Urinary System - Notes

Components of the Urinary System

• Kidney

• Ureters

• Bladder

• Urethra

Components of the Urinary System: Kidneys

• Filter blood

• Remove waste products and convert filtrate into urine

Components of the Urinary System: Urethers

• Transport urine

• From kidneys to urinary bladder

Components of the Urinary System: Bladder

• Expandable muscular sac

• Stores as much as 1L urine

Components of the Urinary System: Urethra

Eliminates urine

Anterior View of Urinary System

Posterior of Urinary System

Function of the Urinary System

• Filtration

• Elimination of Waste

• Regulation of Ion Levels

• Acid-Base Balance

• Blood Pressure Regulation

• Hormone Production

• Gluconeogensis

Function of the Urinary System: Filtration

Kidneys filter blood and produce urine

Function of the Urinary System: Elimination of Waste

Removes metabolic wastes, toxins, excess substances

Function of the Urinary System: Regulation of Ion Levels

Maintains Na⁺, K⁺, and Ca²⁺ balance

Function of the Urinary System: Acid-Base Balance

Adjusts H⁺ and HCO₃^- levels

Function of the Urinary System: Blood Pressure Regulation

Controls blood volume and release renin

Function of the Urinary System: Hormone Production

• Calcitriol: Regulates calcium absorption

• Erythropoietin (EPO): Stimulates red blood cell production

Function of the Urinary System: Gluconeogenesis

Produces glucose during fasting

Gross Anatomy of the Kidney

• Location: Retroperitoneal, lateral to vertebral column

• Structure:

  • Hilum: entry/exit for vessels, nerves, ureter

  • Fibrous Capsule: Protects kidney

  • Perinephric Fat: Cushions and support

  • Renal Fascia & Paranephric Fat: Provide stability

Gross Anatomy of the Kidney: Internal Region

• Renal Cortex: Outer layer

• Renal Medulla: Contains renal pyramids

• Renal Columns: Separate pyramids

• Renal Pelvis: Urine collection area

Clinical View: Renal Ptosis

• Inferior movement of kidney within abdominal cavity

• Due to loss of adipose in elderly or individuals with anorexia nervosa

• May kink ureter, which blocks urine flow to urinary bladder

Clinical View: Hydronephrosis

• Urine builds up, results in swelling of kidney (hydronephrosis)

  • Can lead to renal failure

Clinical View: Renal Agenesis

• Failure of kidney to develop

• Often asymptomatic if unilateral; fatal if bilateral

Clinical View: Pelvic Kidney

Developing kidney fails to migrate from pelvic cavity

Clinical View: Horseshoe Kidney

Inferior parts of left and right kidneys fused

Clinical View: Supernumerary Kidney

• Extra kidney develops

• All 3 typically asymptomatic

Urine Drainage Pathway

Minor Calyces → Major Calyces → Renal Pelvis → Ureter → Bladder

Nephron Structures: Renal Corpuscle

• Glomerulus Capillary: Network for filtration

• Glomerular Capsule: Collects filtrate

Nephron Structures: Renal Tubule

• Proximal Convoluted Tubule (PCT): Reabsorption of nutrients and water

• Nephron Loop: Maintains concentration gradient

• Distal Convoluted Tubule (DCT): Regulated secretion and absorption

Nephron Structures: Two Nephron Type

• Cortical Nephrons: Short loops, 85% of nephrons

• Juxtamedullary Nephrons: Long loops, concentrate urine

Juxtaglomerular Apparatus

• Regulates blood pressure and filtration rate

• Granular Cells: Secrete renin

• Macula Densa: Detects NaCl changes

Histology of Renal Cortex and Medulla

Blood Flow Through the Kidney

Renal Artery → Segmental Arteries → Interlobar Arteries → Arcuate Arteries → Interlobular Arteries → Afferent Arterioles → Glomerulus → Efferent Arterioles → Peritubular Capillaries/Vasa Recta → Veins → Renal Veins

