Blood gas transport

Question 1

How does oxygen get from the atmosphere to the cells?

Answer 1

O2 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

What is plasma?

Answer 3

→ Aqueous portion of blood

Question 4

How does O2 from the lungs bind to Hb?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

How much O2 is in Hb?

Answer 6

→ 98%

Question 7

How much O2 is dissolved in plasma?

Answer 7

→ 2%

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

How much CO2 exists as HCO3-?

Answer 10

→ 70%

Question 11

How much CO2 exists as HbCO2?

Answer 11

→ 23%

Question 12

How much CO2 exists in the dissolved form?

Answer 12

→ 7%

Question 13

Why is Hb essential to O2 transport?

Answer 13

→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 14

What does Hb enable?

Answer 14

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

Question 15

What is PaO2?

Answer 15

→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 16

What is CaO2 and how is it expressed?

Answer 16

→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 17

What is SaO2?

Answer 17

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

Question 18

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

Answer 18

→SaO2 = measured directly in arterial blood
→SpO2 = estimated by pulse oximetry
→ expressed as %

Question 19

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

Answer 19

→ Co-operative binding of O2 to Hb

Question 20

What is co-operative binding?

Answer 20

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

Question 21

Why does the ODC plateau?

Answer 21

→ Saturation of O2 binding sites

Question 22

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

Answer 22

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

Question 23

What enables high carrying capacity of Hb?

Answer 23

→ The concentration of heme groups + Hb contained in RBCs

Question 24

How does the ODC change depending on the environment?

Answer 24

→The curve shifts to offload oxygen to demanding tissues

Question 25

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

Answer 25

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

Question 26

How does the curve shift if the pH is high?

Answer 26

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

Question 27

What is O2 demand like in the lungs?

Answer 27

→ 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 28

What is O2 demand like in resting tissues?

Answer 28

→ 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 29

What is O2 demand like in working tissues?

Answer 29

→ 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 30

What does myoglobin do?

Answer 30

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

Question 31

How does O2 transfer occur in fetal Hb?

Answer 31

→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 32

What is cyanosis?

Answer 32

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

Question 33

Describe central cyanosis

Answer 33

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

Question 34

Describe peripheral cyanosis

Answer 34

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

Question 35

What are three causes of anaemia?

Answer 35

→ Insufficient Hb
→ Iron deficiency
→ Haemorrhage

Question 36

What is the main consequence of anaemia?

Answer 36

→ 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 37

How does anaemia affect the ODC curve?

Answer 37

→ Curve is much lower than normal

Question 38

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

Answer 38

→ CO2 has a higher solubility than O2

Question 39

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

Answer 39

→ CO2 binds to Hb at different sites than O2

→ Forms carbamino Hb

Question 40

How is the majority of CO2 transported?

Answer 40

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

Question 41

What is the Haldane effect?

Answer 41

→ Deoxygenated blood carries more CO2

Question 42

How does CO2 saturation differ in venous and arterial blood?

Answer 42

→ Venous blood carries more CO2 than arterial blood

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

Question 43

What are the ways CO2 is transported in the blood?

Answer 43

→ 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 44

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

Answer 44

→ 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 45

Describe the process of CO2 exchange between RBCs and respiring tissues

Answer 45

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 46

Describe the process of exchange of CO2 from the lungs

Answer 46

1) 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-.