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EXCRETORY SYSTEMS. Excretion is the process by which metabolic wastes and excess substances are removed from the organism. The process of excretion also removes excess heat from the body, thus helping to keep the temperature of the body constant.

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EXCRETORY SYSTEMS

Excretion is the process by which metabolic wastes and excess substances are removed from the organism. The process of excretion also removes excess heat from the body, thus helping to keep the temperature of the body constant.

  • In humans and other complex animals, the organs of excretion are;
  • lungs
  • skin
  • kidney
  • liver

These organs work together with the circulatory, nervous and endocrine systems to keep the body’s internal environment constant. (homeostasis)

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Major Metabolic Wastes

  • Nitrogen compounds
  • Ammonia NH3
  • Urea [CO(NH2)2]
  • Uric acid C5H4N4O3

Mineral salts

Sodium chloride and potassium sulphate build up during metabolism

CO2

is formed during cellular respiration

Water

is formed during cellular respiration and dehydration sysnthesis

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Ammonia NH3

  • is produced by cells
  • solubility of NH3 in water is high
  • it is the most poisonous nitrogen compound
  • excretion of NH3 requires high amount of water
  • most of the aquatic animals excrete NH3
  • is produced by deamination of aminoacids
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Urea [CO(NH2)2]

  • is produced by the reaction of CO2 and NH3
  • it is less poisonous than NH3
  • excretion of urea requires less water than NH3
  • solubility of urea in water is less than NH3
  • it is the excretory material of amphibian and mammals
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Uric Acid C5H4N4O3

  • is synthesized from the products of ammonia metabolism
  • can be crystalized
  • is not dissolved in water
  • is the least poisonous
  • removal of uric acid requires less water than the removal of urea and NH3
  • terrestrial insects, reptiles, and birds excrete uric acid
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EXCRETION IN PROTISTS

  • Metabolic wastes are CO2, mineral salts and ammonia. They are diffused out of the cell through the cell membrane into the surrounding medium.
  • Freshwater protist (ameba and paramecium) pumps out excess water with their contractile vacuole by active transport.
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EXCRETION IN HYDRA

  • Metabolic wastes (CO2, mineral salts, ammonia) are diffused out of the cell through the cell membrane to the surrounding medium.
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A series of tubules that run the length of the body make up the excretory system of the planaria. Side branches of the tubules have cells called flame cells.

  • These cells remove excess water and liquid wastes from the body and pass them into ducts. The contents of the ducts pass out of the worm through small excretory pores on the dorsal surface.
  • CO2 and NH3 are excreted from cell surface to the surrounding medium by diffusion.

flame cells

excretory pore

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CO2 and NH3 are excreted from cell surface to the surrounding medium by diffusion.

  • H2O is excreted from protonephridium to the surrounding medium (osmoregulation).

Protonephridium = flame cells + excretory + excretory tubes pore

  • Flame cells receive H2O from the body spaces and by beating of cilia on their structure force H2O to move into the excretory tubes. H2O is then excreted from excretory pores (nephridiopore).
  • A little amount of CO2 and NH3 excreted with H2O.
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EXCRETION IN EARTHWORMS

  • In earthworm most of the cells are not in contact with the surrounding environment, so they need special excretory organs to remove the metabolic wastes.
  • The excretory organs of the earthworm are nephridia. These structures are found in pairs in most segments of the earthworm’s body. They excrete dilute urine.
  • Urine includes water, mineral salts, ammonia and urea.
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EXCRETION IN INSECTS

  • The excretory organs of insects are the malpighian tubes. The slender excretory tubes are bathed directly by blood. Substances from the blood enter the tubules by diffusion and active transport.
  • From the tubules, these materials pass into the intestine.
  • Water, nutrients and other useful substances are reabsorbed from the tubules and the digestive tract. Then they are returned to the body fluids.
  • The dry nitrogenous waste product is uric acid, it passes out of the body with the feces through the anus.
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EXCRETION IN BIRDS

  • Nitrogenous wastes in the form of uric acid are removed from the blood by the kidneys.
  • Since there is no urinary bladder, these wastes pass through the ureters to the cloaca. They combine with the fecal matter in the cloaca to form a whitish, semisolid material.