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Editor's Note: This article introduces a renal physiology continuing education series to run in the Nephrology Nursing Journal. The articles, which are updates of manuscripts that previously appeared in the journal, are written by experts in nephrology and contain the most up-to-date information and research available.
The kidneys are paired vascular organs that perform excretory, regulatory, and secretory functions. In order to understand how these complex organs work, it is necessary to review renal anatomy and understand the renal processes involved in maintaining the body's internal milieu.
The renal system is comprised of the kidneys, ureters, bladder, and urethra (see Figure 1). Urine is formed by the kidneys and then flows through the other structures to be excreted from the body. The kidneys are located bilaterally in the retroperitoneal space at the level of T-12 to L-3. The organs are bean-shaped, measure approximately 12 cm in length, 6 cm in width, and 2.5 cm in thickness, and weigh 120 to 170 grams in the normal adult. The right kidney is slightly lower than the left because of the liver. The kidneys are protected, not only by their anatomical position within the rib cage, but also by the perinephric structures. A tough fibrous capsule covers each kidney. The renal fascia provides support and perirenal fat acts as a cushion.
The ureters are hollow fibromuscular tubes that begin at the renal pelvis, extend downward retroperitoneally, and join the bladder. Urine flows away from the kidneys by peristalsis. The urinary bladder, located in the pelvic region, is a spherical, muscular sac with a capacity of 300 ml to 500 ml in the normal adult. Urine enters via the ureteral orifices and is excreted through the urethra.
Vascular Supply
The kidneys are highly vascularized organs and receive approximately 20% of the resting cardiac output. Thus, renal blood flow is about 1,200 ml/min. Two characteristics of the renal vasculature make it different from most other vascular beds in the body. First, there are two capillary beds in series, the glomerular capillary bed and peritubular capillary bed. These capillary beds are separated by the efferent arteriole. This arrangement, a capillary bed surrounded by arterial vasculature, is referred to as a portal system. Second, the capillary bed is more porous. For example,...