General Gas Transport by the Blood II Questions

Question 1

At rest, how much CO2 is created and cleared?

Answer 1

At rest, tissue metabolism produces 200-250ml CO2 per minute

As a resting CO of 5L/min, we remove ~40-50ml of CO2 per litre (thus ~200-250mls per minute)

Question 2

What are the 3 forms of CO2 carried in the blood, and what percentage of total carried CO2 does each constitute?

Answer 2

PCO2 = 7%

Bound to Hb - Carbaminohaemoglobin = 10%

Bicarbonate Ions = 80-90%

Question 3

Describe the formation of carbaminohaemoglobin

What type of Hb binds more CO2 in this way?

How is this useful?

Answer 3

10% total CO2 = carbaminohaemoglobin

CO2 reacts with terminal amine group on Hb to form the following reaction….

Hb + NH2 + CO2 = Hb NH COO- H+

This is a rapid, reversible reaction

Reduced (deoxygenated) Hb can bind more CO2 than oxygenated Hb

Thus, at lungs, the increased levels of O2 facilitate a reversal of the above reaction, resulting in increased CO2 offload

At the tissues, the decreased O2 facilitates the formation of carbaminohaemoglobin, and thus increases uptake of CO2

Question 4

Describe how CO2 is transported as a bicarbonate ion

How does this relate to Chloride shift?

Answer 4

80-90% of CO2 is carried as bicarbonate ions

H2O + CO2 = H2CO3 = H+ HCO3-

The first reaction occurs slowly in the plasma, but quickly in RBCs because Carbonic Anhydrase enzyme is present

The second reaction occurs because H2CO3 spontaneously dissociates into H+ and HCO3-

The Produced H+ could potentially decrease pH, but H+ is well buffered by either Hb or plasma proteins

HCO3- diffuses out of the RBC via the HCO3-/Cl- carrier protein.

Increased HCO3- transport out of the RBC causes a Chloride shift

Thus, Chloride shift is associated with increased uptake of CO2 and subsequent production of HCO3-.
This is why venous plasma Cl- concentrations are lower than arterial plasma Cl- concentrations

Question 5

Describe the Haldene effect and its relevance at the tissues and and the lungs

Answer 5

Haldene effect describes Hb’s increased ability to buffer (carry) H+ in its deoxygenated state.

Thus, as Hb becomes less oxygenated, it becomes a more effective buffer as it can carry (buffer) more H+.

This facilitates the production of HCO3- at the tissues: Deoxygenated Hb has greater buffer capacity for H+, facilitating the reaction that results in HCO3-

H20 + CO2 = H2CO3 = H+ and HCO3-

Oxygenated Hb has a decreased ability to carry/buffer H+.

Thus, the increased Hb-O2 binding that occurs at the lungs results in the release of the excess H+ atoms (acquired during the production of HCO3-), thus displacing CO2 by extension. (reversal of the reaction back to H20 and CO2)

Question 6

Basic explanation of Haldene effect:

Answer 6

At the tissues: De-oxygenated Hb has higher affinity to H+ (increased buffering capacity), and thus facilitates the formation of HCO3- (the form in which 80-90% of CO2 is carried)

At the lungs: Oxygenated Hb has a decreased affinity for H+ (decreased buffering capacity) and thus facilitates the reversal of the previous reaction, to form CO2, which is released by the RBCs

Question 7

What is the alveolar ventilation equation?

Answer 7

Alveolar ventilation
= K (constant) x ((rate of CO2 production) / (PCO2 in the arteries))

= K ( VCO2 / PaCO2 )

There is a strong relationship between alveolar ventilation and the amount of CO2 in the blood. This is because CO2 diffuses very quickly and easily.

Thus, in healthy people Alveolar PCO2 is in equilibrium with Arterial PCO2

Question 8

What is the alveolar gas equation?

Answer 8

Alvolar PO2 = (fraction of inspired air that is O2) x (Atmospheric pressure - saturated vapour pressure at 37 degrees) - (Arterial PCO2 / Respiratory Quotient)

PaO2 = PiO2 (Pb - PH2O) - (PaCO2 / RQ)

Thus, normally (at sea level)…

PaO2 = 0.21 x (760 - 47) - (40 - 0.8)

=~100mmHg

Question 9

Causes of Hypercapnia?

Answer 9

1) Reduced Alveolar ventilation with normal/constant CO2 production

Due to: hypoventilation, increase in dead space (which may be due to rapid, short breaths, or V/Q mismatch)

2) Increased CO2 production without appropriate compensatory changes to ventilation

Due to: control problem - e.g. suppression of the respiratory centre

Or abnormality of respiratory pump (e.g. severe emphysema)

Question 10

What determines the ‘RQ’ of the alveolar gas equation?

Answer 10

RQ = respiratory quotient

Generally = ~0.8

RQ = CO2 expiration / CO2 consumption

*The amount of CO2 produced varies with the fuel source (e.g. carbohydrate metabolism produces more CO2 than fat metabolism)