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Chapter 26
An Overview of the Urinary System
▪The urinary system has three major functions:
1)
2)
3)
▪Composed of the kidneys, ureters, urinary bladder, and urethra
The Kidneys
▪Each kidney is supported and protected by three concentric layers of CT:
▫ fibrous capsule, perinephric fat, and renal fascia
▪Sectional Anatomy of the Kidneys
▫Renal sinus▫Renal cortex▫Renal medulla▫Renal pyramids▫Renal papilla1
▫Renal columns▫Renal lobes▫Minor calyx▫Major calyx▫Renal pelvis▫Nephrons-
▪Blood Supply and Innervation of the Kidneys
▫Kidneys receive 20-25% of total cardiac output, in a healthy adult roughly
1200 ml of blood flows through the kidneys each minute
▫Blood flow through the kidney follows this pattern:
Renal artery segmental arteries interlobar arteries arcuate
arteries cortical radiate arteries afferent arterioles
glomerulus efferent arteriole  peritubular capillaries 
venules cortical radiate veins arcuate veins interlobar veins
renal vein
▫Kidneys and ureters are innervated by renal nerves
-Most of the nerve fibers are sympathetic postganglionic fibers.
This innervation:
1)
2)
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▪The Nephron- the functional unit of the kidney, consists of a renal tubule and
renal corpuscle.
▫The renal tubule consists of a tubular passageway, about 50 mm long
▫The renal corpuscle consists of the glomerular capsule and the
glomerulus
▫Filtrate moves through the renal tubule, which is responsible for three
crucial functions:
1)
2)
3)
▫The renal tubule has two convoluted segments, proximal and distal,
separated by a simple U-shaped tube, the loop of Henle.
-Proximal Convoluted Tubule (PCT)-Nephron Loop-Distal Convoluted Tubule (DCT)▫Each nephron empties into the collecting system, a series of tubes that
carries tubular fluid away from the nephron.
▫Roughly 85% of all nephrons are cortical nephrons, located almost
entirely within the superficial cortex of the kidney
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▫The other 15% of nephrons are called juxtamedullary nephrons, their
loops of Henle extend deep into the medulla of the kidney.
▫The renal corpuscle contains the glomerular capsule and the glomerular
capillaries
-The outer wall of the capsule is lined by a simple squamous
parietal epithelium, which is continuous with the visceral epithelium
that covers the glomerular capillaries.
-The visceral epithelium consists of large cells containing podocytes
-Pedicles form filtration slits around the capillaries
-Mesangial cells- supporting cells between capillaries
-The glomerular capillaries are fenestrated capillaries
▫Proximal convoluted tubule- first segment of the renal tubule.
-Lining is simple cuboidal epithelium, apical surfaces contain
microvilli.
-These tubular cells absorb organic nutrients, ions, water, and
plasma proteins (if any) from the tubular fluid and release it into
the peritubular fluid
-Reabsorption is the primary function of the PCT, but secretion can
take place.
▫Nephron Loop- can be divided into the descending limb and ascending
limb
-Each limb contains a thick and thin segment, functions of loop of
Henle will be discussed in detail later.
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▫Distal convoluted tubule- third segment of the renal tubule, initial portion
passes between the afferent and efferent arterioles.
-Smaller in diameter than the PCT and lacks microvilli
-Three important processes take place at the DCT:
1)
2)
3)
-Juxtaglomerular complex (JGC)- formed by the cells of the
macula densa and JG cells.
-Macula densa-Juxtaglomerular (JG) cells-The JGC is an endocrine structure that secretes
erythropoietin and renin.
▫The collecting system- made up of collecting ducts and papillary ducts
-Individual nephrons drain into nearby collecting ducts, several
collecting ducts empty into larger papillary ducts, which empty into
the minor calyx
-Adjusts tubular fluid composition and determines the final osmotic
concentration and volume of the urine.
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Principles of Renal Physiology
▪The goal of urine production is to maintain homeostasis by regulating the
volume and composition of blood.
▪There are three major organic waste products; urea, creatinine, uric acid.
▪Basic Processes of Urine Formation
▫Filtration▫Reabsorption▫Secretion-Reabsorption and secretion involves a combination of diffusion,
osmosis, and carrier-mediated transport. There are four major
types of carrier-mediated transport:
-facilitated diffusion-active transport-cotransport-countertransport-All types of carrier-mediated transport share five features:
1) a specific substrate binds to a carrier protein that
facilitates movement across the membrane
2) a given carrier protein normally works in one direction
only
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3) the distribution of carrier proteins can vary from one
portion of the cell surface to another
4) the membrane of a single tubular cell contains many
types of carrier protein
5) carrier proteins, like enzymes, can be saturated.
-Tm- transport maximum-Renal threshold▪An Overview of Renal Function
*Use figure 26-9 to see the summary of renal function*
(the details of these processes will be discussed in greater detail later.)
