151 Urinary Phys (New)

8/3/2019 151 Urinary Phys (New)

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Anatomy and Physiology, Seventh Edition

Rod R. SeeleyIdaho State University

Trent D. Stephens

Idaho State University

Philip Tate

Phoenix College

Copyright The McGraw-Hill Companies, Inc. Permission required forreproduction ordisplay.

*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.

Chapter 26Chapter 26

Lecture OutlineLecture Outline**


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Kidney Functions

Filters blood plasma returns useful substances to blood

eliminates waste

Regulates osmolarity of body fluids, blood volume, BP

acid base balance

Secretes renin and erythropoietin

Detoxifies free radicals and drugs

Gluconeogenesis


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Nitrogenous Wastes

Urea

proteinspamino acids pNH2 removedpforms ammonia, liver converts to urea

Uric acid nucleic acid catabolism

Creatinine

creatine phosphate catabolism

Renal failure

azotemia: oBUN, nitrogenous wastes inblood

uremia: toxic effects as wastesaccumulate


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Excretion

Separation of wastes from body fluidsand eliminating them; by four systems

respiratory: CO2 integumentary: water, salts, lactic acid,

urea

digestive: water, salts, CO2, lipids, bile

pigments, cholesterol

urinary: many metabolic wastes, toxins,drugs, hormones, salts, H+ and water


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The Nephron

Functional and histologicalunit of the kidney

Parts of the nephron:Bowmans capsule,proximal tubule, loop ofHenle (nephronic loop),distal tubule

Urine continues from thenephron to collectingducts, papillary ducts,minor calyses, major

calyses, and the renalpelvis

Collecting ducts, parts of theloops ofHenle, and papillaryducts are in the renalmedulla


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Types ofNephrons

Juxtamedullary nephrons.Renal corpuscle near thecortical medullary border.Loops ofHenle extend deep

into the medulla. Cortical nephrons. Renal

corpuscle nearer to theperiphery of the cortex.Loops ofHenle do notextend deep into themedulla.

Renal corpuscle. Bowmanscapsule plus a capillary bedcalled the glomerulus.


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Renal Corpuscle Bowmans capsule:

outer parietal (simplesquamous epithelium)

and visceral (cellscalled podocytes)layers.

Glomerulus: network ofcapillaries. Blood entersthrough afferentarteriole, exits throughefferent arteriole.


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Nephron Functions: Overview

Figure 19-2:Filtration, reabsorption, secretion, and excretion


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Filtration

Movement of fluid, derived from blood flowing through the

glomerulus, across filtration membrane Filtrate: water, small molecules, ions that can pass through

membrane

Pressure difference forces filtrate across filtration membrane

Renal fraction: part oftotal cardiac outputthat passes

through the kidneys. Varies from 12-30%; averages 21% Renal blood flow rate: 1176 mL/min = 21% x 5600 ml

Renal plasma flow rate: renal blood flow rate X fraction ofblood that is plasma: 650 mL/min = 1176 x 55%

Filtration fraction: part of plasma that is filtered into lumenof Bowmans capsules; average 19%

Glomerular filtration rate (GFR): amount of filtrate producedeach minute. 125 ml/min = 180 L/day; 650 ml x 19% =125ml/min

Average urine production/day: 1-2 L. Most of filtrate must be


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Filtration Filtration membrane: filtration barrier. It prevents blood cells and

proteins from entering lumen of Bowmans capsule, but is many timesmore permeable than a typical capillary

Fenestrated endothelium, basement membrane and pores formedby podocytes

Filtration pressure: pressure gradient responsible for filtration; forcesfluid from glomerular capillary across membrane into lumen ofBowmans capsules

Forces that affect movement of fluid into or out of the lumen ofBowmans capsule

Glomerular capillary pressure (GCP): blood pressure insidecapillary tends to move fluid out of capillary into Bowmans capsule