Urine Flow Pathway

Glomerulus → Capsular Space → PCT → Nephron Loop → DCT → Collecting Tubule/Duct → Papillary Duct → Minor & Major Calycles → Renal Pelvis → Ureter → Bladder → Urethra

Filtrate

• Blood flows through glomerulus

• Both water and solutes filtered from blood plasma

• Moves across wall of golmerular capillaries and into capsular space

• Forms filtrate

Tubular Fluid

• New name for filtrate when enters PCT

• Flows through

  • 1.PCT

    2.Nephron loop

    3.DCT

    4.Enters collecting tubules

    5.Empties into collecting ducts

    6.Enters papillary duct within renal papilla; now called urine

Urine Formation

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion

Glomerular Filtration

• In glomerular capillaries

• Separates some water and dissolved solutes from blood plasma

• Water and solutes enter capsular space of renal corpuscle

  • Due To pressure differences across filtration membrane

• Separated fluid is called filtrate

Tubular reabsorption

• Movement of components within tubular fluid

• Move by diffusion, osmosis, or active transport

• Move from lumen or tubules and collecting ducts across walls

• Return to blood within peritubular capillaries and vasa recta

• All vital solutes and most water reabsorbed

• Excess solutes, waste products, some water remaining in tubular fluid

Tubular Secretion

• Movement of solutes, usually by active transport

• Move out of blood within peritubular and vasa recta capillaries

• Move into tubular fluid

• Materials moved selectively into tubules to be excreted

Filtration Membrane Layers

1. Endothelium of glomerulus

2. Basement membrane of glomerulus

3. Visceral layer of glomerular capsule

Endothelium of glomerulus

• Fenestrated, allows plasma and dissolved substances to pass

• Restricts passage of large structures (for example, erythrocytes)

Basement membrane of glomerulus

• Glycoprotein and proteoglycan molecules

• Restricts passage of large plasma proteins

Visceral layer of glomerular capsule

• Outermost layer wrapping around glomerular capillaries

• Composed of specialized cells called podocytes

  • Have long •processes, pedicels

  • Support capillary wall but don’t completely enclose it

  • Separated by thin spaces, filtration slits

  • Restrict passage of most small proteins

Mesangial Cells

• Specialized cells positioned between glomerular capillary loops

• Phagocytic, contractile, and signaling properties

Three Categories of Substances in Blood

1. Freely filtered

2. Not filtered

3. Limited filtration

Free filtered

• Small substances

• For example: water, glucose, amino acids, ions

• Pass easily through filtration membrane

Not filtered

• Formed elements and large proteins

• Cannot pass through filtration membrane

Limited filtration

• Proteins of intermediate size

• Usually blocked from filtration

• Due to size or due to negative charge

Regulation of Glomerular Filtration Rate (GFR): Instrinsic Controls

• Myogenic response (arteriole stretch response)

• Tubulogolmerular feedback (macula densa regulation)

Regulation of Glomerular Filtration Rate (GFR): Extrinsic Controls

• Sympathetic Stimulation: Decreases GFR to conserve water

• Atrial Natriuretic Peptide (ANP): Increase GFR to eliminate excess fluid

Glomerular Hydrostatic (Blood) Pressure (HP_g)

• Blood pressure in glomerulus

  • “Pushes” water and some solutes outo of glomerulus

  • Pushed into capsular space of renal corpuscle

  • Higher than blood pressure of other systemic capillaries

    • Required filtration to occur

    • Large diameter of afferent arteriole

    • Smaller diameter of efferent arteriole

Blood colloid osmotic pressure (OP_g)

• Osmotic pressure exerted by dissolved solutes

  • For example: plasma proteins

• Opposes filtration

• Draws fluid back into glomerulus

Capsular hydrostatic pressure (HP_g)

• Pressure in glomerular capsule due to filtrate

• Impede movement of additional fluid

Pressures that Determine Net Filtration Pressure

Renal Autoregulation: Myogenic Response

Reabsorbed Substances

• Sodium (Na⁺): Controlled by aldosterone & ANP

• Water: Controlled by ADH (facultative reabsorption)

