VT 106
Comparative Anatomy and Physiology
Urinary System
URINARY SYSTEM
kidneys – filter blood and form urine
ureters – carry urine from kidneys to bladder
urinary bladder – stores urine
urethra – voids urine to outside
FUNCTIONS OF URINARY SYSTEM
1) Filtration and Reabsorption of Blood – wastes are excreted and nutrients
are reabsorbed into blood
Excretion of Wastes – excreted in urine
nitrogenous wastes
ammonia and urea – main nitrogenous waste of mammals
uric acid – main nitrogenous waste of birds and reptiles
bilirubin – from hemoglobin
creatinine – from creatine phosphate
foreign drugs and toxins
Regulation of Blood Osmolarity, Ion Concentration, and pH
osmolarity = number of solute particles / liter of solution
ions (electrolyte balance) – Na+, K+, Ca+2, Cl-, phosphate
pH – H+, bicarbonate ions
water – maintains blood volume and pressure
2) Secretion of Hormones and Enzymes
calcitriol – increases absorption of dietary calcium
erythropoetin – stimulates RBC production
renin (enzyme) – activates RAA pathway
KIDNEY GROSS ANATOMY
paired retroperitoneal organs cushioned by perirenal fat
renal capsule – dense irregular CT on surface of kidney
renal hilus – medial indentation where vessels, nerves and ureter enter or exit
renal cortex – outer region of kidney
renal medulla – inner region of kidney, composed of renal pyramids
unipyramidal (1 pyramid) – cat, dog, horse
multipyramidal (multiple pyramids) – cattle, pig
renal papilla – apex of pyramid
drains urine into ureter or into funnel-shaped renal pelvis
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RENAL HISTOLOGY
NEPHRON – basic functional unit of kidney (100s of thousands/kidney)
renal corpuscle – site of filtration
renal tubule – collects filtered fluid, site of reabsorption & secretion
Blood Supply to Nephron
renal artery – carries blood to kidney
afferent arteriole – supplies one renal corpuscle
glomerulus – capillary bed in renal corpuscle
efferent arteriole – drains renal corpuscle
peritubular capillaries – surround renal tubules
renal vein – returns blood from kidney
Renal Corpuscle – found in cortex
1) glomerulus – capillary bed where fluid filters out
2) glomerular (Bowman’s) capsule – simple squamous epithelium
surrounding glomerulus
fluid (filtrate) from glomerulus collects in capsule
Renal Tubule – tubule modifies content of filtrate to form urine
1) proximal convoluted tubule (PCT) – twisted tube in cortex
closest to renal corpuscle
2) Loop of Henle
descending limb – enters medulla
ascending limb – returns to cortex
3) distal convoluted tubule (DCT) – twisted tube in cortex
farthest from renal corpuscle
Collecting Ducts – collect urine from renal tubules and carry it towards ureter
Filtrate Flow Summary
glomerular capsule ---> proximal convoluted tubule ---> descending Loop
of Henle ---> ascending Loop of Henle ---> distal convoluted tubule --->
collecting duct ---> (renal pelvis) ---> ureter
RENAL PHYSIOLOGY
3 Processes Involved in Production of Urine:
1) Glomerular Filtration – fluid filters out of blood, into glomerular capsule
filtrate – blood minus cells, platelets and large plasma proteins
contains water, nutrients, wastes, ions
2) Tubular Reabsorption – filtrate passes through renal tubule and water,
nutrients, and some ions are reabsorbed into the blood
3) Tubular Secretion – renal tubule cells secrete additional wastes from blood
into urine to be eliminate
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GLOMERULAR FILTRATION
glomerular blood pressure drives filtration
normally higher than in other capillary beds because diameter of afferent arteriole
is usually larger than diameter of efferent arteriole
capsular pressure – resistance in glomerular capsule that opposes filtration
normally low as long as urine outflow is not obstructed
Glomerular Filtration Rate (GFR) – total amount of filtrate formed/minute
usually maintained at a constant rate
high GFR – urine formed so quickly there is not time for reabsorption
(nutrients and water lost in urine)
low GFR – wastes filtered out slowly, slow movement allows too much reabsorption
(wastes accumulate in blood)
Regulating GFR – maintaining normal GFR despite fluctuations in arterial
blood pressure
1) Renal Autoregulation – nephrons respond to changes in blood pressure
decreased glomerular blood pressure (would decrease GFR)
reflex dilation of afferent arteriole = increased blood flow into
glomerulus = increased GBP = returns GFR to normal
(increased blood pressure = vasoconstriction of afferent arteriole)
2) Autonomic Regulation – sympathetic n.s. decreases blood flow to kidneys
causes vasoconstriction of afferent arterioles = decreased blood flow into
glomerulus = decreased GBP = decreased GFR
3) Hormonal Regulation
RAA pathway – triggered by low blood pressure in kidney
nephron – constriction of efferent arteriole = increased GBP =
increases GFR back to normal
aldosterone secretion – reabsorption of Na+ and water in kidney
increased blood volume = increased GBP = increased GFR
ADH = increases reabsorption of water by renal tubules
increases blood volume = increased GBP = increased GFR
TUBULAR REABSORPTION AND SECRETION
all nutrients, 99% of water, and many ions are normally reabsorbed into blood
additional wastes are secreted from blood into the urine
Proximal Convoluted Tubule (65% of water and solute reabsorption occurs here)
all organic nutrients are reabsorbed – glucose, amino acids, lipids
primary active transport drives most reabsorption
Na/K pumps pump Na out of tubule cells, back into blood
low Na in tubule cells causes Na to be reabsorbed from filtrate
water follows Na by osmosis and is also reabsorbed
