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Chapter 26
The Urinary System
Lecture Outline
Principles of Human Anatomy and Physiology, 11e
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INTRODUCTION
• The urinary system consists of two kidneys, two ureters, one
urinary bladder, and one urethra (Figure 26.1).
• Urine is excreted from each kidney through its ureter and is
stored in the urinary bladder until it is expelled from the body
through the urethra.
• The specialized branch of medicine that deals with
structure, function, and diseases of the male and female
urinary systems and the male reproductive system is known
as nephrology. The branch of surgery related to male and
female urinary systems and the male reproductive system is
called urology.
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Chapter 26
The Urinary System
• Kidneys, ureters, urinary
bladder & urethra
• Urine flows from each
kidney, down its ureter to the
bladder and to the outside
via the urethra
• Filter the blood and return
most of water and solutes to
the bloodstream
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Overview of Kidney Functions
• Regulation of blood ionic composition
– Na+, K+, Ca+2, Cl- and phosphate ions
• Regulation of blood pH, osmolarity & glucose
• Regulation of blood volume
– conserving or eliminating water
• Regulation of blood pressure
– secreting the enzyme renin
– adjusting renal resistance
• Release of erythropoietin & calcitriol
• Excretion of wastes & foreign substances
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ANATOMY AND HISTOLOGY OF THE KIDNEYS
• The paired kidneys are retroperitoneal organs (Figure 26.2).
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External Anatomy of the Kidney
• Near the center of the concave medial border of the kidney
is a vertical fissure called the hilus, through which the ureter
leaves and blood vessels, lymphatic vessels, and nerves
enter and exit (Figure 26.3).
• Three layers of tissue surround each kidney: the innermost
renal capsule, the adipose capsule, and the outer renal
fascia.
• Nephroptosis is an inferior displacement of the kidneys. It
most often occurs in thin people. This condition is
dangerous because the ureters may kink and block urine
flow (Clinical Application).
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External Anatomy of Kidney
• Paired kidney-bean-shaped organ
• 4-5 in long, 2-3 in wide,
1 in thick
• Found just above the waist
between the peritoneum &
posterior wall of abdomen
– retroperitoneal (along with
adrenal glands & ureters)
• Protected by 11th & 12th ribs with
right kidney lower
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External Anatomy of Kidney
•
•
•
•
Blood vessels & ureter enter hilus of kidney
Renal capsule = transparent membrane maintains organ shape
Adipose capsule that helps protect from trauma
Renal fascia = dense, irregular connective tissue that holds against back
body wall
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External Anatomy of Kidney
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Internal Anatomy of the Kidney
• Internally, the kidneys consist of cortex, medulla, pyramids,
papillae, columns, calyces, and pelves (Figure 26.3).
• The renal cortex and renal pyramids constitute the
functional portion or parenchyma of the kidney.
• The nephron is the functional unit of the kidney.
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Internal Anatomy of the Kidneys
• Parenchyma of kidney
– renal cortex = superficial layer of kidney
– renal medulla
• inner portion consisting of 8-18 cone-shaped
renal pyramids separated by renal columns
• renal papilla point toward center of kidney
• Drainage system fills renal sinus cavity
– cuplike structure (minor calyces) collect urine from
the papillary ducts of the papilla
– minor & major calyces empty into the renal pelvis
which empties into the ureter
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Internal Anatomy of
Kidney
• What is the difference between renal
hilus & renal sinus?
• Outline a major calyx & the border
between cortex & medulla.
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Blood and Nerve Supply of the Kidneys
• Blood enters the kidney through the renal artery and exits
via the renal vein.
– Figures 26.4 and 26.5 show the branching pattern of
renal blood vessels and the path of blood flow through
the kidneys.
• In a kidney transplant a donor kidney is placed in the pelvis
of the recipient through an abdominal incision. The renal
artery, renal vein, and ureter of the donor kidney are
connected to the corresponding structure in the recipient.
The patient is then placed on immunosuppressive drugs to
prevent rejection of the transplanted kidney.
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Blood & Nerve Supply of Kidney
• Abundantly supplied with blood vessels
– receive 25% of resting cardiac output via renal arteries
• Functions of different capillary beds
– glomerular capillaries where filtration of blood occurs
• vasoconstriction & vasodilation of afferent & efferent
arterioles produce large changes in renal filtration
– peritubular capillaries that carry away reabsorbed
substances from filtrate
– vasa recta supplies nutrients to medulla without disrupting
its osmolarity form
• The nerve supply to the kidney is derived from the renal
plexus (sympathetic division of ANS). Sympathetic
vasomotor nerves regulate blood flow & renal resistance by
altering arterioles
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Nephrons
• A nephron consists of a renal corpuscle where fluid is
filtered, and a renal tubule into which the filtered fluid
passes (Figure 26.5).
