The Excretory System—Part 1

Maintaining Homeostasis

• The key function of the excretory system is maintaining homeostasis.
• This is done by:
  • Regulating water balance
  • Removing harmful substances
• Nitrogen cannot be recycled within an animal, so it needs to be removed from the organism.
  • However, getting rid of nitrogen is a huge problem.
  • Nitrogen is found in two of the four groups of macromolecules.
    • Nucleic acids (RNA, DNA):
      • A chain of nucleotides
      • Each nucleotide has a nitrogenous base (A, T, C, G, U).
    • Proteins:
      • A chain of amino acids
      • Each amino acid contains an amino group: —NH₂.

Parts of figure 5.27 and an unlabeled figure, pages 78 and 68, Campbell's Biology, 5th Edition

Eliminating Nitrogenous Wastes

• Animals must maintain their water balance while at the same time eliminating nitrogenous wastes.
• First, proteins and nucleic acids are broken down:
  • Proteins break down into amino acids, which break down to amino groups.
  • Nucleic acids break down into nitrogenous bases, which break down to amino groups.
• Nitrogen from amino groups is converted into one of the following three molecules and then excreted into the environment:
  • Ammonia
    • Very toxic
    • Soluble in water
  • Urea
    • 100 000 times less toxic than ammonia
    • Soluble in water
  • Uric acid
    • A white, pasty material
    • Not soluble in water
• Different groups of animals have solved the problem of nitrogenous waste elimination in different ways depending on their particular circumstances:
  • Most aquatic animals convert amino groups to ammonia.
  • Mammals, amphibians, and sharks convert amino groups to urea.
  • Birds, reptiles, insects convert amino groups to uric acid.

Figure 44.10, page 875, Campbell's Biology, 5th Edition

Nitrogenous Waste Creation Details

• Only aquatic animals can excrete ammonia:
  • It is very toxic, so it must be rapidly eliminated from the organism.
  • Aquatic animals are surrounded by water, and therefore the ammonia is constantly diffusing out of the body through their skin and gills.
• Terrestrial animals have several problems:
  • They can't lose much water.
  • They can't tolerate ammonia because it is so toxic.
  • So, they convert ammonia to urea (in the liver).
    • Urea is 100 000 times less toxic, so it can be transported through the circulatory system.
    • It is finally eliminated by an organ called the kidney.
• Animals that develop in an external egg (birds, reptiles, insects) have a bigger problem:
  • Since the egg is external to the mother's body, the nitrogenous wastes cannot be eliminated through a kidney.
    • Therefore, the nitrogenous wastes build up in the circulatory system.
    • Urea is much less toxic than ammonia, but it is a problem if it can't be eliminated on a regular basis.
  • So, animals which lay eggs convert their nitrogenous wastes into uric acid.
    • Uric acid is thousands of times less soluble in water than urea or ammonia.
    • Therefore, it forms a paste-like substance.
    • It can be safely stored in a place in the egg (the allantois) where it won't harm the embryo.
    • There is almost no water loss in processing the nitrogen.

The Liver and the Kidney

• Liver
  • Converts nasty chemicals into less harmful chemicals.
  • Ammonia (a nasty chemical) is converted into urea (a less nasty chemical).
  • Another example: alcohol and drugs are detoxified in the liver.
    • Alcohol abuse can lead to cirrhosis of the liver.
    • People who eat poisonous mushrooms destroy their liver.
• Kidney
  • Removes waste products from the circulatory system: this process is called excretion.
  • Urea is removed from circulatory system by the kidney.

Excretory System Overview

The excretory system works through a three step process:

1. Filtration
   • Blood and other bodily fluids are exposed to a filtering device made of a selectively permeable membrane.
     • This membrane allows the body to retain proteins, other large molecules, and cells.
     • Hydrostatic pressure (blood pressure in many cases) forces water and smaller solutes like salts, sugars, amino acids, urea, etc. through the filter and out of the circulatory system into the excretory system.
   • The resulting aqueous solution in the excretory system is called the filtrate.
2. Reabsorption
   • Some of the water and smaller solutes are recovered from the filtrate, which is now in the excretory system.
   • These molecules are actively transported back into the circulatory system.
   • These molecules are important and essential for the body. They include:
     • Water
     • Glucose
     • Salts
     • Amino acids
3. Secretion
   • Some solutes are actively transported from the blood (circulatory system) into the filtrate (excretory system).
   • These molecules are not needed. They include:
     • Excess salts
     • Toxins

Figure 44.14, page 879, Campbell's Biology, 5th Edition

The Mammalian Excretory System

• Kidney
  • Removes unwanted chemicals from our circulatory system.
  • Once these unwanted chemicals leave the circulatory system, they are called urine because they contain urea.
• Urine leaves the body through the following sequence of organs:
  • Ureter
  • Urinary bladder
  • Urethra

Figure 44.18(a), page 883, Campbell's Biology, 5th Edition