Chapter 47: Assessment of Kidney & Urinary Function

Assessment of Kidney & Urinary Function details the fundamental structure and essential physiological role of the renal and urinary systems in maintaining the body’s state of homeostasis, primarily by strictly governing fluid volume, electrolyte balance, and acid–base status. The anatomical components include the kidneys, ureters, bladder, and urethra, with the functional unit of the kidney being the nephron, of which there are approximately one million per organ. Urine formation is achieved through a complex, three-step process involving glomerular filtration, which produces the initial filtrate, followed by tubular reabsorption and tubular secretion, processes that selectively move substances like sodium, bicarbonate, glucose, and waste products (urea nitrogen, creatinine) between the filtrate and the blood to produce 1 to 2 liters of urine daily. Critical regulatory functions of the kidneys are tightly controlled by hormones; for instance, antidiuretic hormone (ADH) or vasopressin, released in response to plasma osmolality changes, dictates the reabsorption of water, while aldosterone manages sodium retention and potassium excretion. The kidney is also crucial for long-term blood pressure control through the powerful renin-angiotensin-aldosterone system, initiated by specialized juxtaglomerular cells when renal blood pressure falls below normal. Furthermore, the kidney plays a vital part in red blood cell production by releasing erythropoietin in response to hypoxia and aids in calcium metabolism by synthesizing vitamin D to its active form. The glomerular filtration rate (GFR), accurately estimated by calculating creatinine clearance, provides the best approximation of overall renal function. Assessment of the urinary system involves a thorough health history focusing on characteristics of pain (e.g., kidney, ureteral, bladder), changes in voiding patterns, such as oliguria (output less than 400 mL in 24 hours), anuria (less than 50 mL in 24 hours), dysuria, and nocturia, as well as associated gastrointestinal symptoms. Diagnostic evaluation utilizes urinalysis to determine specific gravity and check for abnormalities like hematuria or proteinuria, along with functional tests including blood urea nitrogen (BUN) and serum creatinine levels. Various imaging and endoscopic procedures, such as KUB x-rays, computed tomography (CT), magnetic resonance imaging (MRI), bladder ultrasonography to measure post-void residuals, and cystoscopy for direct visualization or tissue biopsy, are used to evaluate structure and function. Gerontologic considerations emphasize that decreased GFR and diminished thirst response place older adults at heightened risk for chronic kidney disease and fluid imbalance.