Ankit Yadav: Chapter 19

CHAPTER –19 : EXCRETORY PRODUCTS AND THEIR ELIMINATION

All plants and animals produce harmful substances due to a number of metabolic activities occurring in their body tissues. Carbon dioxide produced during respiration is removed by lungs. Ammonia is the chief nitrogenous waste produced as a result of metabolism of proteins and amino acids. This is a highly toxic substance to the body tissues and is eliminated as such by aquatic animals. While on land, ammonia combines with carbon dioxide to form a less toxic substance called urea which eliminated from the body. Thus the process of excretion can be defined as the elimination of waste products from the body which otherwise are toxic if retained within the system. The organs that are involved in this process constitute the excretory system.

Nitrogen Excretion:

The elimination of nitrogenous waste products is a major function of the excretory system. The nitrogenous products varies from species to species. Most of the nitrogenous wastes are formed due to the catabolism of proteins. Normally, according to the species, proteins are catabolised into ammonia, urea or uric acid.

Ammonotelism:

Ammonotelic organisms are those which eliminate their nitrogenous metabolic wastes mainly as ammonia. Ammonia is constantly produced in the organisms by the deamination of amino acids and it is highly toxic if retained in the system. So it must be immediately removed from the body as soon as it is formed. Elimination of ammonia requires large amounts of water. This can be done only in aquatic forms of life. In aquatic animals (like aquatic invertebrates, body fishes and aquatic amphibians etc.), it is quickly eliminated in the surrounding water because it is highly soluble.

Ureotelism:

Ureotelic organisms are those which eliminate their nitrogenous metabolic wastes mainly as urea. Urea is formed in the liver by combining ammonia with carbon dioxide and is comparatively less toxic than ammonia. The synthesis of urea from ammonia requires the expenditure of energy. The elimination of urea requires less water. It is the main product of excretion in man and all other mammals, aquatic mammals like whales and seals, and desert mammals such as camel and kangaroo, terrestrial and semiaquatic amphibians like toads and frogs, cartilaginous fishes such as sharks and sting rays. Some animals can concentrate the amount of urea present in their urine. Man can concentrate urea in the urine more than hundred times its concentration in the blood. Earthworms excrete ammonia when sufficient water is present but eliminate urea when the conditions are dry. Tadpoles eliminate nitrogenous wastes as ammonia but the adult frog mainly eliminates urea.

Uricotelism:

Uricotelic organisms are those which eliminate their nitrogenous metabolic wastes mainly as uric acid. Uric acid is the least toxic nitrogenous waste product and requires very less water for its elimination. Therefore, uric acid is formed only in those animals which have limited supply of water. In the cloaca of reptiles and birds, uric acid accumulates and is further concentrated there. It passes out of the body as whitish semi-solid substance. Uric acid is also eliminated by insects. In man and other primate mammals, urine contains a little of uric acid in addition to urea.

Excretory System:

The excretory system consists of organs and tissues participating in the removal of waste products. Some of these excretory organs constitute the urinary system which forms and eliminates urine and helps mainly in the excretion of nitrogenous waste-products, water and some mineral salts. Apart from this urinary system, they have some accessory excretory organs and tissues such as the skin, lungs and liver. The mode of excretion varies in different kinds of animals.

Excretory Organs of Invertebrates:

The organs of excretion and osmoregulation vary greatly in different groups of animals. Protozoans like Amoeba and Paramecium have got contractile vacuoles for osmoregulation. In sponges, the metabolic wastes are eliminated from the body by the canal system. In Hydra, cells release waste products into the coelenteron from which it goes out through the hypostomal opening. In planarians and other platyhelminthes worms, there are special cells called flame cells that perform the function of excretion. Annelids like Nereis,earthworm etc, have got nephridia as the organs of excretion. Prawns have got green glands that serve as excretory system for them. All insects, millipedes, spiders and scorpions have fine thread like tubules called malpighian tubules at the junction of midgut and hindgut. These tubules lie freely in the body cavity and can filter metabolic wastes from the hemolymph. These malpighian tubules are 60-80 in number and are arranged in 6 to 8 bundles. Each tubule is a fine hollow tube, about 16 mm long, and is lined by glandular epithelium. The epithelium collects the nitrogenous wastes like uric acid from the hemolymph in which the tubules are bathed. The excretory wastes are then poured into the hindgut from where they are thrown outside the body. Most of the water of the excretory materials are reabsorbed into the rectum. Scorpions also have coxal glands as the organs of excretion.

