Regulating the Internal Environment

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Regulating the Internal Environment

• Campbell 6e Chapter 44 Pages 936-952

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Water A Balancing Act

• Protists that live in fresh water environments
  are subjected to a continuous influx of water.
• The solute concentration inside the cell is
  higher than that in the surrounding water, so
  water continuously diffuses in.

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• This inward diffusion of water does not depend on
  any biological activity of the cell.
• It is a purely physical phenomenon that depends
  on only the difference in solute concentration
  (or more specifically, the osmotic pressure
  difference) between the inside of the cell and
  the medium and the permeability of the plasma
  membrane.

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Paramecium

• Water that diffuses in must be moved out.
• Here, Paramecium is using the contractile vacuole
  to pump out excess water.

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Osmoregulation

• Molecules passively diffuse from regions of high
  to low concentration.
• Aquatic animals are generally hyperosmotic to
  their surroundings their internal solute
  concentration is much higher than their
  surroundings.

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• Because of this, aquatic animals must develop
  physiological mechanisms to prevent excess flow
  of water into their bodies.
• They must also develop mechanisms to prevent the
  loss of solutes as excess water is excreted.

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• The process by which organisms actively maintain
  their internal solute concentration is called
  osmoregulation.
• Animals must actively transport solutes from
  their surroundings into their blood against the
  concentration gradient.

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Osmoregulation Mechanisms

• There are 2 main ways osmoregulation is
  accomplished.
• Osmoconformers
• Osmoregulators

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Osmoconformers

• Animals, such as crabs, have an internal salt
  concentration very similar to that of the
  surrounding ocean.
• Such animals are known as osmoconformers, as
  there is little water transport between the
  inside of the animal and the isotonic outside
  environment.

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Osmoregulators

• There are three main types of osmoregulatory
  environments in which animals live freshwater,
  marine, and terrestrial.
• Aquatic animals are either euryhaline or
  stenohaline, depending on their ability to
  tolerate different salinities.

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Tolerance of Change in Osmolarity

• Both of the following can be osmoconformers or
  osmoregulators.
• Stenohaline organisms cannot tolerate large
  changes in external osmolarity.
• Euryhaline organisms can survive large changes in
  external osmolarity.

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Freshwater Osmoregulators

• Freshwater animals (all osmoregulators) include
  invertebrates, fishes, amphibians, reptiles, and
  mammals. The freshwater animals are generally
  hyperosmotic to their environment. The problems
  that they face because of this are that they are
  subject to swelling by movement of water into
  their bodies owing to the osmotic gradient, and
  they are subject to the continual loss of body
  salts to the surrounding environment (which has a
  low salt content).

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• The way these animals deal with these problems is
  to produce a large volume of dilute urine.
• The kidney absorbs the salts that are needed, and
  the rest of the water is excreted.
• Another way these animals deal with lack of salt
  is by obtaining it from the food they ingest.
• A key salt replacement mechanism for freshwater
  animals is active transport of salt from the
  external dilute medium across the epithelium into
  the interstitial fluid and blood.
• Amphibians skin and fish gills are active in
  this process.
• Freshwater animals tend to take in water
  passively and to remove it actively through
  osmotic work of kidneys (in vertebrates) or
  kidney-like organs (in invertebrates).

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Marine Osmoregulators

• Among marine animals, most invertebrates are
  osmoconformers whereas most vertebrates are
  osmoregulators. 
• Marine animals do not need to expend as much
  energy in regulating the osmolarity of their body
  fluids.
• Marine vertebrates have internal concentrations
  of salt that are about one-third of the
  surrounding seawater.

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• There is a tendency for marine fishes to lose
  water to the environment through the gill
  epithelium.
• Marine fishes obtain water in food and drink sea
  water. The talk intake is disposed of through
  active transport out of the gills.
• Very little urine is produced, an adaptation that
  conserves water.

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Terrestrial Osmoregulators

• Air breathing animals are subject to dehydration
  through their respiratory epithelia.
• Humans and most other air-breathing animals
  require a constant source of fresh drinking water
  to excrete accumulated salts and metabolic waste
  products.

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Nitrogenous Wastes

• The breakdown of proteins produces nitrogenous
  wastes.
• When macromolecules are broken down for energy or
  converted into carbs or fats, enzymes remove
  nitrogen in the form of ammonia.
• Ammonia is very toxic and must be removed from
  the body.

