1st PUC Biology Question Bank Chapter 19 Excretory Products and their Elimination

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Karnataka 1st PUC Biology Question Bank Chapter 19 Excretory Products and their Elimination

1st PUC Biology Excretory Products and their Elimination NCERT Text Book Questions and Answers

Question 1.
Define Glomerular Filtration Rate (GFR)
Answer:
Glomerular filtration rate refers to the quantity of filtrate formed by the kidneys per minute.
It is 125 ml per minute i.e., 180 liters per day.

Question 2.
Explain the autoregulatory mechanism of GFR.
Answer:
The kidneys have built-in mechanisms for the regulation of glomerular filtration rate. This regulation is carried out by the juxtaglomerular apparatus (JGA). JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact. A fall in GFR can activate the JG cells to release renin which can stimulate the glomerular blood flow and thereby the GFR back to normal.

Question 3.
Indicate whether the following statements are true or false:
(a) Micturition is carried out by a reflex.
(b) ADH helps in water elimination, making the urine hypotonic.
(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
(d) Henle’s loop plays an important role In concentrating the urine.
(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
Answer:
(a) True
(b) False
(c) True
(d) True
(e) True

Question 4.
Give a brief account of the countercurrent mechanism.
Answer:
Mammals have the ability to produce concentrated urine. The Henle’s loop and vasa recta play a significant role in this. The flow of filtrate in the two limbs of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood through the two limbs of the 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 m OsmolL^-1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop which is exchanged with the descending limb of the vasa recta. NaCl is returned to the interstitium by the ascending portion of the vasa recta. Similarly, small amounts of urea enter the thin segment of 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 arrange¬ment of Henle’s loop and vasa recta is called the countercurrent mechanism (Fig. 19.1). This mechanism helps to maintain a concentration gradient in the medullary interstitium. The 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.

Diagrammatic representation of a nephron and vasa recta showing counter current mechanisms

Question 5.
Describe the role of the liver, lungs, and skin in excretion.
Answer:
Lungs is responsible for the elimination of large amount (18 liters/day) of CO2 and water vapour. The liver secretes bile, degraded steroid hormones, drugs, and certain vitamins.

The skin excretes certain substances through glands present in it e.g., glands which excrete sweat produce cooling effect and sebaceous glands eliminate wastes through sebnum.

Question 6.
Explain micturition.
Answer:
Micturition or urination is the process of expulsion of urine from the urinary bladder through the urethra.
This is accomplished by the simultaneous contraction of the smooth muscles of the urinary bladder wall and relaxation of the skeletal muscles of the sphincter around the opening of the bladder.
The expulsion of urine from the urinary bladder
Biology is called micturition. It is a reflex process, but in grown-up children and adults, it can be controlled voluntarily to some extent.

Question 7.
Match the items of Column I with those of column II:
Column I – Column II
(a) Ammonotelism- (i) Birds
(b) Bowman’s capsule – (ii) Water reabsorption
(c) Micturition – (iii) Bony fish
(d) Urlco’dlsm – (iv) Urinary bladder
(d) ADH – (v) Renal tubule
Answer:
(a) – (iii)
(b) – (v)
(c) – (iv)
(d) – (i)
(e) – (ii)

Question 8.
What is meant by the term osmoregulation?
Answer:
Osmoregulation. It is the maintenance of a fixed osmotic concentration inside the body cells and the extracellular fluids by controlling the amount of water and salts.

Question 9.
Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?
Answer:
Terrestrial adaptation necessitated the production of lesser toxic nitrogenous wastes like urea and uric acid for the 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 the desired osmolarity.

Reptiles, birds, land snails, and insects excrete nitrogenous waste as uric acid in the form of a pellet or paste with a minimum loss of water and are called uricotelic animals. 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.

Question 10.
What is the significance of the juxtaglomerular apparatus (JGA) in kidney function?
Answer:
The JGA plays a complex regulatory role. A fall in glomerular blood flow/glomerular blood pressure/GFR can activate the JG cells to release renin which converts angiotensinogen in the blood to angiotensin I and further to angiotensin II.

Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR. Angiotensin II also activates the adrenal cortex to release aldosterone.

