Chapter 25 Part A: The Urinary System - Human Anatomy and Physiology

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Human Anatomy and Physiology

  • Edition: Eleventh Edition

  • Chapter: 25

  • Part A: The Urinary System

  • Slides Prepared By: Karen Dunbar Kareiva, Ivy Tech Community College

Function of Kidneys

  • Major Excretory Organ: Kidneys are essential in maintaining the body’s internal environment by performing the following functions:

    • Regulating Water Volume: Controls total water volume and total solute concentration in water.

    • Regulating Ion Concentrations: Maintains ion concentrations in extracellular fluid (ECF).

    • Ensuring Acid-Base Balance: Maintains long-term acid-base balance.

    • Excreting Metabolic Wastes: Eliminates metabolic wastes, toxins, and drugs.

    • Hormone Production:

    • Erythropoietin: Regulates red blood cell (RBC) production.

    • Renin: Regulates blood pressure.

    • Activating Vitamin D: Converts vitamin D to its active form.

    • Gluconeogenesis: Produces glucose during prolonged fasting periods.

Components of the Urinary System

  • Ureters: Transport urine from kidneys to the urinary bladder.

  • Urinary Bladder: Acts as a temporary storage reservoir for urine.

  • Urethra: Responsible for transporting urine out of the body.

Gross Anatomy of the Kidneys

  • Location: Retroperitoneal, located in the superior lumbar region between vertebrae T₁₂ and L₅.

    • Right Kidney: Positioned lower than the left kidney due to the liver crowding its space.

    • Adrenal Glands: Located atop each kidney (also known as suprarenal glands).

  • Surface Features:

    • Convex Lateral Surface

    • Concave Medial Surface: Includes the vertical renal hilum leading to the internal space known as the renal sinus, through which ureters, renal blood vessels, lymphatics, and nerves enter and exit.

Internal Gross Anatomy of the Kidneys

  • Three Distinct Regions:

    1. Renal Cortex: Granular-appearing outer region.

    2. Renal Medulla: Deep to the cortex, composed of cone-shaped medullary pyramids. The broad base of each pyramid faces the cortex, while the papilla (tip) points inward. Renal pyramids are separated by extensions of cortical tissue called renal columns.

    3. Lobes: Each kidney contains approximately eight lobes, each consisting of a medullary pyramid and its surrounding cortical tissue.

Internal Structures of the Kidney

  • Renal Pelvis:

    • Funnel-Shaped Tube: Continuous with the ureter.

    • Minor Calyces: Collect urine from pyramidal papillae, forming cup-shaped areas.

    • Major Calyces: Collect urine from minor calyces and empty it into the renal pelvis.

  • Urine Flow Pathway:

    • Renal pyramid → Minor calyx → Major calyx → Renal pelvis → Ureter.

Blood and Nerve Supply of the Kidneys

  • Rich Blood Supply:

    • Renal arteries deliver approximately one-fourth (1200 ml) of the cardiac output to kidneys every minute.

    • Arterial Flow Pathway:

    • Renal → Segmental → Interlobar → Arcuate → Cortical Radiate (Interlobular).

    • Venous Flow Pathway:

    • Cortical Radiate → Arcuate → Interlobar → Renal Veins (Note: No segmental veins exist).

  • Nerve Supply: Supplied by sympathetic fibers from the renal plexus.

Nephrons

  • Definition: Nephrons are the structural and functional units of the kidneys, essential for urine formation. There are more than 1 million nephrons in each kidney.

  • Two Main Parts:

    1. Renal Corpuscle: Contains the glomerulus and the glomerular capsule (Bowman’s capsule).

    2. Renal Tubule: Continues from the renal corpuscle, consisting of different segments with specific functions.

Renal Corpuscle Structure

  • Components:

    1. Glomerulus: A tuft of capillaries with fenestrated endothelium, allowing for efficient filtrate formation.

    • Filtrate: Plasma-derived fluid processed by renal tubules to form urine.

    1. Glomerular Capsule (Bowman's Capsule):

    • Structure: Cup-shaped, hollow, surrounding the glomerulus consisting of two layers:

      • Parietal Layer: Composed of simple squamous epithelium.

      • Visceral Layer: Clings to glomerular capillaries containing branching epithelial podocytes; foot processes extend to form filtration slits that allow filtrate to pass into the capsular space.

Renal Tubule and Collecting Duct

  • Length: Approximately 3 cm (1.2 in.) long, with a single layer of epithelial cells having distinct histology and function in each segment.

  • Key Segments:

    1. Proximal Convoluted Tubule (PCT):

    • Location: Closest to renal corpuscle.

    • Composition: Composed of cuboidal cells with dense microvilli forming a brush border to increase surface area and large mitochondria; primarily functions in reabsorption and secretion (located in cortex).

    1. Nephron Loop (Loop of Henle):

    • U-shaped, consisting of two limbs:

      • Descending Limb: Continuous with proximal tubule and composed of simple squamous epithelium.

      • Ascending Limb: Thick ascending limb and thin segments in some nephrons with cuboidal or columnar cells.

    1. Distal Convoluted Tubule (DCT):

    • Location: Farthest from renal corpuscle.

    • Composition: Cuboidal cells with few microvilli, functions more in secretion than in reabsorption (located in cortex).

Collecting Ducts

  • Function: Receive filtrate from many nephrons, run through medullary pyramids, and contribute to the striped appearance of the pyramids.

  • Principal Cells: Sparse with short microvilli that maintain water and Na⁺ balance.

