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Chapter 46. Osmoregulation and Disposal of Metabolic Wastes. Osmoregulation The active regulation of osmotic pressure of body fluids so that homeostasis is maintained Excretory systems Help maintain homeostasis by regulating the concentration of body fluids.

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Chapter 46

Chapter 46

Osmoregulation and Disposal of Metabolic Wastes


  • Osmoregulation

    • The active regulation of osmotic pressure of body fluids so that homeostasis is maintained

  • Excretory systems

    • Help maintain homeostasis by regulating the concentration of body fluids



  • Osmoconformers

    • Includes most marine invertebrates

    • Salt concentration of their body fluids varies with changes in the seawater


  • Osmoregulators

    • Marine invertebrates that inhabit coastal habitats

    • Maintain an optimal salt concentration despite changes in the salinity of their surroundings


  • Nephridial organs

    • Include protonephridia and metanephridia

    • Function in osmoregulation and waste disposal


  • Protonephridia

    • Found in flatworms and nemerteans

    • Tubules with no internal openings

    • Interstitial fluid enters their blind ends, which consist of flame cells

    • Beating of the cilia propels fluid through the cilia



  • Metanephridia

    • Found in annelids and mollusks

    • Tubules open at both ends

    • As fluid from the coelom moves through the tubule, materials are reabsorbed by capillaries

    • Urine exits the body through nephridiopores



  • Malpighian tubules

    • Extensions of the insect gut wall

    • Have blind ends that lie in the hemocoel

    • Cells of the tubule actively transport uric acid, etc., into the tubule

    • Water follows by diffusion


  • Malpighian tubules, cont.

    • Contents of the tubule pass into the gut and water and some other solutes are reabsorbed in the rectum

    • Malpighian tubules effectively conserve water



  • Freshwater fishes

    • Take in water osmotically

    • Excrete a large volume of dilute urine

  • Marine bony fishes

    • Lose water osmotically

    • Compensate by drinking seawater and excreting salt through gills

    • Produce a small volume of urine



Osmo-regulationin fishes


  • Marine mammals

    • Ingest seawater with their food

    • Produce a concentrated urine


  • Terrestrial vertebrates

    • Must conserve water

    • Endotherms have high metabolic rate

    • Produce a large volume of nitrogenous wastes

    • Conserve water through efficient kidneys and other adaptations



  • Vertebrate kidney

    • Functions in excretion and osmoregulation

    • Is vital in maintaining homeostasis

    • Its structure and function are adapted to the lifestyle of the animal





  • Structure of the kidney

    • Outer portion is the renal cortex

    • Inner portion is the renal medulla

      • Contains eight to ten renal pyramids

      • Tip of each pyramid is a renal papilla

      • Urine flows into collecting ducts, which empty through a renal papilla into the renal pelvis

    • Functioning unit is a nephron










  • Urine volume is regulated by ADH nephron

    • Released by the pituitary gland in response to an increase in osmotic concentration of the blood

    • ADH increases the permeability of the collecting ducts to water

    • As a result, more water is reabsorbed



  • Aldosterone andatrial natriuretic peptide work antagonistically

    • When blood pressure decreases, cells of the juxtaglomerular apparatus secrete renin, which leads to production of angiotensin II

    • Angiotensin II stimulates aldosterone release, which raises blood pressure


  • When blood pressure increases antagonistically

    • Atrial natriuretic peptide (ANP) increases sodium excretion

    • Inhibits aldosterone secretion

    • These actions increase urine output and lower blood pressure



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