• Nephrons perform three basic functions: glomerular
filtration, tubular reabsorption, and tubular secretion.
• A renal tubule consists of a proximal convoluted tubule
(PCT), loop of Henle (nephron loop), and distal convoluted
tubule (DCT).
• Distal convoluted tubules of several nephrons drain into to a
single collecting duct and many collecting ducts drain into a
small number of papillary ducts.
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Blood Vessels around the Nephron
• Glomerular capillaries are formed between the afferent &
efferent arterioles
• Efferent arterioles give rise to the peritubular capillaries and
vasa recta
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Nephrons
• The loop of Henle consists of a descending limb, a thin
ascending limb, and a thick ascending limb (Figure 26.5).
• There are two types of nephrons that have differing
structure and function.
– A cortical nephron usually has its glomerulus in the outer
portion of the cortex and a short loop of Henle that
penetrates only into the outer region of the medulla
(Figure 26.5a).
– A juxtamedullary nephron usually has its glomerulus
deep in the cortex close to the medulla; its long loop of
Henle stretches through the medulla and almost reaches
the renal papilla (Figure 26.5b).
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Blood Supply to the Nephron
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The
• Kidney has over 1 million nephrons composed
of a corpuscle and tubule
Nephron
• Renal corpuscle = site of plasma filtration
– glomerulus is capillaries where filtration
occurs
– glomerular (Bowman’s) capsule is doublewalled epithelial cup that collects filtrate
• Renal tubule
– proximal convoluted tubule
– loop of Henle dips down into medulla
– distal convoluted tubule
• Collecting ducts and papillary ducts drain urine
to the renal pelvis and ureter.
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Cortical Nephron
• 80-85% of nephrons are cortical nephrons
• Renal corpuscles are in outer cortex and loops of Henle lie
mainly in cortex
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Juxtamedullary
Nephron
• 15-20% of nephrons are juxtamedullary nephrons
• Renal corpuscles close to medulla and long loops of Henle extend into
deepest medulla enabling excretion of dilute or concentrated urine
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Histology of the Nephron and Collecting Duct
• Glomerular Capsule
– The glomerular capsule consists of visceral and parietal
layers (Figure 26.6).
– The visceral layer consists of modified simple squamous
epithelial cells called podocytes.
– The parietal layer consists of simple squamous
epithelium and forms the outer wall of the capsule.
• Fluid filtered from the glomerular capillaries enters the
capsular space, the space between the two layers of the
glomerular capsule.
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Histology of the Nephron & Collecting Duct
• Single layer of epithelial
cells forms walls of entire
tube
• Distinctive features due to
function of each region
– microvilli
– cuboidal versus simple
– hormone receptors
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Renal Tubule and Collecting Duct
• Table 26.1 illustrates the histology of the cells that form the
renal tubule and collecting duct.
• The juxtaglomerular apparatus (JGA) consists of the
juxtaglomerular cells of an afferent arteriole and the macula
densa. The JGA helps regulate blood pressure and the rate
of blood filtration by the kidneys (Figure 26.6).
• Most of the cells of the distal convoluted tubule are principal
cells that have receptors for ADH and aldosterone. A smaller
number are intercalated cells which play a role in the
homeostasis of blood pH.
• The number of nephrons is constant from birth. They may
increase in size, but not in number (Clinical Application).
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Structure of Renal Corpuscle
• Bowman’s capsule surrounds capsular space
– podocytes cover capillaries to form visceral layer
– simple squamous cells form parietal layer of capsule
• Glomerular capillaries arise from afferent arteriole & form a ball before
emptying into efferent arteriole
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Histology of Renal Tubule & Collecting Duct
• Proximal convoluted tubule
– simple cuboidal with brush border of microvilli
that increase surface area
• Descending limb of loop of Henle
– simple squamous
• Ascending limb of loop of Henle
– simple cuboidal to low columnar
– forms juxtaglomerular apparatus where makes
contact with afferent arteriole
• macula densa is special part of ascending
limb
• Distal convoluted & collecting ducts
– simple cuboidal composed of principal &
intercalated cells which have microvilli
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Juxtaglomerular Apparatus
• Structure where afferent arteriole makes contact with ascending limb of
loop of Henle
– macula densa is thickened part of ascending limb
– juxtaglomerular cells are modified muscle cells in arteriole
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Number of Nephrons
• Remains constant from birth
– any increase in size of kidney is size increase of
individual nephrons
• If injured, no replacement occurs
• Dysfunction is not evident until function declines by
25% of normal (other nephrons handle the extra
work)
• Removal of one kidney causes enlargement of the
remaining until it can filter at 80% of normal rate of 2
kidneys
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OVERVIEW OF RENAL PHYSIOLOGY
• Nephrons and collecting ducts perform three basic
processes while producing urine: glomerular filtration,
tubular secretion, and tubular reabsorption (Figure 26.7).