  • Intercalated Cells: Two types (A and B) that assist in maintaining acid-base balance of blood.

Classes of Nephrons

  • Two Major Groups:

    1. Cortical Nephrons: Make up 85% of nephrons, primarily located in the cortex.

    2. Juxtamedullary Nephrons: Have long nephron loops that invade the medulla deeply; critical for producing concentrated urine, particularly prominent in desert animals.

Nephron Capillary Beds

  • Associated Capillary Beds:

    • Glomerulus: Specialized capillaries for filtration; unique because they are fed and drained by arterioles (afferent and efferent arterioles).

    • Peritubular Capillaries: Low-pressure, porous capillaries adapting for the absorption of water and solutes, arise from efferent arterioles.

    • Vasa Recta: Long vessels parallel to juxtamedullary nephron loops, arise from efferent arterioles, and are vital for the formation of concentrated urine.

Juxtaglomerular Complex (JGC)

  • Definition: Each nephron has one juxtaglomerular complex consisting of:

    • Distal Portion of Ascending Limb of Nephron Loop.

    • Afferent Arteriole (sometimes efferent): Important for regulating filtrate formation rate and blood pressure.

  • Cell Populations:

    1. Macula Densa: Tall, closely packed cells in the ascending limb containing chemoreceptors for NaCl in filtrate.

    2. Granular Cells: Smooth muscle cells acting as mechanoreceptors sensing blood pressure in afferent arterioles, contain secretory granules with enzyme renin.

    3. Extraglomerular Mesangial Cells: Located between the arteriole and tubule cells, interconnected via gap junctions, possibly transmitting signals between macula densa and granular cells.

Physiology of Kidney

  • Fluid Processing: The kidneys process 180 L of fluid daily, producing only 1.5 L of urine.

  • Filtration Rate: The kidneys filter the body’s plasma volume approximately 60 times each day, consuming 20–25% of the oxygen utilized by the body at rest.

  • Filtrate Composition: Produced via glomerular filtration, it is essentially blood plasma minus proteins; urine is derived from this filtrate and accounts for less than 1% of the original filtrate, containing metabolic wastes and unneeded substances.

Urine Formation Processes

  • Three Processes Involved:

    1. Glomerular Filtration: Produces filtrate, devoid of cells and proteins.

    2. Tubular Reabsorption: Selectively returns 99% of substances from filtrate back to blood in renal tubules and collecting ducts.

    3. Tubular Secretion: Moves specific substances from blood into the filtrate within renal tubules and collecting ducts.

Glomerular Filtration Process

  • Mechanism: A passive process requiring no metabolic energy; hydrostatic pressure forces fluids and solutes through the filtration membrane into the glomerular capsule.

  • Filtration Membrane Structure: Comprises three layers:

    1. Fenestrated Endothelium: Of glomerular capillaries.

    2. Basement Membrane: Fusion of the basal laminae of the other two layers.

    3. Foot Processes of Podocytes: Contain filtration slits allowing only certain sizes of molecules through.

Pressures Affecting Filtration

  • Outward Pressures: Promote filtrate formation; the hydrostatic pressure in the glomerular capillaries (HPgc) is approximately 55 mm Hg.

  • Inward Pressures: Inhibit filtrate formation, mainly hydrostatic pressure in capsular space (HPcs: 15 mm Hg) and colloid osmotic pressure in capillaries (OPgc: 30 mm Hg).

  • Net Filtration Pressure (NFP): Determined as 55 mm Hg (forcing fluid out) - 45 mm Hg (opposing pressure) = 10 mm Hg, critical for determining glomerular filtration rate (GFR).

Glomerular Filtration Rate (GFR)

  • Definition: Rate at which filtrate is formed by both kidneys, typically 120–125 ml/min.

  • Factors Affecting GFR:

    • Directly proportional to net filtration pressure (NFP) and surface area available for filtration; controlled by mesangial cells.

  • GFR is also influenced by the permeability of the filtration membrane, which is significantly higher than that of typical capillaries.

Regulation of GFR

  • Importance: Constant GFR is vital for kidney function and maintaining extracellular homeostasis. GFR influences systemic blood pressure; increased GFR leads to higher urine output and lower blood pressure.

  • Intrinsic Control (Renal Autoregulation): Maintains nearly constant GFR within a mean arterial pressure (MAP) range of 80-180 mm Hg.

    • Methods of Autoregulation:

    1. Myogenic Mechanism: Local smooth muscle contracts when stretched, protecting glomeruli from high blood pressures.

    2. Tubuloglomerular Feedback Mechanism: Regulated by macula densa cells responsive to NaCl concentration changes in filtrate, adjusts afferent arterioles' dilation or constriction to regulate GFR accordingly.

Extrinsic Controls of GFR

  • Nervous and Hormonal Mechanisms: These controls are aimed at maintaining systemic blood pressure and can override intrinsic controls when necessary to increase blood volume.

  • Sympathetic Nervous System Effects: During extreme low effective circulating fluid volume, norepinephrine and epinephrine are released, causing systemic vasoconstriction and afferent arteriole constriction, resulting in decreased GFR and increased blood volume.

Renin-Angiotensin-Aldosterone Mechanism

  • Functionality: The main regulatory mechanism to increase blood pressure involving three triggering pathways for renin release:

    1. Direct sympathetic nervous system stimulation of granular cells.

    2. Activated macula densa cells sensing low NaCl concentrations.

    3. Reduced stretch of granular cells due to lowered blood volume or pressure.