Excretory and Reproductive Systems

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Excretory and Reproductive Systems

Excretory and reproductive systems each have important but different functions in animals. The excretory system maintains water, ion, and nitrogen balance within the body and eliminates wastes. The reproductive system creates new individuals of a species. Both the excretory and reproductive systems are under endocrine control but are also influenced by the external environment.

The Reproductive System

The reproductive system functions to produce more individuals of a species. The reproductive system is regulated by the endocrine system and is affected by environmental conditions outside of the animal and internal conditions inside the animal. Reproductive cycles are influenced by the time of year, amount of daylight and rainfall, and temperature, in addition to nutrition and general health of the animal.

The main organs in the reproductive system are the gonads, which include the ovary and testis. The ovaries produce eggs and the testes produce sperm. The egg and the sperm are gametes that combine to form a zygote in a process called fertilization. The zygote will grow and develop into a new individual of the species.

The egg and sperm both carry DNA, the material that contains the genetic instructions for the growth and development of a new individual. Ga-metes are produced through meiosis and contain only one-half of the genetic information of the parent. When the gametes come together and form a zygote, the zygote will have half of its genetic information from the egg and half from the sperm.

In vertebrates such as humans, egg and sperm come separately from different individuals. Humans and most other animals (both vertebrates and invertebrates) have different sexes commonly called males and females. In the case of animals without separate sexes, one individual produces both eggs and sperm and is called a hermaphrodite. Most hermaphrodites, including many annelids, pair with another individual and exchange eggs and sperm rather than fertilizing themselves. Some platyhelminthes self fertilize. Other animals switch between being male and female, a condition called sequential hermaphroditism.

In addition to sexual reproduction, the fusion of the egg and sperm, some animals can reproduce asexually. In asexual reproduction, genetic material is not combined. Therefore, it produces an offspring that is genetically identical to the parent. Types of asexual reproduction include budding, fission, and parthenogenesis. Parthenogenesis occurs in rotifers, some bees, wasps, ants, and several species of fish, lizards, and amphibians. Some animals switch between sexual and asexual reproduction, and some reproduce only asexually.

Fertilization can happen internally or externally. External fertilization occurs in most sea-living creatures as well as freshwater fish and amphibians. In external fertilization both eggs and sperm are shed into water and fertilization, development, and growth of the zygote all take place outside the body. Internal fertilization occurs when egg and sperm are joined inside of the body. The zygote can develop inside of the reproductive tract, as in mammals. Birds and reptiles have internal fertilization but lay eggs. The zygote develops inside the egg but outside of the body. Invertebrates that have internal fertilization also lay eggs.

The Excretory System

Vertebrates have a closed circulatory system, which means that they have arteries and veins that transport blood. Cellular metabolism produces waste and uses nutrients and water. The circulatory system carries water and nutrients to cells and carries waste products away from cells. Water and nutrients come from ingestion and digestion. Waste removal is critical to maintaining internal homeostasis.

Nitrogen is the most toxic byproduct of cellular metabolism. The body must excrete nitrogen. Nitrogen is commonly produced as ammonia from cells. Many animals will convert ammonia, which is extremely toxic, into a less toxic form of nitrogenous waste. These include urea and uric acid.

The habitat of an animal determines which kind of nitrogenous waste it produces. Most aquatic animals, both invertebrates and freshwater fish, excrete ammonia. Ammonia dissolves in water and is easy to transport outside of the body by diffusion when an animal is surrounded by water. This happens primarily through the skin in invertebrates and through the gills of fishes. Ammonia must be diluted by a great deal of water to be nontoxic. There is not enough water in land animals to dilute ammonia enough, so ammonia is turned into either urea or uric acid.

Which of these an animal produces is linked to where the offspring develop. Animals characterized by internal development of offspring such as mammals make urea. So do animals that have eggs which develop in water, such as amphibians, sharks, and some fish that live in saltwater.

Urea is not very toxic and can be concentrated by the kidney to conserve water. Urea is dissolved in water, so transport of waste out of a developing egg that is sitting in freshwater is done by diffusion. Uric acid is not water soluble and is excreted in a paste. Birds, reptiles, and insects all produce uric acid. These animals develop in an egg on land, and waste must accumulate in the egg as the individual grows and develops. Because uric acid is not water soluble, it can sit in the egg and not be reabsorbed by the developing zygote.

Water balance in the body is maintained through a process called osmoregulation. The vertebrate kidney is a specialized organ that both concentrates urea or uric acid and maintains water balance, mostly by filtering the blood. When blood passes through the kidney, water is reabsorbed and reused by the body. Additionally, almost all sugar, salts, and other nutrients are reabsorbed by the body from the kidney. Waste products are taken out and eliminated, the main waste product being nitrogenous waste. The kidney contains about 20 percent of the blood volume at any one time.

In invertebrates, a variety of specialized structures exist for waste removal. Flame bulb protonephridia in planaria are the most primitive specialized osmoregulation structure in invertebrates. Annelids have an excretory system called the metanephridium, and it functions for both osmoregulation and nitrogenous waste removal. Insects use Malpighian tubules for both osmoregulation and filtering the hemolymph.

see also Catadromous-Diadromous and Anadromous Fishes; Fertilization; Ontogeny.

Laura A. Higgins