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• There are 3 forms of nitrogenous wastes. The type
  of organism (habitat, diet, etc.) dictates the
  form.
• Ammonia
• Urea
• Uric Acid

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Ammonia

• Ammonia is highly toxic and highly soluble in
  water.
• If the organism has a sufficient source of water
  (aquatic), ammonia can simply be excreted in the
  water.
• Aquatic animals such as bony fishes, aquatic
  invertebrates, and amphibians excrete ammonia
  because it is easily eliminated in the water.

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• This is the course taken by many (if not most)
  aquatic organisms, particularly those in
  freshwater.
• Ammonia will diffuse passively out of respiratory
  structures such as gills.
• It takes a lot of water to dissolve and flush
  ammonia, however, and each ammonia molecule
  carries only one nitrogen.

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Urea

• Organisms with less fresh water available, such
  as some marine organisms and all terrestrial
  organisms, are not as likely to waste water
  excreting nitrogen one atom at a time.
• They will often invest some energy to convert the
  ammonia into urea, which is less toxic, has two
  nitrogen atoms, and therefore takes less water to
  excrete.

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• Because it is less toxic, it can be allowed to
  accumulate in the blood to some extent, and many
  organisms have specialized organs to remove urea
  and other wastes from the blood and excrete them.
  

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Uric Acid

• Some organisms go to greater lengths still to
  deal with nitrogen. Where water is at a real
  premium, even the low toxicity and reduced water
  loss possible with urea excretion is not enough.
• Uric acid is not very toxic and is not very
  soluble in water. Excretion of wastes in the form
  of uric acid conserves water because it can be
  produced in a concentrated form due to its low
  toxicity.

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• Uric acid has 4 nitrogen atoms per molecule and
  is excreted with just enough water so that the
  crystals don't scratch on the way out.
• It has evolved in two groups with major water
  loss problems - terrestrial invertebrates and
  egg-laying vertebrates (obviously an embryo can't
  just step out for a drink, and whatever it
  excretes is going to be very close by until
  hatching).

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Excretory Systems

• Excretory systems regulate the chemical
  composition of body fluids by removing metabolic
  wastes and retaining the proper amounts of water,
  salts, and nutrients.
• Components of this system in vertebrates include
  the kidneys, liver, lungs, and skin.

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Excretory System Functions

• Collect water and filter body fluids.
• Remove and concentrate waste products from body
  fluids and return other substances to body fluids
  as necessary for homeostasis.
• Eliminate excretory products from the body.

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Flame-Bulb System

• Many invertebrates such as flatworms use a
  protonephridium as their excretory organ.
• At the end of each blind tubule of the
  protonephridium is a ciliated flame cell.
• As fluid passes down the tubule, solutes are
  reabsorbed and returned to the body fluids.

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• Planarians have two protonephridia composed of
  branched tubules that empty wastes through
  excretory pores on their surface.

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Metanephridia

• Earthworms have two metanephridia in almost all
  of the body segments.
• Each metanephridium consists of a tubule with
  ciliated opening on one end and an excretory pore
  that opens to the outside of the body at the
  other end.
• Fluid is moved in by cilia. Some substances and
  water are reabsorbed in a network of capillaries
  that surround the tubule.
• This system produces large amount of urine (60
  of body wt./day).

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Malpighian Tubules

• The excretory organs of insects are malpighian
  tubules.
• They collect water and uric acid from surrounding
  hemolymph (blood) and empty it into the gut.
• Water and useful materials are reabsorbed by the
  intestine but wastes remain in the intestine.

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Kidney

• ALL vertebrates have paired kidneys.
• Excretion is not the primary function of kidneys.
  
• Kidneys regulate body fluid levels as a primary
  duty, and remove wastes as a secondary one.

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Kidney Functions

• Maintain volume of extracellular fluid
• Maintain ionic balance in extracellular fluid
• Maintain pH and osmotic concentration of the
  extracellular fluid.
• Excrete toxic metabolic by-products such as urea,
  ammonia, and uric acid.