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Overview of Renal Physiology
• Nephrons and collecting ducts perform 3 basic processes
– glomerular filtration
• a portion of the blood plasma is filtered into the
kidney
– tubular reabsorption
• water & useful substances are reabsorbed into the
blood
– tubular secretion
• wastes are removed from the blood & secreted into
urine
• Rate of excretion of any substance is its rate of
filtration, plus its rate of secretion, minus its rate of
reabsorption
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Overview of Renal Physiology
• Glomerular filtration of plasma
• Tubular reabsorption
• Tubular secretion
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GLOMERULAR FILTRATION
• The fluid that enters the capsular space is termed
glomerular filtrate.
• The fraction of plasma in the afferent arterioles of the
kidneys that becomes filtrate is termed the filtration fraction.
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Glomerular Filtration
• Blood pressure produces glomerular filtrate
• Filtration fraction is 20% of plasma
• 48 Gallons/day
filtrate reabsorbed
to 1-2 qt. urine
• Filtering capacity
enhanced by:
– thinness of membrane
& large surface area of
glomerular capillaries
– glomerular capillary BP is high due to small size of
efferent arteriole
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The Filtration Membrane
• The filtering unit of a nephron is the endothelial-capsular
membrane.
– glomerular endothelium
– glomerular basement membrane
– slit membranes between pedicels of podocytes.
• Filtered substances move from the blood stream through
three barriers: a glomerular endothelial cell, the basal
lamina, and a filtration slit formed by a podocyte (Figure
26.8).
• The principle of filtration - to force fluids and solutes through
a membrane by pressure - is the same in glomerular
capillaries as in capillaries elsewhere in the body.
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Filtration Membrane
• #1 Stops all cells and platelets
• #2 Stops large plasma proteins
• #3 Stops medium-sized proteins, not small ones
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Net Filtration Pressure
• NFP = total pressure that promotes filtration
• NFP = GBHP - (CHP + BCOP) = 10mm Hg
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Net Filtration Pressure
• Glomerular filtration depends on three main pressures, one
that promotes and two that oppose filtration (Figure 26.9).
• Filtration of blood is promoted by glomerular blood
hydrostatic pressure (BGHP) and opposed by capsular
hydrostatic pressure (CHP) and blood colloid osmotic
pressure (BCOP).
– The net filtration pressure (NFP) is about 10 mm Hg.
• In some kidney diseases, damaged glomerular capillaries
become so permeable that plasma proteins enter the filtrate,
causing an increase in NFP and GFR and a decrease in
BCOP. (Clinical Application)
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Glomerular Filtration Rate
• Amount of filtrate formed in all renal corpuscles of both
kidneys / minute
– average adult male rate is 125 mL/min
• Homeostasis requires GFR that is constant
– too high & useful substances are lost due to the speed of
fluid passage through nephron
– too low and sufficient waste products may not be removed
from the body
• Changes in net filtration pressure affects GFR
– filtration stops if GBHP drops to 45mm Hg
– functions normally with mean arterial pressures 80-180
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Regulation of GFR
• The mechanisms that regulate GFR adjust blood flow into
and out of the glomerulus and alter the glomerular capillary
surface area available for filtration.
• The three principal mechanisms that control GFR are renal
autoregulation, neural regulation, and hormonal regulation.
