L28. Blood Gas Transport - O2 and CO2

Question 1

O2 transport?

Answer 1

O2 transport occurs in two forms;
1. Dissolved in plasma –> 3mL; 1.5%
2. Bound to haemoglobin (Hb) –> 1997mL; 98.5%

O2 carrying capacity of blood is ~200mL per L of blood
Combined with cardiac output (5L/min) –> 1000mL/min

Only O2 dissolved in plasma is responsible for the PO2 in blood
* The O2 bound to Hb does not account for the PO2 anymore (no longer a gas)

Question 2

O2 carrying capacity of Hb?

Answer 2

• The concentration of Hb in blood is ~150g/L
• If each gram of Hb is completely saturated with O2 then each gram of Hb combines with 1.34 mL O2
• The maximum amount of O2 combined with Hb (Hb-O2 carrying capacity) in one litre of blood is:
  = 1.34 x Hb concentration (150g/L)
  = 1.34 x 150mL O2/L
  = 201mL O2/L of blood

Question 3

Haemoglobin?

Answer 3

• Protein complex
• 4 subunits (tetramer)
• Subunits known as globin
• 1 heme group attached to each globin subunit
• 1 Hb protein complex has 4 heme groups and 4 globin
• Each heme has an iron atom in the middle so have four iron atoms
• As O2 binds heme groups become more exposed increasing O2 binding
• Lack of iron –> form of anemia, affects O2 binding/carrying capacity

Question 4

Cooperative effect?

Answer 4

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

Question 5

O2 saturation of arterial blood?

Answer 5

= amount of O2 bound to Hb / maximal capacity of Hb to bind O2 x 100%
= 197 / 201 x 100% = 98%

Question 6

O2 saturation of venous blood?

Answer 6

• The percent O2-Hb saturation of arterial blood is 98%
• Venous blood contains 150mL O2/litre blood; the O2-Hb saturation of venous blood is = 150 / 201 x 100% = 75%

Question 7

Determinant of how much O2 is bound to Hb?

Answer 7

The amount of O2 bound to Hb is determined by the PO2 in blood
- Venous has lower PO2 of 40mmHg
- Arterial has higher PO2 of 100mmHg

Question 8

Anemia?

Answer 8

(Saturation vs content)
Condition where Hb concentration is less than normal and therefore is a decreased capacity to carry oxygen
* Low number of red blood cells (reduced Hb)
* Iron deficiency anaemia
- In an anaemic patient the amount of O2 carried by the blood (content) is less because there is less haemoglobin available for binding
- An anaemic patient can have 100% saturation

Question 9

O2-haemoglobin dissociation curve - steep slope?

Answer 9

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

Advantage of the steepness:
- Large quantities of O2 can be off-loaded from Hb with only a small decrease in PO2
- At 60mmHg PO2 –> 90% of total Hb is combined with O2

Question 10

O2-haemoglobin dissociation curve - plateau?

Answer 10

• Has a plateau (flat portion) - PO2 between 60-120mmHg
• An increase in PO2 in this range causes only a modest increase in the Hb saturation % of O2

Advantage of the plateau:
- It 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 11

Affinity of Hb for O2?

Answer 11

• The affinity of Hb for O2 is assessed by the PO2 at which haemoglobin is 50% saturated with O2
• This PO2 is called the P50
• The P50 for arterial blood is ~25mmHg
• Affinity can change

Question 12

Increased affinity of Hb for O2 (left shift)?

Answer 12

Any factor that increases the affinity of Hb for O2 will cause:
- A reduction in P50
- A leftwards shift of the Hb-O2 dissociation curve –> facilitates the loading of O2 on to Hb
- “Higher O2 attraction”

Question 13

Decreased affinity of Hb for O2 (right shift)?

Answer 13

Any factor that decreases the affinity of Hb for O2 will cause:
- An increase in P50
- A rightwards shift of the Hb-O2 dissociation curve –> facilitates the release of O2 from Hb
- “Lower O2 attraction”

Question 14

Bohr effect?

Answer 14

= efficient Hb function

O2 affinity of haemoglobin is dependent on CO2, H+ concentrations and temperature –> Bohr effect

• Left shift –> occurs in lungs (low CO2); favours oxygen ‘loading’
• Right shift –> occurs in tissue (high CO2); favours oxygen ‘offloading’

Hb function adapted to environments (lung vs tissue) to facilitate its function

Question 15

CO2 transport?

Answer 15

CO2 transport occurs in three forms;
1. Dissolved in plasma and in the cytoplasm of the erythrocytes (~10%)
2. Bound reversibly in the erythrocytes forming carbamino compounds (~30%)
3. In the form of the bicarbonate ion (HCO3-) (~60%)

Question 16

CO2 dissolved in plasma?

Answer 16

• A small percentage of CO2 (10%) is transported out of the tissues dissolved in plasma
• Only CO2 dissolved in plasma is responsible for the PCO2 in blood
• The CO2 bound to Hb does not account for the PCO2 anymore (no longer a gas)

Question 17

CO2 bound to haemoglobin?