• Potassium (K⁺): Balance between secretion andreabsorption

• Calcium & Phosphate: Regulated by PTH

• Bicarbonate (HCO₃^-) & Hydrogen Ions (H⁺): pH Balance

Clinical View: Glucosuria

• Excretion of glucose in urine

• Plasma glucose concentration above 300 mg/dL

• Glucose cats as an osmotic diuretic

  • Pulls water into tubular fluid

  • Causes loss of fluid in urine

• Classic symptom of diabetes, along with frequent urination and thirst

Glucose Reabsorption

Eliminated Waste Products

• Urea: Protein metabolism byproduct

• Creatinine: From muscle metabolism

• Uric Acid: From nucleic acid breakdown

Reclaiming Filtered Protein

Sodium Reabsorption

• 98% to 100% of Na⁺ reabsorbed from tubular fluid

• Concentration relatively low inside tubule cell

  • Na⁺ moves down gradient across luminal membrane into tubular cell

• Reabsorption regulated by hormones near end of tubule

Sodium reabsorption: Aldosterone

• Steroid hormone produced by adrenal cortex

• Stimulates protein synthesis of Na⁺ channels Na⁺/K⁺ pumps

• Embedded in plasma membranes of principal cells

• Increase in Na⁺ reabsorption

• Water follows by osmosis

Sodium reabsorption: Atrial Natriuretic Peptide

• Inhibits reabsorption of Na⁺ in PCT and collective tubules

• Inhibits release of aldosterone

• More Na⁺ and water excreted in urine

• Increases GFR

Sodium Transport in PCT

Sodium Transport in DCT, CT, or CD

Water reabsorption

• Reabsorbed by paracellular transport between cells

  • Transcellular transport through water transporter proteins, aquaporins

• 180L filtered daily; all but 1.5L reabsorbed

• Tubule permeability varies along its length

• 65% reabsorbed in PCT

  • Aquaporins constant number

  • Water follow Na⁺ by osmosis, obligatory water reabsorption

Water reabsorption: Nephron Loop

10% of filtered water reabsorbed

Water reabsorption: DCT, collecting tubules, and ducts

Water reabsorption controlled by aldosterone and antidiuretic hormone

Water reabsorption: Aldosterone

Increases Na+/K+ pumps and Na+ channels, increasing both Na+ and water reabsorption

Water reabsorption: Antidiuretic Hormone (ADH)

• Binds to principal cells

  • Adds channels to increase water reabsorption

• Concentration gradient within interstitial fluid

  • Tubular reabsorption, facultative water reabsorption

• Results in smaller volume of more concentrated urine

• Elevated levels during dehydration (Urine noticeably darker)

• Urine range = 1200 mOsm to 50 mOsm

Reabsorption and secretion of potassium

• 60%-80% reabsorbed in tubular fluid (PCT)

• Dependent on movement of Na⁺

• 10% to 20% reabsorbed in thick segment of nephron loop

• Net secretion or reabsorption in collecting tubule

Movement of Na⁺

• Sodium reabsorbed across luminal membrane, water follows

• Increased concentration of remaining solutes in tubular fluid

• Creates gradient between tubular fluid and interstitial fluid

• K⁺ moves down gradient from tubular fluid by paracellular route

• Allows passive reabsorption of other solutes

Net secretion or reabsorption in collecting tubule

• Intercalated cells reabsorb K⁺ continuously

• Principal cells secrete K⁺ at varying rates

  • Based on aldosterone level

  • Stimulates principal cells to secrete K⁺

  • Most powerful stimulant for aldosterone, elevated K⁺ level

Calcium and Phosphate Balance

• 60% in blood goes into filtrate

  • Remainder bound to protein and prevented from filtration

• 90% to 95% filtered as blood passes through glomerular capillaries

Parathyroid Hormone (PTH)

