Chapter 25 - Urinary System

Chapter 25 - Urinary System

Objective 1: Functions of the Kidneys

  • Main Role: Maintenance of internal fluid purity and constancy.

  • Overview: The kidneys filter gallons of fluid from the bloodstream, eliminating metabolic wastes, toxins, and excess ions while returning necessary substances to the blood.

Major Functions

  • Elimination of Wastes:

    • Nitrogenous wastes, toxins, and drugs are expelled from the body.

  • Regulation of Blood Composition:

    • Volume and chemical makeup of the blood are controlled.

  • Maintenance of Balance:

    • Water and salts balanced alongside acids and bases.

Other Functions

  • Renin Production:

    • Helps regulate blood pressure and kidney function.

  • Erythropoietin Production:

    • Stimulates red blood cell production.

  • Vitamin D Metabolism:

    • Converts vitamin D to its active form.

Objective 2: Organs of the Urinary System

  • Components:

    • Kidneys: Primary organs for filtration and urine production.

    • Ureters: Tubes that carry urine from kidneys to bladder.

    • Urinary Bladder: Temporary storage for urine.

    • Urethra: Tube through which urine exits the body.

Objective 3: Kidney Structure and Urine Flow

  • Kidney Characteristics:

    • Bean-shaped, retroperitoneal position in the superior lumbar region.

    • Right kidney is lower due to liver positioning.

    • Contains a medial cleft called the hilus for entry/exit of vessels and nerves.

Supporting Tissues

  1. Renal Capsule:

    • Transparent, strong barrier preventing infection spread.

  2. Adipose Capsule:

    • Fatty tissue providing protection and placement.

  3. Renal Fascia:

    • Dense connective tissue anchoring kidneys.

Internal Anatomy

  • Cortex:

    • Outer region housing nephrons.

  • Medulla:

    • Middle region with renal pyramids.

  • Pelvis:

    • Funnel-shaped tube continuous with ureters, collecting urine via major calyces.

Urine Flow Process

  • From Kidney to Bladder:

    • Urine drains from renal pyramids into calyces, then into pelvis and ureters.

    • Transport Mechanism: Peristalsis contracts walls of calyces, pelvis, and ureters to move urine.

Objective 4: Nephron Structure and Function

  • Nephron: Fundamental unit of kidney function (over one million per kidney).

Parts of the Nephron and Their Functions

  1. Glomerulus:

    • Tuft of capillaries for filtration.

  2. Bowman's Capsule:

    • Surrounds glomerulus, collects filtrate.

  3. Proximal Convoluted Tubule:

    • Responsible for tubular reabsorption.

  4. Loop of Henle:

    • Balances sodium and water.

  5. Distal Convoluted Tubule:

    • Site of tubular secretion.

Objective 5: Urine Formation Process

  • Filtrate Processing:

    • Filtrate from distal convoluted tubule goes to collecting tubules, which converge into larger ducts to deliver urine to minor calyces.

  • Fluid Processed Daily:

    • Kidneys process about 47 gallons (approximately 178.5 liters) daily, with only about 1.5 liters leaving as urine.

Processes Involved in Urine Formation

  1. Glomerular Filtration

  2. Tubular Reabsorption

  3. Tubular Secretion

Objective 6: Glomerular Filtration

  • Definition: Initial phase of urine formation; a passive, non-selective process driven by hydrostatic pressure.

  • Key Features of Glomerulus:

    • Fenestrated Capillaries: Permit free passage of everything except blood cells and plasma proteins.

    • Higher Glomerular Blood Pressure: Ensures efficient filtration.

Net Filtration Pressure (NFP)

  • Definition: Pressure favoring renal filtrate formation derived from opposing forces:

    • Glomerular Hydrostatic Pressure: 55 mm Hg (chief force pushing filtrate out).

    • Glomerular Osmotic Pressure: 30 mm Hg.

    • Capsular Hydrostatic Pressure: 15 mm Hg.

  • Calculation:

    • Net Filtration Pressure = 55 mm Hg - (30 mm Hg + 15 mm Hg) = 10 mm Hg.

Glomerular Filtration Rate (GFR)

  • Definition: Amount of fluid filtered per minute; normal GFR = 120 ml/min.

Factors Affecting GFR

  1. Total Surface Area: Surface available for filtration.

  2. Filtration Membrane Permeability: How permeable the membrane is.

  3. Net Filtration Pressure: Major limiting factor; changes in pressures impact GFR.

Regulation of GFR

  • Autoregulatory Mechanism: Maintains GFR despite blood pressure fluctuations.

  • Juxtaglomerular Apparatus: Contains juxtaglomerular cells (renin production) and macula densa cells (respond to salt content).

Objective 7: Tubular Reabsorption

  • Difference Between Filtrate and Urine:

    • Filtrate is composed of plasma components without proteins, whereas urine is concentrated metabolic waste.

  • Reabsorption Process: Occurs mainly in proximal convoluted tubule.

Active vs. Passive Reabsorption

  • Active Tubular Reabsorption:

    • Involves substances against concentration gradient (e.g., glucose, amino acids, ions).

  • Passive Tubular Reabsorption:

    • Water exits via osmosis due to sodium reabsorption (solvent drag).

Objective 8: Tubular Secretion

  • Function: To dispose of substances not in filtrate (e.g., drugs) and eliminate reabsorbed wastes (urea, uric acid).

  • pH Regulation:

    • Secretion of H⁺ when pH is too low, and HCO₃⁻ when pH is too high.

Objective 9: Loop of Henle

  • Description: Selective reabsorption of sodium and water maintaining solute load.

Permeability Characteristics

  • Descending Loop: Permeable to water, impermeable to sodium.

  • Ascending Loop: Permeable to sodium, impermeable to water.

Objective 10: Formation of Dilute and Concentrated Urine

  • Dilute Urine: Formed in the absence of ADH; distal/collecting tubules are impermeable to water.

  • Concentrated Urine: Formed with the presence of ADH; distal/collecting tubules become permeable, allowing water reabsorption.

  • Diuretics: Medications that inhibit sodium reabsorption, affecting urine volume.

Objective 11: Ureter, Bladder, and Urethra Structure and Function

  • Ureters: Slender tubes transporting urine from kidneys to bladder, aided by peristalsis.

  • Bladder: Smooth, collapsible muscular sac for urine storage; features transitional epithelium.

  • Urethra: Thin muscular tube that drains urine from the bladder to the outside of the body.