D’YOUVILLE COLLEGE
BIOLOGY 108/508 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 20
URINARY SYSTEM I
ANATOMY, URINE STORAGE, AND EXCRETION
1.
Parts of Urinary Tract (figs. 25 - 1 to 25 - 3):
a. Kidneys: retroperitoneal, enclosed in protective fatty mass & tough fibrous
capsule
• concave medially (hilum), convex laterally
• renal sinus: immediately internal to hilum, contains renal pelvis and
calyces, blood supply, and some fatty connective tissue
• parenchyma: consists of medulla (pyramids) and cortex (including renal
columns that intervene between pyramids); apex of each pyramid features a papilla
that empties into minor calyx; several minor calyces unite to form major calyx &
major calyces communicate with renal pelvis in the sinus
• blood supplied by renal arteries that branch from the aorta and
distribute branches into parenchyma (including arcuate arteries, arching around
base of pyramid at junction with cortex; capillary system features two sets of
arterioles (described later); drainage occurs via renal veins that empty into inferior
vena cava (fig. 25 - 4)
b. Ureters: medial tubes exiting via hilum from renal pelvis (figs. 25 - 19 & 25
- 20)
• convey urine to urinary bladder (stretch of muscularis layer in wall of
renal pelvis & ureter triggers peristalsis)
c. Urinary Bladder (fig. 25 - 21): extraperitoneal, immediately behind pubic
symphysis; storage site for urine until suitable time for elimination
• wall layers: mucosa (transitional epithelium), muscle (detrusor muscle),
and adventitia
• trigone: special triangular area on posterior, inferior part of mucosa,
bounded by openings of ureters and urethra
d. Urethra (fig. 25 - 21): inferior tube from urinary bladder, conveys urine to
exterior
• internal (smooth muscle) and external (skeletal muscle) urethral
sphincters control urine discharge
• sex dimorphism: exclusively urinary in females, 1 - 11/2 inches, exits
into vestibule of external genitalia; reproductive and urinary in males, 6 - 8 inches,
passes into prostate gland, then into penis; includes prostatic (within prostate
gland), membranous (traversing pelvic diaphragm), & spongy (within penis)
portions
Bio 108/508
lec. 20 - p. 2
e. Micturition Reflex (fig. 25 - 22): distension of bladder with accumulated
urine, causes sensory signals to spinal reflex & to pons, interpreted as “urgency”
• parasympathetic signals relax internal urethral sphincter and stimulate
contraction of detrusor muscle
• voluntary prevention - contraction of external urethral sphincter
Bio 108/508
lec. 20 - p. 3
2.
Nephron Anatomy:
a. Nephron: functional unit of kidney; renal tubule intimately associated with
blood capillaries; approximately one million per kidney (figs. 25 - 5 & 25 - 7)
• cortical (6/7 of total) & juxtamedullary nephrons (1 out of 7)
b. 2 sets of arterioles and 2 sets of capillaries:
• afferent arteriole supplies glomerulus (filtration capillary), drained by
efferent arteriole that supplies peritubular capillaries (absorbing capillaries)
c. Parts: glomerular (Bowman’s) capsule + glomerulus = renal corpuscle
• filtrate passes from glomerular capsule to proximal convoluted tubule,
nephron loop (of Henle), distal convoluted tubule and finally to collecting duct
• proximal tubule is lined by epithelium with extensive microvilli
(higher absorption capability)
• nephron loop features ascending and descending limbs + thick and
thin segments
• each collecting duct receives fluid from several nephrons
• fluid exits collecting duct (completed urine) via renal papilla, entering a
minor calyx
d. Juxtaglomerular Apparatus (fig. 25 - 8):
• macula densa (cells of ascending limb of Henle's loop or distal tubule) &
juxtaglomerular cells (surrounding afferent arteriole)
• juxtaglomerular cells secrete renin (causes formation of angiotensin in
plasma = renin-angiotensin system)
• angiotensin: vasoconstriction, aldosterone release by adrenal cortex, &
ADH release by posterior pituitary