Blood Gas Transport

Question 1

How does oxygen get from the atmosphere to the cells?

Answer 1

2 inhaled from atmosphere into alveoli within lungs.
⇓
O2 diffuses from alveoli into blood within pulmonary capillaries.
⇓
O2 transported in blood, predominantly bound to haemoglobin.
⇓
O2 diffuses into cells/tissues for use in aerobic respiration.
⇓
CO2 diffuses from respiring tissues to blood – exchanged at lungs.

Question 2

What is the primary form gases are found in the blood?

Answer 2

→ gases carried in the blood first dissolve in the plasma.

Question 3

How does O2 from the lungs bind to Hb?

Answer 3

→ O2 exchange at the lungs
→ O2 dissolved in plasma
→ O2 bound to Hb

Question 4

How does O2 go into the tissues from being bound to Hb?

Answer 4

→ O2 bound to Hb
→ O2 dissolved in plasma
→ O2 diffuses into tissues

Question 5

How much O2 is in Hb?

Answer 5

98%

Question 6

How much O2 is dissolved in plasma?

Answer 6

→ 2%

Question 7

How does CO2 go from the tissues to being bound to Hb?

Answer 7

→ CO2 produced by tissues
→ CO2 dissolved in plasma ( PaCO2)
→ CO2 bound to Hb or transported as HCO3-

Question 8

How does CO2 get to the lungs from Hb or HCO3-?

Answer 8

→ CO2 bound to Hb or being transported as HCO3-
→ CO2 dissolved in plasma
→ CO2 exchange at the lung

Question 9

How much CO2 exists as HCO3-?

Answer 9

→ 70%

Question 10

How much CO2 exists as HbCO2?

Answer 10

→ 23%

Question 11

How much CO2 exists in the dissolved form?

Answer 11

→ 7%

Question 12

Why is Hb essential to O2 transport?

Answer 12

→Oxygen has low solubility in plasma

→ In order to dissolve the amount of O2 needed to supply tissues, an impossibly high alveolar PO2 would be required.

Question 13

What does Hb enable?

Answer 13

→It enables O2 to be concentrated within blood (↑ carrying capacity)

Question 14

What is PaO2?

Answer 14

→The partial pressure of O2 within a gas phase (at a gas-liquid interface) that would yield this much O2 in the plasma at equilibrium

Question 15

What is CaO2 and how is it expressed?

Answer 15

→What volume of O2 is being carried in each litre of blood, including O2 dissolved in the plasma and O2 bound to Hb
→expressed as mL of O2 per L of blood (ml/L)

Question 16

What is SaO2?

Answer 16

→What % of total available haemoglobin binding sites are occupied by oxygen

Question 17

How is SaO2 and SpO2 measured and how are they expressed?

Answer 17

→SaO2 = measured directly in arterial blood
→SpO2 = estimated by pulse oximetry
→ expressed as %. Does not inform on content O2 is available

Question 18

What is the initial steep gradient of an ODC (Oxygen dissociation curve)?

Answer 18

→ Co-operative binding of O2 to Hb

Bound oxygen allows more oxygen

Question 19

What is co-operative binding?

Answer 19

→ Once the first molecule of O2 binds to Hb it gets easier for subsequent molecules of O2 to bind

Question 20

Why does the ODC plateau?

Answer 20

→ Saturation of O2 binding sites

Question 21

Why is Hb so effective at transporting O2 within the body?

Answer 21

→ The structure of Hb produces a high O2 affinity so a high level of O2 binding requires low PO2.

Question 22

What enables high carrying capacity of Hb?

Answer 22

→ The concentration of heme groups + Hb contained in RBCs

Question 23

How does the ODC change depending on the environment?

Answer 23

→The curve shifts to offload oxygen to demanding tissues

Question 24

How does the curve shift if the pH is low and what is this called?

Answer 24

→curve shifts right
→ more CO2
→ Hb gives up O2 more readily
→ more 2,3 DPG
→ higher temperature
→Bohr Shift

Question 25

How does the curve shift if the pH is high?

Answer 25

→curve shifts left
→ CO2 decreases
→ less 2,3 DPG
→Decreased temperature
→ Hb has a higher affinity for O2

Question 26

What is O2 demand like in the lungs?

Answer 26

→ High PO2
→ Low PCO2
→ High pH
→ Hb has high O2 saturation
→ LOW DEMAND = Hb Has high O2 affinity and less dissociation
→ Left shifted curve

Question 27

What is O2 demand like in resting tissues?

Answer 27

→ Low PO2
→ Hb has a low O2 saturation
→ More O2 moves from Hb to tissue
→ MODERATE DEMAND = Hb has lower O2 affinity and more dissociation

Question 28

What is O2 demand like in working tissues?

Answer 28

→ Low PO2
→ Anaerobic respiration + hypoxia produces H+, CO2 and 2,3 DPG
→ Increased O2 demand leads to a high CO2, low pH, high 2,3 DPG
→ More O2 moves from Hb to tissue
→ VERY HIGH DEMAND = Hb has a much lower O2 affinity and a lot more dissociation

Question 29

What does myoglobin do?

