536 UNIT VIII RENAL AND UROLOGICAL DISORDERS
tubule fluid contains waste products that can be toxic to the fragile nephron cells. The nephron cells are at risk if urine outflow is not maintained. Any obstruc- tion to urine outflow, also called obstructive uropa- thy , can cause urine to back up from the ureter into the renal pelvis and cause cellular injury. The causes of kidney dysfunction are divided into three categories based upon the mechanism of injury: 1. Prerenal dysfunction : caused by decreased blood flow and perfusion to the kidney. 2. Intrarenal dysfunction : develops secondary to actual injuries to the kidney itself. 3. Postrenal dysfunction : related to obstruction of urine outflow from the kidneys. Prerenal Dysfunction Prerenal dysfunction of the kidney describes patho- physiological processes that affect GFR and are directly related to blood flow and renal perfusion (see Fig. 22-3). Any condition that directly or indi- rectly decreases renal perfusion may lead to prerenal dysfunction. Prerenal dysfunction occurs because of reduced cardiac output or severe hypovolemia (low blood volume). In any type of shock, the patient is vulnerable to prerenal dysfunction. Maintenance of a
Red Blood Cell Production The kidney secretes erythropoietin , which stimulates synthesis of RBCs in the bone marrow. Erythropoietin is released in response to low oxygen levels in arte- rial blood. The kidney also secretes erythropoietin in response to anemia and cellular hypoxia. CLINICAL CONCEPT Individuals who have chronic hypoxia, such as those with chronic obstructive lung disease, often have higher-than-normal hemoglobin and hematocrit levels because of constant secretion of erythropoietin. Conversely, patients with renal failure have lower hemoglobin and hematocrit levels because of deficient erythropoietin.
Vitamin D Synthesis and Calcium Balance
The kidneys synthesize components that comprise vitamin D. Without kidney function, vitamin D is inac- tive, which affects calcium absorption. In the gastroin- testinal tract, calcium is absorbed with the facilitation of vitamin D. Without vitamin D, calcium absorption is diminished, which disrupts calcium balance in the bloodstream. Glucose Homeostasis The renal tubules reabsorb glucose from the glomeru- lar filtrate up to the renal threshold of a blood glucose level of 180 mg/dL. If the blood glucose level is greater than the renal threshold, the excess glucose is excreted in the urine. Additionally, in states of prolonged fast- ing or starvation, the kidneys can create glucose from amino acids in a process known as gluconeogenesis. The kidneys are also responsible for the degradation of insulin. Patients with renal failure have decreased insulin clearance which affects glucose metabolism. Basic Pathophysiological Concepts of Renal Disorders The kidneys are at risk for injury because they require a large blood flow to function and because they process potentially toxic waste products. For the nephrons to function properly, the blood entering at the glomeru- lus must be at high hydrostatic pressure. The kidneys are susceptible to ischemic injury if not provided with high blood flow. All tubule fluid from the nephrons must travel toward the renal pelvis and out the ureter. The nephrons need high pressure to push tubule fluid out of the kidney without any stasis or backflow. The
Intrarenal (damage to structures within the kidney)
Prerenal (marked decrease in renal blood flow)
Postrenal (obstruction
of urine outflow from the kidney)
FIGURE 22-3. The three basic categories of renal dysfunction are prerenal, intrarenal, and postrenal. These are sometimes referred to as prerenal, intrarenal, or postrenal azotemia. Prerenal azotemia occurs in severe dehydration or hemor- rhage; there is inadequate blood flow to optimally perfuse the kidney. The kidney is not the cause of prenatal azotemia; rather, a circumstance that decreases perfusion of the kidney is the source of the problem. In intrarenal azotemia, there is a problem intrinsically with the kidney, such as trauma to the kidney, infection, or nephrotoxic drugs. Postrenal azotemia occurs when urine outflow is obstructed. Urine needs to flow freely out of the kidney; if backed up, it is toxic to the nephrons. Prostate enlargement, kidney stones, a kinked ureter, or tumors can cause postrenal azotemia.
Powered by FlippingBook