Aldosterone causes reabsorption of Na+ and water from the distal parts of the tubule.

This also leads to an increase in blood pressure and GFR. This complex mechanism is generally known as the Renin-Angiotensin mechanism.

Question 11.
Name the following:

1. A chordate animal having flame cells as excretory structures
2. Cortical portions projecting between the medullary! pyramids in the human kidney
3. A loop of capillary running parallel to Henle’s loop.

Answer:

1. Cephalochordate (Amphioxus)
2. Columns of Bertini
3. Vasa Recta.

Question 12.
Fill in the gaps:

1. Ascending limb of Henle’s loop …………………. is to water whereas the descending limb is to it…………………..
2. Reabsorption of water from distal parts of the tubules is facilitated by hormone ………………….
3. Dialysis fluid contains all the constituents as in plasma except ………………….
4. A healthy adult human excretes (on an average) gm of urea/day…………………..

Answer:

1. Impermeable, permeable
2. ADH
3. Nitrogenous wastes
4. 25 – 30 gm.

1st PUC Biology Excretory Products and their Elimination Additional Questions and Answers

1st PUC Biology Excretory Products and their Elimination One Mark Questions

Question 1.
Define excretion.
Answer:
Excretion is the elimination of unwanted waste substances which have been produced as a result of metabolism within the cells of body.

Question 2.
What are ammonotelic animals?
Answer:
Animals that excrete ammonia are ammonotelic animals.

Question 3.
What are ureotelic animals?
Answer:
Animals excrete urea.

Question 4.
What are uricotelic animals?
Answer:
Animals excrete uric acid.

Question 5.
Which are the excretory organs in humans?
Answer:
Kidneys.

Question 6.
What are nephrons? (July 2007)
Answer:
Structural and functional units of kidneys.

Question 7.
What is the renal capsule?
Answer:
Connective tissue envelop around kidney is renal capsule.

Question 8.
Where do you find brush border in nephron?
Answer:
Proximal Convoluted Tubule (PCT).

Question 9.
What are glomeruli? (Oct. 2004)
Answer:
The network of blood capillaries inside Bowman’s capsule is called glomeruli.

Question 10.
Give an example of a Uricotelic animal. (April 83, March 89)
Answer:
‘Snake’ (Lizard, bird, Insect – anyone can be named)

Question 11.
What is Ureotelism? (April 85)
Answer:
The phenomenon of excretion of urea as the predominant nitrogenous waste material is called as Ureotelism.

Question 12.
Give an example of an ammonotelic animal. (Oct. 85, 92)
Answer:
Tadpoles of amphibia (Octopus, crustaceans, polychaete annelids – any one of these can be named)

Question 13.
What is the excretory unit In the kidney? (April 86)
Answer:
The Nephron is the excretory unit of the kidney.

Question 14.
Name the organ that is involved in Urea formation. (March 88, Oct. 93, 95)
Answer:
Urea is formed in the ‘Liver’.

Question 15.
Which hormones control the output of urine? (Mar. 89)
Answer:
The Anti diuretic hormone (ADH) or Vaso pressing

Question 16.
Name the internal lining of the Bowman’s capsule. (Oct. 86)
Answer:
The internal lining of the Bowman’s capsule is made up of simple squamous epithelium.

Question 17.
Where does ultrafiltration take place in the nephron?
Answer:
The Bowman’s Capsule.

Question 18.
What is dialysis? (Oct. 99, April 01, March 11)
Answer:
Dialysis is a process by which solutes in a solution are separated by diffusion through a semipermeable membrane.

Question 19.
What is primary urine? (Oct. 2002)
Answer:
Blood entering the glomerulus is filtered into the glomerular capsule, this filtrate which contains both essential and nonessential substances like water, glucose, vitamins, amino acids, nitrogenous wastes, ions, and plasma proteins is called primary urine.

Question 20.
What is the difference in the mode of transport between the thin and thick segments of the ascending limb of Henle’s loop?
Answer:
In the thin segment, transport of NaCl is passive while in the thick segment, it is active.

