Sep 27, 2014

Excretion in Animals

Nitrogenous Excretory Products

Nitrogenous waste products are produced by the breakdown of proteins, nucleic acids etc. Ammonia is produced by the removal of amino group (NH2) from amino acids, by a process called deamination. Amino group is transferred to another molecule for removal or reuse. Amino group which is not reused for recycling of amino acids is dissolved in water and excreted to avoid toxic rise in the plasma. Elevated levels of nitrogenous wastes can cause convulsions, coma and eventually death. Ammonia may be excreted immediately or converted to urea or uric acid. Lower quantities of nitrogen are excreted in the form of other compounds such as creatine, a creatinine or trimethylamine oxide and in very small quantities as amino acids, purine and pyrimidine. Metabolism of purine and pyrimidine bases produces significant amount of nitrogenous wastes of hypoxanthine, xanthine, uric acid, allantoin, urea and ammonia.

Nature of Excretory Products in Different Habitats

The exact nature of the excretory product is determined mainly by the availability of water to the organism i.e. habitat, and the extent to which the organism controls the water loss. The correlation with habitat is (a) ammonia-aquatic (b) urea- aquatic and terrestrial (c) uric acid - terrestrial.

Main types of excretory products in animals
Ammonia: The major source of ammonia is the deamination of excess of amino acids. Ammonia is extremely toxic and must be eliminated. Being very soluble, it can be eliminated from the body rapidly and safely if diluted in a sufficient volume of water. This presents no real problems to organisms which have ready access to water but this applies only to those organisms living in freshwater. It is therefore excreted rapidly as ammonium ions (NH4++) in most aquatic organisms, from protozoa to amphibia, fishes, before reaching to concentrations toxic to the organisms. About 500 ml water is needed to excrete 1 gram of ammonia.

Urea: It is less toxic than ammonia. About 50 ml of water is needed to excrete 1 gram of urea.

Uric acid: Uric acid and its salts are ideal excretory products for terrestrial organisms and essential for organisms such as land-living insects and birds which produce shelled eggs. They can be stored in cells, tissues and organs without producing any toxic or harmful osmoregulatory effects. Only 1 ml of water is required to eliminate I gram of uric acid.

Excretion in Various Groups of Animals

Animals excreting ammonia, urea and uric acid are called ammonotelic, ureotelic and uricotelic respectively. There is diversity in the excretory structures in the animals. Excretion in Hydra, Planaria, earthworm, cockroach and man is discussed as the main representative models.

Excretion in Hydra

Hydra is a freshwater organism. For excretion there are no specialized structures in Hydra. Carbon dioxide, salts and ammonia leave the cells by diffusion into the iso- osmotic surroundings.

                            Excretion in Planaria

Excretion in Planaria

Planaria, which lives in fresh water, has two strands of branching excretory tubes that open to the outside of the body through excretory pores. The simple tubular excretory system is called protonephridium, which is a network of closed tubules without internal openings. Tubular system is spread throughout the body. Located along the tubules are bulb-like flame cells.

Flame Cells: The flame cells consist of cytoplasmic body and some cytoplasmic prolongations. The cytoplasm is hollowed out to form a large cavity. The cavity is continuous with the cavity of the finer tubules. As a result nucleus is placed on one side. A tuft of cilia projects into the space (cavity) of the cell. As the movement of the cilia resembles the flickering flame under the microscope so it is called flame cell. The beating of flame-cell’s cilia propels hypotonic fluid through the excretory canal and out of the body through numerous nephridiopores. The system functions in osmoregulation and also in excreting nitrogenous wastes. Fresh water flatworm excretes very dilute urine.

Excretion in Earthworm

The body of earthworm is divided into segments by a series of partitions called septa and each body segment has a pair of tubular excretory structures called nephridia. The nephridia are more complex than that of Planaria and are called metanephridia. Each nephridium is a tubule open at both ends. Each nephridium begins with a ciliated funnel the nephrostome, that opens from the body cavity of a segment in a coiled tubule enlarged bladder that empties to the outside of the body via an opening called the nephridiopore. Fluid from the body cavity flows through the nephrostome into the nephridium. As fluid from the coelom is propelled through the tubule by beating cilia, certain substances are re-absorbed and carried away by a network of capillaries surrounding the tubule. This process results in the formation of urine that contains only metabolic wastes, salts and water. Urine is given out of the body through nephridiopore.

Excretion in Cockroach
Excretion in Cockroach
Insects have an excretory system quite different from other animals. The excretory ducts of insects do not open to the outside of the body. Instead, the system consists of a cluster of long, thin Malpighian tubules. These are the only excretory structures in animal kingdom that are attached to the gut. Together the Malpighian tubules and the gut function as the insect’s excretory system. Malpighian tubules are present between the stomach and large intestine. They originate as an outgrowth, from the beginning of the large intestine. They are lined with glandular cells having a characteristic brush border. The number of Malpighian tubules is variable, usually 15-20. They are blind at the distal end and bathe into the haemolymph of the haemocoel.

Uric acid simply flows from the surrounding haemolymph into the Malpighian tubules, and water follows a salt gradient established by active transport of K. Water and salts are reabsorbed in the rectum, but the uric acid crystals, leave the body at the anus.