Lecture 15. Urinary System

two kidneys, two ureters, one bladder and one urethra

osmoregulation:

regulation of osmotic concentration of blood plasma and hence all other body fluids

counter-current multiplier system:

filtration: ultrafiltrate: product of filtration of blood plasma containing most small molecules

selective reabsorption of water, inorganic ions and organic molecules from filtrate; most water and some other molecules reabsorbed; excess and waste excreted

secretion: some excretory products secreted directly from blood
 

erythropoietin:stimulates erythrocyte production in bone marrow; maintenance of oxygen-carrying capacity of blood

KIDNEY
(e.g., rat: archytypal): one lobe made up of a renal pyramid, base in cortex, apex in medulla (point toward hilum); multiple lobes present in compound kidney (e.g., human)

capsule: fibrous ct; continuous at hilum with supporting tissue

renal cortex: cortical components of lobes fuse forming outer zone; contains renal corpuscles; extends between adjacent medullary pyramids in compound kidney

renal medulla: multiple medullary pyramids in compound (multi-lobed) kidney separated by regions of cortex = medullary pyramids

medullary interstitium: osmotic concentration gradient; high osmotic pressure in medullary extracellular fluid; concentration gradient provides mechanism by which urine later undergoes concentration in the collecting tubules and ducts under the influence of ADH.
lipid laden interstitial cells: at right angles to tubules and vasa recta; sudivide interstitium

medullary (=renal) pyramid: numerous tubules and ducts converging toward tips of renal papillae

renal pelvis: funnel-shaped; envelops renal papilla (apical part of renal pyramid); dilated proximal part of ureter

renal papillae:

cribiform area: openings of large collecting ducts
pelvicalyceal space; lined by proximal end of ureter;
urinary epithelium = transitional epi. or urothelium
smooth muscle continuous with that of ureter

calyx: receding sizes in compound kidney; extension of renal pelvis surrounding each renal papilla of compound kidney;

pelvicalyceal system: whole urinary collecting system within kidney;

CONDUCTING PASSAGES (lined by urinary epithelium)

ureter: muscular tube; urine propelled by peristaltic action; two layers smooth muscle (inner elongated spiral [known as longitudinal]; outer tight spiral [traditionally called circular]; additional outer longitudinal in lower third) lumen lined by transitional epi. thrown into folds in relaxed state; broad collagenous lamina propria, loose collagenous adventitia covers muscles; contains blood vessels, lymphatics and nerves

bladder: three loosely arranged layers smooth muscle; elastic fibers contract during micturition; transitional epithelium thrown into many folds in relaxed state; outer adventitia; impermeable; basal layer deeply indented by strands of underlying lamina propria containing capillaries.

NEPHRON: functional unit of urinary system; arise in cortex and loop down into medulla & return to cortex; arises from nephrogenic blastema

two types: (para)cortical (= short looped nephrons [superficial and medial]) and juxtamedullary (= long looped nephrons)

short looped nephrons: located superficially and mid-cortical regions; loops extend little beyond cortico-medullary junctions.

long looped nephrons: mainly associated with juxtamedullary corpuscles; loops may reach the tips of renal papillae;

renal corpuscle: 200 micro m in diameter; dense, rounded structures
vascular pole afferent arteriole: supplies glomerulus (larger); generally source of juxtaglomerula cells (modified smooth muscle; produce renin)
efferent arteriole: drains glomerulus urinary pole: entrance renal tubule

glomerulus

densely packed, anastomosing capillaries capsule visceral layer of capsule = (modified into) podocytes; phagocytic and secretory; reflected at vascular pole to become continuous with parietal epithelium (capsule) proper; extensive branching, pale stained cytoplasm, bulging, large round, pale stained nuclei; long primary processes embrace one or more capillaries; secondary foot processes = pedicels are extensions interdigitating; separated by filtration slits of uniform width (25 nm) bridged by diaphragm (4 nm thick); electronegative glycocalyx covering

capsular space: filled with glomerula filtrate = ultrafiltrate

parietal layer of capsule: continuous with epithelium of renal tubule

glomerular basement membrane: fused basement membrane of podocyte layer and capillary endothelium = filtration barrier
mesangium (mes-  = support; angium- = vessels: equivalent of mesenteries of capillaries within glomerulus):supporting tissue; fills space between capillaries of
glomerular lobule; extends into glomerulus through vascular pole; mesangial matrix = ECM supporting capillary loops, particularly at branching points; also
extraglomerular mesangium (= lacis cells)
mesangial cells: contractile and phagocytic

renal tubule: lumen continuous with space in capsule; extend from capsule to collecting duct

three distinct histo-physiological zones  (5 in long looped nephrons) (1) proximal tubule: convoluted and straight portions: functions in reabsorption of molecules (> peritubular capillaries

