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The Urinary System
P A R T A
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Kidney Functions
Filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine
Regulate volume and chemical makeup of the blood
Maintain the proper balance between water and salts, and acids and bases
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Other Renal Functions
Gluconeogenesis during prolonged fasting Production of renin to help regulate blood
pressure and erythropoietin to stimulate RBC production
Activation of vitamin D
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Other Urinary System Organs
Urinary bladder – provides a temporary storage reservoir for urine
Paired ureters – transport urine from the kidneys to the bladder
Urethra – transports urine from the bladder out of the body
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Urinary System Organs
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Kidney Location and External Anatomy The kidneys lie in a retroperitoneal position in
the superior lumbar region The right kidney is lower than the left because
it is crowded by the liver The lateral surface is convex; the medial
surface is concave The renal hilus leads to the renal sinus Ureters, renal blood vessels, lymphatics, and
nerves enter and exit at the hilus
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Layers of Tissue Supporting the Kidney Renal capsule – fibrous capsule that surrounds
the kidney Adipose capsule – cushions the kidney and
helps attach it to the body wall Renal fascia – outer layer of dense fibrous
connective tissue that anchors the kidney
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Kidney Location and External Anatomy
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Internal Anatomy (Frontal Section) Cortex – the light colored, outer region Medulla – exhibits cone-shaped medullary
(renal) pyramids separated by columnsThe medullary pyramid and its surrounding
cortex constitute a lobeBaseApex or papilla
Minor calyces- collect the urine from the papilla
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Internal Anatomy
Major calyces Receive the urine from the minor calyces
Renal pelvis Funnel shaped tube that collect urine from
the major calyces Renal sinus Urine flows through the pelvis and ureters to
the bladder
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Internal Anatomy
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Blood and Nerve Supply
Approximately one-fourth (1200 ml) of systemic cardiac output flows through the kidneys each minute
The nerve supply is via the renal plexus
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Renal Vascular Pathway
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The Nephron
Nephrons are the structural and functional units that form urine, consisting of:Glomerulus – a tuft of capillaries associated
with a renal tubuleGlomerular (Bowman’s) capsule – blind,
cup-shaped end of a renal tubule that completely surrounds the glomerulus
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The Nephron
Renal corpuscle – the glomerulus and its Bowman’s capsule
Glomerular endothelium – fenestrated epithelium that allows solute-rich, virtually protein-free filtrate to pass from the blood into the glomerular capsule
Renal tubules
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The Nephron
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Anatomy of the Glomerular Capsule The external parietal layer is a structural layer The visceral layer consists of modified,
branching epithelial podocytesExtensions of the octopus-like podocytes
terminate in foot processes Filtration slits – openings between the foot
processes that allow filtrate to pass into the capsular space
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Renal Tubules
Proximal convoluted tubule (PCT) – composed of cuboidal cells with numerous microvilli and mitochondriaReabsorbs water and solutes from filtrate
and secretes substances into it
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Renal Tubules
Loop of Henle – a loop of the renal tubuleProximal part is similar to the proximal
convoluted tubuleProximal part is followed by the thin
segment (simple squamous cells) and the thick segment (cuboidal to columnar cells)
Distal convoluted tubule (DCT) – cuboidal cells without microvilli that function more in secretion than reabsorption
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Renal Tubule
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Renal Tubules
The distal portion of the distal convoluted tubule and the collecting ducts have two types of cells:
Principal Intercalated
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Renal Tubules
Intercalated cellsCuboidal cells with microvilli Function in maintaining the acid-base
balance of the body Principal cells
Cuboidal cells without microvilliHelp maintain the body’s water and salt
balance
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Nephrons
Cortical nephrons – 85% of nephrons; located in the cortex
Juxtamedullary nephrons:Are located at the cortex-medulla junctionHave loops of Henle that deeply invade the
medulla Have extensive thin segmentsAre involved in the production of
concentrated urine
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Nephron Anatomy
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Capillary Beds of the Nephron
Every nephron has two capillary bedsGlomerulus Peritubular capillaries
Each glomerulus is: Fed by an afferent arteriole Drained by an efferent arteriole
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Capillary Beds of the Nephron
