Key Terms

anatomical sphincter

smooth or skeletal muscle surrounding the lumen of a vessel or hollow organ that can restrict flow when contracted

angiotensin I

protein produced by the enzymatic action of renin on angiotensinogen; inactive precursor of angiotensin II

angiotensin II

protein produced by the enzymatic action of ACE on inactive angiotensin I; actively causes vasoconstriction and stimulates aldosterone release by the adrenal cortex

angiotensin-converting enzyme (ACE)

enzyme produced by the lungs that catalyzes the reaction of inactive angiotensin I into active angiotensin II

angiotensinogen

inactive protein in the circulation produced by the liver; precursor of angiotensin I; must be modified by the enzymes renin and ACE to be activated

anuria

absence of urine produced; production of 50 mL or less per day

aquaporin

protein-forming water channels through the lipid bilayer of the cell; allows water to cross; activation in the collecting ducts is under the control of ADH

Bowman’s capsule

cup-shaped sack lined by a simple squamous epithelium (parietal surface) and specialized cells called podocytes (visceral surface) that participate in the filtration process; receives the filtrate which then passes on to the PCTs

brush border

formed by microvilli on the surface of certain cuboidal cells; in the kidney it is found in the PCT; increases surface area for absorption in the kidney

calyces

cup-like structures receiving urine from the collecting ducts where it passes on to the renal pelvis and ureter

cortical nephrons

nephrons with loops of Henle that do not extend into the renal medulla

countercurrent multiplier system

involves the descending and ascending loops of Henle directing forming urine in opposing directions to create a concentration gradient when combined with variable permeability and sodium pumping

detrusor muscle

smooth muscle in the bladder wall; fibers run in all directions to reduce the size of the organ when emptying it of urine

distal convoluted tubules

portions of the nephron distal to the loop of Henle that receive hyposmotic filtrate from the loop of Henle and empty into collecting ducts

diuretic

compound that increases urine output, leading to decreased water conservation

efferent arteriole

arteriole carrying blood from the glomerulus to the capillary beds around the convoluted tubules and loop of Henle; portion of the portal system

endothelins

group of vasoconstrictive, 21-amino acid peptides; produced by endothelial cells of the renal blood vessels, mesangial cells, and cells of the DCT

external urinary sphincter

skeletal muscle; must be relaxed consciously to void urine

fenestrations

small windows through a cell, allowing rapid filtration based on size; formed in such a way as to allow substances to cross through a cell without mixing with cell contents

filtration slits

formed by pedicels of podocytes; substances filter between the pedicels based on size

forming urine

filtrate undergoing modifications through secretion and reabsorption before true urine is produced

glomerular filtration rate (GFR)

rate of renal filtration

glomerulus

tuft of capillaries surrounded by Bowman’s capsule; filters the blood based on size

glycosuria

presence of glucose in the urine; caused by high blood glucose levels that exceed the ability of the kidneys to reabsorb the glucose; usually the result of untreated or poorly controlled diabetes mellitus

incontinence

loss of ability to control micturition

intercalated cell

specialized cell of the collecting ducts that secrete or absorb acid or bicarbonate; important in acid–base balance

internal urinary sphincter

smooth muscle at the juncture of the bladder and urethra; relaxes as the bladder fills to allow urine into the urethra

inulin

plant polysaccharide injected to determine GFR; is neither secreted nor absorbed by the kidney, so its appearance in the urine is directly proportional to its filtration rate

juxtaglomerular apparatus (JGA)

located at the juncture of the DCT and the afferent and efferent arterioles of the glomerulus; plays a role in the regulation of renal blood flow and GFR

juxtaglomerular cell

modified smooth muscle cells of the afferent arteriole; secretes renin in response to a drop in blood pressure

juxtamedullary nephrons

nephrons adjacent to the border of the cortex and medulla with loops of Henle that extend into the renal medulla

leaky tight junctions

tight junctions in which the sealing strands of proteins between the membranes of adjacent cells are fewer in number and incomplete; allows limited intercellular movement of solvent and solutes

leukocyte esterase

enzyme produced by leukocytes that can be detected in the urine and that serves as an indirect indicator of urinary tract infection

loop of Henle

descending and ascending portions between the proximal and distal convoluted tubules; those of cortical nephrons do not extend into the medulla, whereas those of juxtamedullary nephrons do extend into the medulla

