Gaseous Diffusion & Transport #3

Question 1

What is structure of Hb?

Answer 1

• 4 subunits each containing protein chain, globin + haem group
• HbA contains 2 alpha + 2 beta chains

Question 2

What is haem group?

Answer 2

• Fe-porphyrin compound

- Fe atom in ferrous form + each binds an O2 mol so each Hb can bind up to 4 mol of O2

Question 3

What is sig of Hb structure?

Answer 3

• 3D/quarternary structure of Hb can change an alter accessibility of O2 binding site + therefore carrying capacity of Hb

Question 4

What is O2 capacity of normal blood?

Answer 4

• how much Hb it can pot hold
• normal blood Hb conc: 150g/l
• each gram of Hb can comb with max of 1.34ml O2
• O2 capacity = 150 x1.34 = 200ml/l

Question 5

What is O2 content of pul venous blood?

Answer 5

• as blood passes through pul cap, nearly all binding sites are filled
• nearly 200ml/l i.e. nearly 100% sat

Question 6

What is O2 sat, content + capacity of mixed venous blood?

Answer 6

• blood returning from tissues to R heart has 150 av O2 content of 150ml/l = 75% of 200 so blood 75% sat (how full Hb is with O2)
• still has normal O2 capacity as Hb amount is same but less O2 has occupied it

Question 7

What happens when O2 binds to haem group?

Answer 7

• causes conformational change in shape of Hb - makes it easier for other haem groups to bind to O2

Question 8

How does PP change as Hb binds O2?

Answer 8

• have to inc PP a lot to get O2 bound to haem

- once 1 O2 binds, need PP to get more O2

Question 9

Why does O2-Hb diss curve plataeu?

Answer 9

can’t add any more O2 if Hb 100% sat

Question 10

Compare O2 stats of normal arterial + resting mixed blood

Answer 10

• normal arterial blood:
• pO2: 13.3kPa (100mmHg)
• O2 content: 200ml/l
• O2 sat: 97%
• resting mixed venous blood:
• pO2: 5.3kPa (40mmHg)
• O2 content: 150ml/l
• O2 sat: 75%

Question 11

Which factors cause O2-Hb diss curve to shift to right?

Answer 11

• dec pH (acidosis)
• inc PCO2
• inc temp
• inc 2,3 BPG (Bohr shift - O2 unloaded to exercising muscles)

Question 12

Which factors cause O2-Hb diss curve to shift to left?

Answer 12

• inc pH
• dec PCO2
• dec temp
• dec 2,3 BPG

Question 13

What is effect of altitude on O2 stats?

Answer 13

• O2 capacity: normal (Hb normal)
• arterial O2 content: red (insp PO2 red due to low PB)
• arterial O2 sat of Hb: red (not as many Hb full of O2)

Question 14

What is anaemia?

Answer 14

low RBC count/ low Hb content of blood

Question 15

What is effect of altitude on O2 stats?

Answer 15

• insp, alv + arterial pO2: normal - no point giving extra O2
• O2 capacity of arterial blood: red (normal PaO2 but O2 carrying capacity low as Hb low)
• arterial O2 sat of Hb: normal

Question 16

What is treatment for anaemia?

Answer 16

need to inc amount of Hb in blood

Question 17

What happens when pO2 close to normal restin arterial pO2 of 13kPa? is + why?

Answer 17

• effect oflittle change in arterial O2 content/sat following inc/small to mod fall in pO2
• breathing O2 at sea level, modest hypovent/modest fall in inspired O2 has little effect in phys/mental performance

Question 18

What happens when arterial pO2 below 8kPa (60mmHg)+ why?

Answer 18

• small rises/falls in pO2 give large rises/falls in O2 content + sat
• in tissues (PO2: 4-6kPa at rest) - small inc in O2 consumption leads to small fall in local PO2 + causes large release of O2 from Hb
• unloading of O2 further helped by red affinity of caused by inc pO2, temp etc reflected in R shift of curve (Bohr shift)

Question 19

When is PaO2 >8kPa + how can this be resolved?

Answer 19

• high alt
• severe hypovent
• hypoxic lung disease
• inc arterial PO2 a little using O2 enriched air may cause large improvements in phys + mental performance

Question 20

When is PO2 low + what can be done about it?

Answer 20

• high alt
• severe hypo
• other resp disease
• O2 delivery adequate for immediate survival but subject v. vulnerable - small further fall in arterial pO2 will cause large fall in O2 content + tissue delivery

Question 21

Does O2 bound to Hb contribute to pO2/is pO2 related to dissolved O2?

Answer 21

• pO2 in blood is dep on amount of dissolved blood (constant relating pO2 + content is sol of O2 in plasma/blood)
• O2 bound to Hb does not contribute directly to pO2 but acts as reservoir of O2 which can top dissolved O2 as this diffuses to tissues

Question 22

What would happen if pO2 was 0g/l?

Answer 22

• as plasma flowed through pul cap, it would equ with alv gas + arterial pO2 would be ~ 13kPa at sea level + v. small O2 content of 3ml/l
• as plasma entered tissue cap, O2 diffuse out down PP gradient + pO2 quickly fall to 0
• only 1st/2nd cell receive O2 - rest would be hypoxic

Question 23

What happens to pO2 in presence of normal [Hb]?

Answer 23

• as dissolved O2 diffuses to tissue cells - replenished by O2 released from Hb
• pO2 remains high enough to maintain adequate O2 diffusion along whole length of cap at rest with reserve for exercise

Question 24

What is av tissue cap pO2 + why?

Answer 24

close to venous pO2 as pO2 falls steeply initially in cap

Question 25

What is effect of polycythaemia with inc Hb in severe chronic resp failure on pO2?

Answer 25

• partly compensate for low arterial pO2 + sat
• although pO2 gradient driving diffusing red at arterial end of cap, av tissue pO2 less severely affected
• bc polycythaemia inc O2 reservoir + fall in pO2 along tissue cap less steep than usual

Question 26

Is O2 delivery acceptable in patient with polycythaemia?

Answer 26

• yes but still vulnerable - small further fall in arterial pO2 have large effect on arterial content + so reservoir of O2 maintaining tissue pO2

Question 27

What is effect of anameia on pO2?

Answer 27

av cap pO2 red bc amount of bound O2 available to top up dissolved O2 as it diffuses to tissues red

Question 28

Is O2 delivery acceptable in patient with anaemia?

Answer 28

O2 delivery may be adequate at rest but capacity to inc O2 consumption in exercise red