Chapter 14Chapter 14

The Urinary SystemThe Urinary System

V. Pag. 511-557

VI. Pag. 501-545

Functions of the KidneysWater balance and control of osmolarityin body fluids

Regulation of electrolyte concentration

Maintain acid-base balance by regulating H+ and HCO3

- urinary output

Excretion of waste products (urea, creatinine)

Excretion of drug products

Production of erythropoietin (to stimulate red blood formation)

Renin production to control salt conservation

Cardiac Output/Kidney

On average, 20 to 25 percent of the cardiac output passes through the pair of kidneys each minute. Therefore, over a very short time span the entire blood volume of the body passes through the kidneys for control of its chemical composition.

Each minute, the remaining 75 to 80 percent of the cardiac output is pumped from the heart to the other tissues of the body. This blood supplies cells with their metabolic needs (oxygen, glucose, etc.).

Organs of the Urinary System

Urinary system:1) Urine forming organ-

kidneys

2) Urine storage/release structures: urinary bladder, ureter, urethra

Functional Unit of Kidneys: NephronThere are two regions in the

kidneys:1) Renal cortex2) Renal Medulla

Each nephron consist of two areas:

1) Vascular componenta) Glomerulusb) Afferent/efferent arterioles

2) Tubular componenta) Bowman’s capsuleb) Proximal tubulec) Loop of Henled) Distal tubulee) Collecting tubule

Vascular and Tubular Components of the Nephron

Proximal tubule Distaltubule Collecting

duct

Bowman’scapsule

Glomerulus

CortexMedulla

Loop of Henle

To renalpelvis Overview of Functions of Parts of a Nephron

Peritubularcapillaries

Vein

Artery

Afferentarteriole

Efferentarteriole

Juxtaglomerularapparatus

Juxtamedullary vs. Cortical Nephrons

Proximaltubule

Distaltubule

Distaltubule Glomerulus

Bowman’scapsule

Proximaltubule

Loop of Henle

Other nephrons emptying intothe same collecting duct

Collectingduct

Ascendinglimb ofloop ofHenle

To renalpelvis

Vasa recta

Descendinglimb ofloop ofHenle

Medulla

Cortex

Juxtamedullary vs. Cortical Nephrons

Juxtamedullary Nephron(20 %)

Cortical Nephron(80 %)

Glomerulus located in inner layer of cortex

Glomerulus located in outer layer of cortex

Peritubular capillaries form a straight vessel (or vasa recta)

that runs along the loop of Henle

Branched peritubular capillaries surround loop

of Henle

Involved in the formation of very concentrated urine

Renal Processes

The kidneys regulate the composition of the ECF through three processes that occur in the nephrons. As a result of these three processes we have urine formation and excretion.

These processes are:

1) Glomerular filtration (~20%)

2) Tubular reabsorption

3) Tubular secretion (~80%)

Renal Processes

Glomerular Filtration

Tubular reabsorption

Tubular secretion

Glomerulus & Bowman’s capsule

Proximal tubules & loop of Henle

Involved in the formation of very

concentrated urine

Distal & collecting tubules

Filter 20% of blood plasma

Filter 80% of blood plasma

Sequence of Blood and Fluid Flow

180 L/day 178.5 L/day 1.5L/day

Glomerular FiltrationAfferent arteriole Efferent arteriole

Glomerulus

Bowman’scapsule

Lumen ofBowman’scapsule

Outer layer ofBowman’s capsule

Inner layerof Bowman’s capsule(podocytes)

Proximal convoluted tubule

Lumen ofglomerularcapillary

Endothelialcell

•Basement•membrane

Podocytefoot process

Blood Plasma is Filtered Through Three Layers :

1) Capillaries’ endothelial cells2) Basement membrane - collagen & glycoproteins3) Epithelial cells (podocytes) in Bowman’s capsule

Podocytefoot process

Filtrationslit

Basementmembrane

Capillarypore

(see next slide)

Podocytefoot process

Filtrationslit

•Basement•membrane

Capillarypore

Endothelialcell

Lumen of glomerularcapillary

Lumen ofBowman’s capsule

Forces Driving Glomerular FiltrationThe glomerular filtration rate develops by the interaction of several pressures.

