Hb what groups is it bound to?

Answer 27

The amino groups of globin

Question 28

How is bicarbonate transported and where is it converted from CO2?

Answer 28

* Transported dissolved in erythrocytes and plasma * Conversion occurs in the erythrocytes

Question 29

What is the equation for converting CO2 to HCO3- and what enzyme catalyses the reaction?

Answer 29

CO2 + H2O <-> H2CO3 <-> HCO3- + H+ * HCO3- increases osmolarity * H+ buffered by Hb * Catalysed by carbonic anhydrase

Question 30

How does HCO3- move into the plasma?

Answer 30

1. HCO3- moves out of the RBC down its conc. gradient 2. Cl- moves into RBC to maintain electroneutrality 3. H2O moves into RBC to maintain osmolarity

Question 31

How does CO2 move from the alveolar capillaries and into the alveoli?

Answer 31

Blood PCO2 is higher than alveolar PCO2→CO2 diffuses from plasma into the alveoli down a PCO2 gradient

Question 32

What occurs as CO2 moves out of plasma and into the alveoli?

Answer 32

The fall in PCO2 in plasma reduces PCO2 in erythrocytes and increases the dissociation of CO2 from Hb - Reversal of Cl- shift

Question 33

Is there a CO2-Hb dissociation curve and why?

Answer 33

No CO2-Hb dissociation curve, because most of the CO2 is not transported bound to Hb

Question 33

What is the relationship between the PCO2 of blood and the amount of CO2 in blood (in all 3 forms) called?

Answer 33

CO2-blood dissociation curve/ Haldane effect

Question 34

What happens when the PO2 is low?

Answer 34

* CO2-blood dissociation curve shifted up and left * P50 reduced * For a given PCO2: CO2 binds more readily to the globin part of hemoglobin * Facilitates removal of CO2 from tissues - venous blood

Question 34

What happens when the PO2 is high?

Answer 34

* CO2-blood dissociation curve shifted down and right * P50 increased * For a given PCO2: CO2 binds less readily to globin * Binding of O2 to heme changes the conformation of Hb so it's more difficult for CO2 to bind to globin * Blood flowing through pulmonary capillaries acquiring O2 from alveolar air, changes the conformation of Hb to promote release of CO2 from Hb

Question 35

What is the difference between the Haldane and Bohr effect?

Answer 35

* The Haldane effect describes the effect O2 has on CO2 binding to Hb * Bohr effect describes the effect CO2 and hydrogen ions have on O2 carriage

Question 36

What is similar between the Haldane and Bohr effect?

Answer 36

Together they describe how the carriage of one gas facilitates the release of the other

Question 36

What are the 3 blood buffers for pH maintenance?

Answer 36

1. H+ binding to Hb in erythrocytes 2. The carbonic acid bicarbonate buffer system 3. H+ binding to plasma proteins

Question 37

Describe the Hb buffering of H+:

Answer 37

* DeoxyHb has greater affinity for H+ than oxyHb, so it binds (buffers) most of the H+ ions produced by metabolism: Hb + H+ -> HbH

Question 37

Describe the Hb buffering of H+ when blood passes through the lungs:

Answer 37

All reactions are reversed: H+ dissociates from the Hb: HbH -> Hb + H+ H+ + HCO3- -> H2CO3 -> CO2 + H2O

Question 37

Describe the carbonic acid-bicarbonate buffering system when there's a decrease in [H+]:

Answer 37

* Drives the reaction to the right * Less CO2 is blown off in the lungs during expiration

Question 38

Describe the carbonic acid-bicarbonate buffering system when there's an increase in [H+]:

Answer 38

* Drives the reaction to the left * H2CO3 produced -> CO2 and H2O (non-acidic products) * Excess CO2 is then blown off in the alveoli

Question 39

What is acidosis and what are the 2 types?

Answer 39

* Reduced blood pH 1. Respiratory - alterations in respiration 2. Metabolic - increased H+ production

Question 40

What is respiratory acidosis?

Answer 40

Hypoventilation will lead to: * retention of CO2 in blood * increased CO2 will drive the reaction to the right * [H+] in blood will rise and result in a fall in pH (acidosis)

Question 41

What is alkalosis and what are the 2 types?

Answer 41

* Increased blood pH 1. Respiratory - alterations in respiration 2. Metabolic - decreased H+ production

Question 41

What is respiratory alkalosis?

Answer 41

Hyperventilation will lead to: * increased loss of CO2 from blood * reduced CO2 will drive the reaction to the left * [H+] in blood will fall and result in an increase in pH (alkalosis)