Blood pressure in the glomerulus is high because:Arterioles are high-resistance vesselsAfferent arterioles have larger diameters
than efferent arterioles Fluids and solutes are forced out of the blood
throughout the entire length of the glomerulus
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Capillary Beds
Peritubular beds are low-pressure, porous capillaries adapted for absorption that: Arise from efferent arteriolesCling to adjacent renal tubulesEmpty into the renal venous system
Vasa recta – long, straight capillaries of juxtamedullary nephrons
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Capillary Beds
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Vascular Resistance in Microcirculation Afferent and efferent arterioles offer high
resistance to blood flow Blood pressure declines from 95mm Hg in
renal arteries to 8 mm Hg in renal veins
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Vascular Resistance in Microcirculation Resistance in afferent arterioles:
Protects glomeruli from fluctuations in systemic blood pressure
Resistance in efferent arterioles:Reinforces high glomerular pressureReduces hydrostatic pressure in peritubular
capillaries
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Juxtaglomerular Apparatus (JGA) Juxtaglomerular (granular) cells
Enlarged, smooth muscle cells on the arteriole walls
Secrete renin Macula densa
Tall, closely packed distal tubule cells Lie adjacent to JG cells
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Juxtaglomerular Apparatus (JGA) Main JGA functions:
GFR control Stimulated by high tubular [NaCl]
Renin release Stimulated by low tubular [NaCl]
EPO release
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Juxtaglomerular Apparatus (JGA) When GFR increases:
Macula densa senses the increase of flow and NaCl
Macula densa sends paracrine message to afferent arteriole
Afferent arteriole constricts causing decrease in GFR
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Juxtaglomerular Apparatus (JGA)
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Filtration Membrane
Filter that lies between the blood and the interior of the glomerular capsule
It is composed of three layersFenestrated endothelium of the glomerular
capillariesVisceral membrane of the glomerular
capsule (podocytes) A basement membrane
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Filtration Membrane
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Filtration Membrane
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Filtration Barrier
Mesangial cells: Secrete cytokines associated with
immune and inflammatory processes Have filaments that enable them to
contract and decrease capillary blood flow
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Mechanisms of Urine Formation
The kidneys filter the body’s entire plasma volume 60 times each day
The filtrate:Contains all plasma components except
proteinLoses water, nutrients, and essential ions to
become urine The urine contains metabolic wastes and
unneeded substances
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Mechanisms of Urine Formation
• Urine formation and adjustment of blood composition involves three major processes – Glomerular
filtration– Tubular
reabsorption– Secretion
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The Urinary System
P A R T B
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Glomerular Filtration
Principles of fluid dynamics that account for tissue fluid in all capillary beds apply to the glomerulus as well
The glomerulus is more efficient than other capillary beds because:Its filtration membrane is more permeableGlomerular blood pressure is higher It has a higher net filtration pressure
Plasma proteins are not filtered and are used to maintain oncotic pressure of the blood
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Net Filtration Pressure (NFP)
The pressure responsible for filtrate formation NFP equals the glomerular hydrostatic
pressure (HPg) minus the oncotic pressure of glomerular blood (OPg) combined with the capsular hydrostatic pressure (HPc)
Colloid osmotic pressure in the capsular space
NFP = HPg – (OPg + HPc)
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Glomerular Filtration Rate (GFR)
The total amount of filtrate formed per minute by the kidneys
Factors governing filtration rate at the capillary bed are:Total surface area available for filtrationFiltration membrane permeabilityNet filtration pressure
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Glomerular Filtration Rate (GFR) GFR is directly proportional to the NFP Changes in GFR normally result from changes
in glomerular blood pressure
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Glomerular Filtration Rate (GFR)
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Regulation of Glomerular Filtration If the GFR is too high:
Needed substances cannot be reabsorbed quickly enough and are lost in the urine
If the GFR is too low:Everything is reabsorbed, including wastes
that are normally disposed of
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Regulation of Glomerular Filtration Three mechanisms control the GFR
Renal autoregulationNeural controlsHormonal mechanism
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Renal Autoregulation
Under normal conditions, renal autoregulation maintains a nearly constant glomerular filtration rate
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Renal Autoregulation
Two types of controlMyogenic – increased systemic blood
pressure stimulates stretch receptors on the afferent arterioles that causes its vasoconstriction Important in protecting the kidney from
hypertension-induced glomerular injury
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Renal Autoregulation