Capsule pressure (CP): pressure of filtrate already in the lumen

B

lood colloid osmotic pressure (B

COP): osmotic pressurecaused by proteins in blood. Favors fluid movement into thecapillary from the lumen. BCOP greater at end of glomerularcapillary than at beginning because of fluid leaving capillary andentering lumen

Filtration pressure (10 mm Hg) = GCP (50 mm Hg) CP (10 mmHg) BCOP (30 mm Hg)


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Filtration Pressure


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Filtration High glomerular capillary pressure results from

Low resistance to blood flow in afferent arterioles

Low resistance to blood flow in glomerular capillaries

High resistance to blood flow in efferent arterioles: smalldiameter vessels

Filtrate is forced across filtration membrane Changes in afferent and efferent arteriole diameter

alter filtration pressure Dilation of afferent arterioles/constriction efferent

arterioles increases glomerular capillary pressure,increasing filtration pressure and thus glomerularfiltration


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Regulation of Glomerular

Filtration If the GFR is too high:

Urine output rises p Dehydration, electro-

lyte depletion If the GFR is too low:

Everything is reabsorbed, including wastes thatare normally disposed of

GFR is controlled by adjusting glomerularblood pressure through: Autoregulation

Sympathetic control

Hormonal mechanisms: Renin and Angiotensin


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Autoregulation and Sympathetic

Stimulation Autoregulation

Renal autoregulation maintains a nearly constant glomerular filtration

rate over a wide range of systemic blood pressure

GFR constant as systemic BP changes between 90 and 180 mm

Hg

Involves changes in degree of constriction in afferent arterioles As systemic BP increases, afferent arterioles constrict and prevent

increase in renal blood flow

Increased rate of blood flow of filtrate past cells of macula densa:

signal sent to juxtaglomerular apparatus, afferent arteriole constricts

Sympathetic stimulation: norepinephrine Constricts small arteries and afferent arterioles

Decreases renal blood flow and thus filtrate formation

During shock or intense exercise: intense sympathetic stimulation, rate

of filtrate formation drops to a few mm


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Renal Autoregulation of GFR o BP p constrict afferent

arteriole, dilate efferent

q BP p dilate afferentarteriole, constrictefferent

Stable for BP range of80

to 170 mmHg (systolic) Cannot compensate for

extreme BP


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Renal Autoregulation of GFR Myogenic mechanism

o BP p stretches afferent arteriole p

afferent arteriole constricts p restoresGFR

Tubuloglomerular feedback

Macula densa on DCT monitors tubular

fluid and signals juxtaglomerular cells(smooth muscle, surrounds afferent arteriole) toconstrict afferent arteriole to q GFR


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Negative Feedback Control of GFR


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Sympathetic Control of GFR Strenuous exercise or acute conditions

(circulatory shock) stimulate afferent

arterioles to constrict

q GFR and urine production, redirecting

blood flow to heart, brain and skeletal

muscles


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Renin-Angiotensin Aldosterone


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Effects of Angiotensin II


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Tubular Reabsorption:

Overview Tubular reabsorption: occurs as filtrate flows

through the lumens of proximal tubule, loop ofHenle, distal tubule, and collecting ducts

Results because of Diffusion

Facilitated diffusion

Active transport

Cotransport

Osmosis

Substances transported to interstitial fluid andreabsorbed into peritubular capillaries; 99% offiltrate volume. These substances return to generalcirculation through venous system


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Tubular Reabsorption and

Secretion


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Peritubular Capillaries Blood has unusually high COP here,

and BHP is only 8 mm Hg (or lower when

constricted by angiotensin II); this favorsreabsorption

Water absorbed by osmosis and carries

other solutes with it (solvent drag)


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Proximal Convoluted Tubules

(PCT) Reabsorbs 65% of GF to peritubular capillaries

Great length, prominent microvilli and abundant mitochondria for

active transport

Reabsorbs greater variety of chemicals than other parts ofnephron

transcellular route - through epithelial cells of PCT

paracellular route - between epithelial cells of PCT

Transport maximum: when transport proteins of cell membrane are

saturated; blood glucose > 220 mg/dL some remains in urine(glycosuria); glucose Tm = 320 mg/min


