Osmoregulation

Helps the animal not dry out or dying b/c of own excrement or swelling too much

All waste affects the osmotic gradient


Balancing Water Levels

  • Osmoconformers = isoosmotic w/ the environment

    • don’t gain or lose water

    • match inside environment w/ outside

    • need stable environments→ restricted to 1 type of environment

    • mainly marine invertebrates

  • Osmoregulator = osmolarity of body differs from environment

    • in hyperosmotic→ lose water

    • in hypoosmotic→ gain water

    • higher energy cost but more environmental freedom

    • found in terrestrial, freshwater, and marine environments


Nitrogenous Waste

  • breakdown of proteins and nucleic acids produces nitrogenous waste

  • has to be dissolved in water to be excreted→ more waste = more water needed

  • Endotherms eat more food than ectotherms→ generate more nitrogenous waste

  • Carnivores » Omnivores and Herbivores b/c higher protein diets, others eat more carbs and fat

  • different species produce diff forms of Nitrogenous waste b/c of different evolutionary history and osmotic environment, forms differ based on toxicity and water required

3 Types:

  1. Ammonia- requires high volume of freshwater to dilute b/c very toxic→ can’t stay in body

    • secreted by freshwater fish, juvenile amphibians, and some invertebrates

    • very water soluble

    • constantly excrete across body, NH3 in urine

    • Excrete via gills, exchange for sodium ions

  2. Urea- less toxic than ammonia, less concentrated than uric acid

    • secreted by mammals, adult amphibians, sharks, some bony fishes, and turtles

      • amphibians switch strategies from ammonia to this (urea is kinda like a middle strategy)

    • can be stored in body b/c less toxic

    • higher concentration than ammonia→ less water needed

    • converted from ammonia in liver→ energetically costly

    • concentrated in kidneys to minimize water loss

    • flushed from body in urine (soluble)

  3. Uric Acid- least toxic, but most energetically expensive to produce b/c most concentrated

    • secreted by reptiles (including birds), insects, land snails

    • not very water soluble→ excreted as semisolid→ very little water loss

    • common in animals that need to conserve more water


Anatomical Structures:

  • Large intestine

  • Kidneys + Bladder

  • Gills + Skin

follow this design


Solid Waste Excretion in Humans

  • Large Intestine does most of the work

  • Colon reabsorbs liquids and prepares feces

  • Components:

    • Cecum - fermenting bacteria stored in herbivores

    • appendix- bank for


Liquid Waste:

  • reabsorb water + excrete nitrogenous waste

  • Components:


Nephron- basic unit of vertebrate excretory systems

  • human kidneys have a million of these→ process 180 L of filtrate per day→ less than 1 percent becomes urine, rest is reabsorbed

  • interact with blood vessels along the way

  • blood enters the Bowman’s capsule via blood pressure in the afferent artery

  • Filtration occurs in the glomerulus based on size of molecules (blood plasma minus other stuff)

    • Cells and large proteins stay in the blood vessels

    • Salt, ions, nitrogenous wastes, glucose, amino acids, water, and drugs leave

  • Loop of Henle sets up a concentration gradient to pull out urine in an energy efficient way

    • proximal tubule is where bulk reabsorption happens, ions transported out, reclaim most of water and nutrients

    • Descending Loop of Henle→medulla- region is permeable to water, not to salt→water leaves (reabsorbed)

    • Ascending the loop- permeable to salt, not water→ filters out salt, water left behind, makes filtrate more dilute, increases urine concentration and salt reabsorption

    • counter-current system

    • more reabsorption and secretion in the distal tubule

    • Collecting duct- last spot for reabsorption and secretion

      • controlled by hormones

  • deeper into the medulla→ more ions pumped out→ requires more energy→ creates a gradient


Dehydration- losing water

  • blood becomes more concentrated, hypothalamus monitors this

  • increase in blood osmolarity→ hypothalamus signals pituitary gland→ Antidiuretic Hormone (ADH) released

  • increases number of aquaporins in walls of collecting duct→ more water reabsorbed→ less urine produced→ more concentrated

  • negative feedback loop

  • opposite happens when hydrated

  • Alcohol is an antagonist for ADH→ pee more→ dehydration

  • ADH acts in distal tubule and collecting duct


Low Blood Pressure

  • filtration rate decreases b/c less volume of filtrate pushed into nephron

  • Aldosterone secreted from adrenal gland→ increases Sodium ion absorption in collecting duct→ increased water reabsorption b/c water follows salt→ blood volume increases→ blood pressure increases

  • dehydration and low blood pressure have similar symptoms b/c of this

  • if both ADH and Aldosterone high→ blood pressure increases


Comparing Excretory Systems and Osmoregulation

  • Chondrichthyes- sharks, rays, chimeras

    • diff from other marine animals b/c raise internal osmolarity

    • retain urea in body fluids

    • water moves in by osmosis

    • rectal gland

    • instead of always drinking seawater, they absorb it?

    • match ocean using urea

  • Marine bony fishes (salt water)

    • drink sea water constantly→ replaces water lost by osmosis

    • get rid of excess salts using gills (main salt excretion organ)

    • Tissues hypoosmotic to environment→ lose water to hyperosmotic surroundings

    • Kidneys reabsorb ions and prevent excretion and water loss

  • Freshwater fishes- opposite problem

    • hyperosmotic tissues→ gain water from hypoosmotic environment

    • must excrete excess water to preserve salt levels

    • gills transport salt into body

    • kidneys lose some ions to enable water excretion

    • try not to drink water constantly b/c passively getting water all the time

  • Birds

    • independently evolved short loops of Henle

    • can’t concentrate urine as well as mammals

    • excrete concentrated uric acid instead of urea

  • Marine Reptiles (inc birds)

    • nasal gland removes excess salt, nostril excretes it

    • active transport of NaCl into tubule, out of blood

    • countercurrent exchange

  • Nonavian Reptiles

    • lack loops of Henle

    • can’t concentrate urine more than already have

    • produce mostly uric acid

    • still conserve water, reabsorb it later

  • Mammals

    • access to lots of freshwater→ short loops of Henle

    • Desert/marine mammals→ longer loops of Henle