1. Copyright © 2011 Pearson Education, Inc.
Urinary System II Overview
 Nephron filtration
Urine composition
Organs of the urinary system
Ureters, urinary bladder, urethra
Urination

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Kidney Physiology:
Mechanisms of Urine Formation
• The kidneys filter entire plasma volume
• ~60 times each day
• Filtrate
• Blood plasma minus large proteins
• Urine
• <1% of total filtrate
• Contains metabolic wastes and unneeded
substances

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Mechanisms of Urine Formation
1. Glomerular filtration
2. Tubular reabsorption
• Returns glucose and amino acids,
• 99% of water and electrolytes
1. Tubular secretion
• Reverse of reabsoprtion
• selective addition to urine

4. Copyright © 2011 Pearson Education, Inc. Figure 24.10
Cortical
radiate
artery
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Glomerular capsule
Rest of renal tubule
containing filtrate
Peritubular
capillary
To cortical radiate vein
Urine
Glomerular filtration
Tubular reabsorption
Tubular secretion
Three major
renal processes:
Mechanisms of Urine Formation

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Glomerular Filtration
• Filters large particles from plasma
• RBC, WBC and large proteins are too big
• Everything else comes through fenestrated
capillaries
• Efficient filter
• Larger molecules are not filtered
• Hormones help regulate filtration rate

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Osmosis

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Tubular Reabsorption
• Selectively returns molecules to blood
• Microvilli increase absorption
No fenestrated capillaries
• Material travels through endothelial cells
• Glucose and other needed components
• transcellular
lumen
blood

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Tubular Reabsoption
1. PCT (proximal convoluted tubule)
• Site of most reabsorption
• Glucose & amino acids
• 65% of Na+
and water
• Sodium (Na+) reabsorption via
sodium/potassium pump
• Generates energy for other transports

9. Copyright © 2011 Pearson Education, Inc. Figure 25.18a
Cortex
Outer
medulla
Inner
medulla
(a)
(b)
(c)
(e)
(d)
Na+
(65%)
Glucose
Amino acids
H2O (65%) and
many ions (e.g.
Cl–
and K+
)
300
Milliosmols
600
1200
Blood pH regulation
H+
,
NH4
+
HCO3
–
Some
drugs
Active transport
(primary or
secondary)
Passive transport
(a) Proximal convoluted tubule:
• 65% of filtrate volume reabsorbed
• Na+
, glucose, amino acids, and other nutrients actively
transported; H2O and many ions follow passively
• H+
and NH4
+
secretion and HCO3
–
reabsorption to
maintain blood pH (see Chapter 26)
• Some drugs are secreted

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Tubular Reabsoption
1. Loop of Henle
• Extends into kidney medulla
• Descending limb:
• Permeable to H2O
• Ascending limb:
• Permeable to Na+
, K+
, Cl−

11. Copyright © 2011 Pearson Education, Inc. Figure 25.18b
H2O
(b) Descending limb of loop
of Henle
• Freely permeable to H2O
• Not permeable to NaCl
• Filtrate becomes increasingly
concentrated as H2O leaves by
osmosis
(a)
(b)
(c)
(e)
(d)
Cortex
Outer
medulla
Inner
medulla
300
Milliosmols
600
1200
Active transport
(primary or
secondary)
Passive transport

12. Copyright © 2011 Pearson Education, Inc. Figure 25.18c
Na+
Urea
Cl–
Na+
Cl–
K+
(c) Ascending limb of loop of Henle
• Impermeable to H2O
• Permeable to NaCl
• Filtrate becomes
increasingly dilute as salt is
reabsorbed
(a)
(b)
(c)
(e)
(d)
Cortex
Outer
medulla
Inner
medulla
300
Milliosmols
600
1200
Active transport
(primary or
secondary)
Passive transport

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Tubular Reabsoption
3. DCT (distal convoluted tubule) and
collecting duct
• Reabsorption is hormonally regulated
• ADH (antidiuretic hormone)
• Can reclaim all water if needed
• ADH needed to reclaim water

14. Copyright © 2011 Pearson Education, Inc. Figure 25.18d
Na+
; aldosterone-regulated
Ca2+
; PTH-regulated
Cl–
; follows Na+
(d) Distal convoluted tubule
• Na+
and Ca2+
reabsortion regulated by
hormones
• Cl–
cotransported with Na+
(a)
(b)
(c)
(e)
(d)
Cortex
Outer
medulla
Inner
medulla
300
Milliosmols
600
1200
Active transport
(primary or
secondary)
Passive transport

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Tubular Secretion
• Reabsorption in reverse
• Eliminates molecules from plasma that
passively reabsorbed
• urea and uric acid
• Potassium (K+)
• Controls blood pH
• altering [H+
] or [HCO3
–
]

