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Physiology of the Urinary System
Functions of the Urinary System
Major Nitrogenous Wastes
Formation of Urine
Filtration
Reabsorption
Secretion
Functions of the Urinary System
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Remove waste products from blood
Maintain water balance
Maintain salt balance
Regulate blood pressure
Major Nitrogen-containing Wastes
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Urea: results from catabolism of amino acids
- amino acids => ammonia => urea
- protein is 16% nitrogen
- 100 g of protein => 16 g of waste nitrogen
- of these 16 g, 14 g is converted to urea
- most abundant nitrogenous waste product (21 g/day)
Ammonia salts: Minor component of urine. Of 16 g of
waste nitrogen, 2 g are converted to ammonia salt
Major Nitrogen-containing Wastes (cont.)
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Uric acid: results from breakdown of nucleic acids (RNA),
about 0.5 g/day
Creatine: generated in muscle tissue from breakdown of
creatine phosphate (1.8 g/day, depending on muscle mass)
Processes involved in Urine Formation
Three processes are involved:
 Filtration: forcing water and solutes across plasma
membrane (renal corpuscle). Selection by size.
 Reabsorption: Taking back water and important solutes
(nutrients, salts) back into the bloodstream. Very specific.
 Secretion: Transporting substances into urine. Specific.
Filtration
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All filtration occurs at the renal corpuscle
Recall that the corpuscle if formed from the glomerulus
(capillary) and Bowman’s capsule (continuous with the
tubules of the nephron)
DCT
efferent
arteriole
parietal epithelium
podocyte
macula densa
juxtaglomerular
cells
afferent
arteriole
glomerular
capillary
Process of Filtration
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Blood pressure forces water across the glomerular
endothelium, basement membrane, and filtration slits of
podocyte cells (visceral layer of Bowman’s capsule) into
the capsular space
filtration
Podocytes
filtration slits
fenestra
capsular space
endothelial wall
of capillary
basement membrane
Size Selection at Different Components of
Filtration Apparatus
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Fenestrated capillaries of glomerulus: pores allow water
and most solutes through (but NOT blood cells)
Basement membrane: permits smaller proteins, nutrients,
and ions through
Filtration slits of podocytes: prevent passage of most
proteins
- The filtrate contains dissolved ions and small organic
molecules, including nutrients.
- Filtration is selective for size only
Some Definitions regarding Filtration
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Renal Fraction: that part of the total cardiac output which
passes through the kidneys (about 20%)
The renal flow rate is 1.2 liters of blood per minute
Filtration Fraction: the amount of plasma going to the
kidney which is filtered and becomes filtrate
- on average, 20% of renal fraction
- blood is about 50% plasma, so the renal flow rate is
0.6 liters/min
- 0.6 liters/min x 20% = 0.12 ml filtrate/min
Some More Definitions regarding
Filtration
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Glomerular Filtration Rate: how much filtrate is produced
per minute (120 ml/min, or 170 liters/day)
- about 99% of this must be reabsorbed
- urine output = 1.7 liters
Driving Force of Filtration
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The filtration across membranes is driven by the net
filtration pressure
The net filtration pressure = net hydrostatic pressure minus
the net colloid osmotic pressure
The net hydrostatic pressure is determined by the
glomerular hydrostatic pressure (GHP) minus the capsular
hydrostatic pressure (CHP)
Hydrostatic Pressures
The GHP is the blood pressure in the glomerular capillaries
- tendency to push water and solutes out of plasma,
across membranes
- since efferent arteriole is smaller than afferent
arteriole, GHP is relatively high (50 mm Hg)
 The CHP is the resistance to flow along nephron tubules
and ducts
- tendency to push water and solutes out of filtrate, into
plasma
- CHP is normally low (15 mm Hg)
Thus, net hydrostatic pressure = 50 - 15 = 35 mm Hg

Colloid Osmotic Pressure (COP)
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The colloid osmotic pressure is the osmotic pressure
resulting from the presence of proteins in a solution
The COP of blood is about 25 mm Hg
The COP of filtrate is normally 0
Thus, total COP is 25 mm Hg
Net Filtration Pressure
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Thus, the net filtration pressure =
net hydrostatic pressure - colloid osmotic pressure
= 35 mm Hg - 25 mm Hg = 10 mm Hg
Abnormal changes in either net hydrostatic pressure or
colloid osmotic pressure will affect filtration rate
- damage to glomerulus will allow proteins into the
filtrate, decreasing net COP, and increasing filtration rate
- increasing capsular hydrostatic pressure (obstruction
of tubules, ducts) will markedly decrease net hydrostatic
pressure, decreasing filtration rate
Reabsorption
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Reabsorption takes place in the proximal convoluted tubule
(PCT; 65%), loop of Henle (20%), distal convoluted tubule
(DCT; 5%), and collecting ducts (10%)
In the PCT:
- Over 99% of organic nutrients (glucose, amino acids) are
resorbed
- Active ion resorption
- Water resorption by osmosis
- Other solvents (urea, lipids, Cl- ions) resorbed by solvent
drag
At the end of the PCT, filtrate contains no glucose, no
amino acids, 12% of NaCl, 25% of volume, and increased
urea, uric acid (no change in osmolarity)
Resorption in the Loop of Henle
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In the loop of Henle, half of the remaining water and 2/3rds
of the remaining NaCl will be resorbed
The loop of Henle utilizes a countercurrent multiplication
exchange system to reabsorb water and NaCl
Countercurrent: exchange occurs between fluids moving in
opposite directions
Multiplication: the effect of exchange between the limbs
increasing as fluid movement occurs
Countercurrent Exchange: Loop of Henle
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The walls of the descending and
ascending limbs have different
permeability characteristics
The descending limb is permeable to
water, impermeable to solutes
The ascending limb is impermeable to
water and solutes
Na+ and Cl- are actively pumped out
of ascending limb
Osmotic concentration of peritubular
fluid rises
Water leaves descending limb by
osmosis
Increased solute concentration causes
increased Na+ and Cl- transport
Results of Countercurrent Exchange
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Get resorption of water and NaCl, with filtrate at the end of
the loop of Henle with lower osmolarity than at the
beginning of the loop
A concentration gradient is built up in the peritubular space,
which allows subsequent resorption of water from
collecting duct
Reabsorption in the DCT and Collecting Ducts
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In the distal convoluted tubule, small adjustments in
composition of filtrate take place
- active transport of Na+ and Cl- continues
- water reabsorption occurs under influence of ADH
(5% of total water reabsorption)
In the collecting ducts, water and sodium are reabsorbed
- water is reabsorbed under regulation by ADH (10%
of water reabsorption)
- sodium reabsorbed under regulation by aldosterone
- some reabsorption of bicarbonate and urea
Secretion into Urinary Filtrate
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Secretion plays a relatively small role in production of
urine
Occurs in tubules and ducts
There is active secretion of a number of substances into the
filtrate:
- potassium, hydrogen ions (in exchange for sodium ions)
- creatinine, penicillin, neurotransmitters, organic acids and
bases
Summary
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Reviewed major nitrogenous wastes
Defined terminology related to filtration
Reviewed processes of filtration, absorption, and secretion
(mechanisms, site of action)
Next Lecture......
Regulation of the Urinary System