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Osmoregulation

Big Questions:

Why do organisms need to regulate their internal conditions?

How is regulation accomplished?

Make Sure You Can:

Explain why organisms need to regulate their internal osmolarity.

Describe the similarities and differences of the osmoregulatory/nitrogenous waste excretion systems of different lineages of animals.

Describe the structure and function of the mammalian excretory system in general and the nephron in specific.

Explain how all of the hormonal controls discussed in this presentation work to maintain osmoregulatory homeostasis in mammals.

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Osmoregulation, Osmolarity & Excretion

Animals regulate their osmolarity by controlling the amount of solute that they retain in their bodily fluids.

Inake of water, Excretion of fluid and dissolved solute is the major way that animals control internal osmolarity.

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Animals

To Review:

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Plants

Excretory systems depend on speicalized transport epithelia to move specific solutes either in to or out of bodily fluids.

Osmoconformers vs. Osmoregulators

The production of salt secretions in the nasal glands of the albatross allow it to survive by drinking seawater.

animals that remain isoosmotic with their surroundings.

Must be surrounded by saltwater

animals that regulate their internal osmolarity.

Can live in many different environments.

Nitrogenous Waste

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Due to their inability to move, plants are essentially osmoconformers.

However, internal osmolarity has serious consequences for plant physiology.

Waste molecules produced by cells from the breakdown of proteins and nucleic acids.

Three Major Kinds:

  • Ammonia: Most toxic. Only produced by aquatic animals

  • Urea: Formed by combining ammonia with carbon dioxide. Not as toxic, so it can be tolerated at higher concentrations than ammonia, and released with less water.

  • Uric Acid: Least soluble. Can be excreted with the least amount of water. More energetically expensive to produce than Urea.

There is an inverse relationship between solute concentration and water concentration.

Osmolarity- Total solute concentration expressed as (moles of solute/liters of solution)

Ex. Human Blood is ~300 milliOsmoles/liter (mOsm/L). Seawater is ~1000 mOsm/L.

The osmoregulatory adaptations of marine and freshwater fish accomplish opposite purposes.

Cnidarians are osmoregulators

The form of an animal's nitrogenous waste reflects its phylogeny and habitat.

Terrestrial animals constantly lose water to the atmosphere.

Salmon can tolerate both freshwater and marine environments ("euryhaline").

Most animals can only tolerate a narrow range of external osmolarities ("stenohaline")

The Vertebrate Excretory System

Bird guano is a commercially valuable source of nitrogen. Wars have been fought over the stuff.

Tardigrades live in environments that are only hydrated temporarily (drops of water, seasonal ponds). To allow for survival in these kinds of environments, they can survive in a dormant, dehydrated state for decades ("anhydrobiosis")

A comparison of the daily water balance of a kangaroo rat (dessert inhabitant) and a human demonstrates some interesting differences.

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Nephrons

The Kidney

Kidneys:

Excretory Systems

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Filtrate collection and urine production

The interface between the circulatory system and the excretory system

Responsible for filtration, reabsorption and secretion.

A tube surrounded by capillaries:

  • Glomerulus: ball of capillaries that passes filtrate into the nephron at the "Bowmans capsule"
  • Proximal tubule: Reabsorption of water, salt, and bicarbonate ions.
  • Loop of Henle: Reabsorption of water (descending) and salt (ascending).
  • Distal tubule: Reabsorption of water, salt and urea.
  • Collecting Duct: Excretion of concentrated urine.

Ureters:

The organ responsible for filtration and urine production.

Contain ~1,000,000 filtration units ("nephrons")

All Excretory systems involve 4 major processes:

  • Filtration: Initial movement of fluid and solutes from the body to the system ("filtrate")
  • Reabsorption: Water and desirable solutes are reclaimed by transport epithelim.
  • Secretion: Excess waste solute is sent to the filtrate.
  • Excretion: The modified filtrate ("urine") is expelled from the body.

Transport urine to the bladder

Dialysis:

Bladder:

By actively transporting salt ions, the nephron creates a hyperosmotic environment that leads to the passive transport of water from the filtrate and results in the production of hyperosmotic, concentrated urine.

This is called the "Two-Solute" Model of nephron function.

Urine storage

All excretory systems utilize tubules for collection of filtrate.

Urethra:

For individuals with kidney failure. Do the same thing that your kidneys do, but:

  • more expensive.
  • Time consuming.
  • Not a long-term solution for kidney falure.

Protonephridia: Platyhelminthes

Urine Excretion

Control

Adaptations:

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Interstitial fluid moves in to the protonephridia.

The filtrate is produced through the action of ciliated "Flame Bulb" cells.

Filtrate then leaves the animal through openings in the body wall.

The Vampire Bat

The RAAS System:

ADH:

Metanephridia: Annelids

Malpighian Tubules: Arthropods

Feeds at night on large animals.

Drinks a lot of blood per feeding.

When blood osmolarity increases, the pituitary gland releases antidiuretic hormone (ADH), which increases water reabsorption in the collecting duct of the nephron.

ADH also triggers a thirst response in the animal.

These effects decrease osmolarity.

Filtrate moves from hemolymph into the malpighian tubules.

From the tubules, filtrate is combined with undigested food and eliminated from the body through the rectum.

"I Drink your blood, and then I pee on you!"

Fluid from the coelom moves into the metanephridium.

Reabsorption and secretion are accomplished by transport epithelium that line the border of the metanephridium and the capillary network.

Stored urine can be excreted through external openings.

When blood volume or blood pressure decreases, a region of the nephron (the "juxtaglomerular apparatus") releases the hormone renin.

Renin production results in the conversion of angiotensinogen to angiotensin II.

Angiotensin II causes arterioles to constrict, and causes the adrenal glands to produce aldosterone.

Aldosterone triggers increased reabsorption of sodium ions and water in the distal tubule of the nephron.

The effects of the RAAS system increase blood volume (and blood pressure).

"Pee side"

To compensate for the massive influx of fluid, the vampire bat excretes up to 25% of it's body mass in urine per hour while feeding.

ADH triggers a cellular response in collecting duct cells which leads to an increase in the number of aquaporins in cell membranes, increasing water reabsorption.

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