Urinary System and Nephrons — Quick Reference Notes

Role of the Urinary System in Excretion

  • Excretion removes wastes from the body; organs involved include skin, lungs, GI tract; urinary system eliminates water, salts, and water-soluble metabolites including nitrogenous wastes.
  • Nitrogenous wastes:
    • Urea — from amino acid breakdown; waste product of dietary protein; untreated uremia may cause seizures, coma, cardiac arrest.
    • Creatinine — produced from muscle.
    • Uric acid — high levels linked to gout.

Functions of the Kidneys

  • Regulate total body water volume and total solute concentration.
  • Regulate ion concentrations in extracellular fluid.
  • Maintain long-term acid–base balance.
  • Excrete metabolic wastes, toxins, and drugs.
  • Produce erythropoietin and renin.
  • Activate vitamin D.
  • Carry out gluconeogenesis if needed.

Components of the Urinary System

  • Kidneys
  • Ureters
  • Urinary bladder
  • Urethra

Gross Anatomy of the Kidney: Location and External Anatomy

  • Retroperitoneal in the superior lumbar region.
  • Right kidney lower than left due to liver crowding.
  • Adrenal gland atop each kidney.
  • Convex lateral surface; concave medial surface with renal hilum leading to renal sinus.

Supporting Tissues Surrounding the Kidney

  • Renal fascia: anchoring outer dense fibrous tissue.
  • Perirenal fat capsule: fatty cushion.
  • Fibrous capsule: transparent capsule preventing spread of infection to the kidney.

Internal Gross Anatomy

  • Renal cortex: superficial region.
  • Renal medulla: cone-shaped medullary (renal) pyramids; papilla at the tip points internally.
  • Renal pyramids separated by renal columns (extensions of cortical tissue).
  • Lobe: medullary pyramid and surrounding cortical tissue; about 88 lobes/kidney.

Renal Pelvis and Calyces

  • Renal pelvis: funnel-shaped tube continuous with the ureter.
  • Minor calyces collect urine from pyramidal papillae.
  • Major calyces collect urine from minor calyces and drain into the renal pelvis.
  • Urine flow: Renal pyramid → minor calyx → major calyx → renal pelvis → ureter.

Blood and Nerve Supply

  • Renal arteries deliver about 14\frac{1}{4} of cardiac output to kidneys each minute; approximately 1200 mL/min1200\,\text{mL/min}.
  • Renal veins drain the kidneys.
  • Nerve supply: sympathetic fibers from the renal plexus.

Nephrons

  • Structural and functional units that form urine.
  • More than >10^6 per kidney.
  • Two main parts: renal corpuscle and renal tubule.

Renal Corpuscle

  • Glomerulus: tuft of highly porous capillaries; filtration forms filtrate (plasma-derived fluid).
  • Glomerular capsule (Bowman’s capsule): cup-shaped hollow structure surrounding the glomerulus.

Renal Tubule and Collecting Duct

  • Renal tubule length: about 3 cm3\,\text{cm}.
  • Lined by a single layer of epithelial cells.
  • Three major parts:
    • Proximal convoluted tubule (PCT)
    • Nephron loop (Loop of Henle)
    • Distal convoluted tubule (DCT)
  • Distal tubule drains into the collecting duct.

Nephron Classes

  • Cortical nephrons: ~85%85\%; located mostly in the cortex.
  • Juxtamedullary nephrons: long nephron loops invading the medulla; thick and thin segments; important for production of concentrated urine.

Nephron Capillary Beds

  • Glomerulus: capillaries specialized for filtration; fed and drained by arterioles (afferent in, efferent out).
  • Peritubular capillaries: low-pressure, porous; absorb water and solutes; arise from efferent arterioles; cling to renal tubules; drain into venules.
  • Vasa recta: long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons; arise from efferent arterioles; participate in formation of concentrated urine.

Urine Transport and Micturition

  • Ureters: transport urine from kidneys to bladder.
  • Urinary bladder: temporary storage reservoir for urine.
  • Urethra: transports urine out of the body.
  • Micturition: neural control of urination.

Clinical – Homeostatic Imbalance

  • Pyelitis: infection of the renal pelvis and calyces.
  • Pyelonephritis: infection/inflammation of the entire kidney; severe cases can cause swelling, abscess formation, and pus in the renal pelvis; untreated may damage the kidney; typically treated with antibiotics.