Vertebrate Urinary System:

In vertebrates, kidneys are the urine-forming organs. In the case mammals, the urinary system consists of two kidneys, two ureters, a urinary bladder and a urethra.

Organs of excretion in man:

The excretory organs in man consist of the following parts:

1.      Kidneys: They are a pair off bean-shaped structures lying in the abdomen, one on each side of the vertebral column below the diaphragm. The left kidney is placed a little higher than the right kidney. Human kidney is about 10 cm in length, 5 cm in breadth and 9 cm in thickness. The outer surface of the kidney is convex while the inner surface is concave. In the concave depression there is an opening called hilum through which blood vessels (renal artery and renal veins), nerves, lymphatic ducts and ureters enter or leave the kidney. The hilum, inside the kidney, expands into a funnel-shaped area called renal pelvis.

                  Two distinct regions can be seen in the kidney – an outer granular portion called the renal cortex and an inner medulla. In the cortex are present the malpighian bodies which filter the waste products from the blood. The medulla portion contains the collecting ducts of nephrons and thus passes the urine to the pelvis of the kidney. Conical pyramid-shaped masses of medulla project into the pelvis and are called as medullary pyramids. They form the major calyces and minor calyces.

2.      Ureters: From each kidney arises a thin muscular tube called ureter. It emerges out from the hilum of each kidney. It is about 30 cm in length. Urine enters the ureter from the renal pelvis and is passed down the ureters. These two tubes bring the urine downwards and open into urinary bladder.

3.      Urinary bladder: It is a single sac-like structure in which urine is stored for some time before it is voided out. It is present in the pelvic region of the body. Both bladder and ureters are lined by transitional epithelium, which may be considerably stretched and do not get torned out even when bladder and ureters are completely filled with urine.

4.      Urethra: The urinary bladder opens to the outside through a tube called as urethra. It arises from the neck of the bladder and conducts urine to the outside of the body. In females, this tube is about 4 cm long and serves as a passage for urine only; while in males, it measures about 20 cm and functions as a common passage for urine and spermatic fluid. A muscular sphincter keeps the urethra closed except during voiding of urine.

            Micturition is a process of the accumulation of formed urine in the urinary bladder and then elimination of the same from the bladder from time to time. It is controlled by the nervous mechanism. Micturition may be voluntarily inhibited for a prolonged interval until the bladder pressure rises too high. On the contrary, micturition may also be voluntarily initiated even before sufficient urine has collected in the bladder.

Accessory Excretory Organs:

The urinary system is the main excretory organ. Apart from this, some other organs and tissues like the skin, lungs and liver function as accessory excretory organs.

Excretory role of skin/integument:

In aquatic animals, integument is more permeable to nitrogenous wastes than in terrestrial animals. Ammonia is mainly excreted out into the surrounding water by diffusion through the skin. Terrestrial animals can prevent the loss of water through skin mainly in deserts. Mammalian skin, including man, has two types of glands for secreting two fluids on its surface, viz. Sebum from sebaceous glands and sweat from sweat glands.

            Sweat is an aqueous fluid contains sodium chloride, lactic acid, urea, amino acids and glucose. It excretes mainly water and sodium chloride, and small amounts of urea and lactic acid.

            Sebum is a wax-like secretion, which eliminates some lipids such as waxes, sterols, other hydrocarbons and fatty acids on the skin.

Excretory role of lungs:

They help to eliminate the waste products of respiration-the carbon dioxide from the body. Along with carbon dioxide, some water in the form of vapour is also eliminated from the body.

Excretory role of liver:

The liver plays an important role in the excretion of cholesterol, bile pigments inactivated products of steroid hormones, some vitamins and drugs. The bile pigments are produced by the degradation of hemoglobin, some vitamins and drugs. The bile pigments are produced by the degradation of hemoglobin of the dead red blood cells in the liver. Through the bile ducts, the bile pigments are passed into the intestine and are eliminated along with the faecal matter.

Urinary elimination of waste Products in Man:

Kidneys excrete nitrogenous waste products in the form urine. Hence kidneys secrete urine continuously in their nephrons.