Question 21.
Glomerular filtrate has water, glucose, amino acids, and creatinine. Which of them are rapidly reabsorbed actively by blood? (Delhi 2003 C)
Answer:
Glucose and amino acids.

Question 22.
Longer the loop of Henle, the more hypertonic is the urine produced. Is this statement true or false? If false, rewrite correctly.
Answer:
It is true. (All India 2003 C)

Question 23.
In which part of nephron does filtration take place? (Delhi 2001 C)
Answer:
Bowman’s Capsule.

Question 24.
What Is the difference between the ascending and descending limbs of Henle’s loop with reference to permeability to water? (All India 2000)
Answer:

• Ascending limb is impermeable to water and permeable to solutes.
• Descending limb is permeable to water and impermeable to solutes.

Question 25.
What happens to the walls of the distal convoluted tubule (DOT) of a nephron, when vasopressin is released by the pituitary into the bloodstream? (Delhi 1999 C)
Answer:
DCT becomes permeable to water and reabsorb water from the filtrate and makes the urine hypertonic.

Question 26.
Name the excretory structures of
(1) Planaria
(2) Earthworm
Answer:

1. Flame cells
2. Nephridia

Question 27.
Name any two constituents of sweat, besides water. (Delhi 1998 C)
Answer:
Sodium chloride and lactic acid.

Question 28.
What are columns of Bertini in a kidney?
Answer:
The extensions of cortex in between the medullary pyramids as renal columns, are called columns of Bertini.

Question 29.
What is glomerulus?
Answer:
Glomerulus is a tuft of capillaries formed by the afferent arteriole in the hollow of Bowman’s capsule.

Question 30.
Name the parts of nephron that are present in the cortex of the kidney.
Answer:
Bowman’s capsule, Proximal Convoluted tubule and distal convoluted tubule.

Question 31.
What are peritubular capillaries ?
Answer:
The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule called the peritubular capillaries.

Question 32.
What is vasa recta?
Answer:
The U- shaped peritubular capillary that runs parallel to the Henle’s loop is called vasa recta.

Question 33.
What is the driving force for glomerular filtration?
Answer:
Blood pressure in the glomerular capillaries is the driving force for glomerular filtration.

Question 34.
Name the layers that form the filtration membrane in the nephron.
Answer:
It is formed by three layers:

• Endothelium of glomerular capillaries
• Epithelium of Bowman’s capsule
• Basement membrane between the two.

Question 35.
What are podocytes?
Answer:
The epithelial cells of the Bowman’s capsule are called podocytes.

Question 36.
How are the filtration slits formed?
Answer:
The podocytes are arranged in an intricate manner so as to leave some minute spaces called filtration slits.

Question 37.
Why glomerular filtration called ultra filtration?
Answer:
Since the blood is filtered so finely through the membranes, that almost all the constituents of the plasma except the proteins are filtered, it is called ultra filtration.

Question 38.
Where is JGA located in the kidney?
Answer:
JGA is located where the distal convoluted tubule is in contact with the afferent arteriole in cortex.

Question 39.
Expand JGA.
Answer:
Juxta Glomerular Apparatus.

Question 40.
What is JGA?
Answer:
JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the location of their contact.

Question 41.
Name the type of cells that form the Inner lining of PCT.
Answer:
Brush bordered cuboidal epithelium.

Question 42.
Why does filtrate become concentrated as it passes through the descending limb of Henle’s loop?
Answer:
Descending limb is permeable to water but not to solutes, so the filtrate becomes concentrated.

Question 43.
Name two parts in the nephron, when tubular secretion of H⁺ and K⁺ ions takes place.
Answer:
Proximal Convoluted Tubule and Collecting Duct.

Question 44.
Name the site of nephron from where urea comes out.
Answer:
Collecting Duct.

Question 45.
Name the compounds that maintain the concentration gradient in the medullary interstitium.
Answer:
Sodium Chloride, urea.

Question 46.
What causes the release of ANF ?
Answer:
An increase in the blood pressure / blood volume in the atria, causes the release of ANF.

Question 47.
Name the mechanism that acts as a check for the Renin angiotensin mechanism.
Answer:
Atrial Natriuretic Factor (ANF) mechanism.

