urinary

Learning Objectives

• Describe the structure and function of the urinary system

• Explain kidney roles in fluid, electrolyte, pH, and blood pressure regulation

• Describe nephron structure and urine formation

• Explain regulation of glomerular filtration and urine concentration

• Describe urine transport, storage, and elimination

Overview of the Urinary System

• The urinary system consists of the:
    - Kidneys
    - Ureters
    - Bladder
    - Urethra

• Its primary function is to maintain homeostasis by managing the volume and composition of fluid reservoirs, primarily blood.

Homeostatic Functions of the Kidneys

Regulation Roles

• Blood Ionic Composition
    - Regulates ions:
      - Sodium (Na⁺)
      - Potassium (K⁺)
      - Chloride (Cl⁻)

• Blood pH Regulation
    - Regulates:
      - Hydrogen ions (H⁺)
      - Bicarbonate (HCO₃⁻)

• Blood Volume Regulation
    - Regulation of water (H₂O)

• Blood Pressure Regulation

Additional Functions

• Maintenance of blood osmolarity

• Production of hormones
    - Calcitrol
    - Erythropoietin

• Regulation of Blood Glucose Level

• Excretion of Metabolic Wastes and Foreign Substances
    - Includes drugs or toxins

Renal Anatomy

• The kidneys are characterized by the hilum, an indentation that serves as an entrance for the following:
    - Renal Artery
    - Renal Vein
    - Ureter
    - Nerves
    - Lymphatics

Internal Renal Anatomy

• Urine Processing Structures
    - Papillary Ducts: Empty urine into the calyces
    - Calyces: Transfer urine to the ureter

Nephron Structure and Function

• Nephron: The functional unit of the kidney
    - Renal Corpuscle: Responsible for filtering blood plasma
    - Renal Tubule: Modifies the filtrate

Components of a Nephron

• Proximal Convoluted Tubule (PCT)
    - Reabsorbs water, ions, and all organic nutrients

• Distal Convoluted Tubule (DCT)
    - Secretes ions, acids, drugs, and toxins

• Loop of Henle
    - Descending Limb: Water reabsorption
    - Ascending Limb: Reabsorption of sodium and chloride ions

• Collecting Duct
    - Variable reabsorption of water and secretion of ions

Types of Nephrons

• Cortical Nephrons
    - Comprise ~85% of all nephrons
    - Located almost entirely in the cortex with the bottom of the loop of Henle in the medulla

• Juxtamedullary Nephrons
    - Found closer to the renal medulla
    - Their loop of Henle extends deep into renal pyramids, specialized for urine concentration and water conservation
    - Contain vasa recta capillaries

The Renal Corpuscle

• Glomerulus
    - A mass of capillaries that filters blood plasma
    - It is fed by the Afferent Arteriole and drains into the Efferent Arteriole

• Bowman's Capsule
    - Comprised of two layers:
      - Visceral Layer: Contains podocytes wrapping around the capillaries
      - Parietal Layer: Simple squamous epithelium
    - Filtrate is collected between these layers

Renal Blood Flow

Blood Supply to the Nephron

• Afferent Arteriole: Supplies blood to glomerulus

• Efferent Arteriole: Drains blood from glomerulus

• Blood circulation in the glomerulus is influenced by smooth muscle layers in both arterioles

Juxtaglomerular Apparatus (JGA)

• Juxtaglomerular Cells: Smooth muscle ring in the afferent arteriole; involved in renin production

• Macula Densa: Specialized tubule cells of the distal convoluted tubule that monitor sodium levels and act to constrict the afferent arteriole when necessary

Urine Formation and Regulation

Phases of Urine Formation

1. Glomerular Filtration
     - Driven by blood pressure and results in the movement of water and small molecules out of the glomerulus
     - Opposed by:
       - Capsular Hydrostatic Pressure
       - Blood Colloid Osmotic Pressure

2. Tubular Reabsorption
     - Essential nutrients and water are reabsorbed back into the bloodstream

3. Tubular Secretion
     - Additional waste products are secreted into the filtrate

Glomerular Filtration Rate (GFR)

• Definition: The amount of filtrate formed by both kidneys each minute

• Homeostasis Requirements:
    - A constant GFR is necessary to avoid too high (rapid flow preventing absorption) or too low (negligible excretion of waste) conditions

Regulation of GFR

• Autoregulation: Adjustments within the kidney

• Neural Regulation: Sympathetic fibers can cause afferent arteriole constriction during stress (e.g., exercise or hemorrhage)

• Hormonal Regulation:
    - Angiotensin II: Reduces GFR by constricting both arterioles
    - Atrial Natriuretic Peptide (ANP): Increases GFR in response to cardiac atrial stretch

Tubular Reabsorption and Secretion

• Major reabsorption occurs for water, glucose, amino acids, and ions

• Reabsorption in PCT: Limited by the number of carrier molecules available
    - Example: In diabetes mellitus, excess glucose in filtrate is reflected in urine due to exceeded transport rates

Reabsorption in Loop of Henle

• Descending Limb: Permeable to water, leading to volume reduction

• Ascending Limb: Permeable to solutes, enhances filtrate dilution

Late DCT and Collecting Duct

• Principal Cells: Sensitive to ADH, absorb Na⁺
    - Intercalated Cells: Acid-base balance through K⁺ and HCO₃⁻ reabsorption and H⁺ secretion

Urine Production

• Fluid intake variability affects urine output; homeostasis maintains fluid volumes

• In the presence of ADH, collecting ducts increase permeability to water, concentrating urine

Evaluation of Kidney Function

• Routine urinalysis checks for the presence of:
    - Albumin
    - Glucose
    - Red Blood Cells
    - Ketone Bodies
    - Microbes

Urine Transport and Storage

• Ureters: Transport urine via peristaltic waves, hydrostatic pressure, and gravity

• Urine transport begins at renal pelvis; no anatomical valve is present against backflow into ureters due to bladder compression

• Bladder: Hollow muscular organ that stores urine until micturition

Micturition (Urination)

• Involves both voluntary and involuntary muscle contractions triggered by stretch receptors

• The urethra transports urine out of the body and serves a dual function in males (urination and semen discharge).