NCERT Solutions for Class 11 Science Biology Chapter 19 - Excretory Products And Their Elimination

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Ammonia, urea and uric acid are the major nitrogenous wastes excreted in animals. Ammonia being the most toxic and uric acid being the least toxic.

Hence, the correct answer is option (b).

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Blood enters the nephron through affernt arteriole into the glomerulus which is fitted into a cup shaped structure called Bowman's capsule. Glomerulus and Bowman's capsule together form Malpighian body where filtration of blood takes place.

Hence, the correct answer is option (d).

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ADH or vasopressin facilitates water reabsorption from distal parts of the tubule. Aldosterone causes reabsorption of Na⁺ and water from nephron's distal part. Atrial Natriuretic Factor (ANF) cause vasodialation and thereby decrease blood pressure. Renin is secreted by the JG cells and convert angiotensinogen into angiotensin II.

Hence, the correct answer is option (b). 

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Lungs are also accessory excretory organs. It removes large amounts of CO₂ (approximately 200 ml/min) and a significant quantity of water every day.

Hence, the correct answer is option (c).

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The human urine is a light yellow coloured watery fluid which is slightly acidic (pH - 6.0) and has a characteristic odor.

Hence, the correct answer is option (c).

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Birds and reptiles excrete nitrogenous wastes as uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals.

Hence, the correct answer is option (d).

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Aquatic insects are ammonotelic in nature as they excrete ammonia from their body. Land insects excrete uric acid in the form of pellet or paste with a minimum loss of water and are called uricotelic animals. Thus, insects do not excrete urea. 

Hence, the correct answer is option (b).

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The cortex extends in between the medudary pyramids as renal columns called columns of Bertini.

Hence, the correct answer is option (b).

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If the kidneys are malfunctioning, urea starts to accumulate in the blood. This condition is called uremia.

Hence, the correct answer is option (c).

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Vasopressin is known as antiduretic hormone or ADH as it prevents diuresis by facilitating water reabsorption from the latter parts of tubule.

Hence, the correct answer is option (b).

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Mammals have the unique ability to produce concentrated urine. The concentration of urine can be regulated by counter current mechanism  in Henle's loop/vasa recta.

Hence, the correct answer is option (d).

Disclaimer: As given in the NCERT, the correct answer can be option b also because ADH/vasopressin also acts on the collecting duct for water reabsorption making the urine concentrated/diluted.

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Dialysing unit (artificial kidney) contains a fluid which is almost same as plasma except that it has no urea.

Hence, the correct answer is option (c).

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The selective reabsorption of glomerular filtrate takes place in the proximal convoluted tubule (PCT) and distal convoluted tubule (DCT).

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Reptiles excrete uric acid from their kidney as the excretory material in the form of pellet or paste which supports minimum loss of water. Reptiles are uricotelic organisms.

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Sweat is produced by sweat glands. Sweat is a watery fluid containing NaCl, small amounts of urea, lactic acid etc.

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Antennal glands or green glands perform excretory function in prawns.

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Contractile vacuole acts as the excretory organ in Amoeba. It also helps in osmoregulation.

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a. ANF - Atrial Natriuretic Factor
b. ADH - Antidiuretic hormone
c. GFR - Glomerular filtration rate
d. DCT - Distal convoluted tubule

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Presence of glucose in urine is called glycosuria and if ketone bodies are present in urine, then it is called ketonuria.

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Sebaceous glands are present on the skin and excrete certain substances like sterols, hydrocarbons and waxes through sebum. Sebum provides a protective oily layer on the skin.

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Glucose and amino acids are two substances that are actively transported in the glomerular filtrate.

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Two metabolic disorders, which can be diagnosed by analysis of urine are glycosuria (presence of glucose in urine) and ketonuria (presence of ketone bodies in urine).

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There are three main steps of urine formation -
1. Glomerular filtration
2. Reabsorption
3. Secretion

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Actively transported substances during reabsorption of GFR are: Glucose, amino acids, Na⁺ ion
Passively transported substances during reabsorption of GFR are: Nitrogenous wastes, water.
 

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a) Urinary excretion = tubular reabsorption + tubular secretion - glomerular filtration
b) Dialysis fluid = plasma — nitrogenous wastes
 

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NaCl and urea exit from the tubules in order to maintain a concentration gradient in the medullary interstitium.

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When glomerular filtration rate (GFR) falls below normal, the juxtaglomerular cells become activated and release renin. Renin converts blood angiotensinogen into angiotensin I and further to angiotensin II. Angiotensin II is a powerful vasoconstrictor, it increases the glomerular blood pressure and thereby GFR. Angiotensin II also stimulates adrenal cortex to release aldosterone which reabsorbs Na⁺ and water from the distal parts of the tubule. This increases the blood pressure as well as GFR. This mechanism is known as Renin-Angiotensin mechanism.

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Animals which excrete ammonia are ammonotelic in nature. Many bony fishes, aquatic amphibians and aquatic insects are ammonotelic. Ammonia is the most toxic nitrogenous waste and is readily soluble in water. It is generally excreted by diffusion across body surfaces or through gill surfaces (in fish) as ammonium ions. On the other hand, terrestrial organisms excrete lesser toxic nitrogenous wastes like urea or uric acid. They are les soluble in water and thus helpful in water conservation.

