Urinary System
A. Kidneys
1. External anatomy
2. Internal anatomy
3. Nephron
a. Parts of the nephron
b. Cortical and juxtamedullary
nephrons
c. Histology of the nephron
4. Blood and nerve supply
B. Physiology of urine formation
1. Glomerular filtration
a. Net filtration pressure
b. Glomerular filtration rate
c. Regulation of GFR
2. Tubular reabsorption
3. Countercurrent multiplier and
exchange system
4. Tubular secretion
5. Renal clearance
C. Ureters
1. Structure
2. Histology
3. Physiology
D. Urinary bladder
1. Structure
2. Histology
3. Physiology
E. Urethra
1. Histology
2. Physiology
What is excretion?
How is it different from secretion?
Why do we need a urinary system?
1. protein catabolism yields toxic
nitrogenous wastes
(ammonia and urea)
2. water and essential ions tend to
accumulate
Organs that contribute to the function of
waste elimination from the body include:
1.
2.
3.
4.
kidneys
lungs
sudoriferous glands
gastrointestinal tract
The urinary system consists of:
1.
2.
3.
4.
kidneys (2)
ureters (2)
urinary bladder
urethra
The kidneys have several
general functions:
1. regulate blood volume and
composition
2. regulate blood pressure (renin)
3. secrete erythropoietin
4. vitamin D synthesis
Kidney-external
1. location
a. T12 - L3
b. retroperitoneal
2. gross appearance
3. hilus
4. coverings
a. renal fascia
b. adipose capsule
c. renal capsule
Kidney-internal
1. medulla
a. renal pyramids
b. renal papillae
2. cortex
a. renal columns
b. cortical vs
juxtamedullary nephrons
3. renal pelvis and calyces
Nephrons, the functional units of the
kidneys, have three basic functions:
1. filtration
2. reabsorption
3. secretion
Types of Nephrons
juxtamedullary nephron
(with very long loop of Henle)
1. cortical (85%)
2. juxtamedullary (15%)
cortical nephron
(with relatively short loop of Henle)
Parts of a Nephron
1. renal corpuscle
a. glomerulus
b. Bowman's capsule
(parietal vs visceral)
2. renal tubule
a. proximal convoluted
b. loop of Henle
c. distal convoluted
Bowman's capsule and the endothelialcapsular membrane
1. parietal vs. visceral layers
2. fenestrated endothelial cells
(restrict formed elements)
3. fused basement membranes
(restrict plasma proteins)
4. visceral layer = podocytes
a. pedicels
b. filtration slits
Renal Tubule Histology
1. PCT = simple cuboidal + brush border
2. loop of Henle = simple squamous
3. DCT = simple cuboidal + hormone receptors
4. Collecting Ducts= Simple cuboidal
Renal Blood Supply
1. ~25% of CO (1,250 ml/min)
via the renal arteries
segmental arteries 
interlobar arteries 
arcuate arteries 
2. afferent arteriole --->
glomerulus --->
efferent arteriole--->
3. peritubular capillaries
and vasa recta
Renal Blood Supply
1. ~25% of CO (1,250 ml/min)
via the renal arteries
segmental arteries 
interlobar arteries 
arcuate arteries 
2. afferent arteriole --->
glomerulus --->
efferent arteriole--->
3. peritubular capillaries
and vasa recta
Juxtaglomerular Apparatus
1. location
2. macula densa cells
3. juxtaglomerular cells
___________________
4. decreased BP =
renin secretion
5. vasoconstrictor substance
Filtrate formation high-increased
Filtrate formation low-decreased
Nerve supply of the kidneys
Renal nerves arise from the superior
mesenteric ganglion- mostly
sympathetic fibers
Regulate blood flow and stimulate
secretion of renin
Nephrons carry out three
important functions:
1. control blood concentration and
volume
2. regulate blood pH
3. remove toxic wastes from the blood
What do nephrons accomplish?
Urine formation requires three
processes:
1. glomerular filtration by the renal corpuscle
2. tubular reabsorption by the renal tubule
3. tubular secretion by the renal tubule
Glomerular filtration is the first
step in urine formation.
1.
2.
3.
4.
principle of filtration
endothelial-capsular membrane
blood hydrostatic pressure is the driving force
filtrate = all blood components except formed
elements and plasma proteins
5. filtrate = 180 L/day
urine = 1 - 2 L/day
filtrate reabsorbed = 178 - 179 L/day
Features of the renal corpuscle that
enhance its blood-filtering capacity:
1. glomerular capillaries are long
2. endothelial-capsular membrane is
porous and thin
3. glomerular blood hydrostatic
pressure is high due to relationship
between afferent and efferent arterioles
Net filtration pressure (NFP)
1. glomerular blood hydrostatic pr.
2. capsular hydrostatic pr.
3. blood colloid osmotic pr.
___________________________
4. NFP = GBHP - (CHP + BCOP)
= 60 - (18 + 32) or (15 + 27)
= +10 mm Hg
___________________________
5. filtration fraction = 10%
(1,250 ml/min x 0.1=
6. GFR = 125 ml/min)
Glomerular filtration rate (GFR)
1. GFR = 125 ml filtrate produced every
minute
2. GFR is directly related to the NFP.
3. Homeostasis requires a relatively
constant GFR:
a. if GFR is too high, then...
b. if GFR is too low, then...
