Excretory System
Tony Serino, Ph.D.
Assistant Professor of Biology
Misericordia Univ.
Excretory
System
• Remove wastes from internal environment
• Wastes: water, heat, salts, urea, etc.
• Excretory organs include: Lungs, Skin, Liver, GI
tract, and Kidneys
• Urinary system account for bulk of excretion
Fluid Input & Output
Urinary System
Ureter Histolgy
-about 25 cm long, retroperitoneal, moves urine by peristalsis;
volume of urine moved is called a jet (1-5 jets/min)
-ureters enter the bladder wall obliquely, allowing them to
remain closed except during peristalsis
Adventitia
Mucosa
Muscularis
Urinary Bladder
(Remanent of Allantois)
Urinary Bladder Histology
Mucosa
Submucosa
Muscularis
(Detrusor Muscle)
(Serosa)
Urinary Bladder Filling
• Highly distensible
• 10-600ml normally
• Capable of 2-3X that
volume
• Under normal
conditions, the
pressure does not
significantly increase
until at least 300 ml
volume is reached
Urethra
Urethra Histology
-epithelium changes from transitional to stratified squamous along its length
-large numbers of mucous glands present
Bladder (Storage) Reflex
Voluntary control
• As urine accumulates, the
bladder wall thins and
rugae disappear
• Innervation (sympathetic)
to the sphincter muscles
(particularly the internal
sphincter) keeps the
bladder closed and
depresses bladder
contraction
Micturition Reflex (Voiding)
• Urine volume increases, and
the smooth muscle increases
pressure in bladder
• Stretch receptors in detrusor
muscle, increase
parasympathetic activity in
the splanchnic nerve cause
increase bladder contraction
and internal sphincter
relaxation
• Voluntary relaxation of
external sphincter by a
decrease in firing of the
pudendal nerve
Kidney Location (x.s.)
(Retroperitoneal)
Human Kidney
Hilus
Cortex vs. Medulla
Capsule
Anatomy of
Kidney
Major and Minor Calyx
Arterial Supply
Venous Drainage
Renal
Circulation
Renal Cortex Blood Flow
Glomerulus
BP in Renal Vessels
Nephron (two types)
Epithelium of Nephron
Urine Formation Overveiw
a.
b.
c.
d.
d
Pressure Filtration
Reabsorption
Secretion
Reabsorption of water
Glomerulus (SEM)
Glomerulus
Bowman’s
Capsule
Podocytes
Filtration in Glomerulus
Capillary Lumen
Endothelium
Basement Mem.
Pedicels
Slit pores
Glomerular Filtrate
Fenestration
Glomerular Filtration
• A pressure filtration produced by the BP, fenestrated
capillaries of glomerulus, and the podocytes creates
the glomerular filtrate
• Slit size allows filtration of any substance smaller
than a protein
• Blood proteins create an osmotic gradient to prevent
complete loss of water in blood,
• Pressure in Bowman’s capsule also works against
filtration
• Volume of filtrate produced per minute is the
Glomerular Filtration Rate (GFR)
• Average GFR = 120-125 ml/min
Forces controlling
Glomerular Filtration
PCT and DCT (H&E stain)
Tubular Reabsorption
• 75-85% of glomerular filtrate reabsorbed in PCT
• Some of the reabsorption is by passive diffusion
– Example: Na+
• Much of the reabsorption is active, most linked to
the transport of Na+; known as co-transport
• The amount of transporter proteins is limited; so
most actively transported substances have a
maximum tubular transport rate (Tm)
Reabsorption
Loop of Henle and CD
• Provides mechanism where water can be
conserved; capable of producing a low
volume, concentrated urine
• Loop of Henle acts as a counter-current
multiplier to maintain a high salt
concentration in medulla
• CD has variable water permeability and
must pass through the medulla
• Allows for the passive absorption of water
Counter-current Multiplier
• Descending is permeable to
water but not salt; loss of water
concentrates urine in tube
• Ascending is permeable to
NaCl but not water; Salt now
higher in tube than interstitium;
first passively diffuses out then
near top is actively transported
out
• Results in a self-perpetuating
mechanism; maintaining a high
salt concentration in center of
kidney
Vasa Recta
• Supply long loops of
Henle
• Provide mechanism to
prevent accumulation
of water in interstitial
space
• Passive diffusion
allows the blood to
equilibrate with
osmotic gradient in
extracellular space
Counter-current Exchange
Tubular Secretion
• PCT and DCT both actively involved in
secretion (active transport of substances
from the blood to the urine)
• Both ducts play important roles in
controlling amount of H+/HCO3- lost in
urine and therefore blood pH
• DCT actively controls Na+ reabsorption
upon stimulation by aldosterone (controls
final 2% of Na+ in urine)
Summary
Re-absorption
Water Re-absorption
with ADH present
Loses water
Loses NaCl
Selective Secretion & Re-absorption
Dehydration & Water Intoxication
Thirst
ADH release
Reabsorption of
Water in CD
Hypertonic, low
volume urine
Juxtaglomerular Apparatus
Renin-Angiotensin-Aldosterone
iBP
Decreased Stretch
in JG cells
Decreased Na in Urine in DCT
h Renin Release
h stimulation of Macula Densa
Angiotensinogen g Angiotensin I
h arteriolar constriction
Converting Enzyme
h BP
Angiotensin II
h Aldosterone Release
h Na+ reabsorption
h water retention and BV
Declining BP Regulation
Stimulates
thirst
Increase BP Regulation (ANP)
Acid/Base
Transport