Answer 29

→ Acts as an O2 reservoir within muscle tissue and only releases O2 at a low PO2

Question 30

How does O2 transfer occur in fetal Hb?

Answer 30

→Where fetal + maternal Hb come into contact in the placenta
→ O2 is transferred to the high affinity fetal Hb
→effectively ‘steals’ O2 from maternal Hb

Question 31

What is cyanosis?

Answer 31

→purple discoloration of the tissue or skin that occurs when deoxyhaemoglobin becomes excessive

Question 32

Describe central cyanosis

Answer 32

→Bluish discoloration of core, mucous membranes and extremities
→ Inadequate oxygenation of blood
→ Hypoventilation or V/Q mismatch

Question 33

Describe peripheral cyanosis

Answer 33

→ Bluish discoloration confined to extremities
→ Inadequate O2 supply to extremities
→small vessel circulation issues

Question 34

What are three causes of anaemia?

Answer 34

→ Insufficient Hb
→ Iron deficiency
→ Haemorrhage

Question 35

What is the main consequence of anaemia?

Answer 35

→ Hypoxia can occur despite adequate ventilation + perfusion

→ blood is not able to carry sufficient O2 to meet tissue demands due to a lack of Hb

Question 36

How does anaemia affect the ODC curve?

Answer 36

→ Curve is much lower than normal

Question 37

Why is a greater % of CO2 transported in the plasma?

Answer 37

→ CO2 has a higher solubility than O2

Question 38

Why is there a smaller % of CO2 transported bound to Hb?

Answer 38

→ CO2 binds to Hb at different sites than O2

→ Forms carbamino Hb

Question 39

How is the majority of CO2 transported?

Answer 39

→ Reacts with water to form carbonic acid which accounts for 70% of CO2 transported

Question 40

What is the Haldane effect?

Answer 40

→ Deoxygenated blood carries more CO2

Question 41

How does CO2 saturation differ in venous and arterial blood?

Answer 41

→ Venous blood carries more CO2 than arterial blood

→ Arterial blood Hb is saturated with CO2 at a much lower PCO2 than venous blood

Question 42

What are the ways CO2 is transported in the blood?

Answer 42

→ CO2 is produced in tissues
→ Dissolved in plasma
→ Dissolved in RBCs
( forms H2CO3 which dissociates to HCO3- and H+, the H+ can be bound to Hb)
→ CO2 is bound to Hb

Question 43

What happens to the CO2 when oxygen is bound to Hb?

Answer 43

→ The O2 'displaces' the CO2 from Hb
→ CO2 goes back into the RBCs
→ CO2 goes back into the plasma
→ Removed by the lungs
→ H+ bound to Hb joins with HCO3- and forms carbonic acid

Question 44

Describe the process of CO2 exchange between RBCs and respiring tissues

Answer 44

1) CO2 is produced by respiring cells which dissolves in the plasma ( 7%)
2) Conversion of CO2 + H2O to H2CO3 takes place within the RBCs, catalysed by carbonic anhydrase.
3) The effective removal of CO2 by step (2) enables further CO2 to diffuse into the RBC, so that more can enter into the plasma.

4) H2CO3 ionises into HCO3- and H+.
The RBC membrane is impermeable to H+, therefore H+ cannot leave.

5) There is an accumulation of H+ within the cell, and therefore a cessation of step (2).
This can be prevented by haemoglobin acting as a buffer and binding H+.
Also, the movement of O2 from the RBCs to the tissues increases the concentration of deoxyhaemoglobin, thus enabling more CO2 to be transported.

6) The increased HCO3- concentration creates a diffusion gradient for HCO3- to leave the cell. It is exchanged for Cl- to maintain electrical neutrality.

Question 45

Describe the process of exchange of CO2 from the lungs

Answer 45

A low PACO2 creates a diffusion gradient for CO2 to diffuse out of the blood into the airspace.

2) An increased PAO2 leads to oxygen-haemoglobin binding. Oxyhaemoglobin binds less H+ than deoxyhaemoglobin, increasing the concentration of free H+.

3) Increasing the concentration of free H+ leads to
increased H2CO3 and, ultimately, CO2, which contributes to CO2 plasma saturation.

4) The changing equilibrium of the carbonic acid reaction also leads to decreased HCO3- concentration, as it binds the free H+.
This creates a diffusion gradient that allows HCO3- ions to enter the RBC in exchange for Cl-.

Question 46

What happens to COPD patients when oxygen levels suddenly increase?

Answer 46

→the CO2 is displaced from the blood as it can carry less CO2 bound to Hb and as HCO3-.
→leads to sudden very high levels of CO2 within the body, potentially leading to a dangerous acidaemia.
→Whilst a healthy individual would simply increase their level of ventilation to get rid of the excess CO2, this is not possible in the COPD patient due to the hypoventilation and deterioration in lung function associated with the disease

Question 47

Why is there a slight increase in O2 affinity in CO poisoning?

Answer 47

CO inhibits the production of 2,3-DPG, shifting the curve to the left

Question 48

What are the changes in CO poisoning?

Answer 48

reduced O2 content because CO displaces O2
SaO2 reduced
SpO2 normal 
PaO2 normal 
Conc of Hb normal