23 Urinary System

8/13/2019 23 Urinary System

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Chapter 23

The Urinary System

Functions of the urinary system

Anatomy of the kidney

Urine formationglomerular filtration

tubular reabsorption

water conservation Urine and renal function tests

Urine storage and elimination


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Urinary System

Two kidneys

Two ureters

Urethra


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Kidney Functions

Filters blood plasma, eliminates waste, returnsuseful chemicals to blood

Regulates blood volume and pressure

Regulates osmolarity of body fluids Secretes renin, activates angiotensin, aldosterone

controls BP, electrolyte balance

Secretes erythropoietin, controls RBC count Regulates PCO2

and acid base balance

Detoxifies free radicals and drugs

Gluconeogenesis (under starvation)


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Nitrogenous Wastes

Ureaproteinsamino acids NH2removed

forms ammonia, liver converts to urea

Uric acidnucleic acid catabolism

Creatinine

creatinine phosphate catabolism Renal failure

azotemia: nitrogenous wastes in blood

uremia: toxic effects as wastes accumulate


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Excretion

Separation of wastes from body fluids andeliminating them

respiratorysystem: CO2

integumentarysystem: water, salts, lactic acid, ureadigestivesystem: water, salts, CO2, lipids, bile

pigments, cholesterol

urinarysystem: many metabolic wastes, toxins, drugs,

hormones, salts, H+and water


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Anatomy of Kidney

Position, weight and sizeretroperitoneal, level of T12to L3

about 160 g each

about size of a bar of soap (12x6x3 cm) Shape

lateral surface - convex; medial - concave

CT coveringsrenal fascia: binds to abdominal wall

adipose capsule: cushions kidney

renal capsule: encloses kidney like cellophane wrap


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Anatomy of Kidney

Renal cortex: outer 1 cm

Renal medulla: renal columns, pyramids - papilla

Renal lobe: pyramid and its overlying cortex


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Lobe of Kidney


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Kidney: Frontal Section

Minor calyx: cup over papilla, collects urine


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Path of Blood Through Kidney

Renal artery (from descending aorta)

interlobararteries(up renal columns, between lobes)

arcuatearteries(over pyramids)

interlobulararteries(up into cortex)afferentarterioles

glomerulus(cluster of capillaries in the cortex)

efferentarterioles(near medulla vasa recta)

peritubular capillaries

interlobular veins arcuate veins interlobar veins

Renal vein (to inferior vena cava)


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Blood Supply Diagram


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Renal Corpuscle

Glomerular filtrate collects in capsular

space, flows into renal tubule


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Renal (Uriniferous) Tubule Proximal convoluted tubule

(PCT) longest, most coiled, simplecuboidal with brush border (many

microvilli)

Nephron loop - U shaped;descending + ascending limbs

thick segment(simple cuboidal)

initial part of descending limb

and part or all of ascending limb,

active transport of salts

thin segment(simple squamous)very water permeable

Distal convoluted tubule (DCT) cuboidal, minimal microvilli


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Renal (Uriniferous) Tubule

Juxtaglomerular apparatus =DCT +afferent + efferent arteriole Collecting duct: several DCTs join Flow of glomerular filtrate:

glomerular capsule PCT nephron loop DCT collectingduct papillary duct minor calyxmajor calyx renal pelvis ureter urinary bladder urethra


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Nephron Diagram

Peritubular

capillaries

shown only onright


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Nephrons

True proportions of nephron loopsto convoluted tubules shown

Cortical nephrons (85%)

short nephron loopsefferent arterioles branch off

peritubular capillaries

Juxtamedullary nephrons (15%)very long nephron loops, maintainsalt gradient, help conserve water

efferent arterioles branch off vasa

recta, blood supply for medulla


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Urine Formation Preview


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Filtration Membrane Diagram


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Filtration Membrane

Fenestrated endothelium 70-90nm pores exclude blood cells

Basement membrane

proteoglycan gel, negative charge

excludes molecules that are > 8nm (most

proteins are excluded)

blood plasma 7% protein, glomerular

filtrate 0.03%

Filtration slits

podocyte arms have pedicels with

negatively charged filtration slits: allow

particles < 3nm to pass, but not

negatively charged macromolecules


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Filtration Pressure


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Glomerular filtration rate = fluid flow rate between glomerular

capillaries and Bowmans capsule

Kfis called the filtration constant; defined as the product ofthe hydraulic conductivity and the surface area of the

glomerular capillaries. PGis the hydrostatic pressure within the glomerularcapillaries.

