Answer:

Carbon dioxide is produced by cell metabolism

in the mitochondria. The amount produced

depends on the rate of metabolism and the rel-

ative amounts of carbohydrate, fat and protein

metabolized.The amountis about 200mlmin1

when at rest and eating a mixed diet; this utilises

80% of the oxygen consumed, giving a respir-

atory quotient of 0.8 (respiratory quotient ¼

rate of carbon dioxide production divided by

rate of oxygen consumption). A carbohydrate

diet gives a quotient of 1 and a fat diet 0.7.

Carbon dioxide transport in

the blood

Carbon dioxide is transported in the blood

from the tissue to the lungs in three ways:1

(i) dissolved in solution; (ii) buffered with

water as carbonic ACID; (iii) bound to proteins,

particularly haemoglobin.

Approximately 75% of carbon dioxide is

transport in the red blood cell and 25% in the

plasma. The relatively small amount in plasma

is attributable to a lack of carbonic anhydrase

in plasma so association with water is slow;

plasma plays little role in buffering and com-

bination with plasma proteins is poor.

There is a DIFFERENCE between the percentage

of the total carbon dioxide carried in each

form and the percentage exhaled from them.

For example, 5% of the total is in solution but

10% of exhaled carbon dioxide comes from this

source; 10% is protein bound, particularly with

haemoglobin, but this supplies 30% of the

exhaled amount.

Dissolved carbon dioxide

Carbon dioxide is 20 times more soluble than

oxygen; it obeys Henry’s law, which states that

the number of molecules in solution is pro-

portional to the partial pressure at the liquid

surface. The carbon dioxide solubility coeffi-

cient is 0.0308 mmol litre–1 mm Hg–1 or

0.231 mmol litre–1 kPa–1 at 37-

C. (Solubility

increases as the temperature FALLS.) This corres-

ponds to 0.5 ml kPa–1 carbon dioxide in 100 ml

blood at 37-

C. The partial pressure of carbon

dioxide is 5.3 pKa in arterial blood and 6.1 kPa

in mixed venous blood; therefore, arterial

blood will contain about 2.5 ml per 100 ml

of dissolved carbon dioxide and venous

blood 3 ml per 100 ml. A cardiac output of

5 litre min1 will carry 150 ml of dissolved car-

bon dioxide to the lung, of which 25 ml will be

exhaled. Because of this high solubility and

diffusing capacity, carbon dioxide partial pres-

sure of alveolar and pulmonary end-capillary

blood are virtually the same. Even a large shunt

of 50% will only cause an end-pulmonary capil-

lary/arterial carbon dioxide gradient of about

0.4 kPa.

Carbonic acid

Carbon dioxide combines with water to form

carbonic acid, a reaction accelerated by car-

bonic anhydrase. The carbonic acid then freely

dissociates (Equation 1).

CO2 þ H2O Ð

carbonic anhydrase

H2CO3

Ð Hþ þ HCO

3 ð1Þ

The enzyme carbonic anhydrase is present

in a number of organs of the body including

the eye, kidney and brain; however, for this

purpose, it is the red blood cell carbonic anhyd-

rase that is important. Once carbonic acid is

formed it dissociates easily so that the ratio of

H2CO3 to HCO3

– is 1:20 (Equation 2).

CO2

H2CO3

¼ 1000

1

H2CO3

HCO

3

¼ 1

20 ð2Þ

Carbon dioxide and water diffuse freely

into the red blood cell and are converted to

carbonic acid, which dissociates into hydrogen

and bicarbonate ions. Hydrogen ions do not

pass through cell membranes but carbon

dioxide passes readily. This situation cannot

be sustained as the intracellular hydrogen ion

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