Diagram: Diagrammatic representation of a nephron and vasa recta showing the countercurrent mechanism.

Question 5.
Describe the role of liver, lungs and skin in excretion.
Answer:

• Lungs: Eliminate large amounts of carbon dioxide (≈18 litres/day) and water vapor, helping maintain acid-base and water balance.
• Liver: Secretes bile, which contains degraded steroid hormones, drugs, certain vitamins, and other waste products.
• Skin:

• • Sweat glands excrete water, salts, and small amounts of urea, while also producing a cooling effect.
  • Sebaceous glands eliminate certain wastes through sebum, providing a protective oily covering for the skin.

Question 6.
Explain micturition.
Answer:
Micturition or urination is the process of expelling urine from the urinary bladder through the urethra.

• It occurs by the simultaneous contraction of the smooth muscles of the bladder wall and relaxation of the skeletal muscles of the urethral sphincter.
• Micturition is primarily a reflex process, but in grown-up children and adults, it can be voluntarily controlled 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) Uricotelism                                         (iv) Urinary bladder

(d) ADH                                                    (v) Renal tubule

Question 8.
What is meant by the term osmoregulation?
Answer:
Osmoregulation is the maintenance of a constant osmotic concentration in the body cells and extracellular fluids by regulating the amount of water and salts in the body.

Question 9.
Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?
Answer:
Terrestrial adaptation requires conservation of water, so animals produce less toxic nitrogenous wastes.

• Ureotelic animals (e.g., mammals, many terrestrial amphibians) convert ammonia into urea in the liver. Urea is less toxic and excreted via the kidneys, sometimes retained in the kidney matrix to maintain osmolarity.
• Uricotelic animals (e.g., reptiles, birds, land snails, insects) excrete uric acid as a paste or pellet, minimizing water loss.

In contrast, ammonotelic animals (e.g., bony fishes, aquatic amphibians, aquatic insects) excrete ammonia directly through diffusion or gills, which requires large amounts of water and is unsuitable for terrestrial life.

Question 10.
What is the significance of juxta glomerular apparatus (JGA) in kidney function?
Answer:
The juxtaglomerular apparatus (JGA) plays a crucial role in regulating kidney function:

• A fall in glomerular blood flow, blood pressure, or GFR activates the juxtaglomerular (JG) cells to release renin.
• Renin converts angiotensinogen in the blood into angiotensin I, which is further converted to angiotensin II, a powerful vasoconstrictor that increases glomerular pressure and restores GFR.
• Angiotensin II also stimulates the adrenal cortex to release aldosterone, promoting reabsorption of Na⁺ and water from distal tubules.
• This Renin-Angiotensin mechanism helps maintain blood pressure, GFR, and fluid-electrolyte balance in the body.

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:

• Cephalochordate (Amphioxus)
• Columns of Bertini
• Vasa recta

Question 12:
Fill in the gaps:
(a) Ascending limb of Henle’s loop is limb is to it. to water whereas the descending
(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.

Answer:
(a) Ascending limb of Henle’s loop is impermeable to water whereas the descending limb is permeable to water.
(b) Reabsorption of water from distal parts of the tubules is facilitated by antidiuretic hormone (ADH).
(c) Dialysis fluid contains all the constituents as in plasma except nitrogenous wastes.
(d) A healthy adult human excretes (on average) 25–30 g of urea per day.

Additional Questions and Answers

Question 1:
Differentiate between cortical nephrons and juxtamedullary nephrons.
Answer:

Question 2:
What are the main nitrogenous wastes excreted by animals?

Answer:
The main nitrogenous wastes are:

1. Ammonia – highly toxic, excreted by ammonotelic animals (e.g., bony fishes, aquatic amphibians).
2. Urea – less toxic, excreted by ureotelic animals (e.g., mammals, many terrestrial amphibians).
3. Uric acid – least toxic, excreted by uricotelic animals (e.g., reptiles, birds, insects) to conserve water.

Question 3:
Name the excretory structures in the following animals:
(a) Flatworms
(b) Earthworms
(c) Insects
(d) Crustaceans

Answer:
(a) Protonephridia (flame cells) – Flatworms
(b) Nephridia – Earthworms
(c) Malpighian tubules – Insects
(d) Antennal glands (green glands) – Crustaceans

Question 4:
What is the role of the proximal convoluted tubule (PCT) in urine formation?

Answer:

• Lined with brush border epithelium to increase surface area for reabsorption.
• Reabsorbs nearly all essential nutrients, 70–80% of electrolytes and water.
• Helps in pH and ionic balance by secreting H⁺, NH₄⁺, and K⁺ into the filtrate and reabsorbing HCO₃⁻.

Question 5:
How does hemodialysis help in kidney failure?

Answer:

• Blood is withdrawn from an artery and passed through a dialyzing unit (artificial kidney).
• The dialysis fluid has the same composition as plasma except nitrogenous wastes.
• Nitrogenous wastes like urea diffuse out of blood into the dialysis fluid, clearing the blood.
• Cleaned blood is returned to the body after adding anti-heparin.

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