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Regulation of GFR
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Renal Autoregulation of GFR
• Mechanisms that maintain a constant GFR despite changes
in arterial BP
– myogenic mechanism
• systemic increases in BP, stretch the afferent arteriole
• smooth muscle contraction reduces the diameter of
the arteriole returning the GFR to its previous level in
seconds
– tubuloglomerular feedback
• elevated systemic BP raises the GFR so that fluid
flows too rapidly through the renal tubule & Na+, Cland water are not reabsorbed
• macula densa detects that difference & releases a
vasoconstrictor from the juxtaglomerular apparatus
• afferent arterioles constrict & reduce GFR
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Neural Regulation of GFR
• Blood vessels of the kidney are supplied by sympathetic fibers that cause
vasoconstriction of afferent arterioles
• At rest, renal BV are maximally dilated because sympathetic activity is
minimal
– renal autoregulation prevails
• With moderate sympathetic stimulation, both afferent & efferent arterioles
constrict equally
– decreasing GFR equally
• With extreme sympathetic stimulation (exercise or hemorrhage),
vasoconstriction of afferent arterioles reduces GFR
– lowers urine output & permits blood flow to other tissues
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Hormonal Regulation of GFR
• Atrial natriuretic peptide (ANP) increases GFR
– stretching of the atria that occurs with an increase
in blood volume causes hormonal release
• relaxes glomerular mesangial cells increasing
capillary surface area and increasing GFR
• Angiotensin II reduces GFR
– potent vasoconstrictor that narrows both afferent &
efferent arterioles reducing GFR
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TUBULAR REABSORPTION AND TUBULAR
SECRETION
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Tubular Reabsorption & Secretion
• Normal GFR is so high that volume of filtrate in capsular space in
half an hour is greater than the total plasma volume
• Nephron must reabsorb 99% of the filtrate
– PCT with their microvilli do most of work with rest of nephron
doing just the fine-tuning
• solutes reabsorbed by active & passive processes
• water follows by osmosis
• small proteins by pinocytosis
• Important function of nephron is tubular secretion
– transfer of materials from blood into tubular fluid
• helps control blood pH because of secretion of H+
• helps eliminate certain substances (NH4+, creatinine, K+)
• Table 26.3 compares the amounts of substances that are filtered,
reabsorbed, and excreted in urine with the amounts present in
blood plasma.
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Reabsorption Routes
• A substance being reabsorbed can move between adjacent
tubule cells or through an individual tubule cell before
entering a peritubular capillary (Figure 26.11).
• Fluid leakage between cells is known as paracellular
reabsorption.
• In transcellular reabsorption, a substance passes from the
fluid in the tubule lumen through the apical membrane of a
tubule cell, across the cytosol, and out into interstitial fluid
through the basolateral membrane.
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Reabsorption Routes
• Paracellular reabsorption
– 50% of reabsorbed material
moves between cells by
diffusion in some parts of
tubule
• Transcellular reabsorption
– material moves through
both the apical and basal
membranes of the tubule
cell by active transport
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Transport Mechanisms
• Solute reabsorption drives water reabsorption. The
mechanisms that accomplish Na+ reabsorption in each
portion of the renal tubule and collecting duct recover not
only filtered Na+ but also other electrolytes, nutrients, and
water.
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Transport Mechanisms
• Apical and basolateral membranes of tubule cells have
different types of transport proteins
• Reabsorption of Na+ is important
– several transport systems exist to reabsorb Na+
– Na+/K+ ATPase pumps sodium from tubule cell
cytosol through the basolateral membrane only
• Water is only reabsorbed by osmosis
– obligatory water reabsorption occurs when water is
“obliged” to follow the solutes being reabsorbed
– facultative water reabsorption occurs in collecting
duct under the control of antidiuretic hormone
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Active and Passive Transport Processes
• Transport across membranes can be either active or
passive (See Chapter 3).
• In primary active transport the energy derived from ATP is
used to “pump” a substance across a membrane.
• In secondary active transport the energy stored in an ion’s
electrochemical gradient drives another substance across
the membrane.
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Transport Maximum (Tm)
• Each type of symporter has an upper limit on how fast it can
work, called the transport maximum (Tm).
• The mechanism for water reabsorption by the renal tubule
and collecting duct is osmosis.
• About 90% of the filtered water reabsorbed by the kidneys
occurs together with the reabsorption of solutes such as
Na+, Cl-, and glucose.
• Water reabsorption together with solutes in tubular fluid is
called obligatory water reabsorption.
• Reabsorption of the final water, facultative reabsorption, is
based on need and occurs in the collecting ducts and is
regulated by ADH.
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Glucosuria
• Renal symporters can not reabsorb glucose fast
enough if blood glucose level is above 200 mg/mL
– some glucose remains in the urine (glucosuria)
• Common cause is diabetes mellitis because insulin
activity is deficient and blood sugar is too high
• Rare genetic disorder produces defect in symporter
that reduces its effectiveness
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Reabsorption and Secretion in the Proximal
Convoluted Tubule
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Reabsorption in the Proximal Convoluted Tubule
• The majority of solute and water reabsorption from filtered
fluid occurs in the proximal convoluted tubules and most
absorptive processes involve Na+.
• Proximal convoluted tubule Na+ transporters promote
reabsorption of 100% of most organic solutes, such as
glucose and amino acids; 80-90% of bicarbonate ions; 65%
of water, Na+, and K+; 50% of Cl-; and a variable amount of
Ca+2, Mg+2, and HPO4-2.
