Algae

Ch 45 Body Fluid Regulation and Excretion, Part I.

Body fluids are mostly H₂O. Water is maintained at an internal equilibrium within the body of most animals. For most marine invertebrates (clams, lobsters, starfish, etc.) the water content of the body is equal to that of seawater, so there are no organ systems in marine invertebrates devoted to regulating internal water balance. These animals are said to be osmoconformers. Most marine vertebrates and all terrestrial vertebrates must osmoregulate through various physiological means. Terrestrial mammals are constantly losing water through perspiration, breathing, and urination. The kidneys of terrestrial mammals are typically involved in water conservation. At the same time kidneys eliminate metabolic waste using water to carry the waste away during urination. Details of kidney function are covered below under “Urinary System in Humans.” In contrast to terrestrial vertebrates freshwater fish have the opposite problem of too much water gain. Because there is less water in normal body fluids compared to the freshwater that they swim in, fish in freshwater lakes, ponds, and streams constantly gain water through their gills and skin. Kidneys of freshwater fish produce lots of watery urine. Most saltwater fish are surrounded by an ocean of salty water. The salty water constantly draws water out of the marine fish’s body. To counteract this water loss, most marine fish drink ocean water and rid themselves of the imbibed salts by excreting the salt through their gills. Marine fish also produce limited amounts of urine.

For terrestrial vertebrates, nitrogenous wastes are removed from body fluids by action of the kidneys, the same organ involved in maintaining water balance. Nitrogenous waste takes three forms:

ammonia - the direct waste produced as a byproduct of protein metabolism. Produced by all animals. Because of its toxicity, many animals convert ammonia into a less toxic form. Any conversion of ammonia into other forms comes at a price, for animals must expend cellular energy (ATP) to convert ammonia to either form of nitrogenous waste listed below.
urea - formed in the liver from the conversion of ammonia. Urea is the most common form of nitrogenous waste in human body fluids. Urea is less toxic than ammonia but like ammonia, urea is in solution and, for terrestrial animals, its elimination from the body requires some loss of water as water is used to flush it from the body (urea and ammonia are both found in urine).
uric acid - in reptiles and birds uric acid is formed in the liver from the conversion of ammonia. Uric acid is not very soluble. It is a white, pasty solid as seen in bird droppings. As a solid nitrogenous waste, uric acid does not contaminate the fluid confined within a shelled egg. Thus, reptiles and birds during their stage of development within a shelled egg, produce uric acid as their final form of nitrogenous waste. Mature birds retain this habit as to many reptiles. [The uric acid in humans is the result of nucleic acid breakdown and is not produced from ammonia conversion.]

Organs of Excretion

In order for an organ to selective remove metabolic waste molecules it most possess transport epithelia, epithelial tissue with cells possessing numerous transport proteins embedded in their plasma membranes. Organs with transport epithelia involved in excreting (eliminating) waste molecules include:

Gills in fish. In addition to respiratory gas exchange gills excrete the nitrogenous waste ammonia directly into the surrounding water where it quickly dissipates (unless the fish is confined in a small, stagnant body of water).
Nephridia of various sorts in invertebrates such as flatworms and earthworms (see figs. 45.4). Nephridia are tubular structures, the cells that make up the tube possess the transport epithelium.
Kidneys in vertebrates. Interestingly, one of the main functional units of the kidney is the nephron, a microscopic tubular structure reminiscent of the nephridia in earthworms.

Go to Part II of Ch 45 notes