section 28.2

Functional Aspects of Hemoglobin

655

in the acid-citrate-dextrose (ACD) medium used by many

blood banks. As a result, oxygen affinity is increased and

the ability of blood that is transfused to supply oxygen

to the tissues is decreased. Volunteers who received such

blood had an increase in oxygen affinity that did not return

to normal for 6-24 hours. The therapeutic significance of

these changes is not clear. The greatest effects should occur

in patients who receive numerous transfusions over a pe-

riod of about

6

hours, so that a significant fraction of their

circulating erythrocytes has increased oxygen affinity.

Traditionally, red cell survival has been the main crite-

rion of the quality of stored blood. However, cell survival

does not necessarily correspond to maintenance of ade-

quate organic phosphate levels. Studies on the composition

of the storage medium needed to prevent this metabolic

loss of organic phosphates show the following:

1. Citrate-phosphate-dextrose (CPD) medium is better

than ACD for maintaining organic phosphate levels

and for preventing a reduction in

P50,

probably

because of the higher pH of CPD. In 1971, 90% of

blood banks in the United States used ACD and only

10% used CPD; by 1975, the reverse was true. The

shelf life of cells is the same for both media

(21 days), but cells stored in CPD function better

physiologically when transfused.

2. Supplementation with inosine generates a supply of

ribose

1

-phosphate and provides a potential glycolytic

substrate that can be metabolized to 2,3-DPG.

Pyruvate and fructose, which help maintain a supply

of oxidized NAD+, potentiate the effect of inosine.

This modification must be balanced against the

possibility of hyperuricemia (Chapter 27) caused by

transfusion of large amounts of inosine-containing

blood.

3. Supplementation with dihydroxy acetone phosphate

could provide a glycolytic substrate without the risk

of hyperuricemia.

CO

2

Transport

In addition to carrying oxygen from the lungs to the tissues,

hemoglobin helps move carbon dioxide from tissues back

to the lungs, where it can be eliminated. This CO

2

move-

ment is accomplished with little or no change in blood pH,

owing to the buffering capacity of hemoglobin (Chapter 1).

Figure 28-10 summarizes the pathways and intermediates

involved. Of particular importance are the following:

F IG U R E 2 8 -1 0

Interchange of carbon dioxide and oxygen between tissues and lungs. The reactions at right occur in plasma and

erythrocytes in tissue capillaries during oxygen delivery to tissue cells; the reactions at left occur in the lungs during

CO

2

release and O

2

uptake. The cyclic nature of these changes (lungs to capillaries to lungs) should be noted.