This chapter describes the physiological and chemical characteristics of blood plasma. The blood plasma constitutes the intravascular portion of the extracellular fluid of the body. It amounts to some 3.5 liters, and represents, therefore, about one quarter of the total volume of extracellular fluid and about 5% of total body weight. The main cation in the plasma, as in extracellular fluids generally, is sodium. The effective plasma osmotic activity exerted across the walls of the capillaries is much less than 700 kPa as the capillary walls are permeable to small molecules and to water. Plasma differs from the other extracellular fluids in having an appreciable content of proteins. The protein content of plasma amounts to about 75 g/1 of plasma. The distribution of fluid throughout the body depends largely upon the osmotic pressure differences between fluid compartments. Plasma proteins may bind substances and facilitate their transport in the blood. Chronologically, the first stage in blood clotting is the production of the thrombokinase complex. Normal human serum contains an α2 globulin that can exhibit activated C’1 esterase. Deficiency of this inhibitor may be a factor in the hypersensitivity reaction of angioneurotic oedema.
The blood plasma constitutes the intravascular portion of the extracellular fluid of the body. It amounts to some 3.5 litres, and represents, therefore, about one quarter of the total volume of extracellular fluid, and about 5% of total body weight. Plasma is separated by centrifuging whole blood: it is the fluid layer above the packed cells (Fig. 3.1). It normally constitutes around 55% of the blood volume. Plasma differs from serum, the fluid obtained after centrifuging blood that has been allowed to clot, in its greater content of fibrinogen and some of the clotting factors.
Figure 3.1 A haematocrit tube after centrifugation. The proportions of cells (denser than plasma, and therefore at the bottom), and plasma (at the top), can be read off as a percentage of the blood volume. The buffy layer includes the white cells and the platelets.
Although the concentrations of inorganic ions in plasma may vary in different parts of the body, they are in general little different from those in extracellular fluid in the tissues (Table 1.4). The blood serves principally as a transport medium, carrying materials around the body, and acts, therefore, as an equilibrating system throughout the body. The values for plasma concentrations given in this chapter are reasonably representative values: they might be different, for example, in blood leaving the absorptive areas of the gut, or in blood entering or leaving the kidneys. It should be remembered here and throughout the book that most values given as normal are figures which are somewhere towards the middle of the normal range but are rounded out or chosen as being fairly easy to remember, rather than precise as in a physical science. It would often be preferable to give normal ranges, but these are less convenient to hold in one’ mind, and even then may vary among different populations.
The main cation in the plasma, as in extracellular fluids generally, is sodium. The usually accepted value for its concentration is 152 mmol/l of plasma water, or 140 mmol/l of plasma. This distinction is made because the proteins of plasma contribute to its volume, so that 1 litre of plasma contains only about 930 ml of water. Potassium concentrations in extracellular fluids are low; the values usually given for plasma are around 5 mmol/l of plasma water, and around 4.7 mmol/l of plasma. The other major cations of plasma are calcium, at 2.45 mmol/l of plasma, and magnesium, at 1.5 mmol/l of plasma. The pH of plasma is around 7.40, the hydrogen ions being completely balanced by an excess of hydroxyl ions. Variation in pH occurs as carbon dioxide concentration varies: arterial blood in the lungs has a pH of about 7.40, and mixed venous blood returning to the heart, carrying more carbon dioxide in solution, has a pH of about 7.36. The inorganic ions of plasma are important in maintaining fluid balance in the body since they contribute to the total plasma osmotic pressure exerted across the effectively semi-permeable walls of the cells. Some of the plasma ions help to maintain the concentrations of ions in cells and fluids at optimum levels for the correct functioning of specific body systems – calcium ion concentrations, for example, affect the activity of nerves and muscles.
