The Urinary System
The Urinary System
• Paired kidney
• A ureter for each kidney
• Urinary bladder
• Urethra
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Main Functions of Urinary System
• Kidneys filter blood to keep it pure
– Toxins, Metabolic wastes, Excess water
– Excess ions
• Dispose of nitrogenous wastes from blood
– Urea, Uric acid
– Creatinine
• Regulate the balance of water and electrolytes, acids
and bases
• The urinary system eliminates wastes; regulates
blood volume, ion concentration, and pH; and is
involved with red blood cell and vitamin D
production.
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Location and External Anatomy of the
Kidneys
1. A kidney lies behind the peritoneum on the
posterior abdominal wall on each side of the
vertebral column.
2. The renal capsule surrounds each kidney, and the
perirenal fat and the renal fascia surround each
kidney and anchor it to the abdominal wall.
3. The hilum, on the medial side of each kidney,
where blood vessels and nerves enter and exit the
kidney, opens into the renal sinus, containing fat
and connective tissue.
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Internal Anatomy and Histology of
the Kidneys
1. The two layers of the kidney are the cortex
and the medulla.
■ The renal columns extend toward the medulla
between the renal pyramids.
■ The renal pyramids of the medulla project to
the minor calyces.
2. The minor calyces open into the major
calyces, which open into the renal pelvis. The
renal pelvis leads to the ureter.
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3. The functional unit of the kidney is the
nephron. The parts of a nephron are the renal
corpuscle, the proximal convoluted tubule, the
loop of Henle, and the distal convoluted tubule.
■ The renal corpuscle is Bowman’s capsule and
the glomerulus. Materials leave the blood in the
glomerulus and enter Bowman’s capsule
through the filtration membrane.
■ The nephron empties through the distal
convoluted tubule into a collecting duct.
Uriniferous tubule (anatomical unit for forming urine)
Nephron
• Renal corpuscle
• Tubular section
Nephron
Renal corpuscle (in cortex)
Glomerulus (tuft of capillaries)
Glomerular (Bowman’s) capsule
Tubular section
Proximal convoluted tubule
Loop of Henle
Distal convoluted tubule
Collecting duct
(processes the filtrate)
– Proximal convoluted
tubule
– Loop of Henle
– Distal convoluted tubule
(ends by joining
collecting duct)
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4. The juxtaglomerular apparatus consists of the
macula dense (part of the distal convoluted
tubule) and the juxtaglomerular cells of the
afferent arteriole.
Arteries and Veins of the Kidneys
1. Arteries branch as follows: renal artery to
segmental artery
to interlobar artery to arcuate artery to interlobular
artery to afferent arteriole.
2. Afferent arterioles supply the glomeruli.
3. Efferent arteries from the glomeruli supply the
peritubular capillaries (vasa recta).
4. Veins form from the peritubular capillaries as
follows: interlobular vein to arcuate vein to
interlobar vein to renal vein.
Urine Production
• Urine is produced by the processes of
filtration, tubular reabsorption, and tubular
secretion
Filtration
1.The renal filtrate is plasma minus blood cells and blood
proteins. Most (99%) of the filtrate is reabsorbed.
2. The filtration membrane is fenestrated endothelium,
basement membrane, and the slit like pores formed by
podocytes.
3. Filtration pressure is responsible for filtrate formation.
■ Filtration pressure is glomerular capillary pressure minus
capsule pressure minus blood colloid osmotic pressure.
■ Filtration pressure changes are primarily caused by changes in
glomerular capillary pressure.
glomerular filtration rate (GFR
The volume of filtrate formed by both kidneys each minute is
called the glomerular filtration rate (GFR).
55 - (30 + 15) = 10
mmHg.
Tubular Reabsorption
1. Filtrate is reabsorbed by passive transport, including simple
diffusion, facilitated diffusion, active transport, and symport from
the nephron into the peritubular capillaries.
2. Specialization of tubule segments
• 
The thin segment of the loop of Henle is specialized for
passive transport.
• 
The rest of the nephron and collecting ducts perform active
transport, symport, and passive transport.
3. Substances transported
• 
Active transport moves mainly Na across the wall of the
nephron.
Other ions and molecules are moved primarily by symport.
• 
Passive transport moves water, urea, and lipid-soluble,
nonpolar compounds.
Tubular Secretion
1. Substances enter the proximal or distal
convoluted tubules and the collecting ducts.
2. Hydrogen ions, K , and some substances not
produced in the body are secreted by antiport
mechanisms.
Understand at least this much:
Filtration
a. Fluid is squeezed out of the
glomerular capillary bed
Resorption
b. Most nutrients, water ad
essential ions are returned
to the blood of the
peritubular capillaries
Secretion
c. Moves additional
undesirable molecules
into tubule from blood of
peritubular capillaries
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Production of urine
■ In the proximal convoluted tubule, Na and other
substances are removed by active transport. Water
follows passively, filtrate volume is reduced 65%, and the
filtrate concentration is 300 mOsm/L.
