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Human Excretory System

  • All organs involved in excretion constitute the excretory system.
  • The process by which excretory wastes are removed from the body is called excretion.
  • The excretory wastes that are released during respiration and other activities of the body are carbon IV oxide, excess water, salts, and urea.
  • The organs which collectively ensure the removal of different waste products are the kidney, skin, lungs, and liver.
  • The Kidney
    • The kidney shows three regions; the outer region called the cortex, the inner region called the medulla, and the funnel-shaped tubule that leads to the urinary duct called the ureter.
    • The nephron, which are the unit of excretion is made up of the Bowman’s capsule containing the network of the blood vessels called glomeruli.

Sectional View of a Kidney (Photo Credit: Britannica.com)

How it Works
    • The excretory unit in the kidney is called the nephron.
    • Blood reaching the kidney enters into the glomerulus at high pressure that sieves out protein and blood cells at the Bowman’s capsule.
    • Excess mineral salts, water, as well as urea flow into the space in the tubule.
    • Some water and mineral salt are reabsorbed into the tubule and excess salt, water, and urea are passed into the bladder as urine.
    • Urine is then passed outside the body from the bladder through the urethra.
  • The Skin
    • The skin is made up of the dermis, epidermis, and malpighian layer.
    • The dermis is made up of many structures, including sweat glands, sweat ducts, and sweat pores, that are directly involved in the excretion of sweat from the body.

Sectional View of the Skin (Photo Credit: LivingBio.net)

How the Skin Works
    • The sweat gland collects excess water, salts, and urea from the blood capillaries in the dermis to form a salty liquid called sweat.
    • The sweat flows into the sweat ducts through the contraction of the muscles and pass out through the sweat pore.
    • The passing out of sweat through the skin helps maintain the temperature of the body.
  • The Lungs
    • The lungs are used for the removal of carbon IV oxide that results from cell respiration in the body.
    • Carbon IV oxide dissolves in the blood plasma and is carried to the lungs through the blood vessels.
    • The gas diffuses through the capillaries to the alveoli.
    • When contraction of the chest cavity takes place, carbon IV oxide flows from the alveolus to the bronchi, then to the trachea and out through the nostrils.

The Lungs (Photo Credit: EduShore)

  • The Liver and its Functions
    • The liver does not pass out excretory waste directly but it is involved in changing harmful substances in the body to non-harmful.
    • For example, conversion of dead blood cells into bile that is useful in the digestion process, conversion of excess amino acids into ammonia, and consequently urea that can be easily removed from the body through the kidney, conversion of poisonous alcohol and pesticide to non-poisonous substances.

The Anatomy of the Liver (Photo Credit: Hopkins Medicine)

The Need for Excretion
  • Excretion is the removal of metabolic activities from the body.
  • If the process of excretion does not take place, waste accumulates in the body and may be poisonous or can destroy the body resulting in sickness or death.
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