Chapter 24: Urinary System

Chapter 24: Urinary System

24.1 Introduction to the Urinary System

  • Components of the Urinary System:

    • Kidneys: Function to filter blood, remove waste products, and convert filtrate into urine.

    • Ureters: Tubes that transport urine from the kidneys to the urinary bladder.

    • Urinary Bladder: Expandable muscular sac that stores urine, capable of holding up to 1 liter.

    • Urethra: Conducts urine to exit the body.

    • Urinary Tract: Consists of the ureters, urinary bladder, and urethra; facilitates urine transport from kidneys to the exterior of the body.

24.1 Additional Functions of the Urinary System

  • Processes in Filtrate Conversion to Urine:

    • Elimination of Metabolic Wastes.

    • Regulation of Ion Levels: Includes ions like sodium ($Na^+$), potassium ($K^+$), calcium ($Ca^{2+}$), and bicarbonate ($HCO_3^-$).

    • Regulation of Acid-Base Balance: Alters H$^+$ levels.

    • Regulating Blood Pressure: Through volume and composition adjustments.

    • Elimination of Biologically Active Molecules: Such as hormones and drugs.

  • Other Functions of the Kidney:

    • Formation of Calcitriol: Increases calcium absorption from the small intestine.

    • Production of Erythropoietin (EPO): Secreted in response to low blood oxygen levels, stimulating erythrocyte production in red bone marrow.

    • Potential for Gluconeogenesis: Particularly during fasting or starvation, producing glucose from non-carbohydrate sources to maintain glucose levels.

24.2 Gross Anatomy of the Kidney

  • Kidney Description:

    • Two symmetrical, bean-shaped organs, approximately the size of a hand to the second knuckle.

    • Exhibit a concave medial border (hilum) where vessels, nerves, and ureters connect to the kidney, and a convex lateral border.

    • Adrenal Gland: Rests on the superior aspect of each kidney.

Location and Support

  • Kidney Position:

    • Located on the posterior abdominal wall, lateral to the vertebral column.

    • Left kidney is situated between T12 and L3 vertebrae; right kidney is approximately 2 cm inferior to the left kidney to accommodate the liver.

    • Partially protected by the rib cage but still vulnerable to trauma.

    • Kidneys are retroperitoneal, with their anterior surfaces covered by the parietal peritoneum.

  • Supportive Structures:

    • Fibrous Capsule: Dense irregular connective tissue directly covering kidney, maintains shape, protects against trauma, and prevents pathogen penetration.

    • Perinephric Fat: Cushions and supports the kidney, located external to the fibrous capsule.

    • Renal Fascia: Dense irregular connective tissue that anchors the kidney to surrounding structures, external to the perinephric fat.

    • Paranephric Fat: Outermost layer surrounding the kidney, also providing cushioning support.

Clinical Notes

  • Renal Ptosis: Inferior movement of the kidney can occur due to loss of adipose tissue, particularly in the elderly or those with anorexia nervosa, potentially kinking the ureter, leading to urine backup and hydronephrosis (swelling) of the kidney, risking renal failure.

  • Variations and Anomalies:

    • Renal Agenesis: Failure of a kidney to develop, often asymptomatic if unilateral but fatal if bilateral.

    • Pelvic Kidney: Kidney fails to migrate from the pelvic cavity.

    • Horseshoe Kidney: Fusion of the inferior parts of the left and right kidneys.

    • Supernumerary Kidney: Development of an additional kidney, typically asymptomatic.

24.2b Sectional Anatomy of the Kidney

  • Regions of Functional Tissue:

    • Outer Renal Cortex: Contains renal corpuscles and convoluted tubules.

    • Inner Renal Medulla: Contains renal pyramids and columns.

    • Corticomedullary Junction: Where the cortex and medulla meet.

  • Renal Lobes:

    • Composed of renal pyramids and portions of adjacent renal columns.

  • Renal Sinus:

    • Located medially, containing the urine drainage area organized into minor and major calyces and renal pelvis.

  • Minor Calyces: Funnel-shaped structures that merge to form major calyx.

    • Renal pelvis formed from merged major calyces.

Innervation of the Kidney

  • Each kidney is innervated by both the sympathetic and parasympathetic divisions of the autonomic nervous system.

  • Sympathetic Nerves: Originate from T10–T12 and primarily affect blood vessels of the kidney and juxtaglomerular apparatus, decreasing urine production.

  • Parasympathetic Nerves: Arise from cranial nerve X (vagus); effects are unknown but do not have a known impact on urine production.

24.3 Nephron

  • Functional Anatomy:

    • Nephrons are the microscopic functional filtration units of the kidney, composed of:

    • Renal Corpuscle: Contains glomerulus (tangled capillary loops) and glomerular capsule.

      • Glomerulus: Receives blood via afferent arterioles and releases it via efferent arterioles.

      • Glomerular Capsule: Contains visceral (permeable) and parietal (impermeable) layers with a capsular space that receives filtrate.

    • Renal Tubule: Divided into:

      • Proximal Convoluted Tubule (PCT): First region, lined with simple cuboidal epithelium and has microvilli for reabsorption.