Answer 17

Approximately 30% of CO2 is transported bound reversibly to the amino groups of globin within Hb

The reaction is:
CO2 + Hb <=> HbCO2

CO2 bound to haemoglobin is called a carbamino haemoglobin (HbCO2)

Question 18

CO2 transported in the form of a bicarbonate ion - HCO3-?

Answer 18

• Most of the CO2 produced (60%) in the tissue is converted to the bicarbonate ion (HCO3-)
• HCO3- is transported to the alveoli (dissolved in erythrocytes and plasma)
• CO2 conversion occurs in the erythrocytes

Question 19

Conversion of CO2 to HCO3- equation?

Answer 19

CO2 + H2O <–> H2CO3 <–> HCO3- + H+
- A chemical reaction catalysed by the enzyme carbonic anhydrase
- Most of the bicarbonate produced does not remain in the erythrocytes but moves out into the plasma

Question 20

“Chloride shift”?

Answer 20

Exchange of Cl- for HCO3-
1. CO2 + H2O = produces two osmotically active particles within the RBC
i) H+ = buffered by Hb
ii) HCO3-
–> Increases osmolarity
2. HCO3- moves out of the RBC down its conc. gradient
3. Cl- moves into RBC to maintain electroneutrality
4. H2O moves into RBC to maintain osmolarity

Question 21

CO2 at the alveolar capillaries?

Answer 21

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

The fall in PCO2 in plasma reduces PCO2 in erythrocytes and increases the dissociation of CO2 from Hb

Reversal of Cl- shift

Question 22

CO2-Hb dissociation curve?

Answer 22

• There is no CO2-haemoglobin dissociation curve, because most of the CO2 is not transported bound to Hb
• The relationship between the PCO2 of blood and the amount of CO2 in blood (in all 3 forms, dissolved CO2, HbCO2, bicarbonate) is called the CO2-blood dissociation curve

Question 23

CO2-blood dissociation curve (Haldane effect) - PO2 low?

Answer 23

• When the PO2 is low, the CO2-blood dissociation curve is shifted up and to the left (P50 reduced)
• For a given PCO2: CO2 binds more readily to the globin part of haemoglobin
• This effect facilitates the removal of CO2 from tissues, an obvious advantage to venous blood

Question 24

CO2-blood dissociation curve (Haldane effect) - PO2 high?

Answer 24

• When the PO2 is high, the CO2-blood dissociation curve is shifted down and to the right (P50 increased)
• For a given PCO2: CO2 binds less readily to globin
• The binding of O2 to heme changes the conformation of the haemoglobin molecule so that it is more difficult for CO2 to bind to globin
• Thus, as blood is flowing through the pulmonary capillaries acquiring O2 from alveolar air, the change in the conformation of Hb promotes the release of CO2 from Hb

Question 25

Haldane vs Bohr effect?

Answer 25

- The Haldane effect describes the effect O2 has on CO2 binding to Hb, while Bohr effect describes the effect CO2 and hydrogen ions have on O2 carriage - Together the Bohr and Haldane effect describe how the carriage of one gas facilitates the release of the other

Question 26

Blood buffers for pH maintainence?

Answer 26

1. H+ binding to haemoglobin in erythrocytes 2. The carbonic acid-bicarbonate buffer system 3. H+ binding to plasma proteins (not discussed)

Question 27

Hb buffering of H+?

Answer 27

- DeoxyHb has greater affinity for H+ than does oxyHb, so it binds (buffers) most of the H+ ions produced by metabolism: Hb + H+ --> HbH - When blood passes through the lungs, all reactions are reversed: H+ dissociates from the Hb: HbH --> Hb + H+ and then H+ combines with HCO3- --> H2CO3 --> CO2 + H2O

Question 28

Carbonic acid - bicarbonate buffer system?

Answer 28

A very efficient buffer of H+ in plasma - An increase in [H+] drives the reaction to the left * H2CO3 is produced which leads to the production of CO2 and H2O (non-acidic products) - The excess CO2 is then blown off in the alveoli - A fall in [H+] drives the reaction to the right - Less CO2 is blown off in the lungs during expiration

Question 29

Respiratory acidosis?

Answer 29

- Reduced blood pH is called acidosis - Acidosis (increased [H+], decreased pH) can be either: * Respiratory (caused by alterations in respiration) * Metabolic (caused by increased H+ production) Respiratory acidosis: - Hypoventilation will lead to: * Retention of CO2 in blood * The increased CO2 will drive the reaction to the right * The [H+] in blood will rise and a fall in pH (acidosis) will result

Question 30

Respiratory alkalosis?

Answer 30

- Increased blood pH is called alkalosis - Alkalosis (decreased [H+], increased pH) can be either: * Respiratory (caused by alterations in respiration) * Metabolic (caused by decreased H+ production) Respiratory alkalosis: - Hyperventilation will lead to: * Increased loss of CO2 from blood * The reduced CO2 will drive the reaction to the left * The [H+] in blood will fall and an increase in pH (alkalosis) will result