PBis the hydrostatic pressure within the Bowman's capsule.

Gis the colloid osmotic pressure within the glomerularcapillaries.

and Bis the colloid osmotic pressure within the Bowman'scapsule (normally 0, virtually no proteins).


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Glomerular Filtration Rate (GFR)

Filtrate formed per minute = rate of fluid flow intoglomerular capsule

Filtration coefficient (Kf) depends on permeability

and surface area of filtration barrier GFR = NFP x Kf 125 ml/min or 180 L/day

NFP = net filtration pressure

Filtration fraction = GFP/renal plasma flow 99% of filtrate reabsorbed, 1 to 2 L urine excreted


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Constriction/relaxation of efferent arteriole

Things to keep in mind

r and v, where v = velocity of flow

Efferent arteriole:

constriction (mild):

renal blood flow BHP in glomerulus GFR (in severe constriction, colloid osmotic pressure will oppose this)

flow thru renal tubules,

filtration fraction (because GFR increases, while renal plasma flow remainsthe same or decreases)

dilation renal blood flow

BHP

GFR,

flow thru renal tubules

filtration fraction


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Constriction/relaxation of afferent arteriole

Afferent arteriole: constriction ( resistance):

renal blood flow

blood hydrostatic pressure (BHP) in glomerulus

GFR flow thru renal tubules,

normal filtration fraction (because both the GFR, which is plasma entering

the renal tubules, and the renal plasma flow decreased)

dilation ( resistance)

renal blood flow

BHP

GFR,

flow thru renal tubules

normal filtration fraction

I /d i BP


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Increase/decrease in BP

Increase in BP (up to 180 mmHg):

renal blood flow, GBP, GFR and normal FF Kidney response

myogenic mechanism: stretch receptors in afferent arterioles sense

dilation and cause a contraction reflex

tubuloglomerular feedback: increased renal tubule flow and NaClcontent stimulate specialized epithelial cells in the macula densa (distal

convoluted tubule) to secrete substances, such as ATP, resulting in

afferent arteriole vasoconstriction

Modest decrease in BP ( 80 mmHg):

renal blood flow, GBP, GFR and normal FF

Kidney response

myogenic mechanism (less stretching) and tubuloglomerular feedback

( macula densa activity)


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Severe hypotension

Strong decrease in BP ( 60 mmHg) Baroreceptors stimulate sympathetic nervous system and general

vasoconstriction (not good for the kidney)

Vasoconstriction is attenuated at the afferent arteriole by local

secretion of endogenous prostaglandins, which cause partialvasodilation

juxtaglomerular cells secrete renin:

production of active angiotensin II,

constriction of the efferent arteriole; colloid osmotic pressure of the blood and hydrostatic pressure in the

peritubular capillaries, resulting in water reabsorption from the proximal

tubule (tubuloglomerular balance)

NaCl and water reabsorption in the distal convoluted tubules and

collecting ducts (via aldosterone)


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Effects of GFR Abnormalities

GFR, urine output rises dehydration,electrolyte depletion

GFR wastes reabsorbed (azotemia possible)

GFR controlled by adjusting glomerular bloodpressure

Autoregulation

sympathetic controlhormonal mechanism: renin and angiotensin


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Juxtaglomerular Apparatus

- vasomotion

- monitor salinity

phagocytosis


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Renal Autoregulation of GFR

BP constrict afferentarteriole, dilate efferent

BP dilate afferent

arteriole, constrict efferent Stable for BP range of 80 to

170 mmHg (systolic)

Cannot compensate forextreme BP


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Negative Feedback Control of GFR