Question 48.
What is micturition?
Answer:
The process of release of urine is called micturition.

Question 49.
Name two accessory excretory organs in man.
Answer:
Lungs, liver and skin.

Question 50.
What is uremia?
Answer:
Uremia is a condition in which urea is accumulated in blood due to malfunctioning of kidneys.

Question 51.
What is the term for inflammation of glomerules in nephron ?
Answer:
Glomerulonephritis.

1st PUC Biology Excretory Products and their Elimination Two Marks Questions

Question 1.
Explain amonotelism with an example. (Oct. 83)
Answer:
Amonotelism is a phenomenon wherein the nitrogenous waste material is predominantly excreted in the form of Ammonia Example: Tadpole of Frog.

Question 2.
Write short notes on Urlcotelism. (April 87, Oct. 90)
Answer:
Uricoteiism is a phenomenon in which the nitrogenous waste material is predominantly excreted in the form of uric acid. The animals which excrete this nitrogenous waste material predominately are called as the uricotelic animals.

Uricotelism is a adaptation to conserve water in animals which face acute scarcity of water and have to maintain a normal level of water inside then body.Examples: Insects, Reptiles and birds excrete Uric acid. These animals use minimal amount of water or absolutely no water for excretion of uric acid. Specially water is used only for flushing.

Question 3.
Write a note on ultra-filtrationl (Oct. 87)
Answer:
‘Ultra filtration’ is the first step in the formation of urine. It is the process wherein substances from the blood brought into the glomerulus are filtered into the Bowman’s capsule of the nephron, through the ultra fine endothelia-capsular barrier or membrane. It is made possible by the high blood pressure in the glomarular capillaries and the osmotic pressure exerted by the contents on either side (glomerular lumen + Bowman capsular lumen) of the endothelia capsular membrane. The glomerular filtrate or primary urine resembles the blood plasma in its chemical composition, except for the high molecular weight (large molecules) organic molecules.

Question 4.
What la excretion? Name the types of nitrogenous wastes. (April 97,2002)
Answer:

• Excretion is a process in which toxic wastematerials produced during biochemical reactions (metabolic activity) in the cells are eliminated from the body.
• One of the Excretory product which is toxic to the body and has to be eliminated, is the nitrogenous waste material formed from either the nitrogenous compounds got by deamination of amino acids or breakdown of nucleic acids.

They are three types of nitrogenous waste materials formed pre-dominantly in animals, they are

• Ammonia
• Urea
• Uric acid.

Question 5.
Write a note on the glomerulus. (April 98)
Answer:
The glomerulus is a tuft a capillaries found inside the Bowman’s capsule of the nephron. It is formed by the renal artery which branches into arteriole which in turn supply each and every Bowman’s capsule forming the capillary mass, the glomerulus. Blood is brought for purification into the glomerulus by the afferent arteriole and leave it through the efferent arteriole. The glomerular membrane forms an intricate connection with membrane of the Bowman’s capsule forming the endothelia capsular membrane for filtration.

Question 6.
Write four functions of Kidneys. (April 2000, Oct. 2000)
Answer:
The main functions of Kidneys are:

• Maintaining levels of urea and excretory materials in the blood. Kidneys also remove toxins.
• Maintaining acid-base balance of body by controlled excretion of H⁺ ions.
• Maintaining levels of various salts and ions.
• Maintaining optimum amounts of water in the blood and body.

Question 7.
Write a note on renal calculi. (Oct. 2002, M.Q.P.)
Answer:
Renal calculi or Kidney stones are usually composed of calcium oxalate, calcium phosphate, Magnesium phosphate or uric acid. Calcium oxalate stones are the most common. They develop within the kidney or its collecting system. High blood calcium levels, alkaline urine, bacterial infection and diet are some of the reasons for this condition.

Question 8.
What are the two modes of tubular reabsorption from nephrons ? Name the substances absorbed by each of these modes. (All India 2000)
Answer:
Two modes of reabsorption are:

1. Active absorption: It involves expenditure of energy.
   Eg: glucose, amino acids, Na⁺ etc. are absorbed actively.
2. Passive absorption: It does not involve any energy expenditure and follows concentration gradient. Eg: nitrogenous waste, water, Cl^– are absorbed passively.

