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Chapter 26

Urinary System

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Urinary System Functions

Filtering of blood: involves three processes-filtration, reabsorption, secretion.Regulation of

Blood volumeConcentration of blood solutes: Na+, Cl-, K+, Ca2+, HPO4

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pH of extracellular fluid: secrete H+

Blood cell synthesis

Synthesis of vitamin D

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Urinary System Anatomy

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Internal Anatomy of KidneysCortex: outer area

Renal columns: part of cortical tissue that extends into medulla

Medulla: inner areaRenal pyramids: cone-shaped. Base is boundary between cortex and medulla. Apex of pyramid is renal papilla, points toward sinus.

CalycesMinor: papillae extend into funnel of minor calyxMajor: converge to form pelvis

Pelvis: enlarged chamber formed by major calycesUreter: exits at the hilum; connects to urinary bladder

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The Nephron

Functional and histological unit of the kidneyParts of the nephron: Bowman’s capsule, proximal tubule, loop of Henle (nephronic loop), distal tubuleUrine continues from the nephron to collecting ducts, papillary ducts, minor calyses, major calyses, and the renal pelvis(Note what is in the renal medulla & 2 types of nephrons)

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Renal Corpuscle

Bowman’s capsule: outer parietal (simple squamous epithelium) and visceral (cells called podocytes) layers.Glomerulus: network of capillaries. Blood enters through afferent arteriole, exits through efferent arteriole.

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Filtration Membrane

Fenestrae: window-like openings in the endothelial cells of the glomerular capillaries. Filtrations slits: gaps between the cell processes of the podocytes. Filtration membrane: capillary endothelium, basement membrane and podocytes.First stage- occurs here when fluid from blood in capillaries moves across filtration membrane into the lumen of Bowman’s capsule.

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Circulation in the GlomerulusAfferent arteriole: supplies blood to glomerulusEfferent arteriole: drains glomerulusBoth vessels have a layer of smooth muscleJuxtaglomerular apparatus: site of renin production

Juxtaglomerular cells-Macula densa-.

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Histology of the NephronProximal tubule: simple cuboidal epithelium with many microvilli—WHY?

Loops of HenleDescending limbAscending limb

Distal tubule: shorter than proximal tubule. Simple cuboidal, but smaller cells and very few microvilli

Collecting ducts: (form where many distal tubules come together. Larger in diameter, form rays and lead to papillary ducts

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Circulation Through the KidneyArterial supply: ((LAB))

1. Renal arteries branch from abdominal aorta

2. Segmental arteriesbranch from renal

3. Interlobar arteriesascend within renal columns toward cortex

4. Arcuate arteriesbranch and arch over the base of the pyramids

5. Interlobular arteriesproject into cortex and give rise to afferent arterioles

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Circulation Through the Kidney

6. Afferent arteriolessupply blood to glomerulus

7. Glomerulus8. Efferent arterioles exit

the renal corpuscle9. Peritubular capillaries

form a plexus around the proximal and distal tubules

10. Vasa recta: specialized parts of peritubular capillaries that course into medulla along with loops of Henle, then back toward cortex

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Circulation Through the Kidney

Venous drainage (LAB)

11.Peritubular capillaries drain into interlobular veinsand lead to

12.Arcuates13. Interlobar veins14.Renal veins

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Anatomy and Histology of Ureters and Bladder

Ureters: bring urine from renal pelvis to urinary bladder. Lined by transitional epitheliumUrinary bladder: hollow in pelvic cavity posterior to symphysis pubis. ( muscle part of wall is detrusor)Trigone: Triangular area between the entry of the two ureters and the exit of the urethra.( Area expands less than rest of bladder during filling)

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Anatomy and Histology of Urethra

Male: extends from the inferior part of the urinary bladder through the penisFemale: shorter; opens into vestibule anterior to vaginal openingInternal urinary sphincter: in males, elastic connective tissue and smooth muscle keep semen from entering urinary bladder during ejaculationExternal urinary sphincter: skeletal muscle surrounds urethra as it extends through pelvic floor. Acts as a valve

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Urine Formation—Outline for LAB Nephrons considered functional units the kidney: smallest

structural component capable of producing urine

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FiltrationFiltrate: water, small molecules, ions that can pass through membranePressure difference forces filtrate across filtration membrane

