Kidney Stones: Kidney stones, or calculi, are made of substances, such as calcium salts or uric acid, that precipitate out of the urine instead of remaining in solution. They usually form in the renal pelvis, but may also form in the bladder. The causes of stone formation include dehydration, stasis (stagnation) of the urine, and infection of the urinary tract. The stones may vary in size from tiny grains resembling bits of gravel up to large masses that fill the renal pelvis and extend into the calyces. The latter are described as staghorn calculi. There is no way of dissolving these stones because substances that could do so would also destroy kidney tissue. Sometimes, instruments can be used to crush small stones and thus allow them to be expelled with the urine, but more often, surgical removal is required. A lithotriptor, literally a “stone-cracker,” is a device that employs external shock waves to shatter kidney stones. The procedure is called lithotripsy.
Renal Failure: Acute renal failure may result from a medical or surgical emergency or from toxins that damage the tubules. This condition is characterized by a sudden, serious decrease in kidney function accompanied by electrolyte and acid-base imbalances. Acute renal failure occurs as a serious complication of other severe illness and may be fatal. Chronic renal failure results from a gradual loss of nephrons. As more and more nephrons are destroyed, the kidneys gradually lose the ability to perform their normal functions.
As the disease progresses, nitrogenous waste products accumulate to high levels in the blood, causing a condition known as uremia. In many cases, there is a lesser decrease in renal function, known as renal insufficiency, that produces fewer symptoms. Two of the characteristic signs and symptoms of chronic renal failure are the following:
* Dehydration. Excessive loss of body fluid may occur early in renal failure, when the kidneys cannot concentrate the urine and large amounts of water are eliminated.
* Edema. Accumulation of fluid in the tissue spaces may occur late in chronic renal disease, when the kidneys cannot eliminate water in adequate amounts.
* Electrolyte imbalance, including retention of sodium and accumulation of potassium.
* Hypertension may occur as the result of fluid overload and the increased production of renin.
* Anemia occurs when the kidneys cannot produce the hormone erythropoietin to activate red blood cell production in bone marrow.
* Uremia, an excess of nitrogenous waste products in the blood. When these levels are very high, urea can be changed into ammonia in the stomach and intestine and cause ulcerations and bleeding.
Renal Dialysis and Kidney Transplantation: Dialysis means “the separation of dissolved molecules based on their ability to pass through a semipermeable membrane” (Fig. 18-11 A). Molecules that can pass through the membrane move from an area of greater concentration to one of lesser concentration. In patients who have defective kidney function, the accumulation of urea and other nitrogenous waste products can be reduced by passage of the patient’s blood through a dialysis machine. The principle of “molecules leaving the area of greater concentration” thus operates to remove wastes from the blood. The fluid in the dialysis machine, the dialysate, can be adjusted to regulate the flow of substances out of the blood. There are two methods of dialysis in use: hemodialysis (blood dialysis) and peritoneal dialysis (dialysis in the abdominal cavity). In hemodialysis, the dialysis membrane is made of cellophane or other synthetic material. In peritoneal dialysis, the surface area of the peritoneum acts as the membrane (see Fig. 18-11 B). Dialysis fluid is introduced into the peritoneal cavity and then periodically removed along with waste products. This procedure may be done at intervals through the day or during the night. A 1973 amendment to the Social Security Act provides federal financial assistance for people who have chronic renal disease and require dialysis. Most hemodialysis is performed in freestanding clinics. Treatment time has been reduced; a typical schedule involves 2 to 3 hours, three times a week. Access to the bloodstream has been made safer and easier through surgical establishment of a permanent exchange site (shunt). Peritoneal dialysis also has been improved and simplified, enabling patients to manage treatment at home. The final option for treatment of renal failure is kidney transplantation. Surgeons have successfully performed many of these procedures. Kidneys have so much extra functioning tissue that the loss of one kidney normally poses no problem to the donor. Records show that transplantation success is greatest when surgeons use a kidney from a living donor who is closely related to the patient. Organs from deceased donors have also proved satisfactory in many cases.
Figure 18-11 A hemodialysis system and a peritoneal dialysis system. (A) In hemodialysis, a cellophane membrane separates the blood compartment and dialysis fluid compartment. This membrane is porous enough to allow all of the constituents except the plasma proteins (PRO) and blood cells (WBC, RBC) to diffuse between the two compartments. (B) In peritoneal dialysis, a semipermeable membrane richly supplied with blood vessels lines the peritoneal cavity. With dialysate dwelling in the peritoneal cavity, waste products diffuse from the network of blood vessels into the dialysate.