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Sympathetic Control of GFR

Strenuous exercise or acute conditions (circulatoryshock) stimulate afferent arterioles to constrict

GFR and urine production, redirecting blood

flow to heart, brain and skeletal muscles


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Hormonal Control of GFR

+ constriction of eff. arterioles


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Effects of Angiotensin II


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Tubular Reabsorption and Secretion


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Proximal Convoluted Tubules (PCT) Reabsorption of 65% of GF to

peritubular capillaries Great length, prominent microvilli

and abundant mitochondria foractive transport

Reabsorbs greater variety ofchemicals than other parts ofnephron transcellular route - through

epithelial cells of PCT

paracellular route - betweenepithelial cells of PCT

Transport maximum: whentransport proteins of plasma membraneare saturated; glucose > 220 mg/dL

remains in urine (glycosuria)


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T b l S ti f PCT


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Tubular Secretion of PCT

and Nephron Loop

Waste removal

urea, uric acid, bile salts, ammonia, catecholamines,

many drugs

Acid-base balancesecretion of H+ and bicarbonate ions regulates pH of

body fluids

Primary function of nephron loopwater conservation,

also involved in electrolyte reabsorption


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DCT and Collecting Duct

Effect of aldosteroneBP results in angiotensin II formation

angiotensin II stimulates adrenal cortex

adrenal cortex secretes aldosteronealdosterone promotes Na+reabsorption

Na+reabsorption promotes water reabsorption

water reabsorption and

urine volumeBP drops less rapidly


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DCT and Collecting Duct

Effect of atrial natriuretic factor (ANF)BP stimulates right atrium

atrium secretes ANF

ANF promotes Na+and water excretion

BP drops

Effect of ADH

dehydration stimulates hypothalamus

hypothalamus stimulates posterior pituitary

posterior pituitary releases ADH

ADH water reabsorption (more aquaporins)

urine volume


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Water/electrolyte regulation in DCT and collecting duct

Hormone Effect Reason

Aldosterone Na+ and water reabsorption Prevent hypotension

ANP or ANF Na+ and water excretion Prevent hypertension

ADH or vasopressin Na+ and water reabsorption Prevent dehydration and

subsequent hypotension


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Collecting Duct Concentrates Urine

Osmolarity 4x as highdeep in medulla

Medullary portion of

collecting duct is

permeable to water but

not to NaCl


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Control of Water Loss

Producing hypotonic urineNaCl reabsorbed bycorticalcollecting duct

water remains in urine

Producing hypertonic urine (e.g in strenuousexercise)

GFR drops

tubular reabsorption of Na+ and water increases ADH increases collecting ducts water permeability

more water is reabsorbed

urine is more concentrated


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Countercurrent Multiplier

Purpose: to minimize solute washout frommedullary interstitium

Recaptures NaCl and returns it to renal medulla

Descending limb

reabsorbs water/urea but not salt

concentrates tubular fluid

Ascending limb

reabsorbs Na+, K+, and Cl-

maintains high osmolarity of renal medulla

impermeable to water

tubular fluid becomes hypotonic



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of Nephron Loop Diagram


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Countercurrent Exchange System

Formed by vasa rectaprovides blood supply to medulla O2exchange occurs here

arteries become veins

does not remove NaCl from medulla

movement of solute and waterdepends on gradient established bythe multiplier system

Descending capillaries

water diffuses out of bloodNaCl diffuses into blood

Ascending capillarieswater diffuses into blood

NaCl diffuses out of blood

Maintenance of Osmolarity


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Maintenance of Osmolarity

in Renal Medulla

Summary of Tubular


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Summary of Tubular

Reabsorptionand Secretion


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Composition and Properties of Urine

Appearance

almost colorless to deep amber; yellow color due to

urochrome, from breakdown of hemoglobin (RBCs)