The anionic components of plasma (excluding the plasma proteins, which are anions at blood pH) are again those found in extracellular fluids generally. Thus the main anion is chloride, present at about 113 mmol/l of plasma water, or 108 mmol/l of plasma. Next in concentration after this is hydrogen carbonate, at up to some 27 mmol/l of plasma water. Here, the proviso about variation in concentration in different parts of the body applies: in a subject in a resting state, breathing quietly, the arterial blood leaving the lungs will contain around 25 mmol/l of hydrogen carbonate in the plasma whilst the mixed venous blood may contain around 27 mmol/l in the plasma. The remainder of the anionic charge is made up of 1 mmol/l of phosphate – mainly monohydrogen phosphate at this pH −0.5mmol/l of sulphate ion and some 6 mmol/l of organic acids such as citrate and lactate. The concentrations are per litre of plasma. All these latter components may vary in concentration as a result of dietary intake or tissue metabolism. Clinical chemists often express these concentrations as equivalents rather than moles so that the balance between positively and negatively charged ions will be apparent. The final balancing of charge when expressed in this way is due to some 16 mEq/l of anionic charge contributed by the plasma proteins.
The blood plasma contains many small organic molecules, acts as a transport system for many larger ones, and is itself characterised by its content of the plasma proteins. These are described later, but a few of the other organic components may be mentioned here. Their concentrations are given per litre of plasma. The main non-protein nitrogen-containing compound in the plasma is urea, at a concentration of about 4.2 mmol/l. Free aminoacids together make up about 500mg/l. Glucose is the principal carbohydrate in the plasma, normally at about 5 mmol/l but varying according to the subject’ absorptive state. In a fasting subject it may be as low as 4 mmol/l, whilst after a carbohydrate meal it may be as high as 10 mmol/l. Most fats in the plasma are present in a bound form. Hormones, vitamins and enzymes are all found in the plasma in varying amounts.
The total osmotic pressure of plasma (that is, the pressure required to prevent water passing through a membrane permeable only to water, from pure water on one side to plasma on the other) is given by an osmolal concentration of 290 mOsmol/l, or about 700 kPa. This value is usually determined by measuring another colligative property (a property dependent upon the number of particles in solution) – the depression of freezing point in comparison with that of pure water. Plasma freezes at −0.54°C, a depression of freezing point less than that predicted from the total concentration of the constituents. This implies that interactions between ions are effectively changing their size and number.
The effective plasma osmotic activity exerted across the walls of the capillaries is much less than 700 kPa since the capillary walls are permeable to small molecules as well as to water (see p. 22, and the section below on functions of plasma proteins).
Plasma differs from the other extracellular fluids in having an appreciable content of proteins. It is not true to say that plasma alone contains plasma proteins since studies of labelled proteins have shown that there is rapid passage of plasma proteins through the extracellular spaces outside the circulation; nevertheless, the concentrations present outside the circulation at any one time are low because of the limited permeability of most capillaries to molecules of this size and shape.
The protein content of plasma amounts to about 75 g/l of plasma. The plasma proteins contribute little to the total osmotic activity of the blood because their large molecular size means that relatively few molecules, or solute particles, are present in that total – in comparison, for example, with sodium ions whose molecular weight is about 3000 times smaller but whose concentration is only 20 times smaller (see Fig. 3.2).
Figure 3.2 The relative sizes of the plasma constituents. The relation of these sizes to that of the apparent pores in the capillary walls is shown by the size of pore indicated at the bottom of the diagram.
The original classification of proteins into albumin and globulins was based on fractionation of serum by precipitation, at first by ammonium sulphate, later by various alcohol mixtures. Since serum was used, this classification excluded the fibrinogen present in plasma. The use of increasingly sophisticated electrophoretic techniques, which separate molecules on a basis of charge and size, has led to the separation and identification of more and more fractions. This in turn has allowed identification of proteins by function as well as by physical properties. As a result, the globulins have been subdivided into α1 and α2, β and γ globulins, and pre-albumin fractions have been described.
