SH

Urinary System Review

Chapter 24: Urinary System

24.1 Introduction to the Urinary System

  • Structures of the Urinary System:

    • Kidneys

    • Ureters

    • Urinary bladder

    • Urethra

  • General Functions:

    • Remove waste products from the blood

    • Regulate blood volume and blood pressure

    • Regulate electrolytes and metabolites

    • Regulate acid-base balance

    • Produce hormones such as calcitriol and erythropoietin (EPO)

  • Functions of the Kidneys:

    • Elimination of Substances:

      • Urea

      • Creatinine

      • Urinary acids

      • Electrolytes

    • Functions of Calcitriol and EPO:

      • Calcitriol: Active form of Vitamin D that regulates calcium and phosphate in the blood.

      • EPO: Stimulates red blood cell production in the bone marrow.

    • Ions Regulated by Kidneys:

      • Sodium (Na+)

      • Potassium (K+)

      • Calcium (Ca2+)

      • Phosphorus (HPO4^2-)

      • Chloride (Cl-)

    • Acid-Base Regulation:

      • By excreting hydrogen ions (H+) and reabsorbing bicarbonate (HCO3-), kidneys maintain blood pH.

    • Blood Pressure Regulation:

      • By regulating the volume of blood (via water retention and excretion) and secreting renin which increases blood pressure.

    • Gluconeogenesis:

      • The process of glucose production from non-carbohydrate sources, primarily during fasting or low carbohydrate intake; kidneys contribute notably during prolonged fasting.

24.2 Gross Anatomy of the Kidney

  • Regions of the Kidney:

    • Cortex: Outside layer of the kidney

    • Medulla: Inner layer consisting of renal pyramids and columns

  • Urine Flow Tracing:

    • Urine flows from minor calyces - major calyces - renal pelvis - ureter - urinary bladder - urethra.

24.3 Functional Anatomy of the Kidney

  • Nephron Definition:

    • The functional unit of the kidney responsible for urine formation through filtration, reabsorption, and secretion.

  • Major Structures of the Nephron:

    • Renal Corpuscle: Composed of glomerulus and Bowman's capsule

    • Renal Tubule: Composed of proximal convoluted tubule (PCT), nephron loop (loop of Henle), and distal convoluted tubule (DCT)

  • Renal Corpuscle Structure and Components:

    • Glomerulus: A network of capillaries where filtration occurs.

    • Bowman's Capsule: A cup-shaped sac that encases the glomerulus.

  • Renal Tubule Structure:

    • Proximal Convoluted Tubule (PCT): First segment that reabsorbs nutrients and water.

    • Nephron Loop: Involved in the concentration of urine.

    • Distal Convoluted Tubule (DCT): Final adjustments to the filtrate before it enters the collecting ducts.

  • Epithelial Tissues in the Nephron:

    • PCT: Simple cuboidal epithelium with microvilli, facilitates absorption.

    • Nephron Loop: Simple squamous epithelium in the descending limb and cuboidal in the ascending limb for permeability functions.

    • DCT: Similar to PCT but with fewer microvilli; manages final filtrate composition.

  • Types of Nephrons:

    • Cortical Nephrons: Located mainly in the cortex, involved in most of the kidney's functions.

    • Juxtamedullary Nephrons: Located near the medulla, important for establishing a concentration gradient for urine concentration.

  • Juxtaglomerular Apparatus Location and Structure:

    • Located next to the glomerulus and includes granular cells and the macula densa; essential for regulating blood pressure and filtration rate.

  • Actions of Granular Cells:

    • Secrete renin in response to low blood pressure.

  • Function of the Macula Densa:

    • Detects sodium chloride levels in the filtrate, regulating glomerular filtration rate.

  • Function of Extraglomerular Mesangial Cells:

    • Support the glomerulus structurally and help regulate filtration by altering blood flow.

24.4 Blood Flow and Filtered Fluid Flow

  • Peritubular Capillaries vs. Vasa Recta:

    • Peritubular Capillaries: Surround nephron tubules, facilitate nutrient and waste exchange.

    • Vasa Recta: Capillaries associated with juxtamedullary nephrons, critical for maintaining the concentration gradient of the medulla.

  • Differentiating Filtrate, Tubular Fluid, and Urine:

    • Filtrate: Fluid that collects in the Bowman's capsule.

    • Tubular Fluid: Filtrate after it enters the renal tubules, goes through modifications (reabsorption and secretion).

    • Urine: Final product that exits the kidneys, containing waste products.

  • Fluid Flow from Renal Corpuscle to Exit:

    • Filtrate is formed in the renal corpuscle, passes through the nephron (PCT, nephron loop, DCT), enters collecting ducts, travels to the papillary ducts, and exits via the urethra.

24.5 Production of Filtrate Within the Renal Corpuscle

  • Definitions:

    • Filtration: Movement of water and solutes across a membrane due to pressure.

    • Reabsorption: Movement of substances from the tubular fluid back into the blood.

    • Secretion: Movement of substances from the blood into the tubular fluid.