Question 9.
Describe the blood vessels called vasa recta. What is their function ? (Delhi 2000 C)
Answer:
Vasa Recta are ‘U’ shaped, thin walled capillaries that arise from efferent arteriole and run parallel to the Henle’s loop.

They retain the reabsorbed ions in the medullary tissue fluid and maintain its high osmolarity and are involved in the counter current mechanism.

Question 10.
Mention two advantages of uricotelism In birds. (All India 1999C)
Answer:
Birds excrete uric acid and are called uricotelic.
Advantages:

• Uric acid requires very little or no water for its elimination.
• Uric acid is far less toxic and can be eliminated slowly.

Question 11.
What are the functions of nephridia? Name an animal having photonephridia.
Answer:

• Nephridia help to eliminate nitrogenous waste.
• They maintain the fluid and ionic balance in the body.
  Phtonephridia are present in planaria, rotifers etc.

Question 12.
Name two types of excretory organs found among arthropods, with an example for each.
Answer:

• Insects like cockroach, grasshopper etc. have Malpighian tubules as excretory structures.
• Crustaceans like prawn, crab etc. have antennal (green) glands as excretory structures.

Question 13.
What is the importance of tubular secretion? Name two substances that are secreted into the filtrate.
Answer:
‘Tubular secretion is an important step in urine formation as it helps in the maintenance of ionic and acid base balance of body fluids. The tubular cells secrete substances like H⁺, K⁺ and ammonia into the filtrate.

Question 14.
Mention the role of DCT in urine formation.
Answer:

• Conditional reabsorption of Na⁺ and water takes place in this segment.
• It is also reabsorbed HCO₃
• There is selective secretion of hydrogen and potassium ions to maintain the pH and sodium-potassium balance in the blood.

Question 15.
What is the role of ADH in kidney functioning ?
Answer:

• ADH renders the wall of distal convoluted tubule (DCT), collecting tubule and collecting duct permeable to water and facilitates reabsorption of water from the filtrate, thereby preventing diuresis.
• It also affects the kidney function by its constrictory effect on the blood vessels, this causes increase in the blood pressure and glomerular blood flow.

Question 16.
What Is meant by Glycosuria and Ketonuria?
Answer:

• Presence of glucose in the urine is termed as Glycosuria.
• Presence of Ketone bodies in the urine is termed as Ketonuria.
• Both these conditions indicates the presence of diabetes mellitus.

Question 17
What is the ultimate method of correcting acute renal failure? Describe.
Answer:
Kidney transplantation is the ultimate method in the correction of acute renal failures. A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimise its chances of rejection by the immune system of the host.

1st PUC Biology Excretory Products and their Elimination Three Marks Questions

Question 1.
Draw a sketch showing human urinary system and label the parts.
Answer:

Question 2.
Draw a neat labelled diagram of longitudinal section of Kidney.
Answer:

Question 3.
Give a diagrammatic representation of a nephron showing blood vessels, duct and tubule.
Answer:

Question 4.
Describe glomerular filtration In human nephrons. (Delhi 2003)
Answer:
Glomerular filtration is the first step in urine formation, which is the filtration of blood carried out by the glomerulus. 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. Blood is filtered 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. On an average 1100 – 1200 ml of blood is filtered by the kidneys per minute. The glomerular filtration rate Is about 125 ml /minute.

Question 5.
What is the full form of ADH? How does ADH control osmoregulation in human kidney? (Foreign 2001)
Answer:
ADH – Antidiuretic Hormone. ADH is secreted when there is excessive loss of fluid from the body. ADH facilitates water reabsorption from distal convoluted tubule, collecting tubule and collecting duct, thereby preventing diuresis. ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure, which increases the glomerular blood flow and thereby the GFR.

Question 6.
Describe the general structure of renai corpuscle. What happens when blood flows through it?
Answer:
The Bowman’s capsule along with the glomerulus is known as Malpighian corpuscle or Renal corpuscle. Bowman’s capsule is a double-walled cup-like structure. The wall is made of epithelial tissue. The hollow of the cup is occupied by the glomerulus.