Odor - as it stands bacteria degrade urea to ammonia

Specific gravity

density of urine ranges from 1.000 -1.035

Osmolarity - (blood - 300 mOsm/L) ranges from

50 mOsm/L to 1,200 mOsm/L in dehydrated person

pH - range: 4.5 - 8.2, usually 6.0

Chemical composition: 95% water, 5% solutes

urea, NaCl, KCl, creatinine, uric acid


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Urine Volume

Normal volume - 1 to 2 L/day Polyuria > 2L/day

Oliguria < 500 mL/day

Anuria - 0 to 100 mL (usually renal failure orobstruction)


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Diabetes

Chronic polyuria of metabolic origin With hyperglycemia and glycosuria

diabetes mellitus I and II, insulin hyposecretion/insensitivity

gestational diabetes, 1 to 3% of pregnancies

pituitary diabetes, hypersecretion of GH

adrenal diabetes, hypersecretion of cortisol

With glycosuria but no hyperglycemia

renal diabetes, hereditary deficiency of glucose transporters

With no hyperglycemia or glycosuria, but polyuria

diabetes insipidus, ADH hyposecretion or insensitivity

unrelated to diabetes mellitus


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Diuretics

Effectsurine output

blood volume

Useshypertension and congestive heart failure

Mechanisms of action

GFRtubular reabsorption


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Renal Function Tests

Renal clearance volume of blood plasma cleared ofa waste per time

Excretion = filtrationreabsorption + secretion

Determine renal clearance (C) by assessing blood and

urine samples under steady-state conditions: C = (cU/cP)QcU: waste concentration in urine

Q: urine flow (volume/time)

cP: waste concentration in plasma Determine GFR: inulinis neither reabsorbed or

secreted (all is excreted) so for this solute GFR =Cinulin

creatinine is also excreted only by filtration, GFR = Ccreatinine


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Urine Storage and Elimination

Uretersfrom renal pelvis passes dorsal to bladder and enters it

from below, about 25 cm long

3 layers

adventitia - CT

muscularis - 2 layers of smooth muscle

urine enters, it stretches and contracts in peristaltic wave

mucosa - transitional epithelium

lumen very narrow, easily obstructed

i l dd d h l


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Urinary Bladder and Urethra - Female

i l dd


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Urinary Bladder

Located in pelvic cavity, posterior to pubicsymphysis

3 layers

parietal peritoneum, superiorly; fibrous adventitia restmuscularis: detrusor muscle, 3 layers of smooth

muscle

mucosa: transitional epithelium

trigone: openings of ureters and urethra, triangular

rugae: relaxed bladder wrinkled, highly distensible

capacity: moderately full - 500 ml, max. - 800 ml

F l U h


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Female Urethra

3 to 4 cm long External urethral orifice

between vaginal orifice and

clitoris Internal urethral sphincter

detrusor muscle thickened,

smooth muscle,

involuntary control

External urethral sphincter

skeletal muscle, voluntary

control

M l Bl dd d U h


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Male Bladder and Urethra

18 cm long

Internal urethral sphincter

External urethral sphincter

3 regions

prostatic urethra

during orgasm receives semen

membranous urethra

passes through pelvic cavity

penile urethra

V idi U i Mi i i


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Voiding Urine - Micturition

Micturition reflex1) 200 ml urine in bladder, stretch receptors send signal

to spinal cord (S2, S3)

2) parasympathetic reflex arc from spinal cord, stimulates

contraction of detrusor muscle

3) relaxation of internal urethral sphincter

4) this reflex predominates in infants

I f Mi i i R fl Di


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Infant Micturition Reflex Diagram

X

V l t C t l f Mi t iti


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Voluntary Control of Micturition

5) micturition center in ponsreceives stretchsignals and integrates cortical input (voluntarycontrol)

6) sends signal for stimulation of detrussor and

relaxes internal urethral sphincter7) to delay urination impulses sent through pudendal

nerve to external urethral sphincter keep it

contracted until you wish to urinate8) valsalva maneuver

aids in expulsion of urine by pressure on bladder

can also activate micturition reflex voluntarily

Ad lt Mi t iti R fl Di


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Adult Micturition Reflex Diagram

H di l i


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Hemodialysis

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23 Urinary System