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Mechanisms of Reabsorption in the Proximal Convoluted Tubule


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Tubular

Maximum

Maximum rate at which asubstance can be activelyabsorbed

Each substance has its owntubular maximum

Normally, glucoseconcentration in the plasma(and thus filtrate) is lower thanthe tubular maximum and all

of it is reabsorbed; none of itis found in the urine

In diabetes mellitus tubularload exceeds tubularmaximum and glucoseappears in urine. Urine

volume increases becauseglucose in filtrate increasesosmolality of filtrate reducingthe effectiveness of waterreabsorption (osmoticdiuresis).

Accounts for dehydration of


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Reabsorption in Loop ofHenle

Loop of Henle descends intomedulla; interstitial fluid is highin solutes.

Water reabsorption is NOTcoupled to solute reabsorption!

Descending thin segment ishighly permeable to water.

Water moves out of nephron,reducing the volume of filtrateby another 15% and increasingits osmolarity (moreconcentrated).

Ascending thin segment is notpermeable to water, but is

permeable to solutes. Solutesdiffuse out of the tubule and intothe more dilute interstitial fluidas the ascending limb projectstoward the cortex.

At the end of the loop ofHenle,inside of nephron is 100 mOsm


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Reabsorption in Distal Tubule and

Collecting Duct

Active transport ofNa+ out of tubule cells into interstitialfluid with cotransport of Cl-

Na+ moves from filtrate into tubule cells due to

concentration gradient

Collecting ducts extend from cortex (interstitial fluid 300mOsm/kg) through medulla (interstitial fluid very high)

Water moves by osmosis from distal tubule and collecting

duct into more concentrated interstitial fluid

Permeability of wall of distal tubule and collecting ductshave variable permeability to water

Urine can vary in concentration from low volume of high

concentration to high volume of low concentration


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Tubular Secretion of PCT

and Nephron Loop

Waste removal urea, uric acid, bile salts, ammonia, catecholamines, many

drugs

Acid-base balance secretion of hydrogen and bicarbonate ions regulates pH of

body fluids

Primary function of nephron loop

water conservation generates salinity gradient, allows CD to conc. urine

also involved in electrolyte reabsorption


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DCT

and Collecting Duct Principal cells receptors for hormones;

involved in salt/water balance

Intercalated cells involved in

acid/base balance

Function

fluid reabsorption here is variable,

regulated by hormonal action


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DCT

and Collecting Duct Aldosterone effects

q BP p renin release p angiotensin II

formation

angiotensin II stimulates adrenal cortex

adrenal cortex secretes aldosterone

promotes Na+

reabsorption p promotes waterreabsorption p q urine volume p maintains

BP


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DCT

and Collecting Duct Effect of ADH

dehydration stimulates hypothalamus

hypothalamus stimulates posterior pituitary

posterior pituitary releases ADH

ADHo water reabsorption

q urine volume


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DCT and Collecting Duct

Atrial natriuretic peptide (ANP) atria secrete ANP in response to o BP

has four actions:

1. dilates afferent arteriole, constricts efferent

arteriole - o GFR

2. inhibits renin/angiotensin/aldosterone pathway

3. inhibits secretion and action of ADH

4. inhibits NaCl reabsorption

Promotes Na+ and water excretion, o urine

volume, q blood volume and BP


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DCT

and Collecting Duct Effect of PTH

o calcium reabsorption in DCT - o blood Ca2+

o

phosphate excretion in PCT

,q

new boneformation

stimulates kidney production of calcitriol


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Collecting Duct Concentrates

Urine

Osmolarity 4x as concentra-

ted deep in medulla

Medullary portion of CD ismore permeable to water than

to NaCl


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Control of Water Loss Producing hypotonic urine

NaCl reabsorbed by cortical CD

water remains in urine

Producing hypertonic urine

dehydration p oADHp o aquaporinchannels, o CDs water permeability

more water is reabsorbed

urine is more concentrated


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Medullary Concentration Gradient