• Regulates excretion of Ca²⁺ and PO₄³⁻

• Inhibits PO₄^3- reabsorption in PCT

• Stimulates Ca²⁺ reabsorption in DCT

• Less phosphate available to form calcium phosphate

• Calcium deposition in bone decreased

• Ca ²⁺ blood levels increased

Bicarbonate ions, hydrogen ions, and pH

• Moves freely across filtration membrane

• If filtered HCO₃⁻ not reabsorbed, blood too acidic

• 80% to 90% reclaimed from tubular fluid

• Remaining 10% to 20% taken up from thick segment of ascending limb

• Filtered HCO₃ ⁻  “replaced” not reabsorbed

• pH of urine and blood regulated in collecting tubules

Substances with Regulated Reabsorption: pH (acidic)

• If acidic blood, then

  • Synthesized HCO₃⁻ reabsorbed into blood

  • H⁺ excreted within filtrate by type A intercalated cells

  • Increase blood pH and decrease urine pH

Substances with Regulated Reabsorption: pH (alkaline)

• If alkaline blood, then

  • Type B intercalated cells active

  • Secret HCO₃^- and reabsorb H⁺

  • Lower blood pH and increase urine pH

Nitrogenous waste

• Metabolic waste containing nitrogen

• Main nitrogenous waste products:

  • Urea

  • Uric acid

  • Creatinine

Substances Eliminated as Waste: Urea

• Molecule produced from protein breakdown

• Both reabsorbed and secreted

• 50% excreted in the urine

• Helps establish concentration gradient in the interstitial fluid

Substances Eliminated as Waste: Uric acid

• Produced from nucleic acid breakdown in liver

• Both reabsorbed and secreted

Substances Eliminated as Waste: Creatinine

• Produced from creatinine metabolism in muscle

• Only secreted

Elimination of Drugs and Bioactive substances

• Most secretion occurring in PCT

• Certain Drugs: penicillin, sulfonamides, aspirin

• Other metabolic wastes: urobilin, hormone, metabolites

• Some hormones: human chorionic gonadotropin, epinephrine

Urinary Tract & Micturition: Structures

• Ureters: Transport urine from kidneys to bladder

• Urinary Bladder:

  • Stores urine (up to 1L)

  • Detrusor Muscle: Contracts during urination

• Urethra:

  • Female: only urine transport

  • Male: Longer, transports urine & semen

Micturition (Urination Process)

• Storage Reflex: Sympathetic control inhibits urination

• Micturition Reflex: Parasympathetic stimulation contracts bladder and reflaxes sphincters

• Voluntary Control: Somatic motor control of external urethral sphincter

Common Disorders

• UTI (Urinary Tract Infection): Bacterial infection

• Kidney Stones: Mineral crystallization

• Renal Failure:: Loss of kidney function; may require dialysis

Each nephron loop has two limbs, a(n) ____ limb that extends from the cortex to the medulla and a(n) ____ limb that returns back into the renal cortex

descending, ascending

At least 20%-25% of the resting ____ normally flows through the kidney via the renal artery

cardiac output

Filtrate is produced due to the difference between ___ pressure of the blood in the glomerulus and the opposing pressures of the osmotic blood pressure and fluid pressure in the capsular space.

hydrostatic

A mechanism that is effective in maintaining a normal glomerular blood pressure only if the systemic mean arterial pressure remains between 80 and 180 mm Hg is called ____ autoregulation

renal

Sympathetic stimulation causes the release of renin which will result in the subsequent production of ______ which will result in a(n) ______ in the GFR.

angiotensin II; decrease

Some substances are not normal components of urine because they are completely reabsorbed. Choose the substances that satisfy this requirement under healthy conditions.

• filtered plasma proteins

• lactate

• amino acids

• glucose

All substances that have regulated reabsorption

• K⁺

• Ca²⁺

• Na⁺

• Water

Water is reabsorbed through specific water transporter proteins called ___

Aquaporins

As tubular fluid moves through the collecting tubules and ducts, water reabsorption is controlled primarily by ____ and ____ hormones

aldosterone; antidiuretic

Urine volume can be several liters per day if there is a decrease in the release of ____

ADH

Ureters enter the bladder through the ___ wall of the bladder

posterolateral

The layer of the utretr called the ____ consists of inner and outer layers of smooth msucle

mmuscularis