  • Parts of Nephron Where These Occur:

    • Filtration: Renal corpuscle

    • Reabsorption: PCT, nephron loop, DCT

    • Secretion: PCT, DCT

  • Filtration Membrane Layers:

    • Fenestrated Endothelium: Allows passage of blood plasma but retains cells.

    • Visceral Layer (Podocytes): Specialized cells with pedicels that form filtration slits.

    • Basement Membrane: Acts as a charged barrier, repelling most negatively charged proteins.

  • Substances That Can Pass Through Layers:

    • Water, electrolytes, glucose, amino acids can pass; larger proteins and blood cells are retained.

  • Function of Mesangial Cells:

    • Help regulate glomerular filtration, providing structural support and phagocytizing debris.

  • Components of Filtrate:

    • Mainly water, electrolytes, glucose, amino acids, urea.

  • Glomerular Hydrostatic Pressure (HPg):

    • The pressure exerted by blood within the glomerular capillaries; higher than other capillaries due to the arterioles leading in and out and the high resistance they provide.

  • Opposing Pressures to HPg:

    • Capsular Hydrostatic Pressure (HPc): Pressure exerted by fluid in Bowman's capsule opposing filtration.

    • Blood Colloid Osmotic Pressure (COP): Osmotic pressure from proteins in the blood opposing filtration.

  • Net Filtration Pressure Calculation:

    • NFP = HPg - (HPc + COP)

  • Glomerular Filtration Rate (GFR) Definition:

    • The volume of filtrate formed per minute; influenced by NFP, surface area of glomeruli, and permeability of the filtration membrane.

  • Relationship Between HPg and Filtrate Composition:

    • Higher HPg leads to increased solute and water filtration, while lower HPg may reduce filtrate volume.

  • Intrinsic vs. Extrinsic Controls:

    • Intrinsic Controls: Kidney's local regulation mechanisms (e.g., renal autoregulation).

    • Extrinsic Controls: Hormonal and neural signals affecting systemic blood pressure and GFR.

  • Renal Autoregulation Definition:

    • Mechanisms that help maintain a constant GFR despite changes in systemic blood pressure, primarily through myogenic and tubuloglomerular feedback mechanisms.

  • Myogenic Mechanism:

    • A response to stretch in blood vessels, causing smooth muscle to contract and reduce flow when blood pressure increases.

  • Tubuloglomerular Feedback Mechanism:

    • Macula densa cells sense sodium chloride levels; if high, they signal constriction of afferent arterioles to decrease GFR.

  • Function of Macula Densa in Feedback:

    • Monitors sodium chloride concentration; influences renal blood flow and GFR.

  • Limitations to GFR Maintenance:

    • Extreme drops or rises in systemic blood pressure can overwhelm intrinsic mechanisms, potentially leading to renal damage or inadequate filtration.

  • Effects of Neural and Hormonal Control on GFR:

    • SNS activation causes vasoconstriction of afferent arterioles, reducing GFR; hormonal control (e.g., angiotensin II) raises blood pressure and decreases urine output.

24.6 Reabsorption and Secretion in Tubules and Collecting Ducts

  • Definitions of Reabsorption and Secretion (Review):

    • Reabsorption: Moving substances back into blood from tubules.

    • Secretion: Moving substances from blood into the tubular fluid.

  • Transport Maximum Definition:

    • The rate at which a substance is reabsorbed; exceeded when substance concentration in filtrate is too high to allow complete reabsorption, resulting in loss in urine.

  • Osmotic Diuretic Definition:

    • A substance that increases urine output by increasing osmolarity of the tubular fluid, thus preventing reabsorption of water.

  • Renal Threshold Explanation:

    • The plasma concentration at which a substance begins to appear in the urine, directly related to its transport maximum.

  • Filtrate Substances Review:

    • Important for understanding reabsorption and secretion processes, e.g., water, electrolytes, glucose.

  • Reabsorption in Proximal Convoluted Tubule (PCT):

    • Major site for reabsorbing:

      • Glucose

      • Amino acids

      • Bicarbonate

      • Sodium

      • Water (osmosis)

  • Obligatory Water Reabsorption Definition:

    • Water reabsorption that occurs regardless of hydration status, primarily in the PCT and is driven by solute reabsorption.

  • Reabsorption in Nephron Loop:

    • Involves reabsorption of:

      • Water in the descending limb (permeable to water)

      • Sodium and chloride in the ascending limb (impermeable to water)

  • Descending vs. Ascending Limb Permeability:

    • Descending Limb: Highly permeable to water, not to salts.

    • Ascending Limb: Impermeable to water, actively transports salts.

  • Countercurrent Multiplier in Nephron Loop Explanation:

    • Mechanism that creates a concentration gradient in the medulla, allowing for the production of concentrated urine.

  • Function of the Vasa Recta in Countercurrent Exchange System:

    • Maintains the osmotic gradient established by the nephron loop, allowing the exchange of solutes and water without disrupting the gradient.

  • Contribution of Urea Recycling:

    • Helps maintain osmolarity in the interstitial fluid and contributes to the concentration gradient necessary for water reabsorption.