(1a) convoluted: proximal convoluted tubule (PCT)

dominates labyrinthine portion of cortex; brush border (PAS positive [glycocalyx]); simple, tall cuboidal epi; round nuclei with prominent nucleoli; basal striations (deep infolding + columns of elongated mitochondria); lateral interdigitations with apical junctional complexes: disappearance cell boundaries (1b in long looped nephrons) straight (thick) = pars recta (= straight); descends toward medulla; low cuboidal epithelium; round in cross section; brush border = thick descending limb

(2 in long looped nephrons) nephric loop (of Henle): thin portions: connects thick descending and ascending limbs; low cuboidal epithelium; round in cross section; loses brush border straight, thin-walled; descends toward medulla; simple squamous epithelium; regular round shaped in transverse section thin descending limb (look like capillaries with no blood): loops back; readily permeable to water; loops into ascending limb thin ascending limb (looks like capillaries with no blood): active transport of sodium and chloride ions into medullary interstitium; impermeable to water

(3) distal tubule: convoluted and straight portions; reabsorption of sodium ions; active process controlled by aldosterone and coupled to release of hydrogen or potassium ions

thick ascending limb: longer of straight thick portions; incipient urine hypotonic; returns to cortex

convoluted: distal convoluted tubule (DCT); Na⁺ resorbed; K⁺ and H⁺ secreted; controlled by aldosterone (promotes reabsorption Na⁺ and water from DCT); absence of brush border; clearly defined lumen more nuclei per cross-section; reduced affinity for cytoplasmic stains; lateral interdigitations; large number mitochondria and infolding basal plasma membrane; nuclei close to luminal surface; tend to bulge into lumen; overlying cytoplasm devoid of mitochondria

COLLECTING TUBULES: straight terminal portion of nephron; simple low cuboidal (sparse organelles) epithelium becomes columnar; intercalated cells; mitochondria and microvilli at surface; frequency diminishes distally; but wider and less regular cross section; converge on collecting duct; not permeable to water but become permeable in persence of antidiuretic hormone (ADH)

COLLECTING DUCTS (large = ducts of Bellini: simple, tall columnar epithelium with cell borders, pale stained cytoplasm, no brush boder [cells have single cilium]; poorly stained cytoplasm = clear cells; smooth luminal surface; open at tips of renal papillae to discharge urine into pelvicalyceal system); large diameter and pale stained columnar epithelium; concentrate urine by passive reabsorption of water into medullary interstitium following osmotic gradient; vasa rect return water to general circulation

arise in cortex and descent into medulla to discharge urine from the apex of medullary pyramid; descend through cortex in parallel bundles called medullary rays; arises from ureteral (ureteric) bud

not permeable to water but become permeable in persence of antidiuretic hormone (ADH); urine thus becomes hypertonic with respect to plasma; increases permeability to water resulting in retention of water by body and production of hypertonic urine;

VASCULATURE: note: (resembles portal system but instead of capillaries [sinuses] interrupting veins capillaries interrupt arterioles); rete mirabili (glomerulus) comes between afferent and efferent arterioles; vasa recta interrupt efferent arteriole: afferent arterioles > glomerula capillaries > efferent arterioles > vasa recta > interlobular vein

renal artery: two branches formed in hilum

interlobar arteries
ascend btwn pyramids toward cortico-medullary junction

arcuate arteries:
arched branches run parallel to capsule of kidney; Note: look for arcuate veins wherever you find arcuate arteries and vice versa

cortical radial (= interlobular) arteries
if arcuates are in cross section, then radials should be in longitudinal section

afferent arterioles
bigger than efferent

glomerula capillaries = tuft = rete mirabili (negative charge on luminal cell surface)

endothelial fenestrations (50 to 100 nm in diameter; no diaphragms in adult);

glomerula basement membrane: (thick: 240 to 340 nm); continuous

efferent arterioles
structure of arteriole rather than venule;

peritubular capillaries:
attenuated endothelium resting on very thin basement membrane and intervening supporting tissue layer

vasa recta: parallel, wide capillary loops associated with nephric loops; arise from efferent arterioles near cortico-medullary junction; descend into medulla and loop back to drain into veins at junction; take up water from the medullary intersititum and return it to general circulation

plexus of capillaries:
formed from efferents of short-looped nephrons; surround rubules of renal cortex

cortical radial (= interlobular) veins:

arcuate veins

interlobar veins
descend btwn pyramids from cortico-medullary junction

renal vein

last revised:03-24-03