• Normally, 100% of filtered glucose, amino acids, lactic acid,
water-soluble vitamins, and other nutrients are reabsorbed
in the first half of the PCT by Na+ symporters. Figure 26.12
shows the operation of the main Na+-glucose symporters in
PCT cells.
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Reabsorption in the Proximal Convoluted Tubule
• Na+/H+ antiporters achieve Na+ reabsorption and return
filtered HCO3- and water to the peritubular capillaries (Figure
26.13). PCT cells continually produce the H+ needed to
keep the antiporters running by combining CO2 with water to
produce H2CO3 which dissociates into H+ and HCO3-.
• Diffusion of Cl- into interstitial fluid via the paracellular route
leaves tubular fluid more positive than interstitial fluid. This
electrical potential difference promotes passive paracellular
reabsorption of Na+, K+, Ca+2, and Mg+2 (Figure 26.14).
• Reabsorption of Na+ and other solutes creates an osmotic
gradient that promotes reabsorption of water by osmosis
(Figure 26.15).
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Reabsorption in the PCT
• Na+ symporters help reabsorb
materials from the tubular filtrate
• Glucose, amino acids, lactic acid,
water-soluble vitamins and other
nutrients are completely
reabsorbed in the first half of the
proximal convoluted tubule
• Intracellular sodium levels are kept
low due to Na+/K+ pump
Reabsorption of Nutrients
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Reabsorption of Bicarbonate, Na+ & H+ Ions
• Na+ antiporters reabsorb Na+ and
secrete H+
– PCT cells produce the H+ &
release bicarbonate ion to the
peritubular capillaries
– important buffering system
• For every H+ secreted into the tubular
fluid, one filtered bicarbonate
eventually returns to the blood
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Secretion of NH3 and NH4+ in the Proximal
Convoluted Tubule
• Urea and ammonia in the blood are both filtered at the
glomerulus and secreted by the proximal convoluted tubule
cells into the tubules.
• The deamination of the amino acid glutamine by PCT cells
generates both NH3 and new HCO3- (Figure 26.16).
• At the pH inside tubule cells, most NH3 quickly binds to H+
and becomes NH4+.
• NH4+ can substitute for H+ aboard Na+/H+ antiporters and be
secreted into tubular fluid.
• Na+/HCO3+ symporters provide a route for reabsorbed Na+
and newly formed HCO3- to enter the bloodstream.
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Passive Reabsorption in the 2nd Half of PCT
• Electrochemical gradients
produced by symporters &
antiporters causes passive
reabsorption of other solutes
• Cl-, K+, Ca+2, Mg+2 and urea
passively diffuse into the
peritubular capillaries
• Promotes osmosis in PCT
(especially permeable due to
aquaporin-1 channels
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Reabsorption in the Loop of Henle
• The loop of Henle sets the stage for independent regulation
of both the volume and osmolarity of body fluids.
• Na+-K+-Cl- symporters reclaim Na+, Cl-, and K+ ions from the
tubular lumen fluid (Figure 26.15).
• Because K+ leakage channels return much of the K+ back
into tubular fluid, the main effect of the Na+-K+-Clsymporters is reabsorption of Na+ and Cl-.
• Although about 15% of the filtered water is reabsorbed in
the descending limb, little or no water is reabsorbed in the
ascending limb.
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Symporters in the Loop of Henle
• Thick limb of loop of Henle has
Na+ K- Cl- symporters that
reabsorb these ions
• K+ leaks through K+ channels
back into the tubular fluid leaving
the interstitial fluid and blood with a
negative charge
• Cations passively move to the
vasa recta
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Reabsorption in the DCT
• As fluid flows along the DCT, reabsorption of Na+ and Clcontinues due to Na+-Cl- symporters.
– Na+ and Cl- then reabsorbed into peritubular capillaries
• The DCT serves as the major site where parathyroid
hormone stimulates reabsorption of Ca+2.
• DCT is not very permeable to water so the solutes are
reabsorbed with little accompanying water.
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Reabsorption and Secretion in the Collecting Duct
• By end of DCT, 95% of solutes & water have been reabsorbed and
returned to the bloodstream
• Cells in the collecting duct make the final adjustments
– principal cells reabsorb Na+
• Na+ passes through the apical membrane of principal cells via
Na+ leakage channels. Sodium pumps actively transport Na+
across the basolateral membrane (Figure 26.16).
– Principal cells secrete a variable amount of K+ (Figure 26.16).
• The secretion of K+ through K+ leakage channels in the principal
cells is the main source of K+ that is excreted in urine.