Capillary blood pressure (CBP~55 mm Hg) due to differences in diameter of afferent and efferent arterioles

Plasma osmotic pressure (POP~30 mm Hg)

Bowman’s capsule hydrostatic pressure (HP~15 mm Hg)

Net filtration pressure ~10 mm Hg

CBP

POP HP

Influence of Total Peripheral Resistance on Mean Arterial Pressure

Capillary blood pressure (except kidneys)

Capillary blood pressure in kidneys

11 mm Hg(ultrafiltration)

Interstitial fluid

From arteriole To venule

-9 mm Hg(reabsorption)

Initial lymphaticvessel

Blood capillary

Glomerular capillary

blood pressure(55)

Plasmaosmotic pressure

(30)

Bowman’s capsulehydrostatic pressure

(15)

Differences in the driving forces driving glomerular filtration in kidneys and capillary

filtration in other organs

Glomerular Filtration Rate (GFR)

Net Filtration P= Blood plasma P –(Osmotic P + Hydrostatic P)= 10 mm Hg

Osmotic P: can change as a result of albuminuria, increased albumin filtration

Hydrostatic P: can increase due to urinary tract obstruction (kidney stones)

CBP (55)

POP(30)

HP(15)

Glomerular Filtration Rate (GFR)GFR = K x Net Filtration Pressure

Net Filtration Pressure is mainly determined by Capillary Blood Plasma (CBP)

Where: K is filtration coefficient, proportional to filtration area. Notice K is not a constant because filtration area can change as a result of podocyte contraction

CBP

POP HP

•Glomerular•basement•membrane

•Glomerular•capillary lumen

•Glomerular•capillary•loop

•Podocyte foot processes •Filtration•slit

•Bowman’s•capsule•lumen

Capillary Blood Pressure is the Critical Factor Driving Glomerular Filtration

Regulation of GFR:1) Intrinsic Factors (Autoregulation):

acting at the level of the kidneys to autoregulate GFR within MAP of 80-180 mm Hg

a) Myogenic activity of vascular tissue

b) Activity of the juxtaglomerularapparatus

2) Extrinsic Factor: Sympathetic Stimulation

Slide 12

Fig. 14.11aPage 521

Vasoconstriction(decreases blood flowinto the glomerulus)

Afferent arteriole

Glomerulus

Efferent arteriole

Glomerularcapillaryblood pressure

Net filtrationpressure

GFR

When mean arterial pressure increase

Afferent arteriole

Glomerulus

Efferent arteriole

Glomerularcapillaryblood pressure

Net filtrationpressure

GFR

Vasodilation(increases blood flowinto the glomerulus)

When mean arterial pressure decrease

Juxtaglomerular ApparatusMacula dense cells can detect changes in flow rate and

release vasoactive substances (vasoconstrictors and vasodilators) that alter capillary blood flow.

Pressure sensorsSecrete reninSense changes in

flow rateSecrete vasoactive

substances

Juxtaglomerular Apparatus Regulates GFR

Slide 16

Figure 14.13Page 523

Arterial blood pressure

Driving pressure into glomerulus

Glomerular capillary pressure

GFR

Rate of fluid flowthrough tubules

Stimulation of macula densa cellsto release vasoactive chemicals

Chemicals released that induceafferent arteriolar vasoconstriction

Blood flow into glomerulus

Glomerular capillary pressureto normal

GFR to normal

Regulation of GFR by Baroreceptor Reflex and Sympathetic Activity

The idea here is toincrease blood volume when drop in blood pressure occurs

Slide 17

Short-termadjustment for

Arterial blood pressure

Long-termadjustment for

Arterialblood pressure

Detection by aorticarch and carotid sinusbaroreceptors

Cardiacoutput

Totalperipheralresistance

Sympathetic activity

Generalizedarteriolar vasoconstriction

Afferent arteriolarvasoconstriction

Glomerular capillaryblood pressure

GFR

Urine volume

Conservation of fluid and salt

Arterial blood pressure

Figure 14.14Page 523

Glomerular filtration rate (GFR) can be regulated by changes in the filtration

coefficient (K)

