Urinary System

• Urinary system

  Consists of the paired kidneys and ureters, the bladder, and the urethra
• The two kidneys produce about 125mL of filtrate/min

  
• 124 mL of the filtrate is reabsorbed, and 1 mL is released as urine
• About 1500 mL of urine is formed every 24 hrs
• Functions of the urinary system

  • Helps maintain homeostasis by a complex combination of processes
  • Filtration of cellular wastes from blood (primarily by the glomerulus)
  • Selective reabsorption of important constituents like proteins and glucose along with water and solutes
  • Excretion of wastes and excess water as urine
  • Regulate the fluid and electrolyte balance of the body
  • Site of renin production
  • Produce erythropoietin
  • Hydroxylation of vitamin D

  
• Nephron


  The functional unit of the kidney, 1-1.4 million per kidney

  
• Major divisions of each nephron

  • Renal corpuscle
  • Proximal Convoluted Tubule (PCT)
  • Thin and thick limbs of the nephron loop (Loop of Henle)
  • Distal Convoluted Tubule (DCT)
  • Collecting tubule

  
• Classification of nephrons

  • Cortical (short-looped)
  • Juxtamedullary (long looped)
• Renal corpuscle


  The main site of blood filtration

  
• Renal corpuscle

  • Has 2 layers: parietal and visceral
  • Parietal layer is a simple squamous epithelium
  • Visceral layer contains podocytes

  
• Podocytes


  Cells of the visceral layer that extend foot processes to embrace the glomerular capillaries
• Mesangial cells


  • Provide physical support and contraction
  • Perform phagocytosis
  • Secrete cytokines and prostoglandins
• Glomerular filtration barrier

  Sheet of tissue that separates the blood in the glomerular capillaries from the Bowman's space

  
• Glomerular filtration barrier


  • Consists of fenestrated endothelium, basal lamina, and filtration slits between podocyte foot processes
• Proximal Convoluted Tubule (PCT)


  Reabsorbs 60-65% of the water filtered in the renal corpuscle along with almost all of the nutrients, ions, vitamins, and small plasma proteins

  
• Proximal Convoluted Tubule (PCT)

  • Has acidophilic cytoplasm due to numerous mitochondria
  • Actively reabsorbs glucose, amino acids, sodium, chloride, and other ions
  • Involved in vitamin D hydroxylation
• Nephron loop (Loop of Henle)

  1. shaped structure with a descending limb and an ascending limb
• Segments of the Loop of Henle

  • Thick descending limb
  • Thin descending limb
  • Thin ascending limb (for juxtamedullary nephrons)
  • Thick ascending limb
• Distal Convoluted Tubule (DCT)


  • Cells are smaller and have no brush border, but have basal membrane invaginations and associated mitochondria
  • Rate of sodium absorption and potassium secretion is regulated by aldosterone from the adrenal glands
• Glomerular filtration

  Further concentration of glomerular filtrate
• Thick ascending limb


  • Starts in the medulla then heads for the cortex
  • Straight as it enters the cortex then becomes tortuous as the DCT
  • Histologically and functionally identical with the DCT
  • Transforms from simple squamous to simple cuboidal
• Distal convoluted tubule (DCT)


  • Simple cuboidal cells differ from PCT in being smaller and having no brush border
  • DCT cells are flatter and smaller than PCT, more nuclei seen in sections
  • DCT cells have basal membrane invaginations and associated mitochondria similar to PCT
  • Rate of Na absorption and K secretion regulated by aldosterone from adrenal glands
• Aldosterone secretion by adrenal gland

  Promotes sodium and water absorption, helps decrease blood pressure
• DCT

  • Secretes H and NH4 into tubular urine, essential for maintaining acid-base balance
• Juxtaglomerular apparatus (JGA)


  • Functions in autoregulation of GFR and controlling blood pressure
  • Macula densa senses and releases vasoconstrictors that lower glomerular pressure when arterial/blood pressure is elevated
  • JG cells release renin which cleaves angiotensinogen into angiotensin I

  
• Renin secretion by JG cells

  1. Travels to capillaries, converted to angiotensin I and II

  2. Angiotensin II directly raises systemic blood pressure when blood pressure is low

  
• Juxtaglomerular complex

  • Three groups of closely associated atypical cells at the vascular pole: JG cells, macula densa, lacis/extraglomerular mesangial cells
• JG cells


  • In tunica media of afferent arteriole, modified smooth muscle cells
  • Release renin hormone, possibly also thrombopoietin
  • Polyhedral, larger than ordinary smooth muscle, with spherical nucleus, myofilaments, secretory granules
• Macula densa


  • Epithelial cells of DCT on afferent arteriole, more crowded and narrower, resting on thin basal lamina
  • Sensitive to Na concentration and water volume in DCT, generate signals that promote renin secretion by JG cells
• Lacis/extraglomerular mesangial cells

  • Between macula densa and afferent arteriole, flattened and lightly staining
  • No established function yet, probably involved in signal transmission between macula densa and glomerular mesangial cells
• Collecting tubules and ducts

  • Lined by cuboidal epithelium, 40um diameter
  • Converging ducts more columnar, up to 200um diameter
  • Composed mainly of weakly staining principal cells with few organelles and scanty microvilli
  • Contain basal membrane and folding, role is ion transport
  • Scattered darker intercalated cells with more mitochondria, help regulate acid-base balance by secreting H and absorbing HCO3
• Collecting ducts

  • Major component of urine concentrating mechanism
  • Cells rich in aquaporins, function as selective pores for water passage
  • ADH makes collecting ducts more permeable to water, increasing water reabsorption
• Ureters, bladder, urethra

  • Have same basic histologic structure, walls become thicker closer to bladder
  • Mucosa lined by unique stratified transitional epithelium (urothelium), surrounded by folded lamina propria, submucosa, smooth muscle layers, adventitia
  • Urine moves by peristaltic contractions
• Urinary bladder

  • Mucosa is transitional epithelium (urothelium)
  • Lamina propria underneath mucosa
  • Thick detrusor muscle layer for adjusting when filled
  • Empty - mucosa folded, Full - mucosa extended and flattened
• Urothelium


  • 3 layers: basal cells, intermediate columnar cells, superficial umbrella cells
  • Umbrella cells are highly differentiated to protect underlying cells from cytotoxic effects of hypertonic urine
• Urethra


  • Tube carrying urine from bladder to exterior
  • Mucosa has large longitudinal folds
• Male urethra segments

  • Prostatic urethra (3-4cm, lined by urothelium)
  • Membranous urethra (short, lined by stratified columnar and pseudostratified epithelium)
  • Spongy urethra (15cm, enclosed in penis, lined by stratified columnar, pseudostratified columnar, and distal stratified squamous epithelium)
• Female urethra


  • 4-5cm long, initially lined with transitional epithelium then stratified squamous and some pseudostratified columnar epithelium
  • Middle part surrounded by external striated muscle sphincter
• Renin


  Produced in kidneys, regulates blood pressure
• Renal Corpuscle


  Initial site of blood filtration in the kidney