Urinary System
Introduction to the Kidney
- Gross appearance of the kidney varies considerably with species
 Unilobular kidney (carnivores)
 Multilobular kidney (swine, primates, ruminants)
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Regardless of gross anatomic configuration, the histologic structure of all mammalian kidneys are the same
The functions of the kidney are also universal
 Primarily a regulatory organ that controls extracellular fluid volume, electrolyte composition, and pH
 Also has an endocrine function, secreting various important substances
o Renin – acid protease enzyme important in the control of blood pressure and volume
o Erythropoietin – glycoprotein hormone that regulates RBC formation in response to
decreased blood oxygen concentration
o Hydroxylation of vitamin D3 (calcitrol) – a steroid precursor produced by the liver and
regulated primarily by parathyroid hormone (PTH)
Kidney Structure
- Renal capsule
 The surface of the kidney is covered by a thick, CT capsule
 Consists of two distinct layers
o Outer layer of dense fibroblasts (fibrocytes) and collagen fibers
o Inner layer of myofibroblasts
 These contractile capsule cells may resist volume and pressure variations that
accompany changes in renal function
- Cortex and medulla
 The obvious division of the kidney shows the localized and specialized activities of the kidney
o Outer cortex
 Highly vascular (dark red) area where filtration takes place
 Contains ~90-95% of blood passing through the kidney
o Inner medulla
 Nearly avascular (white) area where urine concentration takes place
 Contains ~5-10% of the blood passing through the kidney
- Nephron
 Structural and functional unit of the kidney
 The kidney exhibits ~1.5 million nephrons, though only ~10% are functional at any given time
o This feature is a survival benefit because only a small number of nephrons are exposed to
potentially-nephrotoxic agents at any one time
 Two types of nephrons exist and are named for the region of the kidney in which they are located
o Medullary nephron
 15% of nephrons
 Long-looped nephron that penetrates deeply into the medulla
o Cortical nephron
 85% of nephrons
 Short-looped nephrons whose limbs barely reach the medulla
Nephron Structure: Renal Corpuscle
- 200 microns in diameter (can be seen on a freshly-cut kidney surface)
- Bowman’s capsule
- Glomerulus
 Filtration apparatus of kidney
 Highly-specialized vascular structure consists of the glomerular basement membrane (GBM)
o ~0.10 microns thick and resembles a thin ribbon
 Lamina densa and lamina rara are probably fixation artifacts
o Composed mostly of type IV collage and laminin (contains much heparin sulfate)
 Laminin is a strong negatively-charged polyanion (charge barrier) that repels albumin
(also negatively-charged) and keeps it out of glomerular filtrate
o Lined by two important cell types
 Parietal endothelial cells line the inner surface
- Single layer of squamous cells
 Podocytes cover the outer surface
- Have primary processes that sprout secondary processes (pedicles), which
encircle capillaries. Pedicles interdigitate with those from adjacent podocytes.