▫Normal kidney function can continue only as long as filtration,
reabsorption, and secretion function within relatively narrow limits.
▫If both kidneys are affected, death will occur within a few days unless
medical attention is provided.
Renal Physiology: Filtration and the Glomerulus
▪Filtration occurs across a membrane that is composed of (1) capillary
endothelium, (2) lamina densa, and (3) the filtration slits…The filtration
membrane.
▪This filtration membrane blocks the passage of molecules larger than 6-9 nm in
size.
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▪Filtration pressures- similar to the pressures involved in solute movement at the
capillaries.
▫There is a balance between hydrostatic pressure and colloid osmotic
pressure on either side of the capillary walls.
▫Glomerular hydrostatic pressure (GHP)▫Capsular hydrostatic pressure (CsHP)▫Net hydrostatic pressure (NHP)▫Blood colloid osmotic pressure (BCOP)▫Net Filtration Pressure (NFP)▫NFP = NHP – BCOP
NFP = 35 mmHg – 25 mmHg
NFP = 10 mmHg
▪The glomerular filtration rate- the amount of filtrate produced by the kidneys
each minute.
▪Controlling the GFR- filtration depends on adequate blood flow to the
glomerulus and the on the maintenance of normal filtration pressures. Three
interacting levels of control stabilized GFR:
▫Autoregulation- maintains an adequate GFR despite changes in local
blood pressure and blood flow.
-Maintenance of the GFR is accomplished by changing the
diameters of the afferent arteriole, the efferent arterioles, and the
glomerular capillaries.
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-A reduction in blood flow and a decline in glomerular blood
pressure trigger (1) the dilation of the afferent arteriole, (2) the
relaxation of supporting cells and the dilation of the glomerular
capillaries, and (3) the constriction of the efferent arteriole.
-This combination increase blood flow and elevates
glomerular blood pressure to normal levels.
-An increase in systemic blood pressure will cause the afferent
arterioles to contract, decreasing the GFR, keeping it within normal
limits.
▫Hormonal regulation- the GFR is regulated by the hormones of the reninangiotensin system and the natriuretic peptides (ANP and BNP)
-Renin is an enzyme released by the JGA when (1) blood pressure
declines at the glomerulus, (2) juxtaglomerular cells are stimulated
by sympathetic innervation, and (3) there is a decline in the
osmotic concentration of the tubular fluid at the macula densa.
*see figure 26-11 to refamiliarize yourself with the renin-angiotensin system*
-ANP and BNP causes the dilation of the afferent arteriole and
constriction of the efferent arteriole, increases GFR, increasing fluid
loss, returning blood volume to normal.
▫Autonomic regulation- most of the autonomic innervation of the kidneys
consists of sympathetic postganglionic fibers
-Sympathetic activation has one direct effect on the GFR; it
produces a powerful vasoconstriction of the afferent arterioles,
decreasing the GFR and slowing the production of filtrate
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-Sympathetic activation can be triggered by a drastic loss in
blood pressure or a heart attack
Renal Physiology: Reabsorption and Secretion
▪Reabsorption and secretion at the PCT- the cells of the PCT normally reabsorb
60-70% of the volume of the filtrate produced in the renal corpuscle.
▫The reabsorbed materials enter the peritubular fluid and diffuse into
peritubular capillaries.
▫The PCT has 5 major functions:
1.
2.
3.
4.
5.
▫Sodium ions may enter the tubular cells by diffusion through leak
channels; by the sodium-linked cotransport of glucose, amino acids, or
other organic solutes; or by countertransport for hydrogen ions.
*see fig. 26-12 to familiarize yourself with the transport activities at the PTC*
▪The Nephron Loop and Countercurrent Multiplication
▫The nephron loop will reabsorb roughly half of the water and two thirds
of the sodium and chloride ions remaining in the tubular fluid.
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▫The thin descending limb and the thick ascending limb are very close
together
▫Countercurrent multiplication- exchange that occurs between these two
limbs.
▫The two parallel segments of the loop of Henle have very different
permeability characteristics.
-The thin descending limb-The thick ascending limb▫Countercurrent multiplication operates as follows:
-Na+ and Cl- are pumped out of the thick ascending limb and into
the peritubular fluid
-this pumping action elevates the osmotic concentration in the
peritubular fluid around the thin descending limb
-the result is an osmotic flow of water out of the thin descending
limb and into the peritubular fluid, increasing the solute
concentration in the thin descending limb.
-the arrival of highly concentrated solution in the thick ascending
limb accelerates the transport of Na+ and Cl- ions into the
peritubular fluid of the medulla.
*see fig. 26-13 a, b, and c to familiarize yourself with countercurrent multiplication*
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▫The maximum concentration gradient of the medulla is about 1200
mOsm/L
-about 750 mOsm/L is from Na and Cl ions, the rest, about 450
mOsm/L is due to the urea released at the papillary duct.
