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Anatomy of the urinary tract and kidney. A) The organs of the urinary system include the kidneys, ureters, bladder, and urethra. The function of the urinary system is to filter the blood and create urine as a waste by-product. B) The kidney has an outer region called the renal cortex and an inner region called the medulla. The renal columns are connective tissue extensions that radiate from the cortex to the medulla and separate the renal pyramids and papillae. The papillae are bundles of collecting ducts that carry urine made by the nephron to the calyces for excretion. The renal hilum is the entry and exit site for the blood vessels, nerves, lymphatics, and ureters.



Diagram of the nephron. The nephron is the functional unit of the kidney that uses four mechanisms to process the filtrate: filtration, reabsorption, secretion, and excretion. First, blood flows through Bowman’s capsule, and fluid and solutes are filtered out to form a glomerular filtrate. The proximal convoluted tubule, the loop of Henle, the distal convoluted tubule, and the collecting ducts are sites for the reabsorption of water and ions. Shown are the sites at which ions are moved in each segment. The site where each diuretic functions along the nephron is also indicated.


  • Important renal concepts and formulas: See Table 6.1

  • Pearls about glomerular filtration rate (GFR):

    • - GFR is measured with exogenous substances (inulin or radioisotopes) or estimated with endogenous substances (creatinine or cystatin C)

    • - In clinical practice, estimated GFR (eGFR) is calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula, which has the best accuracy in persons with normal eGFR, or using the Modification of Diet in Renal Disease (MDRD) equation. These equations have largely replaced the Cockcroft-Gault equation.

    • - eGFR is most accurate when a patient is in a steady state (i.e., has a stable sCr). When interpreting eGFR, consider common scenarios that affect the result, such as muscle mass, medication effect (e.g., TMP/SMX), or dynamic states (e.g., early AKI).

    • - If eGFR accuracy is critical, consider calculating eGFR with cystatin C, a freely filtered molecule that is not affected by muscle mass.

TABLE 6.1Important Renal Concepts and Formulas

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