WEEK 14: Excretory System

• Urinary system

  Consists of the paired kidneys and ureters, the bladder, and the urethra
• Primary role of the urinary system

  To ensure optimal properties of the blood

  
• General role of the kidneys

  • Water and electrolyte regulation; acid-base balance
  • Excretion of metabolic wastes, excess water, and electrolytes in urine
  • Excretion of bioactive substances (e.g. drugs)
  • Secretion of renin, a protease important for regulation of blood pressure
  • Secretion of erythropoietin, stimulating RBC production in red marrow
  • Conversion of vitamin D to the active form 1,25-dihydroxyvitamin D3 or calcitriol
  • Gluconeogenesis during fasting state

  
• Kidney

  • Kidney-bean shaped
  • With thick outer cortex, surrounding a medulla that is divided into 8-12 renal pyramids; all of these comprise the renal lobe
  • The apical papilla of each renal pyramid inserts into a minor calyx, a subdivision of two or three major calyces extending from the renal pelvis
  • The ureter carries urine from the renal pelvis and exits the renal hilum, where the renal artery and vein are also located

  
• Nephrons

  • Functional units of the kidneys
  • Around 4-5 million each kidney

  
• Major divisions of the nephron

  • Renal corpuscle
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting ducts
• Renal function

  • Filtration - water and solutes in the blood leave the vascular space and enter the lumen of the nephron
  • Tubular reabsorption - substances move from the tubular lumen across the epithelium into the interstitium and surrounding capillaries
  • Tubular secretion - substances move from epithelial cells of the tubules into the lumens, usually after uptake from the surrounding interstitium and capillaries
  • Excretion - filtrate receives various secreted molecules and then enters the minor calyces as urine

  
• Renal corpuscle

  Comprises of glomerulus, a tuft of blood capillaries, surrounded by a double-walled epithelial capsule called the Bowman's capsule

  
• Renal corpuscle

  • Inner visceral layer closely envelops the glomerular capillaries
  • Outer parietal layer forms the surface of the capsule
  • Vascular pole - where the afferent arteriole enters and the efferent arteriole leaves
  • Tubular pole - where the proximal convoluted tubule (PCT) begins

  
• Podocytes


  Apparatus for renal filtration
• Pedicels


  Interdigitating processes covering the capillary surface
• Filtration slits


  Spaces between pedicels
• Slit diaphragms


  Bridges of slit pores, modified and specialized occluding or tight junctions composed of nephrins and other proteins important for renal function
• Glomerular filtration barrier

  • Fenestrated capillary endothelium - blocks blood cells and platelets
  • Glomerular basement membrane (GBM) - restricts large proteins and some organic anions
  • Filtration slit diaphragms between pedicels - restrict some small proteins and organic anions

  
• Mesangial cells


  Tissue macrophage of the kidney, have contractile properties and produce components of an external lamina

  
• Functions of the mesangium


  • fills interstices between capillaries that lack podocytes
  • Physical support of capillaries within the glomerulus
  • Adjusted contractions in response to blood pressure changes
  • Phagocytosis of protein aggregates
  • Secretion of several cytokines, prostaglandins, and other factors important for immune defense and repair in the glomerulus
• Proximal convoluted tubule

  • Simple cuboidal epithelium
  • Cells well-stained, with numerous mitochondria
  • Has numerous pits and vesicles near the bases of the microvilli, indicating active endocytosis and pinocytosis
  • Prominent basal folds and lateral interdigitations
  • With long microvilli that form a prominent brush border that facilitates reabsorption, giving the lumens a fuzz-filled appearance
• Major functions of the proximal convoluted tubule

  • Reabsorption of glucose, amino acids, most water and electrolytes
  • Secretion of organic anions and cations, H+ , and NH4 in the peritubular capillaries
  • Hydroxylation of vitamin D
  • Production of erythropoietin
• Thin ascending and thin descending limbs of the loop of Henle

  • Simple squamous epithelium
  • Few mitochondria
  • Lumen is prominent
• Main functions of the thin limbs

  Passive reabsorption of Na+ and Cl–
• The countercurrent flow of the filtrate (descending, then immediately ascending) establishes a gradient of osmolarity, maintaining the hyperosmotic interstitium
• Thick ascending limbs of the loop of Henle

  • Simple cuboidal epithelium
  • No microvilli, but many mitochondria
• Main functions of the thick ascending limbs

  Active reabsorption of various electrolytes
• Distal convoluted tubule

  • Simple cuboidal epithelium
  • Cells smaller than in PCT
• Proximal convoluted tubule

  • Secretion of organic anions and cations, H+, and NH4 in the peritubular capillaries
  • Hydroxylation of vitamin D
  • Production of erythropoietin
• Loop of Henle

  • Passive reabsorption of Na+ and Cl–
  • The countercurrent flow of the filtrate (descending, then immediately ascending) establishes a gradient of osmolarity, maintaining the hyperosmotic interstitium
• Thin ascending and thin descending limbs

  • Simple squamous epithelium
  • Few mitochondria
  • Lumen is prominent
• Thick ascending limbs

  • Active reabsorption of various electrolytes
• Distal convoluted tubule

  • Reabsorption of electrolytes
  • The rate of Na+ absorption is regulated by aldosterone

  
• Juxtaglomerular apparatus


  • Utilizes feedback mechanisms to regulate glomerular blood flow and keep the rate of glomerular filtration relatively constant
  • JG cells produce renin

  
• Macula densa


  • Thickened region where cells of the distal tubule become columnar
• Juxtaglomerular granular (JG) cells


  • Region of cells adjacent to the macula densa where the tunica media of the afferent arteriole is modified to a secretory phenotype, including granules with the protease renin
• Lacis cells


  • Extraglomerular mesangial cells adjacent to the macula densa, afferent arteriole, and efferent arteriole
• Collecting ducts


  • Final site of water reabsorption from the filtrate
  • Connecting tubule extends from each nephron join together in the cortical medullary rays to form collecting ducts
  • These ducts merge further, forming larger and straighter collecting ducts with increasingly columnar cells, and will form papillary ducts, which deliver urine directly into the minor calyx

  
• Principal cells


  • Regulated reabsorption of water & electrolytes
  • Regulated secretion of K+
  • Rich in aquaporins, the integral membrane pore proteins that function as specific channels for water molecules
• Intercalated cells


  • Reabsorption of K+ (low-K+ diet)
  • Help maintain acid-base balance by secreting either H+ (from type A or α intercalated cells) or HCO3 – (from type B or β intercalated cells)
• Ureters, 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
  • Lined by the uniquely stratified urothelium or transitional epithelium

  
• Ureters

  • Longitudinally folded mucosa, surrounded by a thick muscularis that moves urine by regular waves of peristalsis
  • The lamina propria is lined by transitional epithelium or urothelium
• Urinary bladder

  • Umbrella cells are especially well developed
  • The thickness of the full bladder's urothelium is half that of the empty bladder (5-7 cell layers vs. 2-3 layers)
  • Urothelium is surrounded by a folded lamina propria and submucosa, followed by a dense sheath of interwoven smooth muscle layers and adventitia
  • The muscularis consists of three poorly delineated layers, collectively called the detrusor muscle, which contract to empty the bladder
• Urethra

  • Mucosa has large longitudinal folds around the lumen
  • Varies between stratified columnar in some areas and pseudostratified columnar elsewhere, but it becomes stratified squamous at the distal end of the urethra