Brent Cornell

    
Excretion is the removal from the body of the waste products of metabolic activity

• Defecation is not considered part of excretion as faeces are undigested food remnants and not metabolic waste products

There are two key functions that an excretory system performs:

• Removes nitrogenous wastes that may be toxic to the body in large concentrations
• Removes excess water to maintain a suitable osmolarity within the tissues and cells

Removing Nitrogenous Waste

Nitrogenous wastes are produced from the breakdown of nitrogen-containing compounds like amino acids and nucleotides

• Nitrogenous wastes are toxic to the organism and hence excess levels must be eliminated from the body
• The type of nitrogenous waste in animals is correlated with the evolutionary history of the animal and the habitat

Most aquatic animals eliminate their nitrogenous wastes as ammonia (NH₃)

• Ammonia is highly toxic but also very water soluble and hence can be effectively flushed by animals in aquatic habitats

Terrestrial animals have less access to water and hence must package nitrogenous waste in less toxic forms

• Mammals eliminate their nitrogenous wastes as urea, which is less toxic and hence can be stored at higher concentrations
• Reptiles and birds eliminate wastes as uric acid, which requires more energy to make but is relatively non-toxic and requires even less water to flush (it is eliminated as a semi-solid paste)

Nitrogenous Waste Products

    
Removing Excess Water

Water levels within an organism are constantly changing as a result of metabolic activity

• Water is produced via condensation reactions (anabolism) and is consumed during hydrolysis reactions (catabolism)
• The concentration of water within cells (osmolarity) will impact tissue viability (i.e. governs osmotic pressure within cells)

Animals may be either osmoconformers or osmoregulators according to how they manage their internal osmotic conditions:

• Osmoconformers maintain internal conditions that are equal to the osmolarity of their environment
• Osmoregulators keep their body’s osmolarity constant, regardless of environmental conditions

By matching internal osmotic conditions to the environment, osmoconformers minimise water movement in and out of cells

• Less energy is used to maintain internal osmotic conditions within an osmoconformer

While osmoregulation is a more energy-intensive process, it ensures internal osmotic conditions are always tightly controlled

• Osmoregulators can maintain optimal internal conditions whereas osmoconformers are affected by environmental conditions

Osmoconformers versus Osmoregulators