Overview

Volume≈3 L (≈55% of blood)

Total protein60–80 g/L

Albumin35–50 g/L

Fibrinogen2–4 g/L

Oncotic pressure≈25–28 mmHg

Volume and composition. Plasma volume is about 3 L (40–45 mL/kg). By weight, plasma is about 92% water, about 7% protein and about 1% other solutes.

Plasma proteins (60–80 g/L). Most are made in the liver.

Albumin (35–50 g/L) is the most abundant, about 55–60% of the total. It produces most of the plasma oncotic pressure. It also carries bilirubin, fatty acids, calcium, hormones and many drugs. Its half-life is about 20 days.

Globulins come in three groups. The α- and β-globulins include transport proteins (transferrin, haptoglobin, lipoproteins, caeruloplasmin), clotting factors and complement. The γ-globulins are the immunoglobulins, made by plasma cells.

Fibrinogen (2–4 g/L) is about 4–7% of the total. It is converted to fibrin in a clot.

Electrolytes and other solutes. Typical values are Na⁺ 135–145 mmol/L, K⁺ 3.5–5.0 mmol/L, Cl⁻ about 98–106 mmol/L, HCO₃⁻ 22–28 mmol/L and total Ca²⁺ 2.2–2.6 mmol/L (about half ionised). Fasting glucose is 3.9–5.5 mmol/L. Plasma also carries nutrients, wastes (urea, creatinine, bilirubin), hormones and dissolved gases.

Serum versus plasma. Serum is the fluid left after blood clots. It lacks fibrinogen and the clotting factors used up in the clot.

Function

Carries blood cells, nutrients, hormones, wastes, CO₂ (as bicarbonate) and heat.
Maintains oncotic pressure and blood volume.
Provides the clotting factors, immunoglobulins and complement.
Buffers pH (bicarbonate, plasma proteins).

Physiology: Plasma osmolality is about 2 × [Na⁺] + glucose + urea (all in mmol/L), normally 275–295 mOsm/kg. Plasma oncotic pressure is about 25–28 mmHg and holds fluid inside the blood vessels.

Clinical significance

Hypoalbuminaemia

Low albumin lowers plasma oncotic pressure and causes oedema and ascites. Causes include nephrotic syndrome, liver cirrhosis, malnutrition and sepsis.

Plasma products and plasmapheresis

Fresh frozen plasma replaces clotting factors in patients with clotting disorders. In plasmapheresis, plasma is removed and replaced to clear harmful antibodies or other proteins. It is used in TTP, Guillain–Barré syndrome, myasthenic crisis and hyperviscosity.

Serum protein electrophoresis

Electrophoresis separates the plasma proteins into bands. A single sharp band (M-band) in the γ region suggests multiple myeloma. A broad increase in γ-globulins suggests chronic inflammation. A missing α1 band suggests α1-antitrypsin deficiency.

Test yourself

Plasma oncotic (colloid osmotic) pressure is generated mainly by:

Fibrinogen
Globulins
Albumin
Sodium ions

Answer: Albumin

Albumin is the most abundant plasma protein (35–50 g/L) and produces about 70–80% of the plasma oncotic pressure (≈25–28 mmHg). Low albumin, as in nephrotic syndrome or cirrhosis, causes oedema. Sodium crosses the capillary wall freely, so it creates no oncotic pressure across it.