Title Med-RM_Zoo_SP-1_Ch-6_Excretory Products and their Elimination.pdf ✅

Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph. 011-47623456 Chapter Contents The process of removal of metabolic wastes from the body is called excretion. Removal of undigested food is called defaecation or egestion. Carbon dioxide and water are metabolic wastes of carbohydrate and fat metabolism. Their removal is therefore excretion. Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the animals. Animals are classified on the basis of their main excretory product. They could be of the following types : (i) Ammonotelism: Ammonia is the most toxic form and requires large amount of water for its elimination. This process of excreting ammonia is called ammonotelism. Ammonia, as it is readily soluble, is generally excreted by diffusion across body surfaces or through gill surfaces (in fishes) as ammonium ions. Kidneys do not play any significant role in its removal. Aquatic organisms, such as Amoeba and many invertebrates like sponges, Hydra, aquatic insects, cray fish and vertebrates for instance tadpole larva of frog, aquatic amphibians, bony fishes are ammonotelic. (ii) Ureotelism: Terrestrial adaptations necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for conservation of water. Urea can be tolerated in much more concentrated form because it is 100,000 times less toxic than ammonia. Mammals, many terrestrial amphibians and marine fishes mainly excrete urea and are called ureotelic animals. Excretion of urea is known as ureotelism. 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. (iii) Uricotelic: Reptiles, birds, land snails and terrestrial 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. Excretion of uric acid is known as uricotelism. Chapter 6 Excretory Products and their Elimination Human Excretory System Urine Formation Functions of the Tubules Mechanism of Concentration of the Filtrate Regulation of Kidney Function Micturition Role of Other Organs in Excretion Disorders of the Excretory System
180 Excretory Products and their Elimination NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Excretory product is mainly uric acid. Uric acid can be passed out almost in the form of a precipitate since it is almost insoluble in water. This leads to minimum loss of water. Uricotelism is very important for land vertebrates laying shelled eggs. If the embryo within the shelled egg had produced ammonia or urea, that would have accumulated to toxic level. However this problem is solved by being uricotelic, as uric acid being almost insoluble, precipitates and remains with the shell only. This problem is not faced by fishes or amphibians by having shell-less eggs (ammonia or urea can diffuse out) or mammals (as urea carried away by maternal blood across placenta). Urea Synthesis (The Ornithine Cycle) is the biochemical aspect of excretion. Also called Kreb-Henseleit cycle, it occurs in liver and includes : (i) Formation of carbamoyl phosphate by the combination of ammonia, CO2 and ATP. (ii) Carbamoyl phosphate combines with ornithine to form citrulline. (iii) Citrulline joins aspartic acid and changes to argininosuccinic acid. (iv) The latter breaks into fumaric acid and arginine. (v) With the help of enzyme arginase, arginine is hydrolysed to urea and ornithine (which is thus, regenerated and re-used in the cycle). Ornithine cycle Ornithine Fig. : Urea Cycle
NEET Excretory Products and their Elimination 181 Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 Excretory structures found in animal kingdom are 1. Protonephridia or flame cells : These are the excretory structures in platyhelminthes (flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate – Amphioxus. Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation. 2. Nephridia : These are the tubular excretory structures of earthworms and other annelids. Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance. 3. Malpighian tubules : These are the excretory structures of most of the insects including cockroaches. Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation. 4. Antennal glands or green gland : These perform the excretory function in crustaceans like prawns. The regulation of its solute and water movements by osmosis is done by two ways: 1. Osmoconformers are the animals that do not actively control the osmotic concentration of their body fluids. They rather change the osmolarity of body fluids according to the osmolarity of the surrounding medium. All marine invertebrates and some freshwater invertebrates are strictly osmoconformers. Hagfish is a vertebrate osmoconformer. Osmoconformers show an excellent ability to tolerate a wide range of cellular osmotic environments. 