      • Nephron Loop (Loop of Henle): Comes after PCT, consists of descending and ascending limbs; the descending limb is thinner and more permeable to water, while the ascending limb is thicker and more permeable to salts.

      • Distal Convoluted Tubule (DCT): Final segment that connects to collecting ducts.

  • Types of Nephrons:

    • Cortical Nephrons: Located near the cortex, short nephron loops; 85% of nephrons.

    • Juxtamedullary Nephrons: Located adjacent to corticomedullary junction; contain long nephron loops that extend into the medulla, aiding in establishing a salt concentration gradient.

Collecting Tubules and Ducts

  • Pathway: Nephrons drain into collecting tubules, which empty into larger collecting ducts, eventually leading to papillary ducts within the renal papilla.

  • Epithelial Cell Types:

    • Principal Cells: Responsive to aldosterone and ADH; regulate urine volume and concentration.

    • Intercalated Cells (Types A and B): Involved in regulating urine pH and blood pH.

24.3c Juxtaglomerular Apparatus

  • Purpose: Regulates blood filtrate formation and systemic blood pressure.

  • Components:

    • Granular Cells: Modified smooth muscle cells; synthesize and release renin when stimulated (due to stretch or sympathetic input).

    • Macula Densa: Modified epithelial cells in the DCT that detect changes in NaCl concentration; signal granular cells to release renin.

    • Extraglomerular Mesangial Cells: Located outside the glomerulus; unclear function but may aid renin release.

24.4 Blood Flow Through the Kidney

  • Arterial Supply:

    • Renal Artery: Supplies blood to each kidney, branches into segmental arteries.

    • Interlobar Arteries: Branch from segmental arteries and travel through renal columns.

    • Arcuate Arteries: Continue from interlobar arteries at corticomedullary junction; branch into interlobular arteries.

  • Afferent Arteriole: Enters the glomerulus; efferent arterioles exit and can form peritubular capillaries or vasa recta for gas and nutrient exchange.

  • Venous Drainage:

    • Blood drains through interlobular veins, arcuate veins, and interlobar veins, finally entering the renal vein, which connects to the inferior vena cava.

24.5 Overview of Urine Formation

  • Processes of Urine Formation:

    • Filtration: Occurs in glomerular capillaries; separation of water and solutes from blood plasma, producing filtrate.

    • Tubular Reabsorption: Returns vital solutes and water from tubular fluid into blood, primarily in the PCT.

    • Tubular Secretion: Selective movement of solutes from blood into tubular fluid for excretion.

  • Key Features of Glomerular Filtration:

    • Filtration Membrane: Composed of capillary endothelium, basement membrane, and visceral layer of glomerular capsule; allows the passage of small substances while restricting larger ones.

    • Daily Production: Approximately 180 L of filtrate produced.

Pressures in Glomerular Filtration

  • Glomerular Hydrostatic Pressure: The pressure that pushes water and solutes out of the glomerulus.

  • Opposing Pressures: Include blood colloid osmotic pressure (due to solutes) and capsular hydrostatic pressure (due to filtrate); these pressures influence net filtration pressure (NFP).

  • NFP Calculation: Can be expressed as:
    NFP = HPg - (OP + HPc)
    With normal values leading to a calculated NFP indicating efficient filtration.

Regulation of Glomerular Filtration Rate (GFR)

  • GFR Factors: Influenced by adjustments in the luminal diameter of efferent and afferent arterioles, and pressure gradients.

  • Intrinsic Control: Maintains GFR via mechanisms like myogenic response and tubuloglomerular feedback (senses NaCl concentration).

  • Extrinsic Control: Involves neural and hormonal mechanisms to regulate GFR based on bodily needs, adapting it during stress or hydration levels.

24.6 Overview of Transport Processes

  • Tubular Reabsorption and Secretion Mechanisms:

    • Transport Processes: Utilize simple epithelium for substance transport through paracellular (between cells) or transcellular (across cells) movement.

    • Reabsorption Sites: Primarily in the PCT, supported by microvilli to increase surface area and efficiency.

  • Transport Maximum: Indicates maximum rate of substance reabsorption or secretion based on available transport proteins, with the renal threshold defining the plasma concentration that can be effectively transported without appearing in urine.

Substance Reabsorption Summary

  • Sodium Reabsorption: 98% to 100% reabsorbed; regulated by hormones like aldosterone affecting reabsorption and secretion.

  • Water Reabsorption: Via paracellular and transcellular routes; regulated by ADH in response to body's hydration state.

  • Bicarbonate and pH Regulation: Bicarbonate ions reabsorbed to prevent blood from becoming too acidic; regulated by intercalated cells.

Clinical Considerations

  • Urine Characteristics: Volume, composition, and chemical properties influenced by hydration, diet, and metabolic state. Normal urine contains around 95% water and mild solutes; average urinary volume is 1 to 2 L per day.

  • Urinary Tract Components: Comprises Ureters, Urinary Bladder, and Urethra; structured to transport, store, and eliminate urine.

  • Micturition: Reflex involving both voluntary control and autonomic regulation of urination, with alterations during urine storage and expulsion phases.

Note: This study guide captures relevant information categorically, providing an exhaustive detail of the urinary system and its functions. It may serve as a comprehensive resource for students studying this topic.