Chronic Parenchymatous Nephritis may be the result of numerous attacks of acute nephritis, or it may have pursued a chronic course from the outset. It is probably due to the persistent presence in the circulation of some irritating toxic substance. It is characterized by chronic degeneration of the secreting epithelium and by a proliferation of the fibrous connective tissue.

The usual variety is the large white kidney, weighing 250 to 300 gm. The kidney is much increased in size, is smooth, pale, and softer than normal. The capsule strips easily. The exposed surface presents a somewhat mottled appearance. On section the cortex is seen to be much thickened, pale and fatty looking, while the pyramids are usually congested and reddish. The cortex may show scattered punctate hemorrhages.

Microscopically the chief change present is a fatty degeneration of the epithelium in the tubules and the glomeruli. The tubules may be filled with granules from the broken-down cells, and erythrocytes may be present. Round-cell infiltration of varying degrees will be found in the interstitial tissues. The Malpighian bodies generally show a proliferation of their epithelium, the capsule of Bowman may become much thickened, and the walls of the capillaries also increase in thickness. The amount of connective tissue is never as great as in the chronic interstitial form.

The urine will contain granular and hyaline casts and varying amounts of albumin.

If the nephritis has existed for a long time, the kidney may become smaller on account of the contraction of the new-formed connective tissue. The organ is then smaller than normal, pale, and granular, the pale granular kidney. The epithelium becomes markedly degenerated and the connective tissue contracts and compresses the parenchyma. The capsule is adherent and cannot be removed without bringing away portions of the kidney.

Chronic interstitial nephritis is a form of kidney inflammation characterized by a continual increase in the amount of interstitial connective tissue associated with an atrophy of the secreting cells. Is probably due to the presence within the blood of an irritant. It is found in alcoholism, syphilis, chronic lead poisoning, and is frequently associated with general arteriosclerosis.

The kidney is small (50 to 60 gm.), dark red in color, firm, and granular, the red granular kidney. The irregularities are due to the contraction of the connective tissue. The capsule strips with great difficulty, tearing away portions of the renal substance with it. On the surface are seen small cysts filled with clear fluid. These are due to the obstruction of a tubule by the pressure of the connective tissue. The tissue cuts with great difficulty, being almost cartilaginous at times, and presents a typical appearance. The cortex is generally very thin with some places of almost normal thickness. Over the thin areas the capsule is usually considerably thickened. The medulla shows little change.

Fig. 168. - Chronic Interstitial Nephritis (McFarland).

a, Still functional glomerule with (b) mass of newly formed connective tissue surrounding Bowman's capsule; c, totally destroyed glomerule; d, newly formed cellular connective tissue; e, atrophic uriniferous tubules; f, slightly altered uriniferous tubules.

Microscopically the picture is very definite, although all portions of the kidney may not be equally involved. As the glomeruli are the parts first brought into contact with the circulating toxic substances, it is there, as a rule, that the processes begin. The glomerulus becomes slowly transformed into a more or less* homogeneous body that loses all lobulations.. At the same time the capsule becomes greatly thickened and the glomerulus is finally transformed into a minute fibrous nodule. The interlobular connective tissue increases until eventually the tubules may become completely atrophic, through compression. Although many of the tubules are atrophic, others will be found markedly dilated, so much so that small cysts may form. There is also a thickening of the walls of the blood-vessels, endarteritis. All these processes may go on to a point where there is very little renal secretory structure left. The parenchyma also shows some changes, but not so markedly as in the chronic parenchymatous variety. There is some atrophy and fatty degeneration.

The urine is generally increased in quantity, of a low specific gravity, 1005 to 1015, with little or no albumin. A few casts of the hyaline and waxy character are found.

Acute interstitial nephritis or suppurative nephritis is the result usually of hematogenic infection by micro-organisms, or it may be due to extension of an inflammation of the pelvis of the kidney or of a neighboring tissue. The organisms gain entrance to the kidney as emboli and usually become lodged in the capillaries of the glomeruli. There is an extravasation of round-cells and leukocytes into the capsular space, and into the tissues between the tubules, and minute foci of suppuration are formed. The irritating products soon cause destruction, with necrosis of the adjacent cells. There is generally some extravasation of blood surrounding the areas of suppuration. The process may terminate by the absorption of the exudate with connective-tissue formation. If the lesion becomes more extensive distinct abscesses may form which may discharge into the tubules or be absorbed and cicatrized.

If the infection has followed a suppurative pyelitis the kidney will be found on section to contain light yellowish colored streaks in the pyramids and medulla. These are composed of tubules that have become rilled with pus. Sometimes several of these foci may coalesce and form a larger abscess, which may discharge its contents into the pelvis of the kidney or upon the surface of the kidney; or it may become absorbed and undergo cicatrization. The suppurative process may go on to such a point that the entire kidney becomes converted into a sac filled with pus.

Tube casts are peculiar bodies that are formed within the urinary tubules and that are composed of various albuminoid substances, some of which react like fibrin. Hyaline casts are pale, almost transparent, structures reacting like fibrin. They may vary greatly in length and also in thickness; are found most commonly in acute parenchymatous nephritis, but are also present in the chronic parenchymatous and interstitial varieties. The hyaline cast is the foundation of many of the other forms. Its surface is adhesive, and according to the substances upon it, we have granular, epithelial, leukocytic, and blood casts. Some blood casts may, however, be formed by the coagulation of extravasated blood within a tubule. Granular casts may be dark or pale according to the amount and form of the material composing them. Are found usually in chronic nephritis. The waxy cast is a rather large, translucent, and solid appearing body that is found especially in chronic parenchymatous nephritis and in amyloid diseases. At times it may give an amyloid reaction with iodin. Cylindroids resemble hyaline tube casts somewhat in general appearance, but are larger and band-like. They are of renal origin and are closely related to true casts.

The effects of nephritis are particularly noticeable in the cardiovascular'system when the renal disease is of a subacute or chronic type. There is a hypertrophy of the heart, especially of the left ventricle, which may become greatly enlarged. The reason for this hypertrophy is not clearly understood. As the kidneys normally secrete urea, chlorids, phosphates, uric acid, urates, and oxalic acid, the accompanying changes may be due to the retention of these substances within the circulation. As in chronic renal disease there is always more or less arteriosclerosis present, the cardiac enlargement may be due to the extra amount of work required to force the blood through the thickened and less elastic vessels. The blood-pressure is usually high, 170 to 220 mm.

The serous membranes in acute nephritis may show inflammatory changes, such as acute endocarditis, acute pericarditis, and pleuritis. Edema is particularly common in acute parenchymatous nephritis, especially if the glomeruli and vessels are involved. The edema appears first in the eyelids and hands, but as the disease progresses and the bloodvessels degenerate it spreads over the entire body. Death may result from edema of the lungs. Uremia is also a frequent condition, due probably, to the retention within the circulation of various toxic substances. On account of the retention of such substances there is a decrease of the general vitality, so that the subjects of such a retention are likely to succumb to infections of varying sorts.