When nephrons are destroyed—as in chronic glomerulonephritis, infection of the renal pelvis and nephrons (pyelonephritis), or loss of a kidney—or when kidney function is reduced by damage caused by diabetes mellitus, arteriosclerosis, or blockage by kidney stones, a condition of renal insufficiency may develop. This can cause hypertension, which is due primarily to the retention of salt and water, and uremia (high plasma urea concentrations). The inability to excrete urea is accompanied by an elevated plasma H+ concentration (acidosis) and an elevated K+ concentration, which are more immediately dangerous than the high levels of urea. Uremic coma appears to result from these associated changes.
Patients with uremia or the potential for developing uremia are often placed on a dialysis machine. The term dialysis refers to the separation of molecules on the basis of their ability to diffuse through an artificial selectively permeable membrane. This principle is used in the "artificial kidney machine" for hemodialysis. Urea and other wastes in the patient's blood can easily pass through the membrane pores, whereas plasma proteins are left behind (just as occurs across glomerular capillaries). The plasma is thus cleansed of these wastes as they pass from the blood into the dialysis fluid. Unlike the tubules, however, the dialysis membrane cannot reabsorb Na+, K+, glucose, and other needed molecules. These substances are kept in the blood by including them in the dialysis fluid so that there is no concentration gradient that would favor their diffusion through the membrane. Hemodialysis is commonly performed three times a week for several hours each session.
More recent techniques include the use of the patient's own peritoneal membranes (which line the abdominal cavity)
for dialysis. Dialysis fluid is introduced into the peritoneal cavity, and then, after a period of time, discarded after wastes have accumulated. This procedure, called continuous ambulatory peritoneal dialysis (CAPD), can be performed several times a day by the patients themselves on an outpatient basis. Although CAPD is more convenient and less expensive for patients than hemodialysis, it is less efficient in removing wastes and it is more often complicated by infection.
The many dangers presented by renal insufficiency and the difficulties encountered in attempting to compensate for this condition are stark reminders of the importance of renal function in maintaining homeostasis. The ability of the kidneys to regulate blood volume and chemical composition in accordance with the body's changing needs requires great complexity of function. Homeostasis is maintained in large part by coordination of renal functions with those of the cardiovascular and pulmonary systems, as described in the preceding chapters.
Test Yourself Before You Continue
1. List the different categories of clinical diuretics and explain how each exerts its diuretic effect.
2. Explain why most diuretics can cause excessive loss of K+. How is this prevented by the potassium-sparing diuretics?
3. Define uremia and discuss the dangers associated with this condition. Explain how uremia can be corrected through the use of renal dialysis.
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