less water in filtrate = concentration of other solutes increased
other solutes (eg. Cl, K, Ca) diffuse from filtrate into blood (reabsorbed)
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secondary active transport – Na gradient used to move other solutes
Na cotransport – nutrients and bicarbonate reabsorbed with Na
Na countertransport – H+ secreted into urine as Na is reabsorbed
Loop of Henle – 25% of reabsorption, but ascending limb is impermeable to water
countercurrent multiplication – forms a concentration gradient in the medulla
Na and Cl are reabsorbed without water following
high NaCl concentration in the medulla of the kidney
Distal Convoluted Tubule – hormones regulate amount of reabsorption here
PTH – target cells reabsorb Ca
aldosterone – target cells reabsorb Na and some water, and secrete K
Na-H+ countertransport – secretes variable amounts of H+ to maintain pH
reabsorbs bicarbonate – amount depends pH
Collecting Duct – same functions as DCT
most important for facultative reabsorption of water
WATER REABSORPTION
obligatory reabsorption – 85% of water reabsorption
occurs in PCT and loop of Henle
water always follows reabsorbed solutes by osmosis
facultative reabsorption – remaining 15% of water reabsorption
occurs in DCT and collecting duct
variable amounts of water reabsorbed depending on hydration
regulated by ADH
no ADH = no water channels = no water reabsorbed
large volume of dilute urine produced
more ADH = more water channels = more water reabsorption
smaller volume of concentrated urine produced
FLUID BALANCE
Water Loss – urine, perspiration, exhaled water vapor, feces
Water Gain – ingestion (drinking and eating)
metabolic water from aerobic respiration
Dehydration – water loss exceeds water gain = low blood pressure
body’s response to dehydration – less urine produced, thirst
more ADH secreted = small volume of concentrated urine produced
Overhydration – water gain exceeds water loss = high blood pressure
body’s response to overhydration – more urine produced
less ADH is secreted = large volume of dilute urine produced
diabetes insipidus – hyposecretion of ADH = polyuria / polydipsia
diuretics – drugs that inhibit reabsorption of water in kidneys
diuresis – production of large volumes of urine
used to treat hypertension (high BP) and edema
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URINE STORAGE AND ELIMINATION
Ureters – carry urine from kidneys to urinary bladder
run retroperitoneal and enter dorsal bladder
transitional epithelium – lines inner surface of ureters
longitudinal and circular layers of smooth muscle
peristalsis(waves of contraction)move urine down ureters to bladder
Urinary Bladder – collects and stores urine
balloon-shaped sac with a narrow neck attached to urethra
transitional epithelium – lines inner surface
detrusor muscle – 3 layers of smooth muscle in wall
contracts during urination to expel urine
internal sphincter normally contracted to hold urine
Urethra – carries urine from bladder to external urethral orifice
female urethra – short, opens into vestibule of vulva
urethral sphincter – skeletal muscle surrounding urethra at
pelvic outlet; allows voluntary control over urination
male urethra – carries urine and reproductive secretions
long; passes through prostate gland and penis
urethral sphincter at pelvic outlet
MICTURITION REFLEX – autonomic reflex
micturition = urination
stretch receptors in wall of bladder trigger a sacral spinal reflex
parasympathetic motor fibers stimulate contraction of detrusor muscle
and relaxation of internal sphincter
cerebral cortex perceives fullness – has voluntary control
skeletal muscles in urethral sphincter must be voluntarily relaxed
urinary incontinence – lack of voluntary control over urination
AVIAN URINARY SYSTEM
each kidney has 3 lobes
kidneys are located in recesses in dorsal body wall
ureters drain into the cloaca (common opening for digestive, urinary, and repro. tracts)
more water from urine can be reabsorbed by large intestine
urine is eliminated with feces through the vent
urine composition
uric acid – main nitrogenous waste in bird and reptile urine
forms a white paste – little water is used in its formation
adaptation for egg-laying species
urinary waste must be stored in the egg until hatching
volume and toxicity of uric acid are very low
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URINALYSIS – examining urine
Volume – well hydrated = larger volume
poorly hydrated = small volume
diabetes
Color – pale = dilute urine
dark = concentrated urine
blood, hemoglobin, myoglobin
Specific Gravity (density compared to water) – indicates solute concentration
low specific gravity = dilute urine
high specific gravity = concentrated urine
albuminuria (albumin) – glomerular damage, high BP
glucosuria (glucose) – diabetes mellitus
hemoglobinuria (hemoglobin) – intravascular hemolysis
hematuria (RBCs) – inflammation, uroliths, trauma
myoglobinuria (myoglobin) – myositis, muscle trauma
ketonuria (ketones) – diabetes, fasting
bilirubinuria (bilirubin) – liver disease
pyuria (WBCs) – infection
casts – clumps of material that block tubules
microbes – infection
BLOOD TESTS OF RENAL FUNCTION
uremia – build-up of urinary waste in blood from decreased renal function
BUN (blood urea nitrogen), creatinine
prerenal uremia – inadequate blood flow to kidneys
(heart failure, blood loss, dehydration)
renal uremia – kidney damage
2/3 of nephrons must be nonfunctional to see clinical signs
postrenal uremia – obstruction of urine outflow
(eg. uroliths, tumors)
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