2. Osmoregulators, on the other hand, are the animals that maintain an internal osmolarity, different from the surrounding medium in which they inhabit. Many aquatic invertebrates are strict or limited osmoregulators. Most vertebrates are strict osmoregulators, i.e, they maintain the composition of the body fluids within a narrow osmotic range. The notable exception, however, are the hagfish (Myxine, a marine cyclostome) and elasmobranch fish(sharks and rays). Osmoregulators must either eliminate excess water if they are in a hypotonic medium or continuously take in water to compensate for water loss if they are in hypertonic solution. Therefore, osmoregulators have to spend energy to move water in or out and maintain osmotic gradients by manipulating solute concentrations in their body fluids. Water and Solute Regulation in Fresh Water Environment Osmolarity of fresh water is generally much less than 50 mOsm L–1 while the fresh water vertebrates have blood osmolarities in the range of 200 to 300 mOsm L–1. The body fluids of fresh water animals are generally hypertonic to their surrounding environment. The problem faced by the animals will be : (i) Loss of body salt to the outside. (ii) Entry of excess of water. Protozoans like Amoeba, Paramoecium have contractile vacuoles that pump out excess water. Adaptations shown by other animals include: (i) Minimising the gain of water and loss of salts by specialised body cover in the form scales or adipose cover. (ii) Do not drink water to reduce the need to expel excess of water. (iii) Passing out very dilute urine. (iv) Presence of ionocytes or chloride cells which can actively uptake the salts (Na+ and Cl– ) from the surrounding water (Surrounding water has less than 1 mM NaCl and plasma concentration is more than 100 mM, therefore, takenup actively). Water and Solute Regulation in Marine Environment Sea water usually has osmolarity of about 1000 mOsm L–1. Osmolarity of human blood is about 300 mOsm L–1. The osmoregulatory problems in marine situation are opposite to those in freshwater environment. Marine bony fishes have the body fluids hypotonic to seawater, and thereby, they tend to lose water from the body through permeable surfaces (gill membranes, oral and anal membranes). To compensate for the water loss, marine bony fish drink seawater. However, drinking seawater results in a gain of excess salts. The ionocytes or chloride cells of the gill membrane of marine bony fish help to eliminate excess monovalent ions from the body fluid to the seawater. Divalent cations are generally eliminated through faecal matter.
182 Excretory Products and their Elimination NEET Aakash Educational Services Limited - Regd. Office : Aakash Tower, 8, Pusa Road, New Delhi-110005 Ph.011-47623456 In general, the body fluids of marine invertebrates, ascidians and the hagfish are isosmotic to seawater. Osmolarity of the body fluids is raised by accumulating certain organic substances (osmolytes). Retention of osmolytes in body fluids reduces the osmoregulatory challenges. The best known examples of such organic osmolytes are urea and trimethylamine oxide (TMAO). Note: Body fluids of sharks and coelacanths are slightly hyperosmotic to seawater due to retention of urea and TMAO while hypoionic to seawater as they maintain far lower concentration of inorganic ions in the body fluids. Water and Solute Regulation in Terrestrial Environment 1. Humans for example, die if they lose around 12 percent of the body water. Therefore, water loss must be compensated by drinking and eating moist food. 2. Desert mammals are well adapted to minimise water loss. Kangaroo rats, for example lose so little water that they can recover 90 percent of the loss by using metabolic water (water derived from different cellular metabolic processes). The nasal countercurrent mechanism for conserving respiratory moisture is also important. 3. Camels: When water is not available, the camels do not produce urine but store urea in tissues and solely depend on metabolic water. When water is available, they rehydrate themselves by drinking up to 80 litres of water in 10 minutes. HUMAN EXCRETORY SYSTEM In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. Kidneys Shape and Size: Kidneys are reddish brown, bean-shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. Each kidney of an adult human measures 10–12 cm in length, 5–7 cm in width, 2–3 cm in thickness with an average weight of 120–170 g. Left kidney is little higher than the right one because of more space being occupied by the liver on right side. So, only in human beings the right kidney is slightly below the left. But in other mammals, e.g., rabbit, the left kidney is below the right. Adrenal gland Renal artery Renal vein Kidney Dorsal aorta Ureter Urinary bladder Urethra Inferior vena cava Medulla Pelvis Cortex Fig. : Human Urinary System