GFR = Kf x net filtration pressure

Podocytefoot process

Filtrationslit

Basementmembrane

Capillarypore

(see next slide)

Podocytefoot process

Filtrationslit

•Basement•membrane

Capillarypore

Endothelialcell

Lumen of glomerularcapillary

Lumen ofBowman’s capsule

Sequence of Blood and Fluid Flow

Renal Processes-ReabsorptionThe kidneys regulate the

composition of the ECF through three processes that occur in the nephrons. As a result of these three processes urine is formed and excreted

These processes are:1) Glomerular filtration (~20%)

2) Tubular reabsorption- allows the recovery of Na+, Cl-, H2O, glucose, amino acids and other substances

3) Tubular secretion (~80%)

Barriers for Tubular ReabsorptionTrans-epithelial Transport

Transepitheliar TransportPassive reabsorption - mov. of substances down concentration or osmotic gradient (Cl-, H2O, urea)Active reabsorption - requires energy for transport

of Na+, glucose, amino acids

Na+ ReabsorptionNa+/K+ APTase in basolateral membrane

generate a Na+ gradient

© Brooks/Cole - Thomson Learning

Basolateral membrane

Regional Differences in Tubular Na+

ReabsorptionNa+ reabsorption takes place in:1) proximal tubules (67%) drives

reabsorption of glucose, AA and Cl-

2) ascending loop of Henle (25%) produce urine of varying concentrations

3) distal/collecting tubules (8%) subject to hormonal control

Notice: Na+ reabsorption does not occur in descending loop of Henle. In descending loop of Henle water reabsorption takes place

No Na+

reabsorption

Regional Differences in Tubular Na+

Reabsorption

No Na+reabsorption

In proximal tubule, Na+ reabsorption occurs independently of Na+

load (total amount of Na+ not Na+

concentration)

In distal tubules, Na+ reabsorption is controlled by aldosterone

Renin-Angiotensin-Aldosterone Regulates Na+ Reabsorption

Aldosterone acting on distal/collecting tubules, drives the insertion of new Na+ channels and Na+/K+ ATPases in tubular cells

Slide 23

Figure 14.19Page 529

NaCl / ECF volume /

Arterial blood pressure

Liver Kidney Lungs Adrenalcortex Kidney

H2Oconserved

Na+ (and CI–)osmotically holdmore H2O in ECF

Na+ (and CI–)conserved

Na+ reabsorptionby kidney tubules( CI–

reabsorptionfollows passively)

Vasopressin Thirst Arteriolarvasoconstriction

H2O reabsorptionby kidney tubules

Fluid intake

ReninAngiotensin-convertingenzyme

Angiotensinogen Angiotensin I Angiotensin II Aldosterone

Aldosterone FunctionAldosterone stimulates Na+ reabsorption by

promoting the insertion of new Na+ channels in the luminal membrane and additional Na+/K+

ATPase carriers in the basolateral membrane

© Brooks/Cole - Thomson Learning

Granular Cells in JuxtaglomerularApparatus Release Renin

1) Macula densa cells sense changes inNaCl/ECF volume and induce the release of renin by granular cell

2) Granular cells sense changes in blood pressure and release renin

3) Drop in blood pressure activate sympathetic NS, that triggers stimulation of granular cells and renin release

Other Functions of the Renin-Angiotensin-Aldosterone System

Slide 23

Figure 14.19Page 529

NaCl / ECF volume /

Arterial blood pressure

Liver Kidney Lungs Adrenalcortex Kidney

H2Oconserved

Na+ (and CI–)osmotically holdmore H2O in ECF

Na+ (and CI–)conserved

Na+ reabsorptionby kidney tubules( CI–

reabsorptionfollows passively)

Vasopressin Thirst Arteriolarvasoconstriction

H2O reabsorptionby kidney tubules

Fluid intake

ReninAngiotensin-convertingenzyme

Angiotensinogen Angiotensin I Angiotensin II Aldosterone

Water conservation

Arteriolar vasoconstriction

Water/fluid intake

Vasopressin release is controlled by ECF volume, MAP, and aldosterone

Slide 11

Figure 18.10Page 684

Paraventricularnucleus

Neurosecretoryneurons

Supraopticnucleus

Hypothalamic–posteriorpituitary stalk

Hypothalamus

Anteriorpituitary

Posterior pituitary

Systemicarterial inflow

Systemicvenous outflow

= Vasopressin

= Oxytocin

Vasopressin-releasing cells

Vasopressin (or antidiuretic hormone) stimulate H2O retention in distal/collecting tubules

