Renal Insufficiency and Hemodialysis

Renal insufficiency is a state in which the kidneys cannot maintain homeostasis due to extensive destruction of their nephrons. Some causes of nephron destruction include:

• Chronic or repetitive kidney infections.

• Trauma from such causes as blows to the lower back or continual vibration from machinery.

• Prolonged ischemia and hypoxia, as in some long-distance runners and swimmers.

• Poisoning by heavy metals such as mercury and lead and solvents such as carbon tetrachloride, acetone, and paint thinners. These are absorbed into the blood from inhaled fumes or by skin contact and then filtered by the glomeruli. They kill renal tubule cells.

• Blockage of renal tubules with proteins small enough to be filtered by the glomerulus—for example, myoglobin released by skeletal muscle damage and hemoglobin released by a transfusion reaction.

• Atherosclerosis, which reduces blood flow to the kidney.

• Glomerulonephritis, an autoimmune disease of the glomerular capillaries.

Nephrons can regenerate and restore kidney function after short-term injuries. Even when some of the nephrons are irreversibly destroyed, others hypertrophy and compensate for their lost function. Indeed, a person can survive on as little as one-third of one kidney. When 75% of the nephrons are lost, however, urine output may be as low as 30 mL/hr compared with the normal rate of 50 to 60 mL/hr. This is insufficient to maintain homeostasis and is accompanied by azotemia and acidosis. Uremia develops when there is 90% loss of renal function. Renal insufficiency also tends to cause anemia because the diseased kidney produces too little erythropoietin (EPO), the hormone that stimulates red blood cell formation.

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

908 Part Four Regulation and Maintenance

Hemodialysis is a procedure for artificially clearing wastes from the blood when the kidneys are not adequately doing so (fig. 23.22). Blood is pumped from the radial artery to a dialysis machine (artificial kidney) and returned to the patient by way of a vein. In the dialysis machine, the blood flows through a semipermeable cellophane tube surrounded by dialysis fluid. Urea, potassium, and other solutes that are more concentrated in the blood than in the dialysis fluid diffuse through the membrane into the fluid, which is discarded. Glucose, electrolytes, and drugs can be administered by adding them to the dialysis fluid so they will diffuse through the membrane into the blood. People with renal insufficiency also accumulate substantial amounts of body water between treatments, and dialysis serves also to remove this excess water. Patients are typically given erythropoietin (EPO) to compensate for the lack of EPO from the failing kidneys.

Hemodialysis patients typically have three sessions per week for 4 to 8 hours per session. In addition to inconvenience, hemodialysis carries risks of infection and thrombosis. Blood tends to clot when exposed to foreign surfaces, so an anticoagulant such as heparin is added during dialysis. Unfortunately, this inhibits clotting in the patient's body as well, and dialysis patients sometimes suffer internal bleeding.

A procedure called continuous ambulatory peritoneal dialysis (CAPD) is more convenient. It can be carried out at home by the patient, who is provided with plastic bags of dialysis fluid. Fluid is introduced into the abdominal cavity through an indwelling catheter. Here, the peritoneum provides over 2 m2 of blood-rich semipermeable membrane. The fluid is left in the body cavity for 15 to 60 minutes to allow the blood to equilibrate with it; then it is drained, discarded, and replaced with fresh dialysis fluid. The patient is not limited by a stationary dialysis machine and can go about most normal activities. CAPD is less expensive and promotes better morale than conventional hemodialysis, but it is less efficient in removing wastes and it is more often complicated by infection.

Hemodialysis Machine Blood Flow

Figure 23.22 Hemodialysis. Blood Is pumped Into a dialysis chamber, where It flows through selectively permeable dialysis tubing surrounded by dialysis fluid. Blood leaving the chamber passes through a bubble trap to remove air before It Is returned to the patient's body. The dialysis fluid picks up excess water and metabolic wastes from the patient's blood and may contain medications that diffuse into the blood.

Figure 23.22 Hemodialysis. Blood Is pumped Into a dialysis chamber, where It flows through selectively permeable dialysis tubing surrounded by dialysis fluid. Blood leaving the chamber passes through a bubble trap to remove air before It Is returned to the patient's body. The dialysis fluid picks up excess water and metabolic wastes from the patient's blood and may contain medications that diffuse into the blood.

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Post a comment