All animals, whether unicellular or multicellular, perform various metabolic activities known as life processes. In different processes, toxic substances are produced in the body. These waste substances have to be eliminated from the body to avoid accumulation. The excretory system is responsible for removing waste from the body, thereby maintaining homeostasis. Different animals excrete distinct waste products and are categorized as
Certain organisms like amoeba, protozoa, echinoderms, Platyhelminthes, poriferans, cnidarians, and aquatic animals excrete ammonia as their waste product. These organisms are called ammnotelic organisms. The mode of excretion is called ammonotelism. These organisms generally perform diffusion using skin, gills, or kidneys to excrete waste from their body. Ammonia has a small molecular size and readily dissolves in water, so its excretion becomes simple. It can traverse easily across cellular membranes. Ammonia dissolves in water to form ammonium hydroxide, which can cause necrosis in the tissues. Thus its elimination is vital for the proper functioning of the body. Due to this property, ammonotelism requires the least energy and a large amount of water for excretion ( 1gram ammonia requires approx 500ml of water).
Fishes and other aquatic organisms consume dietary-rich protein food. These organism intestines are adapted for the deamination of amino acids as they cannot store them for an extended time.
The deamination reaction (conversion of proteins into carbohydrates) produces uric acid. This uric acid is oxidized to allantoin and allantoic acid.
The hydrolysis of allantoin to allantoate and further hydrolysis results in the formation of urea and glyoxylate.
In ammonotelic organisms, urea is further broken down into ammonia and carbon dioxide.
Much of the ammonia is produced from alpha-amino groups present in dietary feed.
Every organism must remove the toxic by-products formed from metabolic activities occurring in the body. The process of removal of nitrogenous waste from the body is called excretion. The principal system involved in the elimination of toxic waste from the body is called the urinary system. Different organisms have characteristics of organs for the removal of waste. In humans and most chordates, the urinary system consists of a pair of kidneys filtering the blood. The blood has urea as the main nitrogenous waste. The kidney receives 25% of cardiac output. The functional unit of the kidney is nephrons responsible for filtering blood and eliminating urea via the formation of urine.
Other excretory systems present in the different organisms are −
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Regulation of osmotic pressure of bodily fluids to maintain homeostasis of the body’s water content is termed osmoregulation. In marine organisms, the cells are isotonic to seawater; thus, no regulation mechanism is required. However, other organisms must excite and conserve water or salts in order to maintain electrolyte balance within the body. For example, in humans, the ADH hormone(antidiuretic hormone or vasopressin) controls the concentration of urine. In the presence of ADH, more water is reabsorbed, resulting in a decreased amount of urine. If the water content in the body is high, ADH is not released by the pituitary, causing the formation of more urine.
Osmoregulation is divided into two types
These organisms complement their body osmolarity with the environment either actively or passively. Examples include marine invertebrates such as echinoderms, mussels, jellyfish, ascidians (sea squirts - primitive chordates), and scallops.
These organisms maintain the concentration gradient with the outside environment so that the net efflux and influx of water are equilibrated.
The types of ions present inside the body and outside the body environment are different. Seawater has a high sodium ion concentration.
Marine invertebrates use sodium ions for muscle contraction and neural signalling, pairing them with potassium ions present in their internal environment. Thus, Osmoconformers can utilize the ionic composition of their external environment to support important biological functions.
Few osmoconformers, like echinoderms, are stenohaline; they can survive in a limited range of external osmolarities.
On the other hand, some are classified as euryhaline, which means they can survive in a broad range of external osmolarities. Mussels are a major example of a euryhaline osmoconformer. Mussels have the ability to close their shells which allows them to remain concealed from unfavourable external environments.
Other examples of osmoconformers are craniates such as hagfish and sharks. Sharks store a high concentration of urea. This allows a diffusion gradient which helps the shark to absorb water in order to balance the concentration difference.
Osmoregulators maintain the specified concentration of ions in the fluid that surrounds cells. Osmoregulation involves a strict brain-to-body signalling mechanism to know the status of body fluids and the distribution of fluids inside the body. The nervous and endocrine system works in coordination to tightly regulate the water ions balance across the body. The organism that can control the water-salt balance despite the different salt concentrations in the environment is called an osmoregulator. Freshwater fishes do not drink much water in hypotonic environments. On the contrary, they frequently urinate dilute urine to achieve electrolyte balance by active transport of salts through the gills. When these fishes move to a hypertonic environment, they start drinking seawater and excrete excess salts through gills and urine.
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In humans, osmoregulation is achieved by kidneys by regulating the reabsorption of water from glomerular filtrate in kidney tubules. ADH hormone act by inserting the aquaporin-2 water channel into the luminal membrane, thereby allowing transcellular water reabsorption to occur down an osmotic gradient.
Animals receive excess amino acids through diet. When proteins, amino acids, or nucleic acids are metabolized, they produce excretory end-products- ammonia, urea, and uric acid. The organism that releases ammonia as a waste product is ammonotelic. These organisms spend less energy and easily diffuse ammonia via gills, skin, and kidneys. The excretion of waste is important for homeostasis and regulation of water ion balance. All organism maintain their internal environment by regulating electrolyte balance. Thus, two types of organisms, osmoconformers and osmoregulator, regulate ionic balance within their body.
Q1. How osmoregulation is achieved in vertebrates?
Ans. In vertebrates, four well-defined processes occur - filtration, reabsorption secretion, and excretion that tightly regulate water ion balance.
Q2. Define the Micturition reflex.
Ans. Discharge of urine from the urinary bladder via a combination of involuntary and voluntary muscle contraction.
Q3. What are abnormal constituents in urine?
Ans. Glucosuria − the presence of glucose in the urine
Hematuria − the presence of RBC in the urine
Ketonuria − high levels of ketone bodies
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