Renal System

Learning Outcomes
  • Understanding the structure and function of organs in the urinary system.
  • Identifying external and internal features of the kidney.
  • Describing anatomy of the ureters, urinary bladder, and urethra.
  • Outlining blood flow through the kidney.
  • Describing nephron structure and function.
  • Outlining the process of urine formation.
Overview of the Urinary System
  • The urinary system removes most metabolic wastes produced by the body’s cells.
  • Kidneys are responsible for eliminating metabolic wastes from circulation and producing urine.
Organs and Functions of the Urinary System
  • Kidneys: Paired organs that produce urine.
  • Urinary Tract: Eliminates urine.
    • Ureters: Paired tubes transporting urine to the urinary bladder.
    • Urinary Bladder: Muscular sac temporarily storing urine.
    • Urethra: Exit tube through which urine is expelled.
  • Urination/Micturition: The process of eliminating urine. Contraction of the urinary bladder forces urine through the urethra.
Functions of the Urinary System
  1. Excretion: Removal of metabolic wastes from body fluids.
  2. Elimination: Discharge of wastes from the body.
  3. Homeostatic Regulation:
    • Regulates blood volume and pressure by adjusting the water lost in urine.
    • Releases erythropoietin and renin.
    • Regulates plasma ion concentrations (sodium, potassium, chloride, etc.).
    • Synthesizes calcitriol to control calcium ion levels.
  4. Blood pH: Stabilizes blood pH by controlling loss of hydrogen and bicarbonate ions in urine.
  5. Nutrient Conservation: Prevents loss of valuable nutrients while removing metabolic wastes (e.g., urea, uric acid).
  6. Liver Assistance: Helps the liver in detoxifying poisons and deaminating amino acids during starvation.
Structure and Position of Kidneys
  • Kidneys are located on either side of the vertebral column.
    • The left kidney is slightly higher than the right.
  • Capped by the adrenal gland and stabilized by 3 layers:
    1. Fibrous Capsule: Collagen fibers covering the outer surface of the kidney.
    2. Perinephric Fat: Adipose tissue surrounding the fibrous capsule.
    3. Renal Fascia: Dense fibrous layer anchoring kidney to surrounding structures.
  • Function: Each kidney weighs about 150 g and has a typical size of 10 cm long, 5.5 cm wide, and 3 cm thick.
  • Hilum: Entry point for the renal artery and exit point for the renal vein and ureter.
Nephrons: Structure and Function
  • Nephrons: Microscopic functional units of the kidneys, consisting of
    • Renal Corpuscle: Composed of glomerular (Bowman's) capsule and glomerulus (capillary network).
    • Renal Tubule: Has proximal convoluted tubule (PCT) and distal convoluted tubule (DCT), with a nephron loop in between.
  • Filtration: Blood pressure drives the filtration of water and solutes into the capsular space via the filtration membrane (fenestrated endothelium, basement membrane, and podocyte foot processes). Larger solutes, such as plasma proteins, do not pass.
Urine Formation Process
  • Proximal Convoluted Tubule (PCT):
    • Site of significant reabsorption, particularly of ions.
  • Nephron Loop:
    • Divided into descending and ascending limbs, crucial for concentrating urine.
  • Distal Convoluted Tubule (DCT):
    • Smaller luminal diameter; functions in reabsorption of water and ions, and secretion of unwanted substances.
The Collecting System and Nephrons
  • Collecting Ducts: Carry tubular fluid away from nephrons and determine final osmotic concentration and volume of urine.
  • Types of Nephrons:
    • Cortical Nephrons: 85% of nephrons, primarily in the cortex.
    • Juxtamedullary Nephrons: 15% of nephrons, having long loops that extend deep into the medulla with efferent arterioles connecting to the vasa recta.