In order to concentrate urine (and prevent a

large volume of water from being lost), thekidney must maintain a high concentration ofsolutes in the medulla

Interstitial fluid concentration (mOsm/kg) is

300 in the cortical region and graduallyincreases to 1200 at the tip of the pyramids inthe medulla

Maintenance of this gradient depends upon

Functions of loops ofHenle Vasa recta flowing countercurrent to filtrate in

loops ofHenle

Distribution and recycling of urea


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Osmotic Gradient in the Renal

Medulla

Figu

re25

.13


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Countercurrent Exchange

System Formed by vasa recta

provide blood supply to medulla

do not remove NaCl from medulla

Descending capillaries water diffuses out of blood

NaCl diffuses into blood

Ascending capillaries water diffuses into blood

NaCl diffuses out of blood


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Countercurrent Multiplier

Recaptures NaCl and returns it to renal medulla Descending limb

reabsorbs water 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

Recycling of urea: collecting duct-medulla urea accounts for40% of high osmolarity of medulla


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Countercurrent Multiplier

ofNephron Loop Diagram


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Figure 20-10: Countercurrent exchange in the medulla of the kidney

Loops ofHenle and vasa rectafunction together to maintain a highconcentration of solutes in theinterstitial fluids of the medulla andto carry away the water and solutesthat enter the medulla from theloops ofHenle and collecting ducts

Water moves out of descendinglimb and enters vasa recta

Solutes diffuse out of ascending

thin segment and enter vasa recta,but water does not

Solutes transported out of thicksegment of ascending enter thevasa recta

Excess water and solutes carried

away from medulla without reducinghigh concentration of solutes

Concentration of filtrate reduced to100 mOsm/kg by the time it reachesdistal tubule


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Maintenance of Osmolarity

in Renal Medulla


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Summary ofTubular

Reabsorption and Secretion

F i f C d


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Formation of Concentrated

Urine Antidiuretic hormone (ADH) inhibits diuresis In the presence of ADH, 99% of the water in filtrate is

reabsorbed

ADH-dependent water reabsorption is called facultative

water reabsorption ADH is the signal to produce concentrated urine

Stimulates the insertion ofaquaporins into apical membranes of

DCT and collecting ducts

The kidneys ability to respond depends upon the high

medullary osmotic gradient


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Formation of Dilute Urine

Filtrate is diluted in the ascending loop ofHenle.

Water permeability of collecting ducts is extremely low Dilute urine is created by allowing this filtrate to continue into the renal pelvis

(water diuresis)

Ocurrs as long as antidiuretic hormone (ADH) is not being secreted

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)

Diabetes insipidus-condition in which no ADH is secreted or kidney fails to

respond to ADH; 20-25 L of urine excreted per day

Osmotic diuresis- increased urine flow due to increase solute excretion; occurs in

diabetes mellitus or inadequate Na+ reabsorption


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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 1.001 -1.028

Osmolarity - (blood - 300 mOsm/L) ranges from50 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/day


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Diabetes

Chronic polyuria of metabolic origin

With hyperglycemia and glycosuria

diabetes mellitus I and II, insulinhyposecretion/insensitivity

gestational diabetes, 1 to 3% of pregnancies

ADH hyposecretion diabetes insipidus; CD q water

reabsorption


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Diuretics

Effects

o urine output

q blood volume Uses

hypertension and congestive heart failure

Mechanisms of action o GFR q tubular reabsorption


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Voiding Urine - Micturition

200 ml urine in bladder, stretch receptors sendsignal to sacral spinal cord

Signals ascend to inhibitory synapses on sympathetic neurons

micturition center(integrates info from amygdala, cortex) Signals descend to

further inhibit sympathetic neurons

stimulate parasympathetic neurons

Result urinary bladder contraction

relaxation of internal urethral sphincter

External urethral sphincter - corticospinal tracts to

sacral spinal cord inhibit somatic neurons - relaxes


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Neural Control of Micturition

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151 Urinary Phys (New)