  • Reabsorption and Secretion in DCT and Collecting Duct:

    • Substances involved:

      • Sodium

      • Chloride

      • Water (influenced by ADH)

      • Potassium (secreted)

    • Facultative Water Reabsorption Definition:

      • Water reabsorption that is regulated by hormones (e.g., ADH) according to body hydration status.

  • Regulated Reabsorption Explanation:

    • Specific reabsorption processes influenced by hormones, adjusting for conditions such as hydration levels.

  • Hormones in Regulated Reabsorption in DCT and CD:

    • Antidiuretic Hormone (ADH): Promotes water retention by increasing permeability in the collecting duct.

    • Aldosterone: Increases sodium reabsorption in exchange for potassium secretion.

    • Atrial Natriuretic Peptide (ANP): Inhibits sodium reabsorption, promoting diuresis.

  • Urine Types with Varying ADH Levels:

    • High ADH: concentrated urine.

    • Low ADH: diluted urine.

  • pH Regulation in Collecting Duct:

    • H+ and bicarbonate ions are secreted or reabsorbed to maintain acid-base balance.

  • Other Substances Secreted Along Tubule:

    • Hydrogen ions, ammonium, drugs, and toxins.

24.7 Evaluating Kidney Function

  • Importance of Renal Function Tests:

    • Assess overall kidney health and function by evaluating GFR and clearance levels.

  • Procedure for Measuring GFR:

    • Involves collecting blood and urine over time, analyzing for substances like creatinine; formula is not necessary for recall.

  • Normal Adult GFR:

    • Approximately 120-125 mL/min; lower values could indicate renal impairment or dysfunction.

  • Renal Plasma Clearance and Importance:

    • The volume of plasma cleared of a substance per minute; creatinine is commonly used as it is freely filtered and minimally secreted.

  • Other Tests for Diagnosing Kidney Disease:

    • Urinalysis, imaging studies, blood glucose, and electrolyte levels.

  • Kidney Failure Definition:

    • A significant decline in kidney function; could be due to conditions like diabetes, hypertension, or acute injury.

  • Consequences of Renal Failure:

    • Waste accumulation in the body, electrolyte imbalances, hypertension, and potential cardiovascular disease.

24.8 Urine Characteristics, Transport, Storage, and Elimination

  • Urine Composition and Characteristics:

    • Primarily water, urea, creatinine, uric acid, and ions; characteristics include yellow color, odor, and specific gravity.

  • Normal and Abnormal Constituents of Urine:

    • Normal: Urea, creatinine, electrolytes

    • Abnormal: Glucose, protein, ketones, hemoglobin

  • Clinical Term for Abnormal Constituents:

    • Pyuria: Presence of pus or white blood cells (indicating infection)

    • Glycosuria: Presence of glucose

    • Hematuria: Blood in urine

  • Normal Daily Volume of Urine:

    • Approximately 1-2 liters, affected by fluid intake, hydration status, and certain medications or health conditions.

  • Definitions of Polyuria, Oliguria, and Anuria:

    • Polyuria: Excessive urine production, often due to diabetes or use of diuretics.

    • Oliguria: Insufficient urine production, indicating potential kidney damage or dehydration.

    • Anuria: Absence or very low production of urine, can indicate severe kidney failure or obstruction.

  • Normal Urine pH Value:

    • Generally ranges from 4.5 to 8; diet influences pH (high protein = more acidic, vegetarian = more alkaline).

  • Specific Gravity Definition:

    • A measure of urine concentration; normal specific gravity ranges from 1.005 to 1.030.

  • Normal Color and Smell of Urine:

    • Light yellow color and odor of ammonia; dehydration, foods, or infection can alter these.

  • Epithelial Tissue in Bladder:

    • Transitional epithelium allows stretching during filling of the bladder.

  • Detrusor Muscle Location and Importance:

    • Located in the bladder wall; essential for contraction during urination to expel urine.

  • Differences between Female and Male Urethra:

    • Female urethra is shorter, leading to a higher incidence of urinary tract infections (UTIs).

  • Internal vs. External Urethral Sphincter:

    • Internal: Involuntary muscle controlled by the autonomic nervous system.

    • External: Voluntary muscle under somatic control allowing conscious control over urination.

  • Pathogen Causing UTIs:

    • Most commonly Escherichia coli (E. coli).

  • Women More Prone to UTIs Explanation:

    • Due to shorter urethra and proximity to the anus.

  • Symptoms of UTI and Treatments:

    • Symptoms: Frequent urination, burning sensation, cloudy urine. Treatments: Antibiotics and increased fluid intake.

  • Micturition Definition:

    • The process of expelling urine from the bladder.

  • Storage Reflex vs. Micturition Reflex:

    • Storage Reflex: Inhibits urination, allowing bladder filling.

    • Micturition Reflex: Stimulates bladder contraction when full, resulting in urination.

  • Conscious Control Over Micturition:

    • Involves the external sphincter; learned behavior allowing voluntary control of urination.