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• The human kidneys
• are two bean-shaped organs, one on each side of
  the backbone.
• Represent about 0.5 of the total weight of the
  body,
• but receive 2025 of the total arterial blood
  pumped by the heart.
• Each contains from one to two million nephrons.

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Human Excretory

• The urinary system is made-up of the kidneys,
  ureters, bladder, and urethra.
• The nephron, an evolutionary modification of the
  nephridium, is the kidney's functional unit.
• Waste is filtered from the blood and collected as
  urine in each kidney.

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• Urine leaves the kidneys by ureters, and collects
  in the bladder.
• The bladder can distend to store urine that
  eventually leaves through the urethra.

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Nephron Function

• Glomerular filtration of water and solutes from
  the blood.
• Tubular reabsorption of water and conserved
  molecules back into the blood.
• Tubular secretion of ions and other waste
  products from surrounding capillaries into the
  distal tubule.

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• The nephron is a tube closed at one end, open at
  the other. It consists of a
• Bowman's capsule.  Located at the closed end, the
  wall of the nephron is pushed in forming a
  double-walled chamber.
• Glomerulus.  A capillary network within the
  Bowman's capsule. Blood leaving the glomerulus
  passes into a second capillary network
  surrounding the
• Proximal convoluted tubule. Coiled and lined with
  cells carpeted with microvilli and stuffed with
  mitochondria.

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• Loop of Henle.  It makes a hairpin turn and
  returns to the
• Distal convoluted tubule, which is also highly
  coiled and surrounded by capillaries.
• Collecting tubule. It leads to the pelvis of the
  kidney from where urine flows to the bladder and,
  periodically, out to the outside world.

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Formation of Urine

• Blood enters the glomerulus under pressure.
• This causes water, small molecules (but not
  macromolecules like proteins) and ions to filter
  through the capillary walls into the Bowman's
  capsule. This fluid is called nephric filtrate.
• It is simply blood plasma minus almost all of the
  plasma proteins. Essentially it is no different
  from interstitial fluid.

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• Nephric filtrate collects within the Bowman's
  capsule and then flows into the proximal tubule.
• Here all of the glucose, and amino acids, gt90 of
  the uric acid, and 60 of inorganic salts are
  reabsorbed by active transport.
• The active transport of Na out of the proximal
  tubule is controlled by angiotensin II.

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• The active transport of phosphate (PO43-) is
  regulated (suppressed by) the parathyroid
  hormone.
• As these solutes are removed from the nephric
  filtrate, a large volume of the water follows
  them by osmosis (8085 of the 180 liters
  deposited in the Bowman's capsules in 24 hours).

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• As the fluid flows into the descending segment of
  the loop of Henle, water continues to leave by
  osmosis because the interstitial fluid is very
  hypertonic. This is caused by the active
  transport of Na out of the tubular fluid as it
  moves up the ascending segment of the loop of
  Henle.
• In the distal tubules, more sodium is reclaimed
  by active transport, and still more water follows
  by osmosis.

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• Final adjustment of the sodium and water content
  of the body occurs in the collecting tubules.

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Hormone Control

• Water reabsorption is controlled by the
  antidiuretic hormone (ADH) in negative feedback.
• ADH is released from the pituitary gland in the
  brain.
• Dropping levels of fluid in the blood signal the
  hypothalamus to cause the pituitary to release
  ADH into the blood.

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• ADH acts to increase water absorption in the
  kidneys. This puts more water back in the blood,
  increasing the concentration of the urine.
• When too much fluid is present in the blood,
  sensors in the heart signal the hypothalamus to
  cause a reduction of the amounts of ADH in the
  blood.
• This increases the amount of water absorbed by
  the kidneys, producing large quantities of a more
  dilute urine.

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Aldosterone

• Aldosterone, a hormone secreted by the kidneys,
  regulates the transfer of sodium from the nephron
  to the blood.
• When sodium levels in the blood fall, aldosterone
  is released into the blood, causing more sodium
  to pass from the nephron to the blood.
• This causes water to flow into the blood by
  osmosis. Renin is released into the blood to
  control aldosterone.

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Animations

• http//www.bbc.co.uk/schools/gcsebitesize/biology/
  humansasorganisms/6homeostasisrev3.shtml

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Credits

• http//www.zoology.ubc.ca/courses/bio332/Labs/OSMO
  .HTM