Atrial Natriuretic Peptide (ANP) Inhibits Na+ Reabsorption

ANP is released by myocardial cells when they sense an increase in NaCl/ ECF volume

Slide 24

Helps correct Helps correctNaCl / ECF volume /Arterial blood pressure

Cardiacatria

Atrial natriuretic peptide

Na+ reabsorptionby kidney tubules

Salt-conservingrenin-angiotensin-aldosterone system

Smooth muscleof afferent arterioles

Sympatheticnervous system

Afferentarteriolarvasodilation

Cardiacoutput

Totalperipheralresistance

Na+ excretionin urine

H2O excretionin urine

GFR

Na+ and H2O filtered

Arterial bloodpressure

Figure 14.20Page 530

ANP inhibits renin and Aldosterone secretion, & Na+ reabsorption

Vasodilation

Active Transport of Glucose and Amino Acids Is Driven by the Na+ Gradient

Transport of glucose/ amino acids (AA) occurs only in proximal tubules by specialized cotransport systems

© Brooks/Cole - Thomson Learning

Glucose, AA

Na+

Regional Differences in Tubular Glucose Reabsorption

Slide 4

Figure 14.3Page 514

Proximal tubule Distaltubule Collecting

duct

Bowman’scapsule

Glomerulus

CortexMedulla

Loop of Henle

To renalpelvis Overview of Functions of Parts of a Nephron

Peritubularcapillaries

Vein

Artery

Afferentarteriole

Efferentarteriole

Juxtaglomerularapparatus

Glucosereabsorption takes place in:

1) proximal tubules

Passive Cl- Reabsorption is Driven by Na+ Gradient

© Brooks/Cole - Thomson Learning

Cl-

Passive Reabsorption of WaterWater molecules pass through aquaporins or water channels

High osmolarity in interstitial fluid (created by active Na+ transport) drives water movement

The number of waterchannels in distal/collectingtubules is controlled byvasopressin

Passive Urea Reabsorption is Driven by Na+ Gradient

Urea is a waste product formed by the breakdown of proteins

Not all Actively Reabsorbed Substances Are Equally Regulated by

KidneysCarrier saturation limits

transport of a substance in tubules- tubular maximum

If concentration of substance in tubules goes above tubular maximumexcretion will take place-renal threshold

Kidneys do not regulate glucose plasma levels= role of endocrine sys. and liver

Kidneys regulate PO43-

plasma levels

[Glucose] 125 mg

[PO43- ]

300 mg

Renal Processes-SecretionThe kidneys regulate the composition

of the ECF through three processes that occur in the nephrons. As a result of these three processes urine is formed and excreted.

These processes are:

1) Glomerular filtration (~20%)

2) Tubular reabsorption

3) Tubular secretion (~80%) involved in the transepithelial movement of K+, H+ and organic ions

Sequence of Blood and Fluid Flow

Tubular secretion allowsthe movement of K+, H+, organic cations and anions intothe tubules from the blood by transepithelial transport

K+ SecretionK+ ions move out of peritubular capillaries along the

nephron but they are only secreted in distal/collecting tubules because K+ channels are located

in the luminal membrane

K+ SecretionIn proximal tubules K+ ions move back into the interstitial

fluid because K+ channels are found only in the basolateral membrane

© Brooks/Cole - Thomson Learning

K+

Increased K+ Concentration Stimulates Aldosterone Release

Increased K+

concentration in extracellular space can result in changes in resting membrane potential.

K+-induced aldosterone release increases K+ secretion

Transport Processes in Nephron

Glomerular filtrationNa+ K+ Glucose AAUrea Cl- H+ Water

Proximal tubules reabsorptionActive Na+ Glucose AA

Passive Urea Cl-

Water channels always open

Descending loop of HenleH2O reabsorption

No active Na+

Ascending loop of HenleActive Na+

No H2O transport

Distal/collecting tubules Reabsorption:

Active Na+ regulated by aldosterone

Passive Urea Cl-

Water channels only open in response to vasopressin

Secretion:K+ H+ organic ions

Why there is no secretion of K+ in proximal tubules?

