UPSC Biology Excretory Products and their Elimination Excretory Products and their Elimination

 Excretory Products and their Elimination

1.           Excretory Products

• Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by the animals. 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.
• The process of excreting ammonia is Ammonotelism. 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.
• 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.
• An 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.
• 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.

2.           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 are reddish brown, bean shaped structures.
• 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. towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter.
• Each kidney ha& nearly one million complex tubular structures called nephrons, which are the functional units. Each nephron has two parts - the glomerulus and the renal tubule. Glomerulus is a tuft of capillaries formed by the afferent arteriole - a fine branch of renal artery. Blood from the glomerulus is carried away by an efferent arteriole.

3.           Urine Formation

• The first step in urine formation is the filtration of blood, which is carried out by the glomerulus and is called glomerular filtration. On an average, \[1100-1200\]ml of blood is filtered by the kidneys per minute which constitute roughly \[l/5th\]of the blood pumped out by each ventricle of the heart in a minute.
• The glomerular capillary blood pressure causes filtration of blood through 3 layers, i.e., the endothelium of glomerular blood vessels, the epithelium of Bowman's capsule and a basement membrane between these two layers.
• The epithelial cells of Bowman's capsule called podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits or slit pores. Blood is filtered so finely through these membranes, that almost all the constituents of the plasma except the proteins pass onto the lumen of the Bowman's capsule. Therefore, it is considered as a process of ultra filtration.
• The amount of the filtrate formed by the kidneys per minute is called glomerular filtration rate (GFR). GFR in a healthy individual is approximately 125 ml/minute, i.e., 180 litres per day.
• A comparison of the volume of the filtrate formed per day (180 litres per day) with that of the urine released (\[1.5\]litres), suggest that nearly 99 per cent of the filtrate has to be reabsorbed by the renal tubules. This process is called reabsorption. The tubular epithelial cells in different segments of nephron perform this either by active or passive mechanisms.
• The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
• An adult human excretes, on an average, 1 to \[1.5\] litres of urine per day. Urine has a characterestic odour.
• On an average, \[25-30\]gm of urea is excreted out per day. .Various conditions can affect the characteristics of urine.
• Analysis of urine helps in clinical diagnosis of many metabolic discorders as well as malfunctioning of the kidney. For example, presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine are indicative of diabetes mellitus.

4.           Role of other Organs in Excretion

• Other than the kidneys, lungs, liver and skin also help in the elimination of excretory wastes.
• Our lungs remove large amounts of \[C{{O}_{2}}\](approximately 200 mL/minute) and also significant quantities of water every day.
• Liver, the largest gland in our body, secretes bile-containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out alongwith digestive wastes.
• The sweat and sebaceous glands in the skin can eliminate certain substances through their secretions. Sweat produced by the sweat glands is a watery fluid containing NaCL small amounts of urea, lactic acid, etc. Though the primary function of sweat is to facilitate a cooling effect on the body surface, it also helps in the removal of some of the wastes mentioned above.
• Sebaceous glands eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin. Small amounts of nitrogenous wastes could be eliminated through saliva.

5.           Artificial kidney (Hemodialysis)

• Kidneys are vital organs for survival. Several factors like infections, injury or restricted blood flow to kidneys reduce the activity of kidneys.
• This leads to accumulation of poisonous wastes in the body, which can even lead to death. In case of kidney failure, an artificial kidney can be used. An artificial kidney is a device to remove nitrogenous waste products from the blood through dialysis.
• Artificial kidneys contain a number of tubes with a semi-permeable lining, suspended in a tank filled with dialysing fluid. This fluid has the same osmotic pressure as blood, except that it is devoid of nitrogenous wastes.
• The patient's blood is passed through these tubes. During this passage, the waste products from the blood pass into dialysing fluid by diffusion. The purified blood is pumped back into the patient.
• This is similar to the function of the kidney, but it is different since there is no reabsorption involved.
• Normally, m a healthy adult, the initial filtrate in the kidneys is about 180 L daily. However, the volume actually excreted is only a litre or two a day, because the remaining filtrate is reabsorbed in the kidney tubules.