chapter 24: the urinary system - houston community college

Chapter 24: The Urinary System

Copyright 2009, John Wiley & Sons, Inc.

Overview of kidney functions

n Regulation of blood ionic compositionn Regulation of blood pHn Regulation of blood volumen Regulation of blood pressuren Maintenance of blood osmolarityn Production of hormones (calcitrol and erythropoitin)n Regulation of blood glucose leveln Excretion of wastes from metabolic reactions and

foreign substances (drugs or toxins)


Anatomy and histology of the kidneys

n External anatomyq Renal hilium – indent where ureter emerges along

with blood vessels, lymphatic vessels and nervesq 3 layers of tissue

n Renal capsule – deep layer – continuous with outer coat of ureter, barrier against trauma, maintains kidney shape

n Adipose capsule – mass of fatty tissue that protects kidney from trauma and holds it in place

n Renal fascia – superficial layer – thin layer of connective tissue that anchors kidney to surrounding structures and abdominal wall


Organs of the urinary system in a female


Position and coverings of the kidneys


Internal anatomy

q Renal cortex – superficialn Outer cortical zonen Inner juxtamedullary zonen Renal columns – portions of cortex that extend between

renal pyramidsq Renal medulla – inner region

n Several cone shaped renal pyramids – base faces cortex and renal papilla points toward hilium

q Renal lobe – renal pyramid, overlying cortex area, and ½ of each adjacent renal column


Anatomy of the kidneys

n Parenchyma (functional portion) of kidneyq Renal cortex and renal pyramids of medulla

n Nephron – microscopic functional units of kidneyn Urine formed by nephron drains into

q Papillary ductsq Minor and major calycesq Renal pelvisq Ureterq Urinary bladder


Internal anatomy of the kidneys


Blood and nerve supply of the kidneys

n Blood supplyq Although kidneys constitute less than 0.5% of total body mass,

they receive 20-25% of resting cardiac outputq Left and right renal artery enters kidneyq Branches into segmental, interlobar, arcuate, interlobular arteriesq Each nephron receives one afferent arterioleq Divides into glomerulus – capillary ballq Reunite to form efferent arteriole (unique)q Divide to form peritubular capillaries or some have vasa rectaq Peritubular venule, interlobar vein and renal vein exits kidney

n Renal nerves are part of the sympathetic autonomic nervous systemq Most are vasomotor nerves regulating blood flow


Blood supply of the kidneys


The nephron – functional units of kidney

____2 partsn Renal corpuscle – filters blood

plasmaq Glomerulus – capillary networkq Glomerular (Bowman’s) capsule

– double-walled cup surrounding glomerulus

n Renal tubule – filtered fluid passes intoq Proximal convoluted tubuleq Descending and ascending loop

of Henle (nephron loop)q Distal convoluted tubule


Nephronsq Renal corpuscle and both convoluted tubules in

cortex, loop of Henle extend into medullaq Distal convoluted tubule of several nephrons

empty into single collecting ductq Cortical nephrons – 80-85% of nephrons

n Renal corpuscle in outer portion of cortex and short loops of Henle extend only into outer region of medulla

q Juxtamedullary nephrons – other 25-20%n Renal corpuscle deep in cortex and long loops of Henle

extend deep into medullan Receive blood from peritubular capillaries and vasa rectan Ascending limb has thick and thin regionsn Enable kidney to secrete very dilute or very concentrated urine


The structure of nephrons and associated blood vessels


Histology of nephron and collecting duct

q Glomerular capsulen Visceral layer has podocytes that wrap projections

around single layer of endothelial cells of glomerular capillaries and form inner wall of capsule

n Parietal layer forms outer wall of capsulen Fluid filtered from glomerular capillaries enters capsular

(Bowman’s) space


Histology of a renal corpuscle


Renal tubule and collecting duct

n Proximal convoluted tubule cells have microvilli with brush border – increases surface area

n Juxtaglomerular appraratus helps regulate blood pressure in kidneyq Macula densa – cells in final part of ascending loop of Henleq Juxtaglomerular cells – cells of afferent and efferent

arterioles contain modified smooth muscle fibersn Last part of distal convoluted tubule and collecting duct

q Principal cells – receptors for antidiuretic hormone (ADH) and aldosterone

q Intercalated cells – role in blood pH homeostasis


Overview of renal physiology

1. Glomerular filtrationq Water and most solutes in blood plasma move across the wall of

the glomerular capillaries into glomerular capsule and then renal tubule

2. Tubular reabsorptionq As filtered fluid moves along tubule and through collecting duct,

about 99% of water and many useful solutes reabsorbed –returned to blood

3. Tubular secretionq As filtered fluid moves along tubule and through collecting duct,

other material secreted into fluid such as wastes, drugs, and excess ions – removes substances from blood

n Solutes in the fluid that drains into the renal pelvis remain in the fluid and are excreted

n Excretion of any solute = glomerular filtration + secretion - reabsorption


Structures and functions of a nephron

Renal corpuscle Renal tubule and collecting duct

Peritubular capillaries

Urine(containsexcretedsubstances)