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The glomerular filtrate contains large amount of water and other dissolved substances like urea, uric acid, glucose, amino acids, sodium, potassium etc. On the other hand, urine is a transparent, light yellow fluid which forms after reabsorption and secretion in the renal tubules. It contains about 95% water and other 5% contains organic substances like nitrogen, urea, creatine, ammonia, hormones and enzymes and inorganic substances like sodium, potassium, sulphate, chloride etc. Normally, glucose is absent in urine. Thus, the composition of glomerular filtrate and urine are not same.

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Kidney transplantation is the ultimate method to treat acute renal failure (kidney failure). A functioning kidney is transplanted from a healthy donor preferably a close relative, to minimize the chances of rejection by the immune system of the host.

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Terrestrial organisms are either ureotelic (organisms that excrete urea) or uricotelic (organisms that excrete uric acid) in nature. They are 
comparatively less toxic than ammonia and less soluble in water. Ammonia produced by metabolism is converted into urea and excreted through the kidney in ureotelic organisms like in mammals or many terrestrial amphibians. In uricotelic organisms like reptiles, birds, land snails, uric acid is excreted in the form of pellet or paste for conservation of water.

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In a malfunctioning kidney urea is accumulated in the blood, leading to uremia which is harmful for the body. Such patients undergo haemodialysis in which blood is drained from a convenient artery and is pumped into a dialysing unit containing dialysing fluid. This unit or haemodialiser acts an artificial kidney and removes excretory wastes from the blood. Hence, a haemodialysing unit is called artificial kidney.

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Hormonal regulation of selective reabsorption:
1. ADH: ADH or vasopressin is secreted by the hypothalamus and released from the posterior pituitary. It facilitates water reabsorption from the latter parts of the tubule.
2. RAAS mechanism (Renin-angiotensin-aldosterone mechanism): Juxtaglomerular cells secrete renin which activate angiotensinogen into angiotensin II. Angiotensin II stimulates adrenal cortex to release aldosterone which causes reabsorption of Na⁺ and water from the distal parts of the tubule.
3. ANF: ANF or atrial natriuretic factor is produced by the atrea of the heart. It causes vasodialation and increases sodium excretion. It also regulates RAAS mechanism.

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Mammals can produce a concentrated urine. The Henle’s loop and vasa recta play a significant role in this. The filtrate flow in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The blood flow through the two limbs of vasa recta is also in a counter current pattern. The proximity between the Henle’s loop and vasa recta, as well as the counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitium, i.e., from 300 mOsmolL^–1 in the cortex to about 1200 mOsmolL^–1 in the inner medulla. This gradient is maintained by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of vasa recta. NaCl is returned to the interstitium by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of Henle’s loop which is transported back to the interstitium by the collecting tubule. The above described transport of substances facilitated by the special arrangement of Henle’s loop and vasa recta is called the counter current mechanism. This mechanism helps to maintain a concentration gradient in the medullary interstitium. Presence of such interstitial gradient helps in an easy passage of water from the collecting tubule thereby concentrating the filtrate (urine). Human kidneys can produce urine nearly four times concentrated than the initial filtrate formed.

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The process of release of urine is called micturition. When the urinary bladder gets filled with urine, the walls of the urinary bladder stretches. This stimulus is received by the stretch receptors present on the wall and it send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of urethral sphincter causing the release of urine. An adult human excretes 1 to 1.5 litres of urine per day.
Disorders of the excretory system:
1. Uremia: Accumulation of urea in the blood due to malfunctioning of the kidney. A patient suffering from uremia undergoes haemodialysis to remove the excretory substances from the blood.
2. Renal failure: It is a condition where kidneys loose its ability to remove excretory wastes. Kidney transplantation is the ultimate method in the correction of acute renal failures.
3. Renal calculi: Stone or insoluble mass of crystalized salts of oxalates formed within the kidney.
4. Glomerulonephritis: Inflammation of kidney glomeruli.

 

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When metabolic wastes are secreted b tubular cells into the filtrate it is known as tubular secretion. Components that are generally secreted includes H⁺, K⁺ ions, ammonia, creatine, hippuric acid, drugs, pigments etc. It is the only mode of excretion in animals which lack glomeruli, like desert amphibians, marine fishes etc. Tubular secretion is a very important step in urine formation as it maintains
the ionic and acid-base balance in body-fluids.

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The thin wall of descending limb of loop of Henle is permeable to water but almost impermeable to electrolytes. So, the glomerular filtrate concentrates as it moves down through the descending limb. Reabsorption is minimum in the ascending limb. This limb is impermeable to water but electrolytes can be transported actively or passively. So, as the concentrated filtrate moves upward it gets diluted due to the passage of electrolytes to the medullary fluid.

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Humans have one pair of kidneys that help in excretion. Kidneys are reddish brown, bean shaped structures. They are situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity. The length of each adult human  kidney is 10-12 cm, width is 5-7 cm and thickness is 2-3 cm with an average weight of 120-170 g. Kidney has a notch towards the center of the inner concave surface. This is called hilum through which water, blood vessels and nerve enter. Inner to the ihilum, there is a broad funnel shaped space called the renal pelvis with projections called calyces. The outer layer of kidney has tough capsule. Inner kidney has two zones: outer cortex and inner medulla. The medulla is divided into a few conical masses called medullary pyramids which are  projecting into the calyces. The cortex extends in between the medullary pyramids as renal columns called columns of Bertini.
Each kidney has nearly one million complex tubular structures called nephrons which are the functional units of the excretory system.
The malpighian corpuscle, PCT and DCT of the nephrons are situated in the cortex of the kidney whereas loop of Henle extends to the medulla.