• intercalated cells reabsorb K+ & bicarbonate ions and secrete H+
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Actions of the Principal Cells
• Na+ enters principal cells
through leakage channels
• Na+ pumps keep the
concentration of Na+ in
the cytosol low
• Cells secrete variable
amounts of K+, to adjust
for dietary changes in K+
intake
– down concentration gradient due to
Na+/K+ pump
• Aldosterone increases Na+ and water
reabsorption & K+ secretion by principal
cells by stimulating the synthesis of new
pumps and channels.
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Secretion of H+ and Absorption of Bicarbonate by
Intercalated Cells
• Proton pumps (H+ATPases) secrete H+
into tubular fluid
– can secrete against a concentration
gradient so urine can be 1000 times
more acidic than blood
• Cl-/HCO3- antiporters move bicarbonate
ions into the blood
– intercalated cells help regulate pH of
body fluids
• Urine is buffered by HPO4 2- and ammonia,
both of which combine irreversibly with H+
and are excreted
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Hormonal Regulation
• Hormones that affect Na+, Cl- & water reabsorption and K+ secretion
in the tubules
– angiotensin II and aldosterone
• decreases GFR by vasoconstricting afferent arteriole
• enhances absorption of Na+
• promotes aldosterone production which causes principal cells
to reabsorb more Na+ and Cl- and less water
• increases blood volume by increasing water reabsorption
– atrial natriuretic peptide
• inhibits reabsorption of Na+ and water in PCT & suppresses
secretion of aldosterone & ADH
• increase excretion of Na+ which increases urine output and
decreases blood volume
• Table 26.4 summarizes the hormonal regulation of
tubular reabsorption and tubular secretion.
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Antidiuretic Hormone
• Increases water permeability of principal
cells so regulates facultative water
reabsorption
• Stimulates the insertion of aquaporin-2
channels into the membrane
– water molecules move more rapidly
• When osmolarity of plasma & interstitial
fluid decreases, more ADH is secreted
and facultative water reabsorption
increases.
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PRODUCTION OF DILUTE AND CONCENTRATED
URINE
• The rate at which water is lost from the body depends
mainly on ADH, which controls water permeability of
principal cells in the collecting duct (and in the last portion of
the distal convoluted tubule).
• When ADH level is very low, the kidneys produce dilute
urine and excrete excess water; in other words, renal
tubules absorb more solutes than water (Figure 26.18).
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Formation of Concentrated Urine
• Compensation for low water intake or heavy perspiration
• When ADH level is high, the kidneys secrete concentrated
urine and conserve water; a large volume of water is
reabsorbed from the tubular fluid into interstitial fluid, and
the solute concentration of urine is high.
• Production of concentrated urine involves ascending limb
cells of the loop of Henle establishing the osmotic
gradient in the renal medulla, collecting ducts
reabsorbing more water and urea, and urea recycling
causing a build up of urea in the renal medulla (Figure
26.19).
• The countercurrent mechanism also contributes to the
excretion of concentrated urine.
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Formation of Concentrated Urine
• Urine can be up to 4 times greater osmolarity than plasma
• It is possible for principal cells & ADH to remove water from urine to
that extent, if interstitial fluid surrounding the loop of Henle has high
osmolarity
– Long loop juxtamedullary nephrons make that possible
– Na+/K+/Cl- symporters reabsorb Na+ and Cl- from tubular fluid to
create osmotic gradient in the renal medulla
• Cells in the collecting ducts reabsorb more water & urea when ADH is
increased
• Urea recycling causes a buildup of urea in the renal medulla
• Figure 26.20 summarizes the processes of filtration, reabsorption, and
secretion in each segment of the nephron and collecting ducts.
Hormonal effects are also noted.
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Summary
• H2O
Reabsorption
– PCT---65%
– loop---15%
– DCT----1015%
– collecting
duct--5-10% with
ADH
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Formation of Dilute Urine
• Dilute = having fewer solutes
than plasma (300 mOsm/liter).
– diabetes insipidus
• Filtrate and blood have equal
osmolarity in PCT
• Water reabsorbed in thin limb,
but ions reabsorbed in thick limb
of loop of Henle create a filtrate
more dilute than plasma
– can be 4x as dilute as
plasma
– as low as 65 mOsm/liter
• Principal cells do not reabsorb
water if ADH is low
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Countercurrent Mechanism
• Descending limb is very permeable to water
– higher osmolarity of interstitial fluid outside the
descending limb causes water to mover out of the tubule
by osmosis
• at hairpin turn, osmolarity can reach 1200 mOsm/liter
• Ascending limb is impermeable to water, but symporters
remove Na+ and Cl- so osmolarity drops to 100 mOsm/liter,
but less urine is left
• Vasa recta blood flowing in opposite directions than the loop
of Henle -- provides nutrients & O2 without affecting
osmolarity of interstitial fluid
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Reabsorption within Loop of Henle
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Clinical Application
• Diuretics are drugs that increase urine flow rate. They work
by a variety of mechanisms. The most potent ones are the
loop diuretics, such as furosemide, which inhibits the
symporters in the thick ascending limb of the loop of Henle.
