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Urinary System. An overview. Functions of the Urinary System. Functions of the Urinary System. Produces, excretes urine Essential for homeostasis Keeps volume of bodily fluids constant Keeps levels of essential chemicals constant Electrolytes, water, acid/base balance

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urinary system

Urinary System

An overview

functions of the urinary system1
Functions of the Urinary System
  • Produces, excretes urine
  • Essential for homeostasis
    • Keeps volume of bodily fluids constant
    • Keeps levels of essential chemicals constant
      • Electrolytes, water, acid/base balance
  • “Clears” (cleans) blood of waste products produced by cellular metabolism (e.g. nitrogenous wastes)
    • Uremia- toxic buildup of metabolic wastes
  • Blood pressure regulation
major structures of the urinary system
Major structures of the urinary system
  • Kidney
    • Retroperitoneal
  • Ureters
  • Urinary Bladder
  • Urethra
urinary bladder
Urinary Bladder
  • Accumulates, stores urine
  • Stretchable epithelium, w/ smooth muscle (detrusor)
  • Internal urinary sphincter
    • Involuntary, circular muscle
    • Keeps ureter closed
  • Can hold up to 1 liter of urine
  • Stretch receptors signal need to void (micturate) when 20% of bladder capacity reached
  • Single, muscular tube
  • Controlled by external urethral sphincter
    • Voluntary
  • Longer in males
  • More prone to infection in females
  • Urination = micturition
  • Innervated by reflex arc; voluntary control gained as one ages
  • Incontinence – loss of voluntary control
  • Anuria – lack of urine production
  • Urinary retention – failure to expel urine
  • Oliguria – decreased urine production
  • Polyuria – excessive urine production
the kidney
The Kidney
  • Renal cortex – Outer “bark”
  • Renal medulla – Inner
  • Renal pyramids
    • Triangular divisions
  • Renal columns –
    • Extension of cortex
    • Divide pyramids
  • Renal papilla
    • Narrow, innermost point of a pyramid
  • Renal pelvis
    • Expanded end of upper end of ureter
  • Calyx
    • Division of renal pelvis
functional unit nephron
Functional Unit: Nephron
  • > 1 million in a kidney
  • Can be cortical (most) or juxtamedullary
  • “Tiny funnel” with a very long stem
  • Highly convoluted (many bends)
  • 2 principal components:
    • Renal corpuscle
    • Renal tubule
renal corpuscle components
Renal corpuscle components
  • Bowman’s capsule
    • Cup/sac-like-shaped top of a nephron
  • Glomerulus
    • Ball of capillaries, lined with podocytes
    • Found in Bowman’s capsule
    • Afferent, efferent arterioles
renal tubule
Renal Tubule
  • Composed of:
    • Proximal convoluted tubule (PCT)
    • Loop of Henle (longer in Juxtamedullary)
    • Distal convoluted tubule (DCT)
    • Collecting tubule
formation of urine overview
Formation of Urine: Overview
  • Filtration
  • Secretion
  • Reabsorption
  • All occur within the functional unit of the kidney, the nephron
  • Of water, dissolved substances out of blood into Bowman’s capsule
  • Occurs in “renal corpuscles”
    • Contains larger afferent, smaller efferent arterioles
      • Creates high pressure in glomerulus
      • Filters blood across fenestrated capillaries, lined w/ podocytes
      • Hold in proteins, cells
      • Lose water, ions = ultrafiltration
glomerular filtration rate
Glomerular Filtration Rate
  • 120 ml/minute = 180 liters/day!
  • Must maintain a minimum blood pressure for kidneys to filter
    • Hemorrhage, etc., can cause kidney function (and urine output) to cease
180 l of urine a day
180 L of urine a day?
  • No…
  • Luckily, most of the fluid that leaves blood during filtration returns to the blood during reabsorption…
  • Movement of substances out of renal tubules into the blood (into peritubular capillaries)
    • Water
    • Glucose
    • Other nutrients
    • Sodium, other ions
    • Begins in proximal convoluted tubule, continues in loop of Henle, distal convoluted tubule, and collecting tubule
  • Reabsorb ~178 L/day (97-98% of water)


  • Active transport
    • Active transport of sodium ions from tubule into interstitial fluid
  • Passive Transport
    • Creates high concentration of sodium, which diffuses into bloodstream
    • Sodium (Na+) in blood attracts negative ions (Cl-, (PO4)3-), which diffuse into blood
another mechanism
Another Mechanism?

Osmosis! (Flow of water from High to Low)

  • Attracted to blood due to ion flow
another another mechanism
Another, another Mechanism?

Facilitated Diffusion

  • Amino acids
  • Glucose
    • Cotransported/symported with sodium
    • Almost all glucose reabsorbed (340 mg/min)
      • Exceptions exist!
        • Diabetes mellitus – too much sugar in bloodstream, exceeds ability of kidney tubule cells to reabsorb (not enough Na+ to cotransport)
        • glycosuria (glucose in blood)
      • Water is lost along with glucose transport… most people with diabetes mellitus urinate excessively!
loop of henle
Loop of Henle
  • Descending loop:
    • Reabsorbs water (20%), large ions and molecules
  • Ascending loop:
    • Active transport of Na, Cl ions
water reabsorption
Water reabsorption
  • 90% passive thru osmosis in PCT, descending Loop of Henle, DCT
  • 10% occurs in last parts of DCT, collecting ducts
  • Substances move into urine in distal, collecting tubules from blood in capillaries around tubules
  • “Reabsorption in reverse”
  • Urea, H+, K+ ions, ammonia, drugs
  • (Bold = active transport; non-bold = diffusion)
  • Regulated by hormones
    • Antidiuretic hormone (ADH) – increases water reabsorption from collecting duct into bloodstream
    • Angiotensin II – constricts efferent, systemic arterioles – alters filtration rate, increases pressures
    • Aldosterone – Produced by adrenal cortex; retains sodium, excretes potassium – raises blood pressure by incr. water absorption in DCT, Collecting duct
  • Affected by other substances
    • Diuretic (causes great outflow of urine)
      • Caffeine –decreases Na+ absorption, causing diuresis
      • Lasix – works on Loop of Henle, producing massive diuresis