The protein fraction present in the greatest quantity, 35—40 g/l of plasma, is serum albumin. Even this concentration is only about 0.5 mmol/l, since the molecular weight of albumin is 69000 daltons. Albumin was one of the two original sub-fractions of the plasma proteins and has not been further subdivided by later analytical methods. With the exception of the small amounts of pre-albumins, it is the protein which migrates most rapidly towards the cathode on electrophoresis, since it carries a relatively high charge at pH values above 7.0 and is one of the smallest plasma proteins. Calculation of the theoretical molecular diameter of a protein of molecular weight 69000 gives a value slightly less than the effective pore size of the capillaries in the kidney glomeruli −10nm. The shape of the molecule, however, is narrow and elongated so that only small amounts of the protein normally pass through the glomerular capillary wall. The calculated equivalent pore size in capillaries in general is about 7nm diameter – approximately the same as the smallest dimension of the albumin molecule and of many other plasma proteins.
Albumin is synthesised in the liver; its turnover rate gives it a biological half-life of about 18 days. Breakdown of the protein is not localised to any specific tissue: albumin functions as part of the mobile aminoacid or nitrogen store of the body. In adults some 10-12 g are produced each day.
The α2 group of plasma globulins is a larger group than the α1, amounting to 4-8g/l of plasma. A number of identifiable proteins migrate within this group: some mucoproteins, the haptoglobins, prothrombin, caeruloplasmin, complement-inhibiting factor, thyroxine-binding protein, transcortin, and a protein involved in the transport of vitamin D.
The β globulins are present in approximately the same concentration as the α2 globulins i.e. 4-8 g/l of plasma. The group includes several proteins of known function: the β-lipoproteins, transferrin, testosterone-binding protein, some complement factors, blood clotting factors VII, VIII and IX, and plasminogen.
The γ globulin fraction is the second largest of the main protein fractions of plasma; the plasma concentration in a normal healthy adult subject is 6-12 g/l. γ globulins are large molecules, with molecular weights between 160000 and 960000 daltons. Five main classes of γ, or immunoglobulins are recognised, all based on a structure consisting of two pairs of ‘heavy’ and ‘light’ polypeptide chains (Table 3.1). The ‘light’ K and L chains are each built up from two subunits of 110 aminoacids and each of the ‘heavy’ α, γ, δ, ε and μ chains consist of four of these subunits. The proteins differ in the types of chain they contain and in the structure formed by these chains (Fig. 3.3).
Figure 3.3 The structure of the different classes of immunoglobulin. Sulphur bonds are shown as S. Each protein is made up of either K or L light chains together with α, γ, ε or μ heavy chains. IgD, with δ chains, is not shown, but resembles IgE.
Two types of cell are concerned in the synthesis of immunoglobulins: the plasmacytes and the lymphocytes. Any organ with a large population of these cell types may be a site of immunoglobulin synthesis – lymph nodes and the spleen are the best examples.
Fibrinogen is the precursor of fibrin, the protein formed during blood clotting. Its concentration in plasma is between 2 and 4 g/l; it has been removed from serum. The molecule was originally assigned a dumb-bell shape, because of its S-S terminal linkages, but other possibilities have now been suggested. It has a molecular weight of 340000 daltons and is synthesised in the liver. Two fibrinopeptides, with molecular weights of 3000daltons and a high content of glutamic acid, are split off from the fibrinogen molecule at arginyl-glycyl bonds to leave the fibrin monomer which can then polymerise into fibrin strands.
Blood circulates throughout the body. Plasma is therefore the only fluid always able to exchange components with every other part of the extracellular fluid compartment. Even lymph does not achieve a similar widespread distribution. The internal environment can be maintained constant in a large multicellular organism only if there exists a transport system capable of smoothing out local concentration changes, carrying nutrients to metabolically active cells, removing the products of metabolism, and providing a means whereby any attack on the cells by external agents can be resisted or neutralised. The blood plasma, and the proteins which it contains, serves these three functions of preserving the constancy of the internal environment, of transport, and of defence.