Urinary System and Kidney Function Notes

Urinary System Overview

  • Primary Function:
    • Producing urine and removing it from the body.
  • Major Structures:
    • Kidneys: Produce urine.
    • Urinary Bladder: Store urine.
    • Ureters: Transport urine.
    • Urethra: Transport urine.

Functions of the Kidneys

  • Primary Functions:
    • Removing wastes and unwanted substances from blood and excreting them in urine.
  • Secondary Functions:
    • Regulating volume and electrolyte composition of body fluids.
  • Other Functions Include:
    • Regulation of arterial pressure.
    • Regulation of acid-base balance.
    • Secretion, metabolism, and excretion of hormones.
    • Secretion of erythropoietin (stimulates red blood cell production).
    • Production of Vitamin D (calcium deposition and reabsorption).
    • Gluconeogenesis (synthesis of glucose from amino acids during prolonged fasting).

Importance of the Kidneys

  • Excretion of Waste:
    • Urea: From metabolism of amino acids.
    • Creatinine: From breakdown of muscle creatine.
    • Uric Acid: From breakdown of nucleic acids.
  • Other Excretes:
    • End products of hemoglobin breakdown, metabolites of hormones, foreign chemicals, and drugs (e.g., antibiotics, ibuprofen, aspirin).
  • Homeostasis:
    • Regulation of water and electrolytes to maintain balance despite daily fluctuations.

Structure of the Kidney

  • Regions:
    • Cortex: Outer region of kidney.
    • Medulla: Inner region of kidney.
  • Renal Pyramids: 8-10 per kidney that are cone-shaped and striated.
  • Urine Conduction Structures:
    • Minor Calyx: Collects urine from nephron pyramids.
    • Major Calyx: Merging of multiple minor calyces.
    • Renal Pelvis: Collects urine from major calyces; becomes the ureter.
  • Muscle Fibers in calyces and pelvis to propel urine towards the bladder.

Nephron: The Functional Unit of Kidney

  • Definition:
    • Responsible for urine formation; over 1 million nephrons in each kidney.
  • Components:
    • Glomerulus: Filters blood.
    • Bowman’s Capsule: Collects filtrate.
    • Tubule: Converts filtrate to urine (includes proximal tubule, loop of Henle, distal tubule, and collecting duct).

Blood Supply of the Kidney

  • Renal Arteries: Supply blood; each branches into interlobar arteries, arcuate arteries, arterioles.
  • Afferent Arterioles: Lead to glomerular capillaries; Efferent Arterioles: Drain glomerular capillaries.
  • Capillary Beds:
    • Glomerular Capillaries: Filtration.
    • Peritubular Capillaries: Absorption.
  • Venous Return: Interlobar vein -> renal vein.

Types of Nephrons

  • Cortical Nephrons (70-80%):
    • Located in outer cortex, short loops of Henle.
  • Juxtamedullary Nephrons (20-30%):
    • Located deep in cortex near medulla, long loops of Henle, important for urine concentration.

Glomerular Filtration

  • Process of Filtration:
    1. Filtration: Fluid and solutes filtered from blood.
    2. Reabsorption: Substances reabsorbed into blood.
    3. Secretion: Substances secreted from blood into renal tubules.
    4. Excretion: Removal via urine.
  • Factors Influencing GFR:
    • Hydrostatic pressure, colloid osmotic pressure, and net filtration pressure (NFP).

Glomerular Filtration Rate (GFR)

  • GFR Definition: Volume of filtrate formed by glomeruli each minute (typically ~125 mL/min).
  • Calculating GFR:
    • Using NFP and filtration coefficient (K_f).
  • Determinants:
    • Arterial pressure, afferent and efferent arteriolar resistance, surface area of glomerular capillaries.