Science · Medicine & the body

Blood plasma

Fractionation developed from 1940

Medicine — haematology and transfusion

The straw-coloured liquid in which the blood cells are suspended, making up roughly 55 per cent of blood by volume. It is about 92 per cent water; most of the rest is protein — albumin, the globulins and fibrinogen — together with electrolytes, glucose, amino acids, lipids carried as lipoproteins, hormones, dissolved gases and waste products such as urea. Remove the fibrinogen and the clotting factors and what remains is serum. Separated and frozen, it is a transfusion product in its own right, and fractionated it yields albumin, immunoglobulins and the clotting factors used to treat haemophilia.

How to spot them

▸About 55 per cent of blood by volume; about 92 per cent of it is water
▸Three protein fractions: albumin (the most abundant), the globulins, and fibrinogen
▸Serum is this minus fibrinogen and the clotting factors
▸Straw or pale yellow in colour, chiefly from bilirubin; milky after a fatty meal
▸For transfusion the universal donor group is AB — the reverse of the rule for red cells

Key facts

Roughly 55 per cent of the circulating volume, and about 92 per cent water

The cells, mostly red, make up the other 45 per cent — the haematocrit

Albumin, the globulins and fibrinogen — its three protein fractions

Albumin is over half the protein content and maintains colloid osmotic pressure

What remains of the straw-coloured fluid once the fibrinogen and clotting factors have been taken out

Serum — the distinction that matters in every laboratory

Charles Drew's wartime banks of 1940, shipped across the Atlantic when whole cells could not be (1940)

The 'Blood for Britain' programme; Drew showed the liquid fraction could be stored and transported

Irving Langmuir borrowed the name in 1928 for the ionised fourth state of matter (1928)

He saw an analogy with a fluid carrying corpuscles along

Its pale yellow colour, which comes chiefly from bilirubin

After a fatty meal it turns milky — lipaemic

Urea, creatinine and uric acid dissolved in it, on their way to the kidney

The waste fraction, which is what a blood chemistry panel measures

Banned in Britain in 1998 over variant CJD, and allowed again from 2021 (1998)

UK-donated material was barred from making medicines until the MHRA review of 2021

Key ideas

Albumin does the unglamorous work: it holds water inside the vessels by colloid osmotic pressure, and when it falls — in liver disease or severe malnutrition — fluid leaks into the tissues and causes oedema.
Whole blood is perishable, but the liquid fraction can be frozen or dried, which is why it, and not whole blood, was the transfusion product of the Second World War.

Quick-fire facts

•Makes up about 55 per cent of blood volume; roughly 3 litres in a 5-litre circulation
•About 92 per cent water, about 7 per cent protein and about 1 per cent other solutes by weight
•The main proteins are albumin (roughly 55–60 per cent of the total), the alpha, beta and gamma globulins, and fibrinogen
•The gamma globulins are the antibodies; fibrinogen is the soluble precursor of fibrin
•Also carries sodium, potassium, calcium, magnesium, chloride and bicarbonate ions, glucose, amino acids, lipoproteins, vitamins, hormones and dissolved oxygen, nitrogen and carbon dioxide
•Waste products carried include urea, creatinine, uric acid and bilirubin
•Serum is this fluid minus fibrinogen and the clotting factors — what is left once blood has clotted
•Charles R. Drew organised the 'Blood for Britain' programme in 1940 and pioneered large-scale storage and shipping
•Plasmapheresis separates it from the cells and returns the cells to the donor
•Fractionation yields albumin, immunoglobulins and clotting factors such as factor VIII
•For transfusion of this component, group AB donors are the universal donors — the opposite of the red-cell rule, because AB carries neither anti-A nor anti-B antibody
•The UK banned the use of its own donations for making medicines in 1998 over variant CJD risk; the restriction was lifted for immunoglobulins in 2021 and for albumin in 2023
•Irving Langmuir adopted the word in 1928 for ionised gas, the fourth state of matter, by analogy with the blood fluid that carries corpuscles

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Sources

OpenStax, Rice University — Anatomy and Physiology 2e — An Overview of Blood
Medicines and Healthcare products Regulatory Agency / GOV.UK — MHRA safety review gives UK plasma donations go-ahead