Week 4 - O2 in the blood

Question 1

What is the solublity of O2 at 13.3kPa?

Answer 1

-0.13mmol/L

Question 2

What does oxygen bing to in haemoglobin?

Answer 2

-4 haem groups

Question 3

Why does the content of o2 in the blood saturate?

Answer 3

-Only a limited amount of Hb

Question 4

Why is saturation shown as a percentage rather than amount on a dissociation curve?

Answer 4

-Saturation is limited by the amount of pigment present. By representing as a % of total saturation possible it shows saturation independent of pigment concentration

Question 5

Why is myoglobin not an efficient carrier?

Answer 5

-The high affinity of myoglobin for oxygen means that at pO2 of the tissues myoglobin would still be bound to O2, ie it wouldnt be released

Question 6

What is the purpose of myoglobin?

Answer 6

-Act as a store of O2 in tissues when pO2 gets extremely low eg during exercise

Question 7

Describe haemoglobin

Answer 7

• tetramer made of 2a and 2b subunits
• Each subunit contains a haem group bound to a porphyrin ring containing iron (globin)
• Has 2 states; low affinity T when low pO2 and high affinity R which high pO2
• Exhibits cooperative binding ie as each O2 molecule binds Hb becomes more relaxed and binding of the next O2 is easier

Question 8

Describe the Hb dissociation curve (including the axis)

Answer 8

• pO2 on X axis, %saturation on Y
• Sigmoidal shape (due to cooperative binding)
• Saturated above 9kPa

Question 9

What is the pO2 of arterial blood leaving the lungs?

Answer 9

-13.3kPa

Question 10

What is the total content of O2 in arterial blood leaving alveoli?

Answer 10

• Each molecule of Hb binds 8.8mmol/L
• Solubility of O2 at 13.3kPa os 0.13mmol/L
• So total content is 8.93mmol/L

Question 11

What is the pO2 in metabolically active tissues?

Answer 11

-5.3kPa

Question 12

What % O2 saturation is typically in metabolically active tissues?

Answer 12

-65%

Question 13

What effects the % O2 saturation of venous blood?

Answer 13

-How much O2 is removed at the tissues -> lower pO2 the more O2 dissociates from Hb (ie during exercise)

Question 14

Why can pO2 in most tissues not fall below 3kPa?

Answer 14

-Tissue pO2 must be high enough to drive diffusion of O2 into cells. Below 3kPa the tissues become too far away from the capillaries to be sufficiently supplied as the gradient is too shallow to drive diffusion in the required time

Question 15

What effect does capillary density have on pO2?

Answer 15

-Higher capillary density means that pO2 can fall below 3kPa as O2 doesnt have to diffuse as far

Question 16

What is the Bohr shift? Why does this happen?

Answer 16

• The shifting of the Hb dissociation curve to the right due to the presence of H+
• The shift is because the T state is promoted at lower pH so O2 is given up more easily

Question 17

What is the purpose of the bohr shift?

Answer 17

-Allows more O2 to be given up by Hb at higher pO2 levels, in those areas which need it

Question 18

Why does the bohr shift occur in metabolically active tissues?

Answer 18

• pH is lower in metabolically active tissues due to CO2 and lactic acid production
• O2 given up more easily due to low pH

Question 19

What is the effect of temperature on Hb dissociation?

Answer 19

-Higher temperature shifts the curve to the right
-Higher temp promotes the T state -> O2 given up more easily
(higher temp in metabolically active tissues)

Question 20

What is maximum unloading of O2 and where can this occur?

Answer 20

• maximun 70% bound O2 can be given up
• Occurs in tissues where pO2 can fall to a low level (high capillary density) and also in tissues whereby increased metabolic activity causes low pH and high temperature

Question 21

Overall, in mixed venous blood, how much bound O2 has been given up? Why is this beneficial?

Answer 21

• 27%

- There is a reserve of oxygen so extraction can increase during exercise without changing CO so dramatically

Question 22

What effect does 2,3-BPG have on Hb?

Answer 22

• Shifts the Hb dissociation curve to the right

- 2,3-BPG binds to deoxy Hb and keeps it in that state, ie decreases Hb affinity for oxygen so more is given up

Question 23

When would higher 2,3-BPG levels be seen?

Answer 23

• Altitude

- Anaemia

Question 24

Why is CO poisonous?

Answer 24

• Hb has a higher affinity for CO than O2
• Reacts with Hb to form COHB
• Increases affinity of unaffected subunits for O2
• Affinity becomes so great that O2 not given up at tissue pO2
• 50%COHb is fatal

Question 25

What is cyanosis?

Answer 25

-Bluish discolouration of the skin and mucous membranes due to poor saturation (deoxyHb

Question 26

Describe pulse oximetry

Answer 26

• Detects level of Hb saturation by detecting difference in absorption of light between oxyHb and DeoxyHb
• Uses red and infared light
• Only measures pulsatile arterial blood

Question 27

Is O2 soluble in water?

Answer 27

-Not very