Flow-dependent tubuloglomerular feedback –increased amount of NaCl in the DCT is sensed by the macula densa. It then releases paracrine signals that
cause afferent vasoconstriction If the NaCl in the DCT is reduced the
paracrines signals will cause afferent vasodilation
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Sympathetic Nervous System
When the sympathetic nervous system is at rest:Renal blood vessels are maximally dilatedAutoregulation mechanisms prevail
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Sympathetic Nervous System
Sympathetic system – when under severe and acute conditions:Norepinephrine and epinephrine cause
vasoconstriction of the afferent arterioles GFR will then decrease
The sympathetic nervous system also stimulates renin release
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Sympathetic Nervous System Renin-angiotensin mechanism Is triggered when the JG cells release renin Renin converts angiotensinogen into
angiotensin I that is converted to angiotensin II As a result systemic pressure rises
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Hormonal Control
Renin release is triggered by:Decreased NaCl concentration at the
macula densaDirect stimulation of the JG cells via 1-
adrenergic receptors by renal nervesDecreased blood pressure at the glomerulus
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Hormonal Control
Angiotensin II will Causes direct vasoconstriction of the
efferent arteriole increases GFRStimulates reabsorption of Na
Directly and through aldosteroneStimulates thirst center in the hypothalamusStimulates hypothalamic release of ADH
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Hormonal Control
Causes general vasoconstriction Mean arterial pressure rises
Stimulates the adrenal cortex to release aldosterone
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Extrinsic Control - hormonal
Angiotensin II will Causes direct vasoconstrictionStimulates reabsorption of Na
Directly and through aldosteroneStimulates thirst center in the hypothalamusStimulates hypothalamic release of ADH
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Tubular Reabsorption A return of most of the water and solutes
filtered to the blood Mainly at PCT Reabsorption routes
TranscellularLuminal and basolateral membranes of
tubule cellsEndothelium of peritubular capillaries
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Tubular Reabsorption
ParacellularBetween tubular cells
Leaky tight junctions
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Routes of Water and Solute Reabsorption
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Glomerular filtration produces fluid similar to plasma without proteins
The PCT reabsorbs 60-70% of the filtrate produced
Secretion also occurs in the PCT
Reabsorption and secretion at the PCT
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Tubular Reabsorption Tubular cells use active transport to create an
electrochemical gradient Na is the primary driving force for most renal
reabsorptionIt is directly or indirectly involved passive or
active transport of many substances
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Tubular Reabsorption
Active transport of Na from the lumen to the ECF creates an electrochemical gradient Lumen becomes negative then ECFAnions will follow Na out of the lumenWater will follow by osmosis
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Reabsorption by PCT Cells
Sodium-linked secondary active transport causes the absorption of many other substancesGlucose, amino acids, ions, etcSymport /antiportFacilitated diffusion
OsmosisObligatory water absorption
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Sodium Reabsorption
Na+ is transported from the lumen into the tubular cell passively down its electrochemical gradient
Na is actively transported from the tubular cells to the interstitial fluid by a Na+-K+ ATPase pump
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Sodium Reabsorption
From there it moves to peritubular capillaries due to:Low hydrostatic pressureHigh osmotic pressure of the blood
Na+ reabsorption provides the energy and the means for reabsorbing most other solutes
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Reabsorption by PCT Cells
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Nonreabsorbed Substances
A transport maximum (Tm): Reflects the number of carriers in the renal
tubules available Exists for nearly every substance that is
actively reabsorbed When the carriers are saturated, excess of that
substance is excreted
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Nonreabsorbed Substances
Substances are not reabsorbed if they: Lack carriersAre not lipid solubleAre too large to pass through membrane
pores Urea, creatinine, and uric acid are the most
important nonreabsorbed substances
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Absorptive Capabilities of Renal Tubules and Collecting Ducts
Substances reabsorbed in PCT include:Sodium, all nutrients, cations, anions, and
waterUrea and lipid-soluble solutesSmall proteins
Loop of Henle reabsorbs:Only H2O in the descending limbOnly electrolytes in the ascending limb
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Absorptive Capabilities of Renal Tubules and Collecting Ducts
DCT absorbs:Electrolytes and water
Collecting duct absorbs:Water and urea
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Na+ Entry into Tubule Cells
Passive entry: symporter Na-K ATPase creates the ionic gradient for
the symporter In the PCT: facilitated diffusion In the ascending loop of Henle: actively In the DCT: Na+-Cl– mainly active. Under
influence of aldosterone In collecting tubules: primarily active transport.