Renal fraction: part of total cardiac output that passes through the kidneys.( Varies from 12-30%) averages 21%

(Filtration fraction: part of plasma that is filtered into lumen of Bowman’s capsules; average 19%)Glomerular filtration rate (GFR): amount of filtrate produced each minute. 180 L/day

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

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Filtration= # 1Filtration membrane: filtration barrier. It prevents blood cells and proteins from entering lumen of Bowman’s capsule, but is many times more permeable than a typical capillary

Very little protein normally found in urineFiltration pressure: Forces that affect movement of fluid into or out of the lumen of Bowman’s capsule

Glomerular capillary pressure (GCP): blood pressure inside capillary tends to move fluid out of capillary into Bowman’s capsuleCapsule pressure (CP): pressure of filtrate already in the lumenBlood colloid osmotic pressure (BCOP): osmotic pressure caused by proteins in blood. Favors fluid movement into the capillary from the lumen. BCOP greater at end of glomerular capillary than at beginning because of fluid leaving capillary and entering lumenFiltration pressure (10 mm Hg) = GCP (50 mm Hg) – CP (10 mm Hg) – BCOP (30 mm Hg)

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Filtration Pressure

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Tubular Reabsorption: OverviewTubular reabsorption: occurs as filtrate flows through the lumens of proximal tubule, loop of Henle, distal tubule, and collecting ductsResults because of

DiffusionFacilitated diffusionActive transportCotransportOsmosis

Substances transported to interstitial fluid and reabsorbed into peritubular capillaries: inorganic salts, organic molecules, 99% of filtrate volume. /return to general circulation through veins

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Reabsorption in Proximal Tubule

Substances pass through cells of tubule wall. Each cell has

Apical surface: surface that faces filtrate. Apical membraneBasal surface: faces interstitial fluid. Basal membraneLateral surfaces: surfaces between

cells

Active transport of Na+ across the basal membrane from cytoplasm to interstitial fluid linked to reabsorption of most solutes

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Reabsorption in Proximal Tubule

Number of carrier molecules limits rate of transportIn diabetes mellitus

Concentration of glucose in filtrate exceeds rate of transportHigh concentration of glucose in plasma (and thus in filtrate) reflected in glucose in the urine

Diffusion between cells: from lumen of nephron into interstitial fluid

Depends on rate of transport of same solutes through the cells of the tubuleK+, Ca2+, and Mg2+

Filtrate volume reduced by 65% due to osmosis of water

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Reabsorption in Loop of Henle

Loop of Henle descends into medulla; interstitial fluid is high in solutes.Descending thin segment is highly permeable to water and moderately permeable to urea, sodium, most other ions (passive).Water moves out of nephron, solutes in. Volume of filtrate reduced by another 15%.Ascending thin segment is not permeable to water, but is permeable to solutes. Solutes diffuse out of the tubule and into the more dilute interstitial fluid as the ascending limb projects toward the cortex. Solutes diffuse into the descending vasa recta.

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Tubular Secretion

Moves metabolic by-products, drugs, molecules not normally produced by the body into tubule of nephronActive or passiveAmmonia: produced by epithelial cells of nephron. Diffuses into lumenH+, K+, penicillin, and substances such as para-aminohippuric acid (PAH): actively secreted into nephron

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Secretion of Hydrogen and Potassium

A. H+ in at PCT (by countertransport)

B. H+ and K+ secreted into filtrate(future urine)bycountertransport in DCT Na+ and K+ move by active transport

C. Na+ and HCO3-

cotransported into interstitial fluid, then diffuse into peritubular capillaries & away

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Urine Production

In Proximal tubulesNa+ and other substances removedWater follows passivelyFiltrate volume reduced

In descending limb of loop of Henle

Water exits passively, solute entersFiltrate volume reduced 15%

In ascending limb of loop of Henle

Na+, Cl-, K+ transported out of filtrateWater remains

In distal tubules and collecting ducts

Water movement out regulated by ADH

If absent, water not reabsorbed and dilute urine producedIf ADH present, water moves out, results in concentrated urine

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Urine Concentration Mechanism

