urinary

• Urinary System


  • Consists of six principle organs
  • Two kidneys
  • Two ureters
  • Urinary bladder
  • Urethra
• Urinary tract


  Has important spatial relationships with the vagina and uterus in females, and the prostate in males
• Functions of the Urinary System


  • Blood filtration
  • Blood volume, pressure, osmolarity regulation
  • Blood electrolyte balance
  • Stimulation of RBC production
  • Help form calcitriol (bone metabolism)
  • Clear hormones and drugs from blood
  • Detoxify free radicals
  • Produce glucose from amino acids (during extreme starvation)
  • Excrete nitrogenous waste
• Nitrogenous waste


  A waste substance produced by the body, some of which can be toxic
• Nitrogenous wastes


  • Urea (50% of nitrogenous waste)
  • Ammonia
  • Other small nitrogen-containing compounds
• Urea formation


  1. Proteins → hydrolyzed to amino acids

  2. Amino acids → amino group (NH2) removed from each amino acid

  3. NH2 forms ammonia (NH3 → very toxic)

  4. Ammonia → urea (less toxic)

  5. Liver quickly converts ammonia to urea
• Excretion


  The process of separating wastes from the body fluids and eliminating them from the body
• Organ systems involved in excretion


  • Respiratory system
  • Integumentary system
  • Digestive system
  • Urinary system
• Kidneys


  • Lie against the back of the abdominal wall
  • Right kidney sits slightly lower than the left
  • Below diaphragm
  • Surrounded by 3 layers of connective tissue (fibrous capsule, perirenal fat capsule, renal fascia)
• Renal Circulation


  • Kidneys account for about 0.4% of body weight
  • Receive 1.2 L of blood per minute, or 21% of the cardiac output (renal fraction)
• Nephron


  • Functional unit of a kidney
  • Each kidney has about 1.2 million nephrons
  • Composed of a renal corpuscle (filters blood plasma) and a renal tubule (converts filtrate into urine)
• Parts of the Nephron


  1. Glomerulus

  2. Bowman's capsule

  3. Proximal convoluted tubule

  4. Loop of Henle (descending and ascending limbs)

  5. Distal convoluted tubule

  6. Collecting duct
• Renal Corpuscle


  • Consists of the glomerulus and the Bowman's capsule (encloses it)
  • Blood filtration → collection of filtrate
• Proximal Convoluted Tubule (PCT)


  • Simple cuboidal epithelium with prominent microvilli (brush border)
  • Allows for absorption (great amount of absorption happens here)
• Loop of Henle


  • Largely associated with urine concentration
• Distal Convoluted Tubule (DCT)


  • Simple cuboidal epithelium with smooth-surfaced cells
  • No microvilli
  • Secretion, urine concentration
• Collecting Duct


  • Concentration, channels urine
  • Many make up the renal pyramids
  • Numerous collecting ducts converge (merge) toward the tip of the renal pyramids → merges to form a larger papillary duct
  • Urine drains from these pores into the minor calyx that encloses the papilla
• Flow of urine through kidney

  Pyramid (of renal medulla) → Papilla → Minor calyx → Major calyx → Renal pelvis → Ureter → Bladder
• Glomerular filtration


  1. Water and solutes filtered out of blood

  2. Water and solutes in the blood plasma pass from capillaries of the glomerulus into the capsular space of the nephron

  3. Fluid passes through three barriers: fenestrated capillaries, basement membrane, filtration slits
• Filtration pressure


  Afferent arteriole is wide, efferent arteriole is narrow
• Glomerular filtration rate (GFR)


  High blood pressure can rupture glomerular capillaries
• Regulation of filtration


  1. Myogenic (stretch mechanism)

  2. Sympathetic control (aided by epinephrine)

  3. ADH
• Basic stages of urine formation


  1. Glomerular filtration

  2. Reabsorption

  3. Tubular secretion

  4. Water conservation
• Proximal Convoluted Tubule (PCT) reabsorption


  • Transcellular (through cells)
  • Paracellular (between cells)
  • Water and solutes enter fluid outside of tubule and reenter blood
  • Na+ reabsorption via symport proteins and Na+-H+ antiport
• Transport Maximum


  Limit to the amount of solute that the renal tubule can reabsorb due to limited numbers of transport proteins
• Loop of Henle


  • Generates osmotic gradient
  • Descending limb → H2O out
  • Ascending limb → active Na+ and Cl- out
  • Osmotic gradient in medullary tissue concentrates urine
• Countercurrent multiplier


  Juxtaposed, connected, and countercurrent + selective permeability and active transport = osmotic gradient
• DCT and Collecting Duct


  • About 20% water and 7% salts from original filtration
  • Without absorption, 36 L per day excreted
  • Need to reabsorb more!
• Cells in DCT and Collecting Duct


  • Principal cells (most abundant, receptors for ADH and aldosterone, absorb Na+ and H2O, secrete K+)
  • Intercalated cells (absorb K+, secrete H+)
• Aldosterone


  • Secreted by adrenal cortex
  • Stimulates cells of tubules to absorb Na+ in exchange for K+ secretion
  • Increases blood volume (and BP)
• Antidiuretic hormone (ADH)


  • Increases aquaporins in collecting ducts
  • Increases water reabsorption → concentrated urine (up to 4x blood osmolarity)
• Parathyroid hormone (PTH)


  • Targets ascending loop and DCT
  • Absorbs more calcium
  • Stimulates calcitriol synthesis in PCT epithelial
• Ureters


  • Muscular tubes that extend to the urinary bladder
  • Transports urine from kidney to the bladder
  • Enter posteriorly and at bottom of bladder
  • Valves at bladder ends prevent flow of urine back up the ureters
  • Stretch receptors stimulate peristalsis of urine from pelvis down ureters
  • Lumen vary narrow, easily obstructed by kidney stones
• Urinary bladder


  • Muscular sac
  • Detrusor muscle (3 layers of smooth muscle)
  • Transitional epithelium with umbrella cells to protect against urine acidity
  • Rugae when empty