58 Plasma density
Blood and plasma density describe the mass of a given volume of blood. Plasma is about 92% water; the rest is proteins (mainly albumin and globulins), glucose, lipids and electrolytes. Its density is therefore only slightly higher than that of water, about 1.025–1.029 g/mL. Red blood cells are denser, so whole blood has a density of about 1.050–1.060 g/mL, depending mainly on the haematocrit and plasma protein concentration. Changes in density reflect shifts of water between blood and tissues, loss or gain of fluid, and changes in plasma proteins.
Units
Density in g/mL multiplied by 1000 gives kg/m³ (for example 1.050 g/mL = 1050 kg/m³). The range used in this application, 1048–1055, corresponds to these units and to values typical of whole blood; it is a conventional range of the method.
How it is measured
Historically, the specific gravity of blood and plasma was measured by the copper sulphate drop method, still used to screen blood donors for anaemia. Precise research measurements use the mechanical oscillator technique, which can follow rapid changes in blood density and is used to track changes in plasma volume. Density depends on temperature, so measurements are standardised (for example at 37 °C).
Clinical significance
Lower density
Haemodilution: excess water (overhydration, heart or kidney failure, excess antidiuretic hormone), low plasma proteins (malnutrition, liver disease, nephrotic syndrome), anaemia.
Higher density
Haemoconcentration: dehydration (vomiting, diarrhoea, sweating, diuretics, diabetes insipidus or uncontrolled diabetes), burns, high haematocrit (polycythaemia), high plasma proteins.
Plasma volume, and so blood density, are controlled by antidiuretic hormone (vasopressin), aldosterone (for example raised in Conn's syndrome) and natriuretic peptides. Blood density also changes with posture: on standing, fluid moves from the blood into the tissues of the legs, and blood density rises within minutes.
Signs of dehydration (thirst, dry mouth, dark urine, dizziness on standing) or of fluid overload (swelling of the legs, breathlessness, rapid weight gain) need medical assessment.
References
- Trudnowski RJ, Rico RC. Specific gravity of blood and plasma at 4 and 37 °C. Clin Chem. 1974;20(5):615–616.
This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.
Published on 29 April 2024