When large volume of water consumedEliminate excess without losing large amounts of electrolytesResponse is that kidneys produce large volume of dilute urine

When drinking water not availableKidneys produce small volume of concentrated urineRemoves waste and prevents rapid dehydration

Mechanisms that create urine of variable concentration

Maintenance of high concentration of solutes in medullaCountercurrent functions of loops of HenleControl of permeability of distal nephron to water

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Medullary Concentration GradientIn order to concentrate urine (and prevent a large volume of water from being lost), the kidney must maintain a high concentration of solutes in the medullaInterstitial fluid concentration (mOsm/kg) is 300 in the cortical region and gradually increases to 1200 at the tip of the pyramids in the medullaMaintenance of this gradient depends upon

Functions of loops of HenleVasa recta flowing countercurrent to filtrate in loops of HenleDistribution and recycling of urea

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Loops of Henle and vasa recta function together to maintain a high concentration of solutes in the interstitial fluids of the medulla and to carry away the water and solutes that enter the medulla from the loops of Henle and collecting ducts

Water moves out of descending limb and enters vasa rectaSolutes diffuse out of ascending thin segment and enter vasa recta, but water does notSolutes transported out of thick segment of ascending enter the vasa rectaExcess water and solutes carried away from medulla without reducing high concentration of solutesConcentration of filtrate reduced to 100 mOsm/kg by the time it reaches distal tubule

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Water and solutes move out of the collecting duct into the vasa recta

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UreaResponsible for large part of high osmolality in medullaDescending limbs of loops of Henle permeable to urea; urea diffuses into interstitial fluidAscending limbs and distal tubules impermeable to ureaCollecting ducts permeable to urea; some diffuses out into interstitial fluidUrea flows in a cycle maintaining high urea concentration in medulla

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Urine Concentrating Mechanisms

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ADH and the Nephron

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Renin/Angiotensin/Aldosterone

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Other Hormones

Atrial natriuretic hormoneProduced by right atrium of heart when blood volume increases stretching cellsInhibits Na+ reabsorptionInhibits ADH productionIncreases volume of urine producedVenous return is lowered, volume in right atrium decreases

Prostaglandins and kinins: produced in kidney. Role unclear

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Autoregulation and Sympathetic Stimulation

AutoregulationInvolves changes in degree of constriction in afferent arteriolesAs systemic BP increases, afferent arterioles constrict and prevent increase in renal blood flowIncreased rate of blood flow of filtrate past cells of macula densa: signal sent to juxtaglomerular apparatus, afferent arteriole constricts

Sympathetic stimulation: norepinephrineConstricts small arteries and afferent arteriolesDecreases renal blood flow and thus filtrate formation

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Clearance and Tubular LoadPlasma clearance: calculated using substances like inulin

Volume of plasma cleared of a specific substance each minuteUsed to estimate GFR

Tubular loadTotal amount of substance that passes through filtration membrane into nephrons each minute

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Tubular Maximum

Maximum rate at which a substance can be actively absorbed

Each substance has its own tubular maximumNormally, glucose concentration in the plasma (and thus filtrate) is lower than the tubular maximum and all of it is reabsorbed; none of it is found in the urineIn diabetes mellitus tubular load exceeds tubular maximum and glucose appears in urine. Urine volume increases because glucose in filtrate increases osmolality of filtrate reducing the

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Urine Movement

Hydrostatic pressure forces urine through nephronPeristalsis moves urine through ureters from region of renal pelvis to urinary bladder. Occur from once every few seconds to once every 2-3 minutes

Parasympathetic stimulation: increase frequencySympathetic stimulation: decrease frequency

Ureters enter bladder obliquely through trigone. Pressure in bladder compresses ureter and prevents backflow

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Micturition Reflex

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Effects of Aging

Gradual decrease in size of kidneys, but only one-third of one kidney necessary for homeostasisAmount of blood flowing through gradually decreasesNumber of glomeruli decrease and ability to secrete and reabsorb decreasesAbility to concentrate urine declines and kidney becomes less responsive to ADH and aldosteroneReduced ability to participate in vitamin D synthesis contributing to Ca2+ deficiency, osteoporosis, and bone fractures