Homeostasis I: Excretion and Water Balance

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Chapter 37

• Homeostasis I Excretion and Water Balance

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Regulation of Chemical Environment

• Animals are about 70 water
• As animals became multi-cellular during evolution
  they began to produce their own extra cellular
  fluid
• Blood plasma constitutes about 7 of total body
  fluid
• Blood functions as a sanitation medium
• Composition of blood is regulated by special
  excretory organs
• Organs nephridia (mollusks and annelids)
• -Malpighian tubules (insects)
• - Kidney (vertebrates)

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Substances Regulated by the Kidneys

• To regulate internal chemical balance animal has
  to solve three problems
• 1.) Excretion of metabolic wastes
• 2.) Regulate the concentrations of ions and
  other chemicals
• 3.) Maintain water balance

• Waste products are released into blood stream
• -Carbon Dioxide, nitrogenous compounds,
  ammonia, produced by AA.
• CO2 is excreted by diffusion
• Ammonia is made into a non-toxic form and
  excreted in most organisms

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Birds

• Uric Acid and uric acid salts are mixed with
  undigested wastes in the cloaca
• Dropped as a semi-solid paste
• Little water is needed for this excretion method
• This excretion, guano can be used as a
  fertilizer

http//members.aol.com/dstevens/
my_gallary/sea_gull.htm
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Mammals

• In a mammal the waste is converted into urea in
  the liver.
• Then is carried to the kidneys through the blood
  stream
• Urea must be dissolved in water for excretion
• All methods of excretion are highly selective

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Water Balance

• Evolutionary Perspective

• Fresh water fish first function of the kidney
• Kidney was probably used to pump water out and
  conserve salts
• Today kidney uses as a filter and reabsorber of
  salts
• Sea fish (cartilaginous)
• sharks have a high concentration of urea in
  blood stream so they are isotonic with water
• Bony fish
• Drink sea water to make up for water lost by
  osmosis
• Gland in gills excretes excess salt from the fish
  back into the water

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Water Gain and Loss in Land Animals

• gain water by eating and drinking.
• When 1 gram of glucose is oxidized .6 g of water
  is formed
• 1 gram of protein .3 g water
• 1 gram of fat 1.1 g water
• Kangaroo Rat
• Can live without drinking
• Lives of water from the oxidized nutrients
• No sweat glands and is nocturnal
• Conserves its water
• Humans
• 2,300 milliliters of water in food and drink
• 200 milliliters of water from oxidized nutrients
• 1,500 mL of water lost from excretion in a day
• Its is healthy to loose a minimum of 500 mL of
  water a day.

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Water Compartments

• 3 principal water compartments
• Plasma (7 body fluid)
• Interstitial fluid and the lymph (28)
• Intracellular fluid (65)

• Water actively moves through these compartments
  and can be traced by dyes.
• - Major source of water gain is absorbed through
  the digestive tract. Most fluid reenters plasma
  osmotically or through lymph vessels.

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THE KIDNEY

• Nephron
• Functional unit of the kidney
• Consists of a cluster of capillaries called the
  glomerulus and a renal tubule.
• Renal tubule originated from the Bowmans Capsule
• Renal tubule is made up of the proximal (near)
  and distal (far) convoluted tubules and are
  connected by the loop of henle.
• The nephron ends at the collecting duct
• Each of the human kidneys contains about a
  million nephrons

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Function of the Kidney

• There are four major steps in excretion, the
  main function of the kidney.

• Step 1 Filtration
• The fluid that enters the Bowmans Capsule is
  called the filtrate
• At this time the filtrate has the same
  composition as the plasma
• Filters out the larger molecules and sends the
  plasma down the renal tubule

• Step 2 Secretion
• Molecules that were left in the plasma are
  selectively removed and actively secreted into
  the filtrate.

• Step 3 Reabsorption
• Water and solutes that were initially removed
  are transported back into the capillaries
• Most amino acids and vitamins are returned to
  the blood stream
• Step 4 Excretion
• The remaining fluid is removed past the renal
  pelvis and is excreted.

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Water Conservation

• The Loop of Henle
• The shape of the loop of henle is key in water
  conservation
• It is made with an ascending branch and a
  descending branch.
• It is shaped like this in order to make the
  excretion hypertonic to blood.
• The ascending branch is thick and allows NaCl to
  diffuse and be actively transported through.
• The descending branch allows water to be removed
  from the tube.
• The longer the loop of henle the urine is more
  hyper tonic

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Hormones

• The ADH hormone controlled by the hypothalamus
  are released depending on the osmotic
  concentration in the blood and also on the blood
  pressure.
• Aldosterone, produced by the adrenal cortex,
  stimulates the reabsorption of sodium ions from
  the distal tubule and the secretion of potassium
  ions into them.