Nephrons: Nephrons are the functional units of kidneys. Each kidney is made up of numerous delicate uriniferous tubules or nephrons. Nephrons are held together by a little connective tissue in which blood vessels, nerves and lymph vessels are also present. There are about 1.2 million nephrons in each kidney of man. Each nephron consists of two main parts: (1) Malphigian body or renal corpuscle – it has a cup-shaped depression called Bowman’s capsule into which tufts of capillaries called glomerulus are present. An afferent renal arteriole enters the capsule, divide into capillaries forming glomerulus and leaves as efferent renal arteriole enters the capsule, divide into capillaries forming glomerulus and leaves as efferent renal arteriole. Efferent renal arteriole then supplies blood to the remaining part of tubule and join to renal vein. Blood is filtered in the Malphigian body. (2) Secretory tubule starts after malphigian body and is divisible into three distinct regions: 1. Proximal convoluted tubule(PCT) starts immediately after bowman’s capsule, makes a few coils in the cortex and proceed towards medulla of kidney; 2. Loop of Henle-tubules make a U-turn in the medulla forming loop of Henle (thus it has a descending and an ascending portion) ; and 3. Distal convoluted tubule (DCT) starts after loop of Henle and joins the collecting tubule. These collecting ducts unite with each other in the medulla to form the larger duct – the duct of Bellini. These run through the renal pyramids and open into renal pelvis. In these tubules, various substances from the nephric filterate are reabsorbed back. There is a rich supply of blood vessels to the kidneys. It is estimated that total length of blood vessels in the kidneys of man is about 160 kilimetres.   A network of vessels paralleling the loop of Henle is known as vasa recta.

Composition of Urine: Urine is transparent, pale yellow in colour, aquous fluid which is usually acidic in nature. The pale yellow colour of the urine is mainly dependent upon the presence of a pigment urochrome. This colour also varies with the quantity and concentration of the urine voided. In fever, it becomes dark-yellow of brownish in colour. Quantity of urine formed in 24 hours in an adult normal individual varies from 1000 ml to 1800 ml. Normally, it depends upon water intake, diet, environmental temperature, mental state and physiological state of the person.

            During summer months, when the body is exposed to warmth, skin perspires freely because cutaneous blood vessels are dilated whereas the abdominal and renal blood vessels are constricted causing less volume of urine. During winter months, cutaneous blood vessels are constricted and elimination of water through perspiration is ceased but the abdominal and renal blood vessels are dilated resulting in the increased flow of urine. Thus perspiration has got inverse relation with the formation of urine.

            Substances that increases the formation of urine are termed as diuretics. Tea, coffee and alcoholic beverages have got diuretic effects.

            The normal specific gravity of urine varies between 1.003 and 1.040. The odour of the normal urine is slightly aromatic and is due to the presence of large number of volatile organic substances particularly the bad smelling substance – urinod. When allowed to stand for some time, the urine smells of ammonia due to the bacterial decomposition of urea to ammonia.

            Urea is the main nitrogenous constituent of human urine. Besides urea, it contains other nitrogenous substances like ammonia, uric acid, creatinine and hippuric acid. Sodium chloride is the principal mineral salt in urine. Small amount of inorganic salts like chlorides, sulphates and phosphates of potassium, calcium and magnesium are also present. Non-nitrogenous organic substances include small amounts of vitamic C, oxalic acid and phenolic substances. Glucose is normally negligible in amount. Proteins, bile salts, bile pigments, glucose and ketone bodies occur in urine in various pathological conditions.

Formation of Urine:

Urine is formed in the nephron by the following three processes:

1.      Glomerular filtration or ultrafiltration:

Glomerulus is a tuft of capillaries found in the Bowman’s capsule of the nephron. In it there are fine filter pores (millipore filters) that separate the red blood cells from the plasma. The blood that enters into the glomerulus is under a very high pressure from the afferent renal arteriole. It has got urea, glucose, various salts and proteins of plasma and large quantities of water. The lining of Bowman’s capsule has a single layer of flat epithelial cells through which blood is filtered into the lumen of the tubule. The filtrate formed in the tubule is known as glomerular or nephric filtrate and contains urea, large amounts of water, glucose, amino acids and various minerals. Ultrafiltration in the glomerulus is a passive process. Nephric filterate does not have blood proteins as big-sized molecules are not filtered through the lining of the Bowman’s capsule. It is estimated that the effective filtration pressure by which the blood is filtered is about 10 mm Hg and at this pressure, 125 ml of nephric filtrate is formed in the two kidneys after every minute.   It is also estimated that about one fifth of the total volume of blood plasma flowing through the kidneys is filtered out as glomerular filtrate. The filtration occurs across the membrane made of the glomerular capillary wall and the inner membrane of the Bowman’s capsule. The pores of this filtering membrane are impermeable to large molecules or particles. Large particles like blood cells and protein macromolecules do not normally enter into the glomerular filtrate. But smaller molecules like glucose, urea, creatinine, amino-acids and mineral salts are filtered into the Bowman’s capsule in concentrations more or less similar to their respective concentrations in the plasma. Therefore, the filtrate almost resembles the protein-free plasma in composition and osmotic pressure.

2.      Tubular reabsorption:

This occurs by two ways i.e., active and passive reabsorption.   The important substances like glucose, amino acids are reabsorped actively. This active reabsorption is rapid and continues even when the concentration of the substance is far lower in the glomerular filtrate than in the blood.

Some other substances are reabosrbed from the tubules slowly by the physical process of diffusion, This passive reabsorption occurs only when their concentrations in the glomerular filtrate exceed their respective concentrations in the blood. Hence, these substances can never be totally reabosrbed from the urine, e.g. urea, ammonia, creatinine and ketone bodies.

·         In proximal portion of convoluted tubule (PCT) most of the water re-enters into the blood capillaries. About 80% of the water is reabsorbed here. Glucose, amino acids and vitamin C are also reabsorbed back here. But this is an active reabsorption; while the absorption of water is purely passive. About 70% of Na+ ions, 75% of K+ ions and large amounts of Ca++ ions are also reabsorbed here by the active transport mechanism.

·         In descending loop of Henle about 5 per cent of water comes out due to osmosis because of high osmotic pressure of the medullary extracellular fluid. Sodium enters the tubules and it increases the hypertonicity of the nephric filtrate.

·         In ascending loop of Henle sodium and potassium is reabsorbed actively and also some amounts of Cl- ions. There is no reabsorption of water as its walls are thick and impermeable to water. So the filtrate now becomes hypotonic as compared to the plasma when it flows through this part of the nephron.

·         In distal convoluted tubule (DCT) and collecting duct there is active reabsorption of sodium ions from the filtrate. The process of diffusion also reabsorbs some Cl- ions. Large amounts of water are also reabsorbed. This make the nephric filtrate isotonic. This isotonic fluid enters the collecting tubule and here it is more concentrated due to the further reabsorption of water.

3.      Tubular secretion:

In addition to reabsorption, the renal tubules also secrete into their lumen many substances like urea, creatinine, uric acid, para amino hippuric acid etc. Most of the K+ ions eliminated in the mammalian urine are secreted by the distal convoluted tubule (DCT) and the collecting ducts in exchange to the reabsorption of Na+ ions. These substances are then removed from the body along with urine. This tubular secretion is much significance in marine fishes and desert amphibians. Because these animals possess no glomerulus in their nephrons; they form urine by secreting solutes such as urea, creatinine and mineral ions into their tubules. Tubular secretion is of less importance in mammals.

            Thus, urine is formed in the nephron by a combination of glomerular filtration, tubular reabsorption and tubular secretion.

Osmoregulation by Kidney:

            The kidneys maintain the water and osmotic concentration of the blood. This phenomenon is known as osmoregulation. In aquatic animals like fresh water fishes, or when the animal is taking large amounts of water, the urine passed out of the body is more dilute than the plasma of blood even. Thus they eliminate hypotonic urine in order to get rid off excess of water. Most of the vertebrates including mammals can pass out hypotonic urine. In them, first the isotonic glomerular filterate is formed in the Bowman’s capsule and then from that filterate some solutes are reabsorbed by the tubules of the nephron. Therefore, the urine passed out of the body becomes hypotonic. It helps to raise the osmoconcentration of the blood to normal.