The spaces between the pedicles (filtration slits) present a thin membrane (slit
diaphragm) that consists of nephrin (a zipper-like protein) and has important
control over what does and doesn’t enter glomerular filtrate
Renal Corpuscle
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Glomerular mesangial cells
 Resident phagocytic cells of kidney
 Provide structural suppose
 Secrete IL-1 and PDGF
 Can differentiate to juxtaglomerular (JG) cells
Juxtaglomerular (JG) apparatus
Glomerulus
Renal tubular system
- The tubular portion of each nephron begins at the urinary pole of a renal corpuscle and ends at the junction of
the tubule with a collecting duct
- EM examination of renal tubular epithelium illustrates several different regions of the renal tubule
 Proximal convoluted tubules
o Relatively long segment that dominates most sections taken from the cortex
o Composed of proximal cells
 Demonstrates most selective resorption in the kidney
 Cuboidal-shaped, many mitochondria (high metabolic activity)
 Have microvilli (brush border) on luminal surface to increase absorption
 Canaliculi at base of microvilli enable protein resorption from glomerulus
- Protein forms pinocytotic vesicle called “hyaline droplets”
 Tight junctions prevent leakage between adjacent cells
 Distal convoluted tubules
o Cells lack microvilli and canaliculi
o Cells have fewer mitochondria
o Macula densa
 Where tubule rejoins glomerulus
 Integral part of JG apparatus
o Tubules terminate at the collecting duct

Proximal and Distal Tubules
Thin segment
o Consists of Loop of Henle
 Nephron tubule dips into medulla and returns to the cortex
 Thin descending limb
 Thin ascending limb
 Thick ascending limb
- Removes Na+/Cl- from filtrate to allow water resorption & urine concentration
- Water resorption requires high interstitial Na+ (1300 mOsmol)
- Renal interstitium
 Consists of interstitial cell tissue (function unknown) between the tubules
 Prominent in the medulla, inconspicuous in the cortex

Renal Circulation
- Afferent and efferent arterioles enter and leave the renal corpuscle at the vascular pole
- Efferent arteriole branches into a peritubular capillary network after leaving a glomerulus
 This network supplies the rest of the nephron
 Some efferent arterioles form a vasa recta
o Efferent arteriolar loop within the renal medulla of a medullary nephron
- After supplying the nephron, capillaries join venules, which join stellate veins, which empty into larger veins
 Stellate veins are easily seen on feline renal capsule (gives it a unique appearance)
Lower Urinary Tract
- Urothelium
 Transitional epithelium that lines lower urinary tract
 Has secretory activity that can modify urine composition
 Contains ions channels, receptors and ligands
o Important for cell signaling
o Allows for attachment and invasion of bacteria (uropathogenic E. coli)
 Histological appearance differs depending on whether bladder is relaxed or distended
o Plaques (uroplakin) are formed in some regions due to thickened membrane
 In relaxed bladder, plaques invaginate into cell and appear as isolated vesicles (EM)
 In distended bladder, filaments attach to plaques to prevent over-stretching
o Inter-plaque regions consist of non-thickened membrane
- Ureter
 Narrow lumen when not distended (usual state)
 Mucosa has stellate appearance due to numerous longitudinal folds
 Mucosal wall lined by transitional epithelium, smooth muscle, and CT
o Smooth muscle tissue (tunica muscularis) layers are organized in opposite fashion as GI tract
 Inner longitudinal
 Outer circular
 Vesicoureteral valve
o Results from entry of ureter into bladder wall at oblique angle
o Prevents bacterial transmission to ureter and renal pelvis
o Prevents reflux of urine into ureters during bladder distension
o Prevents higher hydrostatic pressure of urinary bladder from reaching renal pelvis
- Urinary bladder
 Distensible reservoir for urine
 Trigone of the bladder is defined by the triangular region formed by 3 openings
o Ureteric openings (2)
o Internal urethral orifice (1)
 Mucosa
o Composed of large, transitional epithelial cells called superficial cells
 Have extensive infoldings that interdigitate with membranes of transitional cells
 Desmosomes anchor transitional epithelial cells and maintain epithelial integrity
o Basement membrane
o Propria-submucosa
 Smooth muscle (tunica muscularis)
o Detrusor m. (bladder wall)
o Internal urethral sphincter m. (ring-like muscle around the internal urethral orifice)
 Innervated by both sympathetic and parasympathetic nerve fibers
- Urethra
 Predominant lining is transitional epithelium
o Variable patches of simple columnar or stratified cuboidal epithelium
 Mucosal surface is arranged into longitudinal folds that flatter or disappear during micturition
 Propria-submucosa consists of loose CT, smooth muscle and lymphoid (diffuse and nodular) tissue
 Male urethra is divided into three segments
o Prostatic – extends from bladder to caudal edge of the prostate gland
o Pelvic – extends from caudal edge of prostate to bulb of the penis
o Penile – courses through the penis and terminates at the external urethra opening
 Female urethra is shorter than that of males
o Vessels (endothelial-lined cavernous spaces) are scattered throughout CT of the propriasubmucosa layer giving the appearance of erectile tissue