Blood flowing through the glomerulus is filtered, which is the first step in urine formation and is called ultrafiltration.

The filtration membrane is made up of

• The endothelium of glomerular capillaries.
• The epithelium of Bowman’s capsule
• The basement membrane between the two.

Question 7.
Describe the process of hemodialysis.
Answer:
Urea can be removed by the process called hemodialysis in patients suffering from uremia. Blood drained from a convenient artery is pumped into a dialysing unit after adding an anti coagulant like heparin. The unit contains a coiled cellophane tube surrounded by a fluid having the same composition as that of plasma except the nitrogenous waste.

The porous ceilophare membrane of the tube allows the passage of molecules based on concentration gradient. As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. The cleared blood is pumped back to the body through a vein after adding anti heparin to it.

Question 8.
Give an account of disorders of excretory system.
Answer:

• Uremia: It is a condition which leads to the accumulation of urea in the blood due to the malfunctioning of kidneys, which is highly harmful and may lead to kidney failure.
• Renal calculi: Stone or insoluble mass of crystallised salts like oxalates etc. formed within the kidney is termed as Renal calculi.
• Glomerulonephritis: It is a condition of inflammation of glomeruli of kidney.

1st PUC Biology Excretory Products and their Elimination Five Marks Questions

Question 1.
Classify animats based on the nature of excretory products, giving examples. (April 84)
Answer:
The animals are classified into 3 types depending on the type of nitrogenous waste material excreted by them. They are:

1. Ammonotelic animals
2. Ureotelic animals
3. Uricotelic animals.

(1) Ammonoteiic animals: are the animals which predominantly excrete their nitrogenous waste material in the form of ammonia. Examples: Crustaceans, Polychaete annelids, Octopus, Sepia, tadpoles of Amphibians and some teleosts fishes (bony fishes)

(2) Ureotelic animals; are the animals which predominantly excrete their nitrogenous waste material in the form of Urea. Examples: Eiasmobranchs, Amphibians and Mammals.

(3) Uricotelic animals: are the animals which predominantly excrete their nitrogenous waste material in the form of uric acid Examples: Reptiles and Birds.

Question 2.
Explain the process of urine formation. (Oct. 84, 91,93, 2000, 2004, April 86, 89, 93, 1995, 99, 2006, 2009, July 2008, 2010)
Answer:
During urine formation, the nitrogenous waste materials are removed as urine from the blood without losing the valuable molecules of blood in the process. It involves three main stages namely.

1. Ultra filtration (Glomerular filtration)
2. Reabsorption (tubular)
3. Tubular secretion.

1. Ultra filtration: Involves the removal or filtration of substances from blood of the glomerular lumen into the Bowman’s capsular lumen through the Ultra fine endothelia capsular membrane. Glomerular filtration is caused due to changes in blood pressure, the pressure of the Bowman’s capsular fluid and osmotic pressures excreted by contents in glomerular and Bowman’s lumen. The glomerular filtrate resembles the blood plasma in its chemical composition except for the molecules of high molecular weight.

2. Tubular reabsorption: This is the second step or stage in urine formation and involves the reabsorption of physiologically important substances into the surrounding capillaries when the primary urine moves down route in the nephron tubules. The reabsorption may be passive cr active at various regions of the convoluted tubule. During this process glucose, amino acids, inorganic salts and some water is absorbed. Some of the absorptions are under the influence of hormones.

3. Tubular secretion: (Active secretion) This is the last stage or step in the urine formation at the end of which a hypertonic or highly concentrated urine is formed for excretion. During this stage, the capillaries surrounding the convoluted tubule actively secrete certain unwanted substances which could not be the filtered in the Bowman’s capsule by the glomerulus into the filtrate of the tubular lumen.

As a result of these 3 processes or stages .in urine formation, homeostasis is blood is restored and waste products remain in the tubular fluid forming concentrated urine for excretion from urine. Two hormones come into play during the process namely ADH – for reabsorption of water when the body has the deficit of water and Aldosterone helping in reabsorption of ions like K+ when the body faces the deficit of these ions.

