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NCERT Solutions · Class 11 Biology Excretory Products and their Elimination

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Exercises 16.1–16.12

  1. Exercise 16.1

    Define Glomerular Filtration Rate (GFR)

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    Glomerular Filtration Rate (GFR) is the amount of filtrate formed by the kidneys per minute.
    In a healthy individual it is about $\displaystyle 125$ mL per minute, i.e. nearly $\displaystyle 180$ litres per day.
    It is the outcome of glomerular filtration, in which $\displaystyle 1100$–$\displaystyle 1200$ mL of blood is filtered by the kidneys every minute — roughly \(\displaystyle 1/5^{th}\) of the blood pumped out by each ventricle in a minute.

  2. Exercise 16.2

    Explain the autoregulatory mechanism of GFR.

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    GFR is autoregulated by the juxta glomerular apparatus (JGA) — the kidneys' own built-in mechanism, needing no external control.
    What JGA is: a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at the point where the two come in contact.
    The trigger: a fall in glomerular blood flow, glomerular blood pressure or GFR.
    The response: the JG cells release renin, which stimulates the glomerular blood flow and brings GFR back to normal.
    How renin works: it converts angiotensinogen in blood to angiotensin I and further to angiotensin II, a powerful vasoconstrictor that raises glomerular blood pressure and hence GFR.
    Angiotensin II also makes the adrenal cortex release aldosterone, which causes reabsorption of \(\displaystyle Na^{+}\) and water from the distal parts of the tubule, again raising blood pressure and GFR. This whole loop is the Renin-Angiotensin mechanism.

  3. Exercise 16.3

    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.

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    (a) True. Micturition is carried out by a reflex — stretching of the filled bladder makes stretch receptors signal the CNS, which sends motor messages contracting the bladder muscles and relaxing the urethral sphincter. This is the micturition reflex.
    (b) False. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis. It conserves water and makes the urine concentrated, not hypotonic.
    (c) True. Blood is filtered so finely that almost all plasma constituents except the proteins pass into the lumen of the Bowman's capsule — this is why it is called ultra filtration.
    (d) True. Henle's loop maintains the high osmolarity of the medullary interstitial fluid ($\displaystyle 300$ to about $\displaystyle 1200$ \(\displaystyle mOsmolL^{-1}\)) through the counter current mechanism, and this gradient concentrates the urine.
    (e) True. Substances like glucose, amino acids and \(\displaystyle Na^{+}\) are reabsorbed actively; the PCT reabsorbs nearly all of the essential nutrients.

  4. Exercise 16.4

    Give a brief account of the counter current mechanism.

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    The counter current mechanism is the transport of substances made possible by the special arrangement of the Henle's loop and the vasa recta; it is what lets mammals produce a concentrated urine.
    The filtrate flows in opposite directions in the two limbs of Henle's loop, and blood flows in a counter current pattern through the two limbs of the vasa recta as well.
    The proximity of Henle's loop and vasa recta, together with these counter currents, maintains an increasing osmolarity towards the inner medullary interstitium — from $\displaystyle 300$ \(\displaystyle mOsmolL^{-1}\) in the cortex to about $\displaystyle 1200$ \(\displaystyle mOsmolL^{-1}\) in the inner medulla.
    This gradient is caused mainly by NaCl and urea.
    NaCl is transported by the ascending limb of Henle's loop and exchanged with the descending limb of the vasa recta; it is returned to the interstitium by the ascending portion of the vasa recta.
    Urea enters the thin segment of the ascending limb of Henle's loop in small amounts and is transported back to the interstitium by the collecting tubule.
    The resulting interstitial gradient allows an easy passage of water out of the collecting tubule, thereby concentrating the filtrate — human kidneys can make urine nearly four times as concentrated as the initial filtrate.
    NCERT_Solution_Class11_Biology_Ch16_Q16-4

  5. Exercise 16.5

    Describe the role of liver, lungs and skin in excretion.

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    Besides the kidneys, the lungs, liver and skin all help eliminate excretory wastes.
    Lungs: remove large amounts of \(\displaystyle CO_{2}\) (approximately $\displaystyle 200$ mL/minute) and also significant quantities of water every day.
    Liver: the largest gland in the body; it secretes bile, which carries out bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these ultimately pass out along with the digestive wastes.
    Skin — sweat glands: sweat is a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. Its primary function is cooling the body surface, but it also removes these wastes.
    Skin — sebaceous glands: eliminate sterols, hydrocarbons and waxes through sebum, a secretion that also provides a protective oily covering for the skin.
    Small amounts of nitrogenous waste are eliminated through saliva too.

  6. Exercise 16.6

    Explain micturition.

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    Micturition is the process of release of urine from the urinary bladder; the neural mechanism causing it is the micturition reflex.
    Urine formed by the nephrons is carried to the urinary bladder and stored there until a voluntary signal is given by the central nervous system (CNS).
    The signal is initiated by the stretching of the bladder as it fills with urine.
    The stretch receptors on the walls of the bladder send signals to the CNS.
    In response, the CNS passes motor messages that cause contraction of the smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, releasing the urine.
    An adult human excretes, on an average, $\displaystyle 1$ to $\displaystyle 1.5$ litres of urine per day.