Regulation of GFR
1. glomerular blood flow depends on 2 factors:
a. systemic blood pressure
b. afferent and efferent arteriolar diameters
2. factors regulated by three mechanisms
a. renal autoregulation
b. hormonal regulation
c. neural regulation
Renal Autoregulation
1. myogenic mechanism
Tendency of smooth muscle of afferent arteriole to contract
when arterial BP is high, relaxes when arterial BP is low
2. tubuloglomerular feedback
(vasoconstrictor substance)
Renal Autoregulation
MYOGENIC MECHANISM
increased blood pressure
TUBULOGLOMERULAR FEEDBACK
increased GFR
increased flow of tubular fluid
stretch of afferent arteriole
arteriolar vasoconstriction
decreased glomerular blood pressure
decreased NFP
macula densa cells secrete vasoconstrictor
substance onto juxtaglomerular cells
vasoconstriction of afferent arteriole
decreased glomerular blood pressure
decreased NFP
decreased filtration
decreased filtration
works the opposite way with decreased BP
works the opposite way with decreased GFR
Renin-Angiotensin System
Atrial Natriuretic Peptide (ANP)
When BP is too high, ANP is produced
by the heart which inhibits sodium and
water reaborption and the secretion of
renin and ADH. Kidneys eliminate
more sodium and water and lower
blood pressure
Neural Regulation of GFR
Sympathetic Nervous System
Tubular Reabsorption of Solutes
1. tubular maximum (TM)
2. active transport
a. receptors (carriers)
b. glucose and amino acids
(100% of the time)
c. Na+ and K+
(aldosterone controlled)
d. Ca++
(PTH and CT controlled)
3. passive transport (diffusion)
a. diffusion or electrical
b. Cl-, PO4--
Tubular Reabsorption of Water
1. obligatory reabsorption (90%)
a. osmosis related to solutes
b. Na+ has major effects
2. facultative reabsorption (10%)
(controlled by ADH)
Negative Feedback Control of ADH
CONTROLLED CONDITION
Blood osmotic pressure (decreased water
concentration) is increased in response to
some stressor
RETURN TO HOMEOSTASIS
In response, there is increased water
reabsorption, and blood osmotic pressure
decreases
RECEPTOR
Hypothalamic osmoreceptors respond to
increased blood osmotic pressure and
send nerve impulses to appropriate
neurons in hypothalamus
EFFECTORS
In response to ADH, aquaporins in distal
tubules and collecting ducts become more
permeable to water
CONTROL CENTER
Hypothalamic neurons, via the posterior
pituitary gland, secrete ADH in the blood
The concentration gradient in
interstitial fluid
Contributing factors:
1. The nephron loop via the countercurrent multiplier
2. The Vasa recta via the countercurrent exchange
system
3. Urea Recycling
The countercurrent multiplier-Responsible for
establishing the salt concentration gradient with the
interstitial fluid.
Countercurrent- tubular fluid reversing directions
Multiplier- positive feedback loop that increases
concentration of salts.
Blood flows in opposite direction to nephron loop
Exchanges salts for water
Countercurrent Exchange System
(Loop of Henle and Vasa Recta)
Blood flows in opposite directions
To Loop of Henle.
As blood flows down alongside
Ascending loop of Henle, water
is lost and salt gained.
As blood flows up alongside
Descending loop of Henle, water
is gained and salt lost.
Overall, vessels absorb more
water on the way out than they
Gave up on the way in.
Osmolarity of medulla maintained
And water and solutes maintained.
The Loop of Henle acts as a Countercurrent multiplier
Tubular secretion
1. cells secrete directly into filtrate
2. two principal effects
a. directly excrete substances
(H+, NH3, K+, creatinine)
b. control body fluid pH
(H+, NH4+)
If H+ secretion = HCO3- filtration then no change occurs in extracellular fluid pH.
If H+ secretion < HCO3- filtration, then extracellular fluid(blood) pH
decreases.
If H+ secretion > HCO3- filtration, then extracellular fluid (blood)pH
increases.
Secretion and Neutralization of
H+ ions in Kidney
Renal Clearance
Renal clearance is the volume of blood
plasma from which a particular waste is
completely removed in 1 minute.
What would you expect the renal
clearance of glucose to be in a normal
individual?
Summary of Nephron Functions
Urine Drainage
1. papillary ducts --->
minor calyces --->
major calyces --->
renal pelvis --->
2. ureters
a. location
b. anatomy
c. function
Urinary Bladder
1. location
2. anatomy
3. histology
a. detrusor muscle
b. sphincters
4. micturition reflex
Urethra
1. anatomy
2. location
a. female
b. male
(1) prostatic
(2) membranous
(3) penile
Micturition reflex (in the adult)
1. stretch of the bladder (1/2 full) stimulates stretch
receptors
2. sacral parasympathetic area output, causing:
a. contraction of the detrusor muscle; and
b. reflex relaxation of the internal urethral sphincter
3. association neuron carries information to cerebrum
for conscious awareness
4. voluntary relaxation of the external urethral
sphincter
(in the infant, this is also reflexive)
5. expulsion of urine through urethra
Neural Control of Micturition