Also under the influence of aldosterone
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Atrial Natriuretic Peptide Activity
ANP reduces blood Na+ which:Decreases blood volumeLowers blood pressure
ANP lowers blood Na+ by:Acting directly on medullary collecting ducts
to inhibit Na+ reabsorptionCounteracting the effects of angiotensin IIIncreasing GFR and reducing water
reabsorption
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Tubular Secretion
Essentially reabsorption in reverse, where substances move from peritubular capillaries or tubule cells into filtrate
Tubular secretion is important for:Disposing of substances not already in the
filtrate Eliminating undesirable substances such as
urea and uric acidRidding the body of excess potassium ionsControlling blood pH
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The Urinary System
P A R T C
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Regulation of Urine Concentration and Volume
OsmolalityThe number of solute particles dissolved in
1L of waterReflects the solution’s ability to cause
osmosis Body fluids are measured in milliosmols
(mOsm) The kidneys keep the solute load of body fluids
constant at about 300 mOsm This is accomplished by the countercurrent
mechanism
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Countercurrent Mechanism Happens in the medulla Countercurrent multiplier in the loop of Henle
CountercurrentFluid flowing in opposite directions in two
adjacent tubulesMultiplier
Because it multiplies the salinity deep in the medulla
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Countercurrent Mechanism
Countercurrent Exchanger in the vasa rectaBlood make passive exchange with the
surrounding interstitial fluid of the medulla It looses water when flowing into the
medulla It gains water and looses NaCl when
blood flows toward the cortex
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Countercurrent Mechanism
The solute concentration in the loop of Henle ranges from 300 mOsm to 1200 mOsm
Dissipation of the medullary osmotic gradient is prevented because the blood in the vasa recta equilibrates with the interstitial fluid
Vasa recta also delivers blood to the cells in the area
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Osmotic Gradient in the Renal Medulla
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Loop of Henle: Countercurrent Multiplier The descending loop of Henle:
Is relatively impermeable to solutesIs permeable to water
Obligatory water absorption The ascending loop of Henle:
Is permeable to solutesIs impermeable to water
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Loop of Henle: Countercurrent Multiplier Urea also contributes to the medullary
osmolalityThin limbs of Henle absorb ureaDCT is impermeable to urea Collecting ducts in the deep medullary
regions are permeable to urea
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Loop of Henle: Countercurrent Mechanism
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Formation of Dilute Urine
Filtrate is diluted in the ascending loop of Henle
Dilute urine is created by allowing this filtrate to continue into the renal pelvis
This will happen as long as antidiuretic hormone (ADH) is not being secreted
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Formation of Dilute Urine
Collecting ducts remain impermeable to water; no further water reabsorption occurs
Sodium and selected ions can be removed by active and passive mechanisms
Urine osmolality can be as low as 50 mOsm (one-sixth that of plasma)
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Formation of Concentrated Urine Antidiuretic hormone (ADH) inhibits diuresis This equalizes the osmolality of the filtrate and
the interstitial fluid In the presence of ADH, 99% of the water in
filtrate is reabsorbed
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Formation of Concentrated Urine ADH-dependent water reabsorption is called
facultative water reabsorption ADH works by inserting aquaporins into the
principal cells of the collecting ducts The kidneys’ ability to respond depends upon
the high medullary osmotic gradient
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Formation of Dilute and Concentrated Urine
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Diuretics
Chemicals that enhance the urinary output include:Any substance not reabsorbedSubstances that exceed the ability of the
renal tubules to reabsorb itSubstances that inhibit Na+ reabsorption
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Diuretics
Osmotic diuretics include:High glucose levels – carries water out with
the glucose Alcohol – inhibits the release of ADHCaffeine and most diuretic drugs – inhibit
sodium ion reabsorptionLasix and Diuril – inhibit Na+-associated
symporters
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Summary of Nephron Function
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Renal Clearance
The volume of plasma that is cleared of a particular substance in a given time
Renal clearance tests are used to:Determine the GFRDetect glomerular damageFollow the progress of diagnosed renal
disease
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Renal Clearance
RC = UV/P
RC = renal clearance rateU = concentration (mg/ml) of the substance
in urineV = flow rate of urine formation (ml/min)P = concentration of the same substance in
plasma
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Physical Characteristics of Urine Color and transparency