  7. Exercise 16.7

    Match the items of column I with those of column II : Column I
    (a)
    Ammonotelism
    (b)
    Bowman’s capsule
    (c)
    Micturition
    (d)
    Uricotelism (d) ADH Column II
    (i)
    Birds
    (ii)
    Water reabsorption
    (iii)
    Bony fish
    (iv)
    Urinary bladder
    (v)
    Renal tubule

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    (a) Ammonotelism – (iii) Bony fish — many bony fishes excrete ammonia by diffusion across gill surfaces as ammonium ions.
    (b) Bowman's capsule – (v) Renal tubule — the renal tubule begins with the double-walled, cup-like Bowman's capsule.
    (c) Micturition – (iv) Urinary bladder — micturition is the release of urine stored in the urinary bladder.
    (d) Uricotelism – (i) Birds — birds excrete uric acid as a pellet or paste with minimum loss of water.
    (e) ADH – (ii) Water reabsorption — ADH (vasopressin) facilitates water reabsorption from the latter parts of the tubule, preventing diuresis.
    Note: the book misprints the fifth item's label as "(d)"; ADH is the fifth entry of Column I.

  8. Exercise 16.8

    What is meant by the term osmoregulation?

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    Osmoregulation is the regulation of the ionic concentration and fluid volume of the body, so that the osmolarity of the body fluids is kept at the desired level.
    It is carried out by the excretory structures: protonephridia (flame cells) are primarily concerned with ionic and fluid volume regulation, nephridia maintain fluid and ionic balance, and Malpighian tubules help in osmoregulation along with waste removal.
    In humans it is a kidney function: the DCT and collecting duct allow extensive reabsorption of water and certain electrolytes, adjusting the volume and concentration of urine.
    ADH, the renin-angiotensin mechanism (via aldosterone) and ANF monitor blood volume, body fluid volume and ionic concentration and correct them by feedback.
    Some ureotelic animals even retain a little urea in the kidney matrix to maintain a desired osmolarity.

  9. Exercise 16.9

    Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why ?

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    Because ammonia is the most toxic nitrogenous waste and needs a large amount of water for its elimination — water that a land animal cannot spare.
    Ammonia is readily soluble and is got rid of by diffusion across body surfaces or gill surfaces as ammonium ions, which is possible only in a water-rich habitat (bony fishes, aquatic amphibians, aquatic insects). Kidneys play no significant role in its removal.
    Terrestrial adaptation therefore necessitated the production of less toxic nitrogenous wastes — urea and uric acid — for conservation of water.
    Ureotelic animals (mammals, many terrestrial amphibians, marine fishes) convert ammonia to urea in the liver; urea is far less toxic, so it can be carried in blood and excreted by the kidneys with much less water.
    Uricotelic animals (reptiles, birds, land snails, insects) excrete uric acid, the least toxic form, as a pellet or paste with a minimum loss of water — the greatest water economy of the three.

  10. Exercise 16.10

    What is the significance of juxta glomerular apparatus (JGA) in kidney function?

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    The juxta glomerular apparatus (JGA) is the kidney's efficient built-in mechanism for the regulation of the glomerular filtration rate (GFR).
    It is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at their point of contact.
    A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin.
    Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II, a powerful vasoconstrictor that increases glomerular blood pressure and thereby GFR.
    Angiotensin II also activates the adrenal cortex to release aldosterone, which causes reabsorption of \(\displaystyle Na^{+}\) and water from the distal parts of the tubule — again raising blood pressure and GFR.
    This complex regulatory role of the JGA is known as the Renin-Angiotensin mechanism, and it is checked by ANF released when blood flow to the atria increases.

  11. Exercise 16.11

    Name the following:
    (a)
    A chordate animal having flame cells as excretory structures
    (b)
    Cortical portions projecting between the medullary pyramids in the human kidney
    (c)
    A loop of capillary running parallel to the Henle’s loop.

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    (a) Amphioxus — the cephalochordate whose excretory structures are protonephridia (flame cells). (Flame cells also occur in flatworms such as Planaria and in rotifers, but among chordates it is Amphioxus.)
    (b) Columns of Bertini — the renal columns, i.e. the extensions of the cortex in between the medullary pyramids.
    (c) Vasa recta — the 'U' shaped minute vessel of the peritubular capillary network that runs parallel to Henle's loop. It is absent or highly reduced in cortical nephrons.

  12. Exercise 16.12

    Fill in the gaps :
    (a)
    Ascending limb of Henle’s loop is _______ to water whereas the descending limb is _______ to it.
    (b)
    Reabsorption of water from distal parts of the tubules is facilitated by hormone _______.
    (c)
    Dialysis fluid contain all the constituents as in plasma except _______.
    (d)
    A healthy adult human excretes (on an average) _______ gm of urea/day.

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    (a) Ascending limb of Henle's loop is impermeable to water whereas the descending limb is permeable to it.
    (b) Reabsorption of water from distal parts of the tubules is facilitated by the hormone ADH (antidiuretic hormone or vasopressin).
    (c) Dialysis fluid contains all the constituents as in plasma except the nitrogenous wastes (such as urea) — their absence is what makes the wastes move freely out of the blood.
    (d) A healthy adult human excretes, on an average, $\displaystyle 25$–$\displaystyle 30$ gm of urea per day.