Excretory Products and their Elimination

NCERT Class 11 Biology • Chapter 16 Solutions

GFR & Regulation Mechanisms

1. Define Glomerular Filtration Rate (GFR).

Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute is called GFR.

Standard Value:
– Approx 125 ml/minute in a healthy individual.
– Approx 180 litres/day.
2. Explain the autoregulatory mechanism of GFR.

The kidneys have a built-in mechanism called the Juxta Glomerular Apparatus (JGA) mechanism to regulate GFR.

  1. Stimulus: A fall in GFR activates the JG cells.
  2. Response: JG cells release the enzyme Renin.
  3. Action: Renin stimulates glomerular blood flow (through the RAAS pathway) and thereby brings the GFR back to normal.
10. Significance of JGA in kidney function.
[Image of Juxta glomerular apparatus]

Significance of JGA:

  • It is a specialized sensitive region formed by the distal convoluted tubule (DCT) and the afferent arteriole.
  • It plays a complex regulatory role. When GFR/Blood Pressure falls, it releases Renin.
  • Renin initiates the Renin-Angiotensin-Aldosterone System (RAAS), which increases blood pressure and reabsorption of $Na^+$ and water, restoring GFR.

Urine Formation & Transport

3. True or False Statements.
  • (a) Micturition is carried out by a reflex. -> True
  • (b) ADH helps in water elimination, making the urine hypotonic. -> False (ADH helps in water reabsorption, making urine hypertonic).
  • (c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule. -> True (Ultrafiltration).
  • (d) Henle’s loop plays an important role in concentrating the urine. -> True.
  • (e) Glucose is actively reabsorbed in the proximal convoluted tubule. -> True.
12. Fill in the gaps.

(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 hormone ADH (Antidiuretic Hormone).

(c) Dialysis fluid contain all the constituents as in plasma except nitrogenous wastes.

(d) A healthy adult human excretes (on an average) 25-30 gm of urea/day.

Concentration Mechanism

4. Brief account of the counter current mechanism.
[Image of Counter current mechanism in nephron]

Objective: To produce concentrated urine (hypertonic) to conserve water.

Components: Henle’s Loop and Vasa Recta.

Mechanism:

  • The flow of filtrate in the two limbs of Henle’s loop is in opposite directions (Counter Current). Same for blood in Vasa Recta.
  • Ascending Limb: Transport $NaCl$ to the interstitium (impermeable to water).
  • Descending Limb: Permeable to water, water moves out to balance the high osmolarity of interstitium created by NaCl.
  • Urea: Also contributes to the osmolarity gradient.
  • This gradient (from 300 to 1200 mOsmol/L) allows water to be reabsorbed freely from the Collecting Duct, concentrating the urine up to 4 times the blood osmolarity.

Excretory Organs & Micturition

5. Role of liver, lungs and skin in excretion.
  • Lungs: Remove large amounts of $CO_2$ (approx 200ml/min) and significant quantities of water vapor daily.
  • Liver: The largest gland. It secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. These pass out with digestive wastes. It also converts toxic Ammonia to Urea (Urea Cycle).
  • Skin:
    • Sweat Glands: Produce sweat (watery fluid containing NaCl, small amounts of urea, lactic acid) for cooling and minor excretion.
    • Sebaceous Glands: Eliminate sterols, hydrocarbons and waxes through sebum.
6. Explain Micturition.

Micturition: The process of release of urine from the urinary bladder.

Mechanism (Micturition Reflex):

  1. Urine fills the bladder, stretching its walls.
  2. Stretch receptors send signals to the CNS.
  3. CNS passes motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter.
  4. Urine is released.

Classification & Identification

7. Match Column I with Column II.
Column IColumn II
(a) Ammonotelism(iii) Bony fish
(b) Bowman’s capsule(v) Renal tubule
(c) Micturition(iv) Urinary bladder
(d) Uricotelism(i) Birds
(e) ADH(ii) Water reabsorption
8. What is meant by the term Osmoregulation?

Osmoregulation is the process by which an organism regulates the water balance and solute (salt) concentration in its body fluids to maintain homeostasis, regardless of environmental conditions.

9. Why are terrestrial animals generally ureotelic or uricotelic, not ammonotelic?
  • Toxicity: Ammonia is highly toxic and requires large amounts of water for its elimination to prevent body damage.
  • Water Conservation: Terrestrial animals need to conserve water. Converting ammonia to less toxic forms like Urea (ureotelic) or Uric Acid (uricotelic) allows them to excrete wastes with minimal water loss.
11. Name the following.
  • (a) A chordate animal having flame cells: Amphioxus (Lancelet).
  • (b) Cortical portions projecting between medullary pyramids: Columns of Bertini.
  • (c) A loop of capillary running parallel to Henle’s loop: Vasa Recta.
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