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Diuretics
• Substances that slow renal reabsorption of water &
cause diuresis (increased urine flow rate)
– caffeine which inhibits Na+ reabsorption
– alcohol which inhibits secretion of ADH
– prescription medicines can act on the PCT, loop of
Henle or DCT
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EVALUATION OF KIDNEY FUNCTION
• An analysis of the volume and physical, chemical, and
microscopic properties of urine, called urinalysis, reveals
much about the state of the body.
• Table 26.5 summarizes the principal physical characteristics
of urine.
• Table 26.6 lists several abnormal constituents of urine that
may be detected as part of a urinalysis.
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EVALUATION OF KIDNEY FUNCTION
• Two blood screening tests can provide information about
kidney function.
– One screening test is the blood urea nitrogen (BUN),
which measures the level of nitrogen in blood that is part
of urea.
– Another test is measurement of plasma creatinine.
• Renal plasma clearance expresses how effectively the
kidneys remove (clear) a substance from blood plasma.
– The clearance of insulin gives the glomerular filtration
rate.
– The clearance of para-aminohippuric acid gives the rate
of renal plasma flow.
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Clinical Application
• Dialysis is the separation of large solutes from smaller ones
through use of a selectively permeable membrane.
• Filtering blood through an artificial kidney machine is called
hemodialysis. This procedure filters the blood of wastes and
adds nutrients.
• A portable method of dialysis is called continuous
ambulatory peritoneal dialysis.
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URINE STORAGE, TRANSPORTATION, AND
ELIMINATION
• Urine drains through papillary ducts into minor calyces,
which joint to become major calyces that unite to form the
renal pelvis (Figure 26.3). From the renal pelvis, urine
drains into the ureters and then into the urinary bladder, and
finally, out of the body by way of the urethra (Figure 26.1).
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Ureters
• Each of the two ureters connects the renal pelvis of one
kidney to the urinary bladder (Figure 26.21).
• The ureters transport urine from the renal pelvis to the
urinary bladder, primarily by peristalsis, but hydrostatic
pressure and gravity also contribute.
• The ureters are retroperitoneal and consist of a mucosa,
muscularis, and fibrous coat.
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Anatomy of Ureters
•
•
•
•
•
•
10 to 12 in long
diameter from 1-10 mm
Extends from renal pelvis to bladder
Retroperitoneal
Enters posterior wall of bladder
Physiological valve only
– bladder wall compresses arterial
opening as it expands during filling
– flow results from peristalsis, gravity &
hydrostatic pressure
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Histology of Ureters
• 3 layers in wall
– mucosa is transitional epithelium & lamina propria
• since organ must inflate & deflate
• mucus prevents the cells from being contacted by urine
– muscularis
• inner longitudinal & outer circular smooth muscle layer
– distal 1/3 has additional longitudinal layer
• peristalsis contributes to urine flow
– adventitia layer of loose connective tissue anchors in place
• contains lymphatics and blood vessels to supply ureter
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Urinary Bladder
• The urinary bladder is a hollow muscular organ situated in
the pelvic cavity posterior to the pubic symphysis.
• Anatomy and Histology of the Urinary Bladder
• In the floor of the urinary bladder is a small, smooth
triangular area, the trigone. The ureters enter the urinary
bladder near two posterior points in the triangle; the urethra
drains the urinary bladder from the anterior point of the
triangle (Figure 26.21).