Clear, pale to deep yellow (due to urochrome)
Concentrated urine has a deeper yellow color
Drugs, vitamin supplements, and diet can change the color of urine
Cloudy urine may indicate infection of the urinary tract
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Physical Characteristics of Urine Odor
Fresh urine is slightly aromaticStanding urine develops an ammonia odorSome drugs and vegetables (asparagus)
alter the usual odor
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Physical Characteristics of Urine pH
Slightly acidic (pH 6) with a range of 4.5 to 8.0
Diet can alter pH Specific gravity
Ranges from 1.001 to 1.035 Is dependent on solute concentration
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Chemical Composition of Urine
Urine is 95% water and 5% solutes Nitrogenous wastes: urea, uric acid, and
creatinine Other normal solutes include:
Sodium, potassium, phosphate, and sulfate ions
Calcium, magnesium, and bicarbonate ions Abnormally high concentrations of any urinary
constituents may indicate pathology
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Ureters
Slender tubes that convey urine from the kidneys to the bladder
Ureters enter the base of the bladder through the posterior wallThis closes their distal ends as bladder
pressure increases and prevents backflow of urine into the ureters
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Ureters
Ureters have a trilayered wall Transitional epithelial mucosaSmooth muscle muscularisFibrous connective tissue adventitia
Ureters actively propel urine to the bladder via response to smooth muscle stretch
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Urinary Bladder
Smooth, collapsible, muscular sac that stores urine
It lies retroperitoneally on the pelvic floor posterior to the pubic symphysisMales – prostate gland surrounds the neck
inferiorlyFemales – anterior to the vagina and
uterus Trigone – triangular area outlined by the
openings for the ureters and the urethraClinically important because infections
tend to persist in this region
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Urinary Bladder
The bladder wall has three layers Transitional epithelial mucosaA thick muscular layerA fibrous adventitia
The bladder is distensible and collapses when empty
As urine accumulates, the bladder expands without significant rise in internal pressure
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Urinary Bladder
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Urethra
Muscular tube that:Drains urine from the bladderConveys it out of the body
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Urethra
Sphincters keep the urethra closed when urine is not being passedInternal urethral sphincter – involuntary
sphincter at the bladder-urethra junctionExternal urethral sphincter – voluntary
sphincter surrounding the urethra as it passes through the urogenital diaphragm
Levator ani muscle – serves as a voluntary constrictor of the urethra
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Urethra The female urethra is tightly bound to the
anterior vaginal wall Its external opening lies anterior to the vaginal
opening and posterior to the clitoris The male urethra has three named regions
Prostatic urethra – runs within the prostate gland
Membranous urethra – runs through the urogenital diaphragm
Spongy (penile) urethra – passes through the penis and opens via the external urethral orifice
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Urine – Storage reflex Distension of bladder walls stimulates stretch
receptors Visceral afferent fibers take the stimulus to the
sacral region of the spinal cord Sympathetic stimulation and parasympathetic
inhibitionRelax the detrusor muscleContracts the internal urethral sphincter
Somatic motor stimulation causes contraction of the external urethral sphincter
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Micturition (Voiding or Urination) The act of emptying the bladder: voiding reflexes Stretch receptors in the bladder wall send stimulus
to the sacral portion of the spinal cordSympathetic neurons are inhibitedParasympathetic neuron are stimulated
Stimulate detrusor muscle to contractCauses internal sphincter to relax
Somatic motor neurons are inhibited
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Micturition (Voiding or Urination)
Also inhibit synapses on the sympathetic neurons
The micturition center integrates information from the bladder with information coming from amygdala and cerebral cortex
When it is appropriate to urinate the external sphincter relaxes
Fear prompts urination
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Micturition (Voiding or Urination)
– Also inhibit synapses on the sympathetic neurons• The micturition center integrates information
from the bladder with information coming from amygdala and cerebral cortex
• When it is appropriate to urinate the external sphincter relaxes
• Fear prompts urination
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Developmental Aspects Infants have small bladders and the kidneys cannot
concentrate urine, resulting in frequent micturition Control of the voluntary urethral sphincter develops
with the nervous system E. coli bacteria account for 80% of all urinary tract
infections Sexually transmitted diseases can also inflame the
urinary tract Kidney function declines with age, with many
elderly becoming incontinent