BIO144 Histology Lecture 11. Urinary System

• Urinary System


  • Functions:
  • Regulation of the balance between water and electrolytes
  • Excretion of metabolic wastes along with excess water and electrolytes in urine
  • Secretion of renin
  • Secretion of erythropoietin
  • Conversion of the steroid prohormone vitamin D to its active forms (calcitriol)
  • Gluconeogenesis during starvation or periods of prolonged fasting
• Kidneys


  • Approximately 12-cm long, 6-cm wide, and 2.5-cm thick in adults
  • Has a concave medial border, the hilum
  • Renal cortex
  • Renal medulla – Aligned linear tubules and ducts
• Nephrons


  Functional units of the kidney, each consisting of a corpuscle and a long, simple epithelial renal tubule with three main parts along its length: Renal Corpuscle, Proximal Tubule, Loop of Henle, Distal Tubule, Connecting Tubule
• Blood Filtration


  Filtration occurs through a structure with three parts: The fenestrations of the capillary endothelium, The thick, combined basal laminae, The filtration slit diaphragms between pedicels
• Secretion into the Filtrate

  Organic anions and cations not filtered in the renal corpuscle may be released in the peritubular capillaries, taken up by the cells of the proximal tubules and undergo secretion into the filtrate
• Mesangium


  • Physical support of capillaries within the glomerulus
  • Adjusted contractions in response to blood pressure changes, which help maintain an optimal filtration rate
  • Phagocytosis of protein aggregates adhering to the glomerular filter, including antibody-antigen complexes abundant in many pathological conditions
  • Secretion of several cytokines, prostaglandins, and other factors important for immune defense and repair in the glomerulus
• Renal Tubule


  Transcellular reabsorption involves both active and passive mechanisms, with the cells having a large variety of transmembrane ion pumps, ion channels, transporters, enzymes, and carrier proteins. Water and certain solutes can also move passively between the cells (paracellular transport) along osmotic gradients through leaky apical tight junctions.
• Cell Structure of the Renal Tubule

  • Proximal Tubule Cells: Central nuclei, very acidophilic cytoplasm due to abundant mitochondria, very many long microvilli forming a prominent brush border
  • Distal Convoluted Tubule Cells: Less tubular reabsorption, smaller cells with no brush border, more discernible nuclei
• Loop of Henle


  1. Thin descending limb – Simple squamous epithelium

  2. Thin ascending limb – Simple squamous epithelium

  3. Thick ascending limb – Simple cuboidal epithelium, actively transport sodium and chloride ions out of the tubule and into the interstitium
• Distal Convoluted Tubule


  Less tubular reabsorption, simple cuboidal cells are smaller and have no brush border and more empty lumens, more discernible nuclei
• Collecting Ducts


  1. The last part of each nephron, the connecting tubule, carries the filtrate into a collecting system that transports it to a minor calyx and in which more water is reabsorbed if needed by the body

  2. Approaching the apex of each renal pyramid, several medullary collecting ducts merge again to form each papillary duct (or duct of Bellini), which deliver urine directly into the minor calyx
• Structure of the Collecting Ducts

  • Composed mainly of pale-staining principal cells with few organelles, sparse microvilli, and unusually distinct cell boundaries
  • The medullary collecting ducts are the final site of water reabsorption from the filtrate. Principal cells are particularly rich in aquaporins
  • Scattered among the principal cells are variably darker intercalated cells with more abundant mitochondria and projecting apical folds
• Ureters, Urinary Bladder, and Urethra

  Urine is transported by the ureters from the renal pelvis to the urinary bladder where it is stored until emptying by micturition via the urethra
• Structure of the Ureters, Urinary Bladder, and Urethra


  • The mucosa of these organs is lined by the uniquely stratified urothelium or transitional epithelium
  • Ureters: The thick muscularis of the ureters moves urine toward the bladder by peristaltic contractions and produces prominent mucosal folds when the lumen is empty
  • Urinary Bladder: Umbrella cells are especially well developed, the lamina propria and dense irregular connective tissue of the submucosa are highly vascularized, the muscularis consists of three poorly delineated layers, collectively called the detrusor muscle
  • Urethra: The urethral mucosa has prominent longitudinal folds, the male urethra is longer and consists of three segments: prostatic urethra, membranous urethra, spongy urethra