▫Benefits of countercurrent multiplication include:
-it efficiently reabsorbs solutes and water before the tubular fluid
reaches the DCT and collecting system.
-it establishes a concentration gradient that permits the passive
reabsorption of water from the tubular fluid in the collecting
system. This reabsorption is regulated by circulating levels of ADH.
▪Reabsorption and secretion at the DCT- selective reabsorption or secretion,
primarily along the DCT, makes the final adjustments in the solute composition
and volume of the tubular fluid.
▫Throughout most of the DCT, the tubular cells actively transport Na+ and
Cl- out of the tubular fluid. There are also exchange pumps that reabsorb
Na+ and secrete K+. Both of these transport mechanisms are controlled
by aldosterone.
▫The DCT is also the primary site of Ca2+ reabsorption, regulated by
circulating levels of PTH and calcitriol.
▫Any rise in undesirable materials in the peritubular fluid will cause tubular
cells to secrete the excess into the tubular fluid. K+ and H+ ions must be
maintained within relatively narrow limits.
-Potassium ions are exchanged for sodium ions and pumped into
the tubular fluid, see fig. 26-14 a and b.
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-Hydrogen ion secretion is also associated with the reabsorption of
sodium, see fig. 26-14 c for the summary of hydrogen ion
secretion.
-During acidosis, PCT and DCT will deaminate amino acids
which ties up H+ and yields NH2 and HCO3-.
-Provides carbon chains suitable for catabolism
-Generates bicarbonate ions that add to the buffering
capabilities of plasma.
▪Reabsorption and secretion along the collecting system
▫The collecting ducts receive tubular fluid from many nephrons and carry
it toward the renal sinus along the concentration gradient in the medulla,
regulated in two ways:
1) by aldosterone
2) by antidiuretic hormone (ADH)
▫Important examples of solute reabsorption in the collecting system
include:
-sodium ion reabsorption
-bicarbonate reabsorption
-urea reabsorption
▫Secretion of hydrogen ions or bicarbonate ions by the collecting system
plays an important role in regulated body fluid pH.
-If the pH of the peritubular fluid declines, carrier proteins pump
hydrogen ions into the tubular fluid and reabsorbs bicarbonate ions
that help restore normal pH
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-If the Ph of the peritubular fluid rises, the collecting system
secretes bicarbonate ions and pumps hydrogen ions into the
peritubular fluid.
▪The control of urine volume and osmotic concentration
▫Urine volume and osmotic concentration are regulated by controlling the
reabsorption of water.
-Obligatory water reabsorption-
-Facultative water reabsorption-
*see fig. 26-15 a and b to familiarize yourself with the affects of ADH on water
reabsorption*
▪The function of the vasa recta- to reabsorb water and solutes from peritubular
fluid and return them to the general circulation, this is accomplished by:
1)
2)
▫Under normal conditions, the removal of solutes and water by the vasa
recta precisely balances the rates of solute reabsorption and osmosis in
the medulla.
▪The composition of normal urine
▫Composition of urine refers to the identity and amounts of certain
compounds.
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▫Concentration of urine refers to the amount of water that is excreted with
these compounds.
▪Summary of renal function
*use fig. 26-16 to help summarize renal function*
Urine Transport, Storage, and Elimination
▪The ureters- a pair of muscular tubes that extend from the kidney to the urinary
bladder, about 30 cm in length.
▪The urinary bladder- a hollow muscular organ that functions as a temporary
reservoir for the storage of urine.
▫Trigone▫The wall of the urinary bladder contains mucosa, submucosa, and
muscularis layers
-Detrusor muscle▪The urethra- extends from the neck of the urinary bladder to the exterior of the
body, differences in male and female urethras
▫Male urethra can be divided into three regions (18-20 cm)
1)
2)
3)
▫Female urethra is quite short (3-5 cm), extends from the bladder to the
vestibule, external urethral meatus is near the anterior wall of the vagina.
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▫In both genders there is a circular band of skeletal muscle, the external
urethral sphincter, that acts as a valve.
▫The lining of the urethra varies from stratified transitional at the neck of
the urinary bladder to stratified columnar at the midpoint, to stratified
squamous near the external urethral meatus.
▪The micturation reflex and urination
▫As the bladder fills, stretch receptors are stimulated, which carry impulses
to the sacral spinal cord.
▫The increased level of activity in the fibers:
1)
2)
▫Micturation does not occur unless both the internal and external urethral
sphincters are relaxed.
▫Once the volume exceeds 500 mL, the micturation reflex may generate
enough pressure to force open the internal urethral sphincter, which
triggers a reflex that opens the external urethral sphincter, urination will
occur despite voluntary opposition.
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