URINARY SYSTEM - Components include: Kidneys, Ureters, Urinary Bladder, Urethra
Function = elimination of nitrogenous wastes produced by protein catabolism,
maintenance of ionic and osmotic balance w/in body
Mechanism = ultrafiltration of blood plasma followed by reabsorption of most
water and small molecules (e.g., glucose) and secretion of certain ions (mostly
ACCESSORY FUNCTION = erythropoietin release; this is a hormone which stimula
tes RBC production SEQUENCE = Urine produced in kidneys, drains toward hilum ("dent" in kidney
bean shape), entering renal pelvis where it leaves the kidneys. Upon exiting
kidney, urine enters ureters which carry it to the urinary bladder
(muscular-walled sac serving as reservoir for urine storage). Urethra drains
urinary bladder to exterior.

- Basic structural unit = nephron (renal tubule or kidney tubule)
Hilum = depression thru which urine exits and blood vessels enter (and exit) the kidney
Renal Pelvis = expansion of upper part of ureter within the hilum, divided into
large and small cups (major and minor calyces).
Collecting Ducts = empty into calyces, these are structures into which renal
tubules drain

As a gross structure, the kidney is surrounded by a dense CT capsule which gives off
trabeculae extending toward the interior that produce a lobular organization. The
kidney is divided into an outer cortex and an inner medulla. The
medulla contains the loops of Henle (found only in mammals and some birds) and the
collecting ducts. Medullary rays extend into cortex



1. Renal Corpuscle = blind end of nephron; consists of a thin capsule of
epithelial tissue (Bowman's Capsule) surrounding a ball of capillaries
(glomerulus). Blood plasma filters from glomerular capillaries into Bowman's
capsule, thereby entering the renal tubule.
- Filtration Barrier = capillary endothelium (fenestrated) + relatively thick glomerular
basement membrane (functions in support of capillary network) + Bowman's
Capsule epithelium (simple squamous cells = podocytes). Filtration slits lie
between podocytes, but are closed by a slit membrane (diaphragm).
- Mesangial Cells = lie in extracellular matrix between glomerular capillaries,
serve a phagocytic function; also can expand or contract to locally modify filtration.

2. Proximal Convoluted Tubule = takes a winding path thru the cortex; leads
from Bowman's capsule to descending loop of Henle.
- To distinguish from distal convoluted tubule:
a) in c.s., tubules are more numerous since proximal tubule is longer
b) consists of larger, more triangular cells with spherical nucleus and indistinct lateral
borders (simple cuboidal epithelium)
c) microvilli are present on the lumenal surface (forms brush border)
- Proximal tubule is active in reabsorption of water, NaCl, glucose, amino acids,
vitamins, etc.

3. Loop of Henle = composed of:
a) thick descending portion, actually is a continuation of proximal tubule
b) thin descending portion
c) thin ascending portion
d) thick ascending portion, leads into distal convoluted tubule
- These loops extend variable distances into medulla, depending on position of renal
- Thick portion is lined by simple cuboidal, thin portion is lined by simple squamous.
- Functions to concentrate urine by removing water and NaCl

4. Distal Convoluted Tubule = takes a winding path to empty into collecting
- lined by cuboidal cells with relatively distinct lateral borders; lack brush border
- functions in sodium reabsorption
5. Collecting Ducts = lie mainly within medulla and medullary rays extending
into cortex
- lined by simple cuboidal/columnar cells with very distinct lateral borders
- main function is to reabsorb water



Countercurrent Multiplier System
1) Each region has a special permeability for water, ions, and/or urea
2) Interstitial spaces accumulate ions and urea to build up a concentration gradient that
increases with depth into the medulla
3) Parallel to nephrons lies a capillary network (vasa recta) which has freely permeable
walls, these capillaries passively participate in maintenance of the concentration
gradient in the interstitium by removal of reabsorbed water and NaCl


Composed of:
1) Modified smooth muscle cells of tunica media of afferent arteriole (Juxtaglomerular Cells)
2) Macula Densa = specialized region of nephron epithelial cells at the junction
of the ascending loop of Henle with the distal convoluted tubule; forms row of cells adjacent to glomerulus.
3) Mesangial cells that are closely associated with Juxtaglomerular cells = Lacis Cells - function is uncertain


- Afferent arteriole senses changes in blood pressure (blood volume)
- Macula densa detects changes in sodium concentration
- Decreased volume or decreased sodium concentration stimulates JG cells to release
renin which splits angiotensinogen (from liver) to form
angiotensin I
. Angiotensin I is converted to angiotensin II by converting
enzyme (from lung). Angiotensin II is the most powerful vasoconstrictor in the body;
causes an increase in blood pressure, but a decrease in blood flow thru glomerulus,
so glomerular filtration rate decreases, as does urine production.

Other Hormones Affecting Kidney Function:
1) Aldosterone = steroid hormone released from adrenal cortex; acts to increase
sodium reabsorption from distal convoluted tubule (Angiotensin II stimulates
release of aldosterone)
2) ADH = released from posterior pituitary; acts to increase water reabsorption by
increasing permeability of the collecting ducts to water


URETER = tube extending from renal pelvis to urinary bladder
1) Mucosa = transitional epithelium + lamina propria of elastic areolar CT;
longitudinal folds are present in mucosa; no submucosa is present
2) Muscularis = bundles of smooth muscle fibers with interspersed CT; Smooth
muscle arrangement is inner longitudinal, outer circular, and in lower end an
outermost longitudinal is also present; these muscle layers are not clearly defined
- Peristaltic contractions of the Muscularis move urine into bladder
3) Adventitia = Fibroelastic CT with blood vessels, lymphatics, nerves

URINARY BLADDER = sac-like structure with wall structure similar to ureter;
differences include:
1) thicker transitional epithelium
2) thick muscularis, same 3 poorly defined layers as in lower ureter; at neck of bladder
there is a smooth muscle coat that forms an involuntary internal sphincter
3) adventitia is covered by mesothelium = serosa

URETHRA = tube emptying bladder to exterior
- Male = relatively lengthy tube, shared with reproductive tract; epithelium
changes from transitional to stratified or pseudostratified columnar; lumen has an
irregular border - pits = Glands of Littre which produce mucus; surrounded at
base of penis by striated muscle = external sphincter, otherwise muscularis is
composed of 2 indistinct layers
- Female = much shorter tube, separate from reproductive tract; epithelium
changes from transitional to stratified or pseudostratified columnar; external sphincter
of skeletal muscle surrounds external urethral orifice

To Lecture 22