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Location of Urinary Bladder
• Posterior to pubic symphysis
• In females is anterior to vagina & inferior to uterus
• In males lies anterior to rectum
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Anatomy of Urinary Bladder
• Hollow, distensible muscular organ with capacity of 700 - 800 mL
• Trigone is smooth flat area bordered by 2 ureteral openings and one urethral
opening
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Histology of Urinary Bladder
• 3 layers in wall
– mucosa is transitional epithelium & lamina propria
• since organ must inflate & deflate
• mucus prevents the cells from being contacted by urine
– muscularis (known as detrusor muscle)
• 3 layers of smooth muscle
– inner longitudinal, middle circular & outer longitudinal
• circular smooth muscle fibers form internal urethral
sphincter
• circular skeletal muscle forms external urethral sphincter
– adventitia layer of loose connective tissue anchors in place
• superior surface has serosal layer (visceral peritoneum)
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Micturition Reflex
• Micturition or urination (voiding)
• Stretch receptors signal spinal cord and brain
– when volume exceeds 200-400 mL
• Impulses sent to micturition center in sacral spinal cord
(S2 and S3) & reflex is triggered
– parasympathetic fibers cause detrusor muscle to
contract, external & internal sphincter muscles to relax
• Filling causes a sensation of fullness that initiates a desire
to urinate before the reflex actually occurs
– conscious control of external sphincter
– cerebral cortex can initiate micturition or delay its
occurrence for a limited period of time
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Anatomy of the Urethra
• Females
– length of 1.5 in., orifice between clitoris & vagina
– histology
• transitional changing to nonkeratinized stratified
squamous epithelium, lamina propria with elastic
fibers & circular smooth muscle
• Males
– tube passes through prostate, UG diaphragm & penis
– 3 regions of urethra
• prostatic urethra, membranous urethra & spongy
urethra
• circular smooth muscle forms internal urethral
sphincter & UG diaphragm forms external urethral
sphincter
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Anatomy of the
Urethra
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Urinary Incontinence
• Lack of voluntary control over micturition
– normal in 2 or 3 year olds because neurons to
sphincter muscle is not developed
• Stress incontinence in adults
– caused by increases in abdominal pressure that
result in leaking of urine from the bladder
• coughing, sneezing, laughing, exercising,
walking
– injury to the nerves, loss of bladder flexibility, or
damage to the sphincter
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Waste Management in Other Body Systems
• Buffers bind excess H+
• Blood transports wastes
• Liver is site for metabolic recycling
– conversion of amino acids into glucose, glucose into
fatty acids or toxic into less toxic substances
• The lungs excrete CO2. H2O, and heat.
• Sweat glands eliminate excess heat, water, and CO2,
plus small quantities of salts and urea.
• The GI tract eliminates solid, undigested foods, waste,
some CO2, H2O, salts and heat.
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DEVELOPMENT OF THE URINARY TRACT
• The kidneys develop from intermediate mesoderm.
• They develop in the following sequence: pronephros,
mesonephros, and metanephros (Figure 26.22).
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Developmental Anatomy
• Mesoderm along the posterior aspect attempts to differentiate
3 times into the kidneys
• Pronephros, mesonephros and metanephros
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Later Developmental Anatomy
• By 5th week, the uteric bud forms the duct system
• Metanephric mesoderm forms the nephrons
• Urogenital sinus forms the bladder and urethra
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Aging and the Urinary System
• After age 40, the effectiveness of kidney function begins
to decrease.
• Anatomical changes
– kidneys shrink in size from 260 g to 200 g
• Functional changes
– lowered blood flow & filter less blood (50%)
– diminished sensation of thirst increases susceptibility
to dehydration
• Diseases common with age
– acute and chronic inflammations & canaliculi
– infections, nocturia, polyuria, dysuria, retention or
incontinence and hematuria
• Cancer of prostate is common in elderly men
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Disorders of Urinary System
•
•
•
•
•
Renal calculi
Urinary tract infections
Glomerular disease
Renal failure
Polycystic kidney disease
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DISORDERS: HOMEOSTATIC IMBALANCES
• Crystals of salts present in urine can precipitate and solidify
into renal calculi or kidney stones. They may block the
ureter and can sometimes be removed by shock wave
lithotripsy.
• The term urinary tract infection (UTI) is used to describe
either an infection of a part of the urinary system or the
presence of large numbers of microbes in urine. UTIs
include urethritis (inflammation of the urethra), cystitis
(inflammation of the urinary bladder), pyelonephritis
(inflammation of the kidneys), and pyelitis (inflammation of
the renal pelvis and its calyces).
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Glomerular Diseases
• Glomerulonephritis (Bright’s disease) is an inflammation of
the glomeruli of the kidney. One of the most common
causes is an allergic reaction to the toxins given off by
steptococcal bacteria that have recently infected another
part of the body, especially the throat. The glomeruli may be
permanently damaged, leading to acute or chronic renal
failure.
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Glomerular Diseases
• Chronic renal failure refers to a progressive and generally
irreversible decline in glomerular filtration rate that may
result from chronic glomerulonephritis, pyelonephritis,
polycystic disease, or traumatic loss of kidney tissue.
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Glomerular Diseases
• Polycystic kidney disease is one of the most common
inherited disorders. In infants it results in death at birth or
shortly thereafter. In adults, it accounts for 6-12% of kidney
transplantations. In this disorder, the kidney tubules become
riddled with hundreds or thousands of cysts, and
inappropriate apoptosis of cells in noncystic tubules leads to
progressive impairment of renal function and eventually to
renal failure.
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end
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