Blood(containsreabsorbedsubstances)

Fluid inrenal tubule

Afferentarteriole

Filtration from bloodplasma into nephron

Efferentarteriole

Glomerularcapsule

1





Tubular reabsorptionfrom fluid into blood


Afferentarteriole


Efferentarteriole

Glomerularcapsule

1

2





Tubular secretionfrom blood into fluid

Tubular reabsorptionfrom fluid into blood


Afferentarteriole


Efferentarteriole

Glomerularcapsule

1

2 3


Glomerular filtration

n Glomerular filtrate – fluid that enters capsular spaceq Daily volume 150-180 liters – more than 99% returned to

blood plasma via tubular reabsorptionn Filtration membrane – endothelial cells of glomerular

capillaries and podocytes encircling capillariesq Permits filtration of water and small solutesq Prevents filtration of most plasma proteins, blood cells and

plateletsq 3 barriers to cross – glomerular endothelial cells

fenestrations, basal lamina between endothelium and podocytes and pedicels of podocytes create filtration slits

q Volume of fluid filtered is large because of large surface area, thin and porous membrane, and high glomerular capillary blood pressure


The filtration membrane

Filtration slitPedicel of podocyte

Fenestration (pore) ofglomerular endothelial cell

Basal lamina

Lumen of glomerulus

(b) Filtration membrane

TEM 78,000x

(a) Details of filtration membrane

Filtration slitPedicel

Fenestration (pore) of glomerularendothelial cell: prevents filtration ofblood cells but allows all componentsof blood plasma to pass through

Podocyte of viscerallayer of glomerular(Bowman’s) capsule

1



Basal lamina

Lumen of glomerulus


TEM 78,000x




Basal lamina of glomerulus:prevents filtration of larger proteins


1

2



Basal lamina

Lumen of glomerulus


TEM 78,000x




Basal lamina of glomerulus:prevents filtration of larger proteins

Slit membrane between pedicels:prevents filtration of medium-sizedproteins


1

2

3


Net filtration pressure

n Net filtration pressure (NFP) is the total pressure that promotes filtrationq NFP = GBHP – CHP – BCOPq Glomerular blood hydrostatic pressure is the blood

pressure of the glomerular capillaries forcing water and solutes through filtration slits

q Capsular hydrostatic pressure is the hydrostatic pressure exerted against the filtration membrane by fluid already in the capsular space and represents “back pressure”

q Blood colloid osmotic pressure due to presence of proteins in blood plasma and also opposes filtration


The pressures that drive glomerular filtration

NET FILTRATION PRESSURE (NFP)=GBHP – CHP – BCOP= 55 mmHg 15 mmHg 30 mmHg= 10 mmHg

GLOMERULAR BLOODHYDROSTATIC PRESSURE(GBHP) = 55 mmHg

Capsularspace

Glomerular(Bowman's)capsule

Efferent arteriole

Afferent arteriole

1

Proximal convoluted tubule


CAPSULAR HYDROSTATICPRESSURE (CHP) = 15 mmHg


Capsularspace


Efferent arteriole

Afferent arteriole

1 2



BLOOD COLLOIDOSMOTIC PRESSURE(BCOP) = 30 mmHg

CAPSULAR HYDROSTATICPRESSURE (CHP) = 15 mmHg


Capsularspace


Efferent arteriole

Afferent arteriole

1 2

3



Glomerular filtration

n Glomerular filtration rate – amount of filtrate formed in all the renal corpuscles of both kidneys each minuteq Homeostasis requires kidneys maintain a

relatively constant GFRn Too high – substances pass too quickly and are not

reabsorbedn Too low – nearly all reabsorbed and some waste

products not adequately excretedq GFR directly related to pressures that determine

net filtration pressure


3 Mechanisms regulating GFR

1. Renal autoregulationn Kidneys themselves maintain constant renal blood

flow and GFR using q Myogenic mechanism – occurs when

stretching triggers contraction of smooth muscle cells in afferent arterioles – reduces GFR

q Tubuloglomerular mechanism – macula densa provides feedback to glomerulus, inhibits release of NO causing afferent arterioles to constrict and decreasing GFR

