Gas Transport

Question 1

How does oxygen travel in the blood?

Answer 1

In solution in plasma (3ml O2).
Bound to Hb in RBCs (197ml O2).

Question 2

How does CO2 travel in the blood?

Answer 2

The bulk (77%) of CO2 is transported in solution in plasma, with the rest being stored within Hb.

Question 3

What is the relationship between arterial O2 content and cardiac output?

Answer 3

Arterial O2 content x CO = O2 delivery to tissues.
The O2 demand of resting tissues = 250ml/min.
Total arterial O2 = 1000ml/min.

Question 4

How is the structure of Hb related to its function?

Answer 4

Each molecule contains 4 haeme groups, each one containing one Fe2+, which binds one O2.

Question 5

What is the major determinant of which Hb is saturated with O2?

Answer 5

PO2 in the blood.
Alveolar ventilation –> Alveolar PO2 –> Plasma PO2 –> O2 in Hb.

Question 6

How does Hb maintain a partial pressure gradient of O2?

Answer 6

Separates O2 from the plasma.
Sucks O2 out of the alveoli until saturated.

Question 7

How saturated is Hb at different values of PO2?

Answer 7

100mmHg (normal) - almost 100%.
60mmHg - ~90%.
Normal venous PO2 - ~75% (reserve).

Hb is easily saturated even when PO2 is low, allowing a relatively normal uptake of O2.

Question 8

What factors decrease the affinity of Hb for O2?

Answer 8

A decrease in pH.
An increase in PCO2 and temperature.
Binding 2,3-DPG (synthesised by erythrocytes).

Question 9

When does 2,3-DPG increase?

Answer 9

In situations with inadequate oxygen supply.
Heart disease, lung disease, high altitudes.
Maintains O2 release in the tissues.

Question 10

What is anaemia?

Answer 10

Any condition which results in a decrease in the oxygen carrying capacity of the blood.
Iron deficiency, haemorrhages, vitamin B12 deficiency.

Question 11

How are PO2 and total blood O2 content affected by anaemia?

Answer 11

PO2 is normal.
Total blood O2 content is low.
Both cannot be low at the same time.

Question 12

How are RBCs saturated in anaemia?

Answer 12

Since PO2 is normal, saturation is normal.
In iron deficiency - the number of O2 binding sites is reduced, but still saturated.

Question 13

How does CO bind to Hb?

Answer 13

Forms COHb - 250x affinity than O2.
Binds readily, and dissociates slowly.

A PCO of 0.4mmHg can cause progressive COHb formation.

Question 14

What is COHb characterised by?

Answer 14

Hypoxia.
Anaemia.
Cherry red skin and mucous membranes.
Nausea and headaches.

Question 15

What are the effects of COHb?

Answer 15

No effect on RR - arterial PCO2 is normal.
Brain damage and death can occur.

Question 16

What is the treatment for COHb?

Answer 16

Treatment - provide 100% O2 to increase PaO2.

Question 17

What is carbonic anhydrase?

Answer 17

Catalyses CO2 + H2O –> HCO3- + H+.
Important in CO2 transport.

Question 18

How is CO2 diffused in the blood?

Answer 18

7% - dissolved in plasma and erythrocytes.
23% - combines in the erythrocytes with deoxyhaemoglobin to form carbamino compounds.
70% - combines in the erythrocytes with H2O to form H2CO3 (which forms HCO3- and H+).

Question 19

What do HCO3- and H+ do in the blood?

Answer 19

HCO3- - most moves from the erythrocytes into the plasma in exchange for Cl-.
H+ - excess binds to deoxyhaemoglobin.

Question 20

What occurs at the pulmonary capillaries, in terms of CO2?

Answer 20

CO2 moves down its concentration gradient from the blood to the alveoli.