■ In the descending limb of the loop of Henle, water exits
passively and solute enters. The filtrate volume is reduced
15%, and the osmolality of the filtrate concentration is
1200 mOsm/kg.
■ In the ascending limb of the loop of Henle, Na , Cl , and
K are transported out of the filtrate, but water remains
because this segment of the nephron is impermeable to
water. The osmolality of the filtrate concentration is 100
mOsm/kg.
Hormonal Mechanisms
1. Aldosterone is produced in the adrenal cortex
and affects Na and Cl transport in the nephron and
collecting ducts.
■ A decrease in aldosterone results in less Na
reabsorption and an increase in urine concentration
and volume. An increase in aldosterone results in
greater Na reabsorption and a decrease in urine
concentration and volume.
■ Aldosterone production is stimulated by
angiotensin II, increased blood K concentration,
and decreased blood Na concentration.
Hormonal Mechanisms conti.
2.Renin, produced by the kidneys, causes the
production of angiotensin II.
■ Angiotensin II acts as a vasoconstrictor and
stimulates aldosterone secretion, causing a
decrease in urine production and an increase in
blood volume.
■ Decreased blood pressure or decreased Na
concentration stimulates renin production
Hormonal Mechanisms conti.
3.ADH is secreted by the posterior pituitary and
increases water permeability in the distal
convoluted tubules and collecting ducts.
■ ADH decreases urine volume, increases blood
volume, and thus increases blood pressure.
■ ADH release is stimulated by increased blood
osmolality or a decrease in blood pressure.
■ Water movement out of the distal convoluted
tubules and collecting ducts is regulated by ADH. If
ADH is absent, water is not reabsorbed and a dilute
urine is produced. If ADH is present, water moves
out and a concentrated urine is produced
Hormonal Mechanisms conti.
4. Atrial natriuretic hormone, produced by the
heart when blood pressure increases, inhibits
ADH production and reduces the kidney’s ability
to concentrate urine.
The Ureters
• Slender tubes about 25
cm (10 “) long leaving
each renal pelvis
• One for each kidney
carrying urine to the
bladder
• Descend
retroperitonealy and
cross pelvic brim
• Enter posterolateral
corners of bladder
• Run medially within
posterior bladder wall
before opening into
interior
• This oblique entry helps
prevent backflow of
urine
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The position of the ureter where it passes
through the bladder wall.
Three basic layers
• Transitional epithelium
of mucosa stretches
when ureters fill
• Muscularis
Ureters play an active role in
transporting urine (it’s not
just by gravity)
– Inner longitudinal, outer
circular layers
– Inferior 3rd with extra
longitudinal layer)
– Stimulated to contract
when urine in ureter:
peristaltic waves to
propel urine to bladder
• Adventitia (external)
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Urinary Bladder
See also brief atlas
• Collapsible muscular
sac
• Stores and expels
urine
• Lies on pelvic floor
posterior to pubic
symphysis
– Males: anterior to
rectum
– Females: just anterior
to the vagina and
uterus
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• If full: bladder is spherical and extends into
abdominal cavity (holds about 500 ml or 1 pt)
• If empty: bladder lies entirely within pelvis with
shape like upside-down pyramid
• Urine exits via the urethra
• Trigone is inside area between ureters and urethra:
prone to infection (see slide 38)
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Bladder wall has three layers (same as ureters)
– Mucosa with distensible transitional epithelium and
lamnia propria (can stretch)
– Thick muscularis called the detrusor muscle
• 3 layers of highly intermingled smooth muscle
• Squeezes urine out
– Fibrous adventitia
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The Urethra
• Smooth muscle with inner mucosa
– Changes from transitional through stages to stratified squamous near end
– Drains urine out of the bladder and body
• Male: about 20 cm (8”) long
• Female: 3-4 cm (1.5”) long
– Short length is why females have more urinary tract infections than males ascending bacteria from stool contamination
urethra
Urethra____
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• Urethral sphincters
– Internal: involuntary sphincter of smooth muscle
– External: skeletal muscle inhibits urination voluntarily until
proper time (levator anni muscle also helps voluntary
constriction)
Males: urethra has
three regions (see
right)
_________trigone
1. Prostatic urethra__________
2. Membranous urethra____
3. Spongy or penile urethra_____
female
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With all the labels
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• Micturition
AKA:
– Voiding
– Urinating
– Emptying the bladder
(See book for diagram
explanation p 701)
KNOW:
Micturition center of brain: pons
(but heavily influenced by higher
centers)
Parasympathetic: to void
Sympathetic: inhibits micturition
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