Tuboglomerular feedback

Mechanisms regulating GFR2. Neural regulation

q Kidney blood vessels supplied by sympathetic ANS fibers that release norepinephrine causing vasoconstriction

q Moderate stimulation – both afferent and efferent arterioles constrict to same degree and GFR decreases

q Greater stimulation constricts afferent arterioles more and GFR drops

3. Hormonal regulationq Angiotensin II reduces GFR – potent vasoconstrictor of both

afferent and efferent arteriolesq Atrial natriuretic peptide increases GFR – stretching of atria

causes release, increases capillary surface area for filtration


Tubular reabsorption and tubular secretion

n Reabsorption – return of most of the filtered water and many solutes to the bloodstreamq About 99% of filtered water reabsorbedq Proximal convoluted tubule cells make largest

contributionq Both active and passive processes

n Secretion – transfer of material from blood into tubular fluidq Helps control blood pHq Helps eliminate substances from the body


Reabsorption routes and transport mechanismsn Reabsorption routes

q Paracellular reabsorptionn Between adjacent tubule cellsn Tight junction do not completely seal off interstitial fluid from

tubule fluidn Passive

q Transcellular reabsorption – through an individual celln Transport mechanisms

q Reabsorption of Na+ especially importantq Primary active transport

n Sodium-potassium pumps in basolateral membrane only q Secondary active transport

n Symporters, antiportersq Transport maximum (Tm)

n Upper limit to how fast it can workq Obligatory vs. facultative water reabsorption

Reabsorption routes: paracellular reabsorption and transcellularreabsorption

Reabsorption and secretion in proximal convoluted tubule (PCT)

Largest amount of solute and water reabsorptionqSecretes variable amounts of H+, NH4

+ and ureaqMost solute reabsorption involves Na+

n Symporters for glucose, amino acids, lactic acid, water-soluble vitamins, phosphate and sulfate

n Na+ / H+ antiporter causes Na+ to be reabsorbed and H+ to be secreted

qSolute reabsorption promotes osmosis – creates osmotic gradient

n Aquaporin-1 in cells lining PCT and descending limb of loop of Henle

n As water leaves tubular fluid, solute concentration increasesqUrea and ammonia in blood are filtered at glomerulus and secreted by proximal convoluted tubule cells


Reabsorption and secretion in the proximal convoluted tubule


Reabsorption in the loop of Henleq Chemical composition of tubular fluid quite different

from filtraten Glucose, amino acids and other nutrients reabsorbedq Osmolarity still close to that of bloodn Reabsorption of water and solutes balancedq For the first time reabsorption of water is NOT

automatically coupled to reabsorption of solutesn Independent regulation of both volume and osmolarity

of body fluidsq Na+-K+-2Cl- symporters function in Na+ and Cl-

reabsorption – promotes reabsorption of cationsq Little or no water is reabsorbed in ascending limb –

osmolarity decreases

Na+–K+-2Cl- symporter in the thick ascending limb of the loop of Henle


Reabsorption and secretion in the late distale convoluted tubule and collecting duct

n Reabsorption on the early distal convoluted tubuleq Na+-Cl- symporters reabsorb Na+ and Cl-q Major site where parathyroid hormone stimulates

reabsorption of Ca+ depending on body’s needsn Reabsorption and secretion in the late distal

convoluted tubule and collecting ductq 90-95% of filtered solutes and fluid have been returned by

nowq Principal cells reabsorb Na+ and secrete K+

q Intercalated cells reabsorb K+ and HCO3- and secrete H+

q Amount of water reabsorption and solute reabsorption and secretion depends on body’s needs


Hormonal regulation of tubular reabsorption and secretion

q Angiotensin II - when blood volume and blood pressure decreasen Decreases GFR, enhances reabsorption of Na+, Cl- and water

in PCTq Aldosterone - when blood volume and blood pressure

decreasen Stimulates principal cells in collecting duct to reabsorb more

Na+ and Cl- and secrete more K+

q Parathyroid hormonen Stimulates cells in DCT to reabsorb more Ca2+


Regulation of facultative water reabsorption by ADH

q Antidiuretic hormone (ADH or vasopressin)n Increases water permeability of

cells by inserting aquaporin-2 in last part of DCT and collecting duct

q Atrial natriuretic peptide (ANP)n Large increase in blood volume

promotes release of ANPn Decreases blood volume and

pressure by inhibiting reabsorption of Na+ and water in PCT and collecting duct, suppress secretion of ADH and aldosterone


Production of dilute and concentrated urine

n Even though your fluid intake can be highly variable, total fluid volume in your body remains stable

n Depends in large part on the kidneys to regulate the rate of water loss in urine

n ADH controls whether dilute or concentrated urine is formedq Absent or low ADH = dilute urineq Higher levels = more concentrated urine through

increased water reabsorption


Formation of dilute urine

q Glomerular filtrate has same osmolarity as blood 300 mOsm/liter

q Fluid leaving PCT is isotonic to plasma

When dilute urine is being formed: q the osmolarity of fluid increases as it goes down the

descending loop of Henleq the osmolarity of fluid decreases as it goes up the

ascending limb, and decreases still more as it flows through the rest of the nephron and collecting duct