16. Copyright © 2011 Pearson Education, Inc. Figure 25.16a
Loop of Henle
Osmolality
of interstitial
fluid
(mOsm)
Inner
medulla
Outer
medulla
CortexActive transport
Passive transport
Water impermeable
The ascending limb:
• Impermeable to H2O
• Permeable to NaCl
Filtrate becomes increasingly dilute as NaCl leaves, eventually becoming
hypotonic to blood.
Filtrate entering the
loop of Henle is
isotonic
The descending limb:
• Permeable to H2O
• Impermeable to NaCl
As filtrate flows, it
becomes increasingly
concentrated as H2O
leaves the tubule by
osmosis.
H2O
H2O
H2O
H2O
H2O
H2O
H2O
NaCI
NaCI
NaCI
NaCI
NaCI

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Hormones
• Antidiuretics hormone (ADH) creates
concentrated urine
• Allows water to leave DCT
• Transports urea into collecting ducts
• Aldosterone
• Made in adrenal glands
• Reabsorption of Na+
• Secretion of K+

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Diuretics
• Chemicals that enhance urination
• Osmotic diuretics
• Substances not reabsorbed
• Causes large water output
• ex. high glucose diabetic
• ADH inhibitors: alcohol
• Substances inhibit Na+
reabsorption
• ex. caffeine and many drugs

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Characteristics of Urine
• Color and transparency
• Pale to deep yellow
• Urochrome – hemoglobin breakdown
• diet can alter color
• Cloudy urine may indicate infection

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Physical Characteristics of Urine
• pH
• Slightly acidic, ~pH 6,
• Odor
• Slightly aromatic
• Ammonia odor may develop
• Altered by drugs

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Chemical Composition of Urine
• 95% water and 5% solutes
• Nitrogenous wastes:
• urea, uric acid, and creatinine
• Other molecules (electrolytes)

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Diabetes Mellitus
• “pass through sweet” – lots of sugary urine
produced
• Cells can not take up excess glucose
• Glucose accumulates in the blood stream
• Kidneys filter out glucose, ends up in urine
• Attempts to dilute it yields large output volume.

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ORGANS OF THE URINARY
SYSTEM
Ureters, urinary bladder, urethra

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Ureters
• Connect kidneys to bladder
• Carry urine
• Enter the base of the bladder
• As bladder fills ureters close, prevents
backflow

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Ureters
Three layers of wall of ureter
1. Lining of transitional
epithelium
2. Smooth muscle muscularis
• Contracts when
stretched
1. Outer adventitia of fibrous
connective tissue

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Renal Calculi “Kidney Stones”
• Kidney stones:
• Crystallized calcium, magnesium, or uric acid
salts
• Form in renal pelvis
• Large stones block uretes
• cause backup and pain in kidneys

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Urinary Bladder
• Muscular sac for urine

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Urinary Bladder
• Layers of the bladder wall
1. Transitional epithelial mucosa
2. Thick detrusor muscle
• three layers of smooth muscle
1. Fibrous adventitia
• peritoneum on superior surface only

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Urinary Bladder
• Trigone – formed by ureters and urethra
• Common infection site
• Rugae – ridges or folds
• Allows bladder expantion

30. Copyright © 2011 Pearson Education, Inc. Figure 25.21b
Ureter
Trigone
Peritoneum
Rugae
Detrusor
muscle
Bladder neck
Internal urethral
sphincter
External urethral
sphincter
Urogenital diaphragm
Urethra
External urethral
orifice
Ureteric orifices
(b) Female.
Urinary Bladder

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Urinary Bladder
• Males—prostate gland surrounds
the neck inferiorly
• Females—anterior to the vagina
and uterus

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Urethra
• Muscular tube drains urine from the bladder
• Mostly pseudostratified columnar epithelium,
• Varies between males and females

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Urethra
• Sphincters
• Internal urethral sphincter
• Involuntary (smooth muscle)
• at bladder-urethra junction
• Contracts to open
• External urethral sphincter
• Voluntary (skeletal) muscle
• surrounds urethra at pelvic floor

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Urethra (Female)
• Female urethra (3–4 cm):
• Bound to anterior vaginal wall
• External urethral orifice –
• anterior to the vaginal opening, posterior to
the clitoris

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Urethra (Male)
• Male urethra
• Carries semen and urine
• Three named regions
1. Prostatic urethra (2.5 cm) — within prostate gland
2. Membranous urethra (2 cm) — passes through
urogenital diaphragm
3. Spongy urethra (15 cm) — passes through penis

36. Copyright © 2011 Pearson Education, Inc. Figure 25.21a
Ureter
Trigone of bladder
Prostate
Membranous urethra
Prostatic urethra
Peritoneum
Rugae
Detrusor muscle
Bladder neck
Internal urethral sphincter
External urethral sphincter
Urogenital diaphragm
Spongy urethra
Erectile tissue of penis
Ureteric orifices
Adventitia
(a) Male. The long male urethra has three
regions: prostatic, membranous and spongy.
External urethral orifice

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Urination
• a.k.a.: urination, mitroturition or voiding”
• Three simultaneous events
1. Contraction of detrusor muscle (involuntary)
2. Opening of internal urethral sphincter
(involuntary)
3. Opening of external urethral sphincter
(voluntary)

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Urination
• Urinary retention – in men a sign of BPH or
prostate cancer.
• BPH – benign prostate hyperplasia
• Prostate growth
• Catheters are inserted to allow drainage of
the urinary bladder