            In those animals living in scarcity of water, urine passed out of the body is hypertonic to the plasma of blood. This reduces the water loss from the body. Mammals and birds can excrete hypertonic urine, which is more concentrated than their blood plasma. In them first an isotonic glomerular filterate is formed in the Bowman’s capsule and then from that filtrate most of the water is reabsorbed by the tubules of nephron. However, some of the solutes are also reabsorbed but this is not proportionate to the amount of water reabsorbed. Therefore, the urine passed out of the body becomes hypertonic. It is a very effective mechanism to reduce the loss of urinary water.

            Normally in summer months, when most of the water is lost from the body during perspiration, the urine passed out is hypertonic, and in winter months as there is no perspiration, the urine passed out is hypotonic. Largely the movement of Na+, Cl- and water regulates the fluid volume and osmolarity at the kidney. When the protein-free fluid is filtered into the Bowman’s capsule from the blood, it has the same osmo-concentration as the blood plasma of the capillaries surrounding uriniferous tubule. In PCT, Na+ gets actively reabsorbed, and Cl- gets reabsorbed passively as it is attracted to the positive charge of Na+.

            The concentration of urine is mainly dependent upon two factors: (i) the loop of Henle – the greater the ability of an animal to excrete hypertonic urine, the longer are its loop of Henle; (ii) presence of vasa rectae in the kidneys – these vessels occur as loops surrounding the loop of Henle. The blood flows in opposite directions in the two limbs of each vasa recta, the blood flowing in descending limb comes close to the blood flowing in ascending limb. This is known as counter-current system and is mainly responsible for making the urine hypertonic.



            The two limbs of loop of Henle also constitute the counter-current system. The nephric filtrate flows in opposite directions in the descending and ascending loops of Henle. Na+ and Cl- are reabsorbed from the ascending limb into the surrounding vasa recta. Moreover, water cannot flow out of this limb as it is impermeable to water. Thus in ascending limb, urine becomes hypotonic. But the walls of descending limb are permeable to both water and salts. Thus the reabsorbed Na+ enters the descending limb and the water moves out. It makes the urine hypertonic.

Role of ADH in forming hypertonic urine:

            Antidiuretic hormone (ADH) or vasopression is released from the posterior lobe of pituitary. It acts on the distal convoluted tubules (DCT) and collecting ducts of the nephron and increases their permeability to water. If the water contents in the body is more than needed, it inhibits the secretion of ADH. As a result, DCT and other collecting tubules remain less permeable to water. So water is not reabsorbed, but the active reabosrption of Na+ continues from the filtrate. Thus the filtrate becomes more and more dilute and hypotonic urine passes out of the kidney.

            If the water contents in the body is low, it stimulates the secretion of ADH that acts on the walls of DCT and other collecting tubules and make them more permeable to water. Further, the surrounding tissue is hypertonic due to active reabsorption of Na+ into it and also due to the retention of Na+ and urea by the counter current system of vasa recta. So water is pregressively reabsorbed from the filtrate while flowing through DCT and collecting tubules. Hence the urine passing out becomes hypertonic.

            Thus, the kidney helps in osmoregulation by eliminating either hypotonic or hypertonic urine, according to the need of the body.

Functions of kidney:

·         It excretes waste products, specially formed due to protein metabolism. These waste products are ammonia, urea and uric acid.

·         It helps to maintain water balance of the body and thereby plasma volume.

·         It helps to maintain the normal pH of the blood and other fluids of the body.

·         It helps to maintain the optimum concentration of certain constituents of blood by the process of selective reabsorption in the kidney tubules.

·         It eliminates drugs and various toxic substances from the body.

·         It helps to maintain the osmotic pressure in blood and tissues.

Disorders of the Excretory System:

1.      Uremia: Malfunctioning of kdneys due to accumulation of urea in blood. It is very harmful and leads to renal failure. For such kind of patients we go for Hemodialysis, where blood is drained from a convenient artery is pumped into a dialyzing unit after adding an anticoagulant like Heparin. Hemodialysis contains a coiled cellophone tube surrounded by a dialysing fluid having the same composition as that of plasma except the nitrogenous wastes. The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient. The cleared blood is pumped back to the body through a vein after adding anti-heparin to it.

2.      Renal failure: a functioning kidney is used in transplantation from a donor of closely relative.

3.      Renal calculi: Stone or insoluble mass of crystallized salts (oxalalates, etc)formed within the kidney.

4.      Glomerulonephritis: Inflammation of glomeruli of kidney.