Composition-inorganic

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.

Composition-organic

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 activity of plasma

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).

The plasma proteins