Problems with Kidney Function
The composition of normal urine is given in Table 16.3. Water accounts for almost all of the volume of urine (95%). The remaining 5% consists of electrolytes and various solutes, including nitrogenous end products and substances derived from drugs. Notice that urine is typically free of proteins and blood cells because they are not filtered at the glomerulus.
Urinalysis is an examination of the physical, chemical, and microscopic properties of the urine. A urinalysis is done to help determine the state of the body. The composition of the urine changes if disease has altered body metabolism or if kidney function is abnormal. Abnormal substances in urine and abnormal quantities of normal constituents are both matters of concern.
Many types of illnesses, especially diabetes, hypertension, and inherited conditions, cause progressive renal disease and renal failure. Infections of the urinary tract are fairly common, particularly in females because the urethra is considerably shorter than that of the male. If the infection is localized in the urethra, it is called urethritis. If the infection invades the urinary bladder, it is called cystitis. Finally, if the kidneys are affected, the infection is called pyelonephritis.
Glomerular damage sometimes leads to blockage of the glomeruli so that glomerular filtration either does not occur or allows large substances to pass through. This is detected when a urinalysis is done. If the glomeruli are too permeable, albumin, white blood cells, or even red blood cells appear in the urine. A trace amount of protein in the urine is not a matter of concern, however.
When glomerular damage is so extensive that more than two-thirds of the nephrons are inoperative, urea and other waste substances accumulate in the blood. This condition is called uremia. Although nitrogenous wastes can cause serious damage, the retention of water and salts is of even greater concern. The latter causes edema, fluid accumulation in the body tissues. Imbalance in the ionic composition of body fluids can lead to loss of consciousness and to heart failure.
Patients with renal failure can undergo hemodialysis, utilizing either an artificial kidney machine or continuous ambulatory peritoneal dialysis (CAPD). Dialysis is defined as the diffusion of dissolved molecules through a semipermeable natural or synthetic membrane having pore sizes that allow only small molecules to pass through. In an artificial kidney machine (Fig. 16.10), the patient’s blood is passed through a membranous tube, which is in contact with a dialysis solution, or dialysate. Substances more concentrated in the blood diffuse into the dialysate, and substances more concentrated in the dialysate diffuse into the blood.
The dialysate is continuously replaced to maintain favorable concentration gradients. In this way, the artificial kidney can be utilized either to extract substances from blood, including waste products or toxic chemicals and drugs, or to add substances to blood-for example, bicarbonate ions (HCO3-) if the blood is acidic.
In the course of a three- to six-hour hemodialysis, from 50 to 250 grams of urea can be removed from a patient, which greatly exceeds the amount excreted by normal kidneys. Therefore, a patient needs to undergo treatment only about twice a week. CAPD is so named because the peritoneal lining of the peritoneal (abdominal) cavity is the dialysis membrane. A fresh amount of dialysate is introduced directly into the abdominal cavity from a bag that is temporarily attached to a permanently implanted plastic tube. The dialysate flows into the peritoneal cavity by gravity. Waste and salt molecules pass from the blood vessels in the abdominal wall into the dialysate before the fluid is collected four or eight hours later. The solution is drained into a bag from the abdominal cavity by gravity, and then it is discarded. One advantage of CAPD over an artificial kidney machine is that the individual can go about his or her normal activities during CAPD.
Replacing a Kidney
Patients with renal failure sometimes undergo a kidney transplant operation during which a functioning kidney from a donor is received. As with all organ transplants, there is the possibility of organ rejection. Receiving a kidney from a close relative has the highest chance of success. The current one-year survival rate is 97% if the kidney is received from a relative and 90% if it is received from a nonrelative. As discussed in the What’s New reading on page 9, it’s possible that kidneys from pigs will eventually be available or that tissue engineering will produce kidneys for transplant in the laboratory.
Figure 16.10 An artificial kidneymachine. As the patient’s blood is pumped through dialysis tubing, it is exposed to a dialysate (dialysis solution). Wastes exit from blood into the solution because of a preestablished concentration gradient. In this way, blood is not only cleansed, but its water-salt and acid-base balances can also be adjusted.
Illnesses Detected by Urinalysis
Urinalysis, or examination of the urine, indicates whether any abnormal substances are present in the urine. The presence of glucose in the urine usually indicates that the individual has diabetes mellitus, a condition in which either the liver fails to store glucose as glycogen or the cells fail to take up glucose. In both cases, the blood glucose level is abnormally high. This makes the filtrate level of glucose high, and because the proximal convoluted tubule cannot absorb all of it, glucose appears in the urine. The presence of albumin and/or blood cells in the urine indicates that the glomerulus is more permeable than usual, as occurs in renal disease. When plasma proteins are excreted in the urine, the blood’s osmotic pressure is reduced, and capillaries fail to take up water. Tissue fluid accumulates, and edema, particularly in the abdomen, occurs. As blood volume, and therefore blood pressure, decreases, the kidneys absorb more salt and water, but this, in the end, serves only to increase the edema. The best treatment is to cure the underlying cause of the edema. Insufficient urine suggests kidney failure, which leads to uremia, or a very high blood urea nitrogen level (BUN). Death from kidney failure, however, is not due to the buildup of nitrogenous wastes; rather, it is due to an imbalance of electrolytes. Studies have shown that if urea is high but can be stabilized at normal levels, the patient usually recovers from the symptoms of uremia. An electrolyte imbalance, however, particularly the accumulation of potassium in the blood, interferes with the heartbeat and leads to heart failure.