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  Excretory Mechanism, Biology tutorial

Introduction to Excretory Mechanism:

Excretion is procedure by which waste products of metabolism are abolished from the organism. In vertebrates this is mainly performed by lungs, kidneys and skin. This is in contrast with secretion, where substance may have particular tasks after leaving cell. Excretion is the necessary process in every forms of life. In mammals urine is carried out through urethra and which is part of excretory system. In single-celled organisms, waste products are released directly through surface of cell.

Every organism, from smallest protist to largest mammal, should free itself of potentially harmful by-products of its own essential activities. This procedure in living things is known as elimination, that may be regarded to encompass all different mechanisms and procedure through which life forms dispose of or throw off waste products, toxic substances, and dead portions of organism. Nature of the procedure and of particular structures developed for waste disposal differs greatly with size and difficulty of organism.

Four terms are usually related with waste-disposal procedures and are frequently utilized interchangeably, although not always properly: secretion, excretion, elimination and egestion.

Excretion is the general term referring to division and throwing off of waste materials or toxic substances from cells and tissues of the plant or animal.

Separation, elaboration, and elimination of definite products arising from cellular functions in multicellular organisms are known as secretion. Although these substances may be the waste product of cell producing them, they are often helpful to other cells of organism. Examples of secretions are digestive enzymes generated by intestinal and pancreatic tissue cells of vertebrate animals, hormones synthesized by particular glandular cells of plants and animals, and sweat secreted by glandular cells in skins of some mammals. Secretion involves that chemical compounds being secreted were synthesized by specialized cells and that they are of functional value to organism. Disposal of common waste products must not, consequently, be regarded to be of secretory nature.

Plants make carbon dioxide and water as respiratory waste products. In green plants, carbon dioxide discharged during respiration gets used during photosynthesis. Oxygen can be considered as waste product produced during photosynthesis, and exits via root cell walls, stomata, and other routes. Plants can get relieve of excess water via transpiration and guttation. It has been illustrated that leaf acts as excretophore and, additionally to being the green plant's main organ of photosynthesis, is also utilized as plant's way of excreting toxic wastes. Other waste materials which are exuded by few plants - resin, saps, latex, etc. are forced from interior of plant by hydrostatic pressures inside plant and by absorptive forces of plant cells. These later processes don't require added energy, they operate passively. Though, during pre-abscission phase, metabolic levels of the leaf are high. Plants also expel some waste substances in soil around them.

Green plants in darkness or plants which don't have chlorophyll generate carbon dioxide and water as respiratory waste products. Carbon dioxide is secreted in same way as oxygen using diffusion by stomata and cell walls. Materials which are exuded by some plants- saps, resins, latexes, etc.-are forced from interior of plant by hydrostatic pressures within plant and via absorptive forces of plant cells. Such forces are passive in nature, and exudation needs no energy expenditure on the part of plant.

As in aquatic animals, excretory system in land animals eliminates nitrogenous waste and assists establish the balance between salt and water in body. Terrestrial animals, though, also run risk of drying out by evaporation from body surface and lungs. Elimination of feces and excretion of urine also bring about water loss. Drinking, foods having large amounts of water, and generating water during cellular respiration assist overcome loss. Animals which generate uric acid require less water than those excreting urine. Flame cells in flatworms, nephridia in segmented worms, Malpighian tubules in insects, and kidneys in vertebrates are all cases of excretory systems.

Planarians and other flatworms contain the excretory system which comprises of two or more longitudinal branching tubules which run length of body. Tubules open to outside of animal by holes or pores in surface. Tubules end in flame cells, bulb-shaped cells which contain cilia. Cilia create currents which move water and wastes through canals and out pores. Flatworms lack the circulatory system, so their flame cells excretory system chooses wastes directly from body tissues.

Egestion is act of excreting not viable or undigested material from cell, as in case of single-celled organisms, or from digestive tract of multicellular animals.

Various mechanisms have evolved which allow different animal species to live in wide range of environments. In animals whose bodies comprise of the single layer of cells, waste disposal is achieved mainly by diffusion from site of waste production to outside environment. This method is efficient when distances over which wastes diffuse are comparatively short, when there is high surface area to volume relationship, and when rate of waste production is comparatively low. In more complex animals, though, waste elimination by diffusion through body wall to exterior is less effective as individual cells are farther removed from exterior surface of organism. Presence of specialized mechanisms of elimination in higher animals allows wastes to be quickly transported to exterior surface of body.

Several unicellular organisms such as Amoeba throw out wastes by diffusion from body surface. Protozoan's contain no organs for excretion. As they dwell in aquatic habitat, their wastes are eradicated by diffusion through plasma membrane.

Simple multicellular organisms such as Hydra throw out solid waste matter by their mouth. Higher multicellular organisms contain well-defined specialized excretory organs. Such organs could be plain tubular structures as in flatworms and leech.

Excretory organs of insects (like housefly, grasshopper and cockroach) are also tubular. They get rid of nitrogenous wastes from body fluid and assist in maintaining water balance in body. In vertebrates, major organs of excretion and maintenance of water balance are kidneys.

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