A. Drugs acting on the Plasma. - Many substances must after absorption exist in solution in the plasma, and purgatives, diuretics and diaphoretics must alter the composition of the plasma by abstracting substances from it; but while drugs are given with the object of acting on the plasma it is in order to render it more alkaline: we know no drugs which will make it acid, or even markedly reduce the natural alkalinity of the plasma, as the mineral acids can only exist in it in the form of neutral salts.

The alkalizers of the plasma are salts of -

(1) Potassium.

(2) Sodium.

(3) Ammonium.

(4) Lithium.

(5) Magnesium.

(6) Calcium.

This is approximately the order of their alkalizing power. Potassium is certainly the most powerful, while calcium is very feeble.

The citrates and tartrates of these metals are decomposed in the plasma into alkaline carbonates. As alkalies unite with uric acid and form soluble urates, they are given with the hope that this will occur in the plasma. The diuretic effect of the alkali aids the excretion of the urates.

Therapeutics

The chief use of alkalies is their administration in gout, in which disease the uric acid is greatly in excess in the plasma. As the treatment has to be continued for some time, a preparation which does not disturb digestion, such as potassium citrate, is usually preferred, or lithium citrate, for the lithium compound of uric acid is very soluble in water; but the value of lithium is doubtful, as it does not increase the solubility of urates in the plasma. For the same purpose the numerous natural alkaline waters are frequently prescribed.

In lead-poisoning the lead is locked up in the tissues in a very sparingly soluble form. Potassium iodide was given because some authorities believed it increased the solubility of lead in the plasma, and consequently facilitated its excretion by the kidneys.

Alkalies have been largely used in rheumatic fever, on the assumption that there is a deleterious agent in the plasma, and that its solubility is increased by increasing the alkalinity of the plasma; but this treatment has now been abandoned in favor of that by salicylates. For the same theoretical reason alkalies have been given in rheumatoid arthritis.

Purgatives, diaphoretics and diuretics necessarily alter the composition of the plasma, and are largely used when there is much oedema of any part, or effusion into serous cavities, in the hope that as fast as these remedies drain off fluid from the plasma it will be replaced by that which is effused pathologically. Also they are given in conditions, as uraemia, in which it is thought that there are poisons in the blood, in order that their excretion may be hastened.

The composition of the plasma can also be altered directly either by venesection or transfusion.

B. Drugs acting on the Red Corpuscles. - The most important are those which can increase the amount of haemoglobin when that is deficient. Strictly speaking, all these have a pathological and not a physiological action, for we know of no drugs which will increase the amount of iron in perfectly healthy blood. These drugs are called haematinics.

They are -

(1) Iron and its salts.

(2) Arsenous acid.

(3) Potassium permanganate (doubtful).

(4) Copper salts

(5) Hydrochloric acid

(6) Potassium salts

(7) Phosphorus

(doubtful).

They not only increase the quantity of haemoglobin in each corpuscle, but also the number of red corpuscles. Their action is much aided by good food, fresh air, and attention to the general health, and especially to the digestive organs. The mode of action of these haematinics is very obscure, and will be discussed under each drug. Iron is by far the most important and effectual.

Indirect haematinics are drugs which benefit the patient by removing some obvious cause for his deficiency in haemoglobin, or anaemia, as it is generally termed. Such are mercury, given for syphilis, quinine for ague, etc.

Alcohol and quinine diminish the oxygenating power of the blood, for they render oxyhaemoglobin a more stable compound than it usually is, but their action in this direction is slight. Citrates and tartrates of the alkaline metals are partially oxidized to carbonates at the expense of the oxygen of the red blood-corpuscles.

The size of the red blood-corpuscles is said to be diminished by carbon dioxide, quinine and morphine, and to be increased by oxygen, hydrocyanic acid, and quinine; and their number is said to be increased by small doses of mercury.

A large amount of sodium chloride causes the red corpuscles to pass rapidly through the walls of the capillaries.

Quinine and hydrocyanic acid diminish the ozonizing power of the blood.

There are some drugs which are not employed therapeutically for their action on the blood, which are nevertheless very important physiologically and toxicologically, for they kill by altering the composition of the haemoglobin, thus preventing its uniting with oxygen. Such are carbonic oxide, which turns out the oxygen from oxyhaemoglobin, hydrocyanic acid, which forms cyano-haemoglobin, and potassium chlorate. Acetanilid, antipyrin, phenacetin, and nitrites, especially amyl nitrite, convert the haemoglobin into methaemoglobin; acetanilid, amyl nitrite, potassium chlorate, and pyrogallic acid destroy the red corpuscles.

Phosphorus, arsenic, hydrogen sulphide, turpentine, iodine, and sulphur also reduce oxyhaemoglobin. Phosphorus is especially destructive to the blood.

When freshly drawn blood is exposed to the air its oxidization is diminished by hydrocyanic acid, alcohol, chloroform, quinine, morphine, nicotine, strychnine, and brucine.

C. Drugs acting on the White Corpuscles. - Most if not all drugs which are poisonous to amoebae are poisons to white corpuscles when applied in sufficient strength, which, however, is rarely the case in the human body. All irritants which set up inflammation cause the white blood-corpuscles to wander through the capillary walls; and all the cinchona alkaloids, viz., quinine, quinidine, cinchonine, and cinchonidine, have the power of arresting this migration; of these, quinine is the most powerful. Berberine sulphate and acetanilid are also powerful. If the quinine is circulating in the capillaries, it prevents the white corpuscles from wandering out; if it is applied to the outside of the vessels, it prevents the corpuscles from wandering away from the vessel through the wall of which they have passed.

Veratrine applied to white corpuscles outside the body kills them.

Camphor, myrrh and other aromatics are said to increase their production by increasing absorption from the intestine.

The following facts do not fall under any of the previous headings: - Poisonous doses of mercury increase the fluidity of the blood, impair its power of coagulation, and diminish the solids in it. Cod-liver oil increases the solids of the blood. Potassium iodide and calcium salts especially the chloride are said to increase the power of coagulation; other substances doing this •will be described under astringents.