macula densa

cells found in the part of the DCT forming the JGA; sense Na⁺ concentration in the forming urine

medulla

inner region of kidney containing the renal pyramids

mesangial

contractile cells found in the glomerulus; can contract or relax to regulate filtration rate

micturition

also called urination or voiding

myogenic mechanism

mechanism by which smooth muscle responds to stretch by contracting; an increase in blood pressure causes vasoconstriction and a decrease in blood pressure causes vasodilation so that blood flow downstream remains steady

nephrons

functional units of the kidney that carry out all filtration and modification to produce urine; consist of renal corpuscles, proximal and distal convoluted tubules, and descending and ascending loops of Henle; drain into collecting ducts

net filtration pressure (NFP)

pressure of fluid across the glomerulus; calculated by taking the hydrostatic pressure of the capillary and subtracting the colloid osmotic pressure of the blood and the hydrostatic pressure of Bowman’s capsule

oliguria

below normal urine production of 400–500 mL/day

osteomalacia

softening of bones due to a lack of mineralization with calcium and phosphate; most often due to lack of vitamin D; in children, osteomalacia is termed rickets; not to be confused with osteoporosis

pedicels

finger-like projections of podocytes surrounding glomerular capillaries; interdigitate to form a filtration membrane

peritubular capillaries

second capillary bed of the renal portal system; surround the proximal and distal convoluted tubules; associated with the vasa recta

physiological sphincter

sphincter consisting of circular smooth muscle indistinguishable from adjacent muscle but possessing differential innervations, permitting its function as a sphincter; structurally weak

podocytes

cells forming finger-like processes; form the visceral layer of Bowman’s capsule; pedicels of the podocytes interdigitate to form a filtration membrane

polyuria

urine production in excess of 2.5 L/day; may be caused by diabetes insipidus, diabetes mellitus, or excessive use of diuretics

principal cell

found in collecting ducts and possess channels for the recovery or loss of sodium and potassium; under the control of aldosterone; also have aquaporin channels under ADH control to regulate recovery of water

proximal convoluted tubules (PCTs)

tortuous tubules receiving filtrate from Bowman’s capsule; most active part of the nephron in reabsorption and secretion

renal columns

extensions of the renal cortex into the renal medulla; separates the renal pyramids; contains blood vessels and connective tissues

renal corpuscle

consists of the glomerulus and Bowman’s capsule

renal cortex

outer part of kidney containing all of the nephrons; some nephrons have loops of Henle extending into the medulla

renal fat pad

adipose tissue between the renal fascia and the renal capsule that provides protective cushioning to the kidney

renal hilum

recessed medial area of the kidney through which the renal artery, renal vein, ureters, lymphatics, and nerves pass

renal papillae

medullary area of the renal pyramids where collecting ducts empty urine into the minor calyces

renal pyramids

six to eight cone-shaped tissues in the medulla of the kidney containing collecting ducts and the loops of Henle of juxtamedullary nephrons

renin

enzyme produced by juxtaglomerular cells in response to decreased blood pressure or sympathetic nervous activity; catalyzes the conversion of angiotensinogen into angiotensin I

retroperitoneal

behind the peritoneum; in the case of the kidney and ureters, between the parietal peritoneum and the abdominal wall

sacral micturition center

group of neurons in the sacral region of the spinal cord that controls urination; acts reflexively unless its action is modified by higher brain centers to allow voluntary urination

specific gravity

weight of a liquid compared to pure water, which has a specific gravity of 1.0; any solute added to water will increase its specific gravity

systemic edema

increased fluid retention in the interstitial spaces and cells of the body; can be seen as swelling over large areas of the body, particularly the lower extremities

trigone

area at the base of the bladder marked by the two ureters in the posterior–lateral aspect and the urethral orifice in the anterior aspect oriented like points on a triangle

tubuloglomerular feedback

feedback mechanism involving the JGA; macula densa cells monitor Na⁺ concentration in the terminal portion of the ascending loop of Henle and act to cause vasoconstriction or vasodilation of afferent and efferent arterioles to alter GFR

urethra

transports urine from the bladder to the outside environment

urinalysis

analysis of urine to diagnose disease

urochrome

heme-derived pigment that imparts the typical yellow color of urine

vasa recta

branches of the efferent arterioles that parallel the course of the loops of Henle and are continuous with the peritubular capillaries; with the glomerulus, form a portal system