Vertical Osmotic GradientIs generated because different parts of the loop of Henle have

different permeabilities to NaCl and water. It allows the kidneys to secrete urine of varying concentrations

Renal medulla

Renal cortex

Factors that Induce Formation of Vertical Osmotic Gradient

Proximaltubule

Distaltubule

DistaltubuleGlomerulus

Bowman’scapsule

Proximaltubule

Loop of Henle

Other nephrons emptying intothe same collecting duct

Collectingduct

Ascendinglimb ofloop ofHenle

To renalpelvis

Vasa recta

Descendinglimb ofloop ofHenle

Medulla

Cortex

Anatomical1) Loop of Henle in

juxtamedullary nephrons go deep into renal medulla

2) Vasa recta and loop of Henle flow in parallel

Physiological1) Different permeabilities of

ascending and descending loop of Henle to Na+ and H2O

http://www.cellphys.ubc.ca/undergrad_files/301KKurine_anim.htmhttp://www.colorado.edu/kines/Class/IPHY3430-200/13urinar.html

Descending loop of Henle-Permeable to water-Does not reabsorb NaCl

Ascending loop of Henle-Impermeable to water-Actively reabsorb NaCl

Permeability to NaCl and Water in Loop of Henle

Functional Consequence of Differential Permeabilities in Loop of Henle:

Countercurrent System

High osmolarity in interstitial fluid of renal medulla can be used to control urine concentration by vasopressin

Functional Significance of Countercurrent System

Regulation of Urine Concentration by Vasopressin

Fromproximaltubule

Filtrate has concentrationof 100 mosm/liter as itenters distal andcollecting tubules

In the face of water deficit: vasopressin stimulates water movement out of distal/collecting tubules

Collectingtubule

Loop ofHenle

Medulla

Cortex

Distal tubule

Concentration ofurine may be upto 1,200 mosm/literas it leavescollecting tubule

= permeability to H2Oincreased by vasopressin

= passive diffusion of H2O

= active transport of NaCl

= portions of tubule impermeable to H2O

*

*

*

*

Fromproximaltubule

Filtrate has concentrationof 100 mosm/liter as itenters distal andcollecting tubules

In the face of water excess: vasopressin secretion decreases and excess water is removed in urine

Collectingtubule

Loop ofHenle

Medulla

Cortex

Distal tubule

Concentration ofurine may be as lowas 100 mosm/literas it leavescollecting tubule

= permeability to H2Oincreased by vasopressin

= passive diffusion of H2O

= active transport of NaCl

= portions of tubule impermeable to H2O

*

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter27/animation__micturition_reflex.htmlhttp://camel2.conncoll.edu/academics/zoology/courses/zoo202/Excretory/micturition.html

MicturationEmptying of the Bladder

Slide 3

Female Male

Urethra

Externalurethralorifice

Ureter

Smoothmuscleof bladderwall

Ureteralopenings

Internalsphincter

Pelvicdiaphragm

Externalsphincter

Prostate gland(an accessorysex gland)

Bulbourethralglands(accessorysex glands)

Externalurethralorifice

Urethra

Figure 14.2Page 513

Bladder consist of inner layer of epithelial tissue and outer layer of smooth muscle innervated by parasympathetic NS

Emptying of bladder requires opening of internal sphincter (smooth muscle) and external sphincter (skeletal muscle)

http://camel2.conncoll.edu/academics/zoology/courses/zoo202/Excretory/micturition.html

MicturationIs driven by the micturation reflex (a spinal reflex) and voluntary

control

Slide 49

Figure 14.32Page 552

Reflex control Voluntary controlBladder fills

Stretch receptors

Parasympathetic nerve

Bladder

Bladder contracts

Internal urethralsphincter mechanicallyopens when bladdercontracts

Cerebral cortex

Motor neuron toexternal sphincter

External urethralsphincter openswhen motor neuronis inhibited

External urethralsphincter remainsclosed when motorneuron is stimulated

No urinationUrination