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CHAPTER 9, EXCRETION

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INTRODUCTION, ● Animals that invaded land had the risk of desiccation and were, unable to excrete metabolic waste directly into the water; hence, there was a need for an alternate pathway to dispose the, nitrogenous wastes., ● Most animals rely on kidneys to control ionic and water balance., ● Some animals depend on external tissues such as the gills, skin, and digestive mucosa to collectively regulate three homeostatic, processes namely, osmotic regulation, ionic regulation and, nitrogen excretion., ● The process by which the body gets rid of the nitrogenous waste, products of protein metabolism is called excretion.

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● Fresh water vertebrates maintain higher salt concentrations in, their body fluids; marine vertebrates maintain lower salt, concentrations in their body fluids and terrestrial animals have, more water in their body than the surrounding hence tend to, lose water by evaporation, ● The major nitrogenous waste products are ammonia, urea and, uric acid. Other waste products of protein metabolism are, trimethyl amine oxide (TMO) in marine teleosts, guanine in, spiders, hippuric acid, allantonin, allantoic acid, ornithuric acid,, creatinine, creatine, purines, pyramidines and pterines.

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MODES OF EXCRETION

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● Excretory system helps in collecting nitrogenous waste and, expelling it into the external environment., ● Animals have evolved different strategies to get rid of these, nitrogenous wastes., ● Ammonia produced during amino acid breakdown is toxic hence, must be excreted either as ammonia, urea or uric acid., ● The type of nitrogenous end product an animal excretes depends, upon the habitat of the animal., ● Ammonia requires large amount of water for its elimination, whereas, uric acid, being the least toxic can be removed with the minimum, loss of water, and urea can be stored in the body for considerable, periods of time, as it is less toxic and less soluble in water than, ammonia.

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● Animals that excrete most of its nitrogen in the form of ammonia, are called ammonoteles. Many fishes, aquatic amphibians and, aquatic insects are ammonotelic., ● Reptiles, birds, land snails and insects excrete uric acid crystals, with, a minimum loss of water and are called uricoteles. In terrestrial, animals, less toxic urea and uric acid are produced to conserve, water., ● Mammals and terrestrial amphibians mainly excrete urea and are, called ureoteles. Earthworms while in soil are ureoteles and when in, water are ammonoteles.

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● Most invertebrates have a simple tubular structure in the form of, primitive kidneys called protonephridia and metanephridia., ● Vertebrates have complex tubular organs called kidneys., Protonephridia are excretory structures with specialized cells in the, form of flame cells (cilia) in Platyhelminthes (example tapeworm), and Solenocytes (flagella) in Amphioxus., ● Nematodes have rennette cells, Metanephridia are the tubular, excretory structures in annelids and molluscs., ● Malpighian tubules are the excretory structures in most insects., ● Antennal glands or green glands perform excretory function in, crustaceans like prawns., ● Vertebrate kidney differs among taxa in relation to the environmental, conditions.

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● Nephron is the structural and functional unit of kidneys., ● Reptiles have reduced glomerulus or lack glomerulus and Henle’s, loop and hence produce very little hypotonic urine, whereas, mammalian kidneys produce concentrated (hyperosmotic) urine due, to the presence of long Henle’s loop., ● The Loop of Henle of the nephron has evolved to form hypertonic, urine., ● A glomerular kidneys of marine fishes produce little urine that is iso, osmotic to the body fluid. Amphibians and fresh water fish lack, Henle’s loop hence produce dilute urine (hypoosmotic)

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STRUCTURE OF KIDNEY, ● Excretory system in human consists of a pair of kidneys, a pair of, ureters, urinary bladder and urethra., ● Kidneys are reddish brown, bean shaped structures that lie in the, superior lumbar region between the levels of the last thoracic and third, lumbar vertebra close to the dorsal inner wall of the abdominal cavity., ● The right kidney is placed slightly lower than the left kidney. Each, kidney weighs an average of 120-170 grams., ● The outer layer of the kidney is covered by three layers of supportive, tissues namely, renal fascia, perirenal fat capsule and fibrous capsule.

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● The longitudinal section of kidney shows, an outer cortex, inner, medulla and pelvis., ● The medulla is divided into a few conical tissue masses called, medullary pyramids or renal pyramids., ● The part of cortex that extends in between the medullary pyramids is, the renal columns of Bertini., ● The centre of the inner concave surface of the kidney has a notch, called the renal hilum, through which ureter, blood vessels and, nerves innervate., ● Inner to the hilum is a broad funnel shaped space called the renal, pelvis with projection called calyces.

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● The renal pelvis is continuous with the ureter once it leaves the, hilum., ● The walls of the calyces, pelvis and ureter have smooth muscles, which contracts rhythmically., ● The calyces collect the urine and empties into the ureter, which is, stored in the urinary bladder temporarily., ● The urinary bladder opens into the urethra through which urine is, expelled out.

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●, ●, ●, ●, , ●, ●, , ●, , Each kidney has nearly one million complex tubular structures called nephron, Each nephron consists of a filtering corpuscle called renal corpuscle, (malpighian body) and a renal tubule., The renal tubule opens into a longer tubule called the collecting duct., The renal tubule beginswith a double walled cup shaped structure called the, Bowman’s capsule, which encloses a ball of capillaries that delivers fluid to, the tubules, called the glomerulus ., The Bowman’s capsule and the glomerulus together constitute the renal, corpuscle., The endothelium of glomerulus has many pores (fenestrae). The external, parietal layer of the Bowman's capsule is made up of simple squamous, epithelium and the visceral layer is made of epithelial cells called podocytes., It ends in foot processes which cling to the basement membrane of the, glomerulus. The openings between the foot processes are called filtration, slits.