Plasma

Introduction

Plasma is the fluid portion of the blood in which the blood cells are suspended. Plasma in combination with the interstitial fluid makes up the extracellular body fluid compartment. Blood plasma can be prepared following centrifugation of a fresh sample that has been placed in a sample pot containing anti-coagulant. Whilst in the centrifuge the red blood cells separate and fall to the bottom leaving a layer of yellow plasma on top. The plasma can subsequently be drawn off.

Blood plasma is to be distinguished from serum which is plasma without fibrinogen which is separated following the clotting process.

Some electrolytes whilst in plasma are bound to plasma proteins and hence are termed protein bound; this must be factored in when calculating plasma (or serum) levels for diagnostic purposes. A good example would be Calcium.

Composition

Plasma is mostly water (90%) but also contains the following substances:

• Plasma Proteins
• Electrolytes
• Nutrients
• Minerals
• Gases
• Waste products
• Hormones and enzymes.

These substances are circulated throughout the body by plasma and diffuse into tissues and cells where needed.

Plasma proteins are large molecules of which there are four main types, albumin, Globulin, fibrinogen and prothrombin. Albumin maintains oncotic pressure within the blood vessels and hence blood volume and blood pressure. Globulins which are produced by lymphocytes act in immune reactions and fibrinogen and prothrombin are required in order for clotting to occur.

Electrolytes include Calcium, Sodium, Chloride and Potassium. Calcium is present in three forms within the plasma: bound to protein (approx. 50%), complexed and ionised. Sodium is an important cation of the extracellular fluid and contributes to plasma oncotic pressure. Potassium is the main intracellular cation, and levels in plasma are rarely reflective of the total body level of this electrolyte.

Nutrients in solution include the breakdown products of digestion, amino acids, glucose and fatty acids.

Mineral Salts in solution include sodium, potassium, calcium, chloride, bicarbonate, sulphate and phosphate. The salts are ions which act to maintain the PH of the blood at 7.4.

Gases in solution include oxygen and carbon dioxide. Oxygen is carried mainly by the red blood cells but a small amount is dissolved in the plasma. Carbon dioxide is carried by the plasma as carbonic acid and bicarbonate ions.

Waste products carried by the plasma include urea and creatinine which are nitrogenous waste. They are taken to the liver and kidneys where they are removed via the bile or urine.

Hormones and Enzymes including cortisol and thyroxine are transported around the body in plasma bound to plasma proteins.