Formation of dilute urinen Osmolarity of interstitial fluid

of renal medulla becomes greater, more water is reabsorbed from tubular fluid so fluid become more concentrated

n Water cannot leave in thick portion of ascending limb but solutes leave making fluid more dilute than blood plasma

n Additional solutes but not much water leaves in DCT

n Low ADH makes late DCT and collecting duct have low water permeability


Mechanism of urine concentration in long-loop juxtamedullary nephrons

(b) Recycling of salts and urea in the vasa recta(a) Reabsorption of Na+CI– and water in a long-loop juxtamedullary nephron

Glomerular (Bowman’s) capsule

Afferentarteriole

Efferentarteriole

Glomerulus

Distal convoluted tubule

Proximalconvolutedtubule

Symporters in thickascending limb causebuildup of Na+ and Cl–

Interstitial fluidin renal medulla

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Osmoticgradient

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H2OH2O

H2O

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980

600780

400580

200380

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Loop of Henle 1200 Concentrated urine

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H2O

Urea

Papillaryduct

Collectingduct

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Na+CI–Blood flow

Flow of tubular fluid

Presense of Na+-K+-2CI–symporters

Interstitialfluid inrenal cortex

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Juxtamedullary nephronand its blood supply together

Vasarecta

Loop ofHenle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–



Afferentarteriole

Efferentarteriole

Glomerulus





300

1200

1000

800

Osmoticgradient

600

400

H2OH2O

H2O

200

1200

980

600780

400580

200380

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100


300

300

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600

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H2O

Urea

Papillaryduct

Collectingduct

Countercurrent flowthrough loop of Henleestablishes an osmoticgradient

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Na+CI–Blood flow




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Vasarecta

Loop ofHenle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–



Afferentarteriole

Efferentarteriole

Glomerulus





300

1200

1000

800

Osmoticgradient

600

400

H2OH2O

H2O

200

1200

980

600780

400580

200380

300

100


300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillaryduct

Collectingduct


Principal cells incollecting ductreabsorb morewater when ADHis present

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Na+CI–Blood flow




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Vasarecta

Loop ofHenle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

3

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–



Afferentarteriole

Efferentarteriole

Glomerulus





300

1200

1000

800

Osmoticgradient

600

400

H2OH2O

H2O

200

1200

980

600780

400580

200380

300

100


300

300

320

400

600

800

1000

1200

800

H2O

Urea

Papillaryduct

Urea recyclingcauses buildupof urea in therenal medulla

Collectingduct


Principal cells incollecting ductreabsorb morewater when ADHis present

300

500

700

900

1100

1200

400

800

1000

600

Na+CI–Blood flow




320


Vasarecta

Loop ofHenle

H2O

H2O

H2O

H2O

H2O

H2O

H2O

1

2

3

4

H2O

H2O

Na+CI–

Na+CI–

H2O

Na+CI–

H2O

Na+CI–


Evaluation of kidney function

n Urinalysisq Analysis of the volume and physical, chemical and

microscopic properties of urineq Water accounts for 95% of total urine volumeq Typical solutes are filtered and secreted substances

that are not reabsorbedq If disease alters metabolism or kidney function,

traces if substances normally not present or normal constituents in abnormal amounts may appear


Evaluation of kidney function

n Blood testsq Blood urea nitrogen (BUN) – measures blood nitrogen that

is part of the urea resulting from catabolism and deamination of amino acids

q Plasma creatinine results from catabolism of creatine phosphate in skeletal muscle – measure of renal function

n Renal plasma clearanceq More useful in diagnosis of kidney problems than aboveq Volume of blood cleared of a substance per unit timeq High renal plasma clearance indicates efficient excretion of

a substance into urineq PAH administered to measure renal plasma flow


Urine transportation, storage, and eliminationn Ureters

q Each of 2 ureters transports urine from renal pelvis of one kidney to the bladder

q Peristaltic waves, hydrostatic pressure and gravity move urine

q No anatomical valve at the opening of the ureter into bladder – when bladder fills it compresses the opening and prevents backflow


Ureters, urinary bladder, and urethra in a female


Urinary bladder and urethran Urinary bladder

q Hollow, distensible muscular organq Capacity averages 700-800mLq Micturition – discharge of urine from bladder

n Combination of voluntary and involuntary muscle contractionsn When volume increases stretch receptors send signals to

micturition center in spinal cord triggering spinal reflex –micturition reflex

n In early childhood we learn to initiate and stop it voluntarilyn Urethra

q Small tube leading from internal urethral orifice in floor of bladder to exterior of the body

q In males discharges semen as well as urine


Comparison between female and male urethras

chapter 24: the urinary system - houston community college

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