Introduction
Q. What is Excretory System?
Ans: When cells perform their functions, certain waste products are released into the blood stream. These are toxic and hence need to be removed from the body. The process of removal of wastes produced in the cells of the living organisms is called Excretory System
Key Points
- The parts involved in excretion forms the excretory system.
- Waste removal is done by the blood capillaries in the kidneys.
- When the blood reaches the two kidneys, it contains both useful and harmful substances. The useful substances are absorbed back into the blood. The wastes are removed as urine.
- From the kidneys, the urine goes into the urinary bladder through tube-like ureters. It is stored in the bladder and is passed out through the urinary opening at the end of a muscular tube called urethra. The kidneys, ureters, bladder, and urethra form the excretory system.
- An adult human being normally passes about 1 – 1.8 L of urine in 24 hours, and the urine consists of 95% water, 2.5 % urea and 2.5% other waste products.
System
- Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions like Na+, K+, Cl-, phosphate, sulphate, either by metabolic activities or by other means like excess ingestion. These substances have to be removed totally or partially.
- Ammonia, urea, and uric acid are the major forms of nitrogenous wastes excreted by the animals.
- The way in which waste chemicals are removed from the body of the animal depends on the availability of water.
- Ammonia is the most toxic form and requires large amount of water for its elimination, whereas uric acid, being the least toxic, can be removed with a minimum loss of water.
- Aquatic animals like fishes, excrete cell waste in gaseous form (ammonia) which directly dissolves in water.
- Some land animals like birds, lizards, snakes excrete a semi-solid, white coloured compound (uric acid).
- The major excretory product in humans is urea which is excreted through urine.
- Sometimes a person’s kidneys may stop working due to infection or injury. As a result of kidney failure, waste products start accumulating in the blood. Such persons cannot survive unless their blood is filtered periodically through an artificial kidney. This process is called dialysis.
- The process of excreting ammonia is Many bony fishes, aquatic amphibians, and aquatic insects are ammonotelic in nature.
- Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. Kidneys do not play any significant role in its removal.
- Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water.
- Mammals, many terrestrial amphibians, and marine fishes mainly excrete urea and are called ureotelic animals. Ammonia produced by metabolism is converted into urea in the liver of these animals and released into the blood which is filtered and excreted out by the kidneys.
- Some amount of urea may be retained in the kidney matrix of some of these animals to maintain a desired osmolarity [the concentration of a solution expressed as the total number of solute particles per litre].
- Reptiles, birds, land snails and insects excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals.
- A survey of animal kingdom presents a variety of excretory structures. In most of the invertebrates, these structures are simple tubular forms whereas vertebrates have complex tubular organs called kidneys. Some of these structures are mentioned here.
- Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate.
- Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation. Nephridia are the tubular excretory structures of earthworms and other annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance.
- Malpighian tubules are the excretory structures of most of the insects including cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation.
- Antennal glands or green glands perform the excretory function in crustaceans like prawns.