Urinary System Terms

Aldosterone: A mineralocorticoid hormone secreted by the adrenal cortex that promotes sodium and water reabsorption and potassium excretion by the kidneys, which increases blood volume.

Angiotensin II: A hormone activated by renin that causes vasoconstriction, which increases blood pressure, and it stimulates release of aldosterone.

Antidiuretic hormone (ADH): A hormone secreted by the hypothalamus and stored in the posterior pituitary gland; it stimulates water reabsorption by the kidneys, inhibits sweat glands, and causes vasoconstriction of arterioles, which increases blood pressure and blood volume. Also called vasopressin.

Atrial natriuretic peptide (ANP): A hormone released by the heart atria in response to high blood pressure; it causes vasodilation, which decreases blood pressure, it inhibits secretion of renin and aldosterone, and increases the kidney GFR, which promotes greater excretion of salt and water, and decreases blood volume.

Collecting duct: A renal tubule that receives filtrate from many nephrons and delivers urine to the papilla; contains intercalated cells with microvilli that maintain the acid-base balance of the body, and principal cells that function in hormone-dependent reabsorption and secretion, which is controlled by aldosterone and antidiuretic hormone.

Distal convoluted tubule (DCT): A renal tubule surrounded by peritubular capillaries which is confined to the cortex; it is made of cuboidal cells with very few microvilli that functions in hormone-dependent reabsorption and secretion, which is controlled by aldosterone and parathyroid hormone.

Erythropoietin (EPO): A hormone released by the kidney and liver that enhances erythropoiesis.

Glomerular capsule: Double-walled cup at a renal tubule that encloses a glomerulus; its inner (visceral) membrane forms part of the filtration membrane. Also called Bowman's capsule

Glomerular filtration rate (GFR): The volume of filtrate that moves out of the plasma, through glomerular capillaries into the glomerular capsules, per unit time; it is assumed to be equivalent to inulin clearance.

Glomerulus: Cluster of capillaries forming part the nephron that is the site of filtrate formation.

Juxtaglomerular Apparatus (JGA): The cells in the nephron that touch the glomerular capsule and regulate filtrate formation and blood pressure; macula densa cells are columnar cells, modified from the ascending limb of the loop of Henle, that act as chemoreceptors and detect osmotic concentration of the filtrate; juxtaglomerular (granular) cells are modified smooth muscle cells of the arterioles with mechanoreceptors that detect blood pressure and secrete renin when blood pressure is too low; mesangial cells are phagocytic cells.

Kidneys: The paired reddish organs located in the lumbar region that produce urine and regulate the composition, volume, and pressure of blood.

Loop of Henle: A U-shaped renal tubule which is mostly in the medulla and surrounded by vasa recta capillaries; it is composed of a thin descending limb made of squamous cells that are freely permeable to water, and a thick ascending limb made of cuboidal cells that are impermeable to water.

Nephron: The basic structural and functional unit of the kidney that maintains the body's water, solute, and pH balance by filtration of the blood, reabsorption of needed substances, and excretion of the remaining material as urine; cortical nephrons are mostly in the cortex and constitute 85% of all nephrons; juxtamedullary nephrons have long loops of Henle that run deeply into the medulla to reabsorb water.

Parathyroid hormone (PTH): Hormone released by the parathyroid glands that raises blood calcium level, increases intestinal absorption of calcium, and promotes kidney reabsorption of calcium and excretion of phosphate; it acts as an antagonist to calcitonin.

Perirenal fat capsule: A fatty cushion that surrounds the kidneys, located between the renal fascia and renal capsule. Also called renal adipose capsule.

Proximal convoluted tubule (PCT): A renal tubule surrounded by peritubular capillaries which is confined to the cortex; it is made of cuboidal cells with many microvilli and mitochondria; it is the primary site of tubular reabsorption and secretion.

Renal calyces: The cup-shaped extensions of the renal pelvis; the minor calyces collect urine from the renal papillae, which flows into the major calyces, then empties into the renal pelvis.

Renal columns: Areas of cortical tissue running between the renal medullary pyramids.

Renal corpuscle: The glomerular capillaries and the glomerular (Bowman's) capsule that encloses them.

Renal cortex: The lighter, superficial portion of the kidney that contains most of the nephron structures.

Renal fascia: Outermost sheath of dense fibrous connective tissue that surrounds the perirenal fat capsule and anchors the kidneys.

Renal fibrous capsule: The fibrous membrane that surrounds the kidney and is tightly adherent to its surface.

Renal hilum: Area of the kidney which allows entry and exit of ureters, blood vessels, lymphatics, and nerves.

Renal medullary pyramid: A dark reddish-brown, conical structure in the renal medulla that contains the renal papillae and the bulk of the loops of Henle and collecting ducts.

Renal papilla: The tip of a renal medullary pyramid that releases urine into a minor calyx.

Renal pelvis: A funnel-shaped cavity in the center of the kidney that is continuous with the ureter, and opens into the major calyces.

Renin: An enzyme released by the kidneys that is involved with activating the hormone angiotensin II, which can increase blood pressure.

Ureter: One of the two tubes that transport urine from the renal pelvis to the urinary bladder by peristalsis; lined with transitional epithelium, surrounded by circular and longitudinal layers of smooth muscle, and covered externally by an adventitia.

Urethra: The duct from the urinary bladder to the exterior of the body that conveys urine in females, and urine or semen in males.

Urinary bladder: A collapsible sac that stores urine temporarily until it is excreted; its lining is transitional epithelium; its wall is smooth muscle called the detrusor that contracts to expel urine; its trigone is a triangular region at the base where the two ureters enter and the urethra exits; it is located in the pelvic cavity posterior to the pubic symphysis.