Question 3.
With a neat diagram explain the structure of nephron. (Apr. 85, 01,2005, June 2009, March 2010)
Answer:

Nephrons are the fundamental units of the kidney. Each nephron is made up of
(a) A Bowman’s capsule
(b) A convoluted tubule

(a) The Bowman’s capsule is cup shaped and double walled enclosing a capsular space. It is lined by the simple squamous epithelium. Its cup shaped depression holds the highly folded tuft of blood capillaries called the glomerulus. The glomerulus brings blood for purification in terms of nitrogenous waste material, removal of excess of ions, salts, water etc.

It is formed by the branch of afferent renal artery namely the renal arteriole. After filtration into the Bowman’s capsule the remaining constituents are drained by the efferent arteriole which again branch and rebranch around the convoluted tubule of the nephron to form the particular capillaries and the peritubular capillaries join to

‘form renal venules which in turn form the renal vein carrying away blood from kidney with reabsorbed materials and reset homeostasis back into the circulation. The glomerular epithelium & Bowman’s capsular epithelium form an intricate association called endothelia capsular barrier.

(b) The convoluted tubule: starts from the hind endo. the cup-shaped Bowman’s capsule. It is thin, elongated, tubular, double-walled enclosing a tubular space. It is lined by cuboidal epithelium. The convoluted tubule is composed of 3 parts, each having a distinct function. They are:

(1) Proximal convoluted tubule: It is wide tube starting from Bowman’ capsule and is involved in absorption of ions, amino acid glucose etc. by active reabsorption and water by passive reabsorption forming the primary urine or filtrate. The epithelium (endothelium) is brush bordered in this region.

(2) Henle’s loop: The proximal convoluted tubules narrows down and dips into a thin ‘U’ shaped tubule (of various length in difference animals) called the Henle’s loop. It helps in reabsorption of water, Na⁺, Cl^– etc. from the filtrate by Active transport.

(3) The Distal convoluted tubule: The Henle’s loop broadens and leads into the distal convoluted tubule. Here active reabsorption & secretion of unwanted wastes takes place. The reabsorption of water and ions is under the influence of Hormones (ADH and Aldosterone). The lining of Distal convoluted tubule is made up of cuboidal epithelium.

The entire convoluted tubule is surrounded by the peritubular capillaries formed by efferent arteriole arising from glomerulus. Required substances are reabsorbed from the tubules into these peritubular capillaries and unwanted substances are secreted from the peritubular capillaries into the distal convoluted tubule.

The Distal convoluted tubules of many nephrons join the collecting tubules and the nephron fluid formed at the final region of the convoluted tubule i.e. the DCT is drained into the collecting tubules from various nephrons.

Question 4.
Explain the role of different regions of a nephron in urine formation.
Answer:
(i) Glomerulus: The first step of urine formation, ultrafiltration, occurs from the glomerular capillaries into the lumen of the Bowman’s capsule.

(ii) Proximal convoluted tubule (PCT): In this segment, nearly 70 – 80% of water and electrolytes are reabsorbed. PCT helps in maintaining the pH and ionic balance of the body fluids, by selective secretion of H⁺, ammonia and K⁺ into the filtrate and by absorbing bicarbonate ions.

(iii) Henle’s loop: This segment plays an important role in maintaining high osmolarity of the medullary interstitial fluid. The descending limb is permeable to water and impermeable to solutes. So filtrate becomes hypertonic. The ascending limb is, impermeable to water but permeable to solutes, so filtrate becomes hypotonic.

(iv) Distal Convoluted tubule (DCT): Reabsorption of Na⁺ and water takes place in this segment. DCT also reabsorbs HCO₃ ions and secretes NH³⁺, hydrogen and potassium ions into the filtrate.

(v) Collecting duct: Large amounts of water is reabsorbed from this region. This segment also allows the transport of small amounts of urea, into the filtrate.

(v) Collecting duct: Large amounts of water is reabsorbed from this region. This segment also allows the transport of small amounts of urea, into the medullary interstitium. It also plays a role in maintaining the pH and ionic balance of the body fluids.