NCERT Solutions: Excretory Products & their Elimination

Q1: Define Glomerular Filtration Rate (GFR)
Ans: Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed by both kidneys per minute. In a healthy adult, it is approximately 125 mL per minute. The filtrate is a protein-free fluid and contains small solutes such as glucose, amino acids, sodium, potassium, urea, uric acid, ketone bodies and a large amount of water.

Fig: Glomerular Filtration Rate

Q2: Explain the autoregulatory mechanism of GFR.
Ans: The kidney maintains a fairly constant GFR through an intrinsic autoregulatory mechanism centred on the juxtaglomerular apparatus (JGA). Key features are:

• The JGA is located where the afferent arteriole reaches the renal corpuscle and where the distal convoluted tubule (DCT) returns near the vascular pole.
• Its main components are the juxtaglomerular (JG) cells in the afferent arteriole, the macula densa region of the DCT, and the extraglomerular mesangial cells.
• When GFR or renal blood flow falls, JG cells secrete the enzyme renin. The macula densa senses changes in Na+ and Cl- concentration in the tubular fluid and signals the JG cells to adjust renin release.
• Renin initiates the renin-angiotensin cascade, producing angiotensin II, which causes vasoconstriction of systemic arterioles and preferential constriction of the efferent arteriole. This raises glomerular hydrostatic pressure and helps restore GFR.
• Thus, the JGA provides a feedback loop (tubuloglomerular feedback) that keeps GFR within a narrow range despite fluctuations in blood pressure.

Fig: Glomerular Filtration rate

Q3: 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.

Ans:
 (a) True.
Explanation: Micturition is initiated by a spinal reflex that is modulated by higher centres; stretching of the bladder wall triggers sensory signals that lead to reflex contraction of the bladder and relaxation of the internal sphincter.
(b) False.
Explanation: Antidiuretic hormone (ADH, also called vasopressin) promotes water reabsorption in the collecting ducts, reducing water loss and making urine more hypertonic, not hypotonic.
(c) True.
Explanation: The glomerular filter normally excludes plasma proteins, so the primary filtrate entering Bowman's capsule is essentially protein-free and similar in composition to plasma minus large proteins.
(d) True.
Explanation: The loop of Henle establishes and helps maintain the osmotic gradient in the medulla (counter-current mechanism), which is essential for concentrating urine.
(e) True.
Explanation: Glucose is reabsorbed from the filtrate in the proximal convoluted tubule by active transport mechanisms so that normally no glucose appears in urine.

Q4: Give a brief account of the counter current mechanism.
Ans:

• The counter-current mechanism in the kidney is a key adaptation for water conservation and concentrating urine. It involves two main structures: the loop of Henle (in nephrons) and the vasa recta (capillary loop).
• The loop of Henle is U-shaped. Its descending limb is permeable to water but relatively impermeable to solutes, while the ascending limb is impermeable to water but actively transports NaCl out into the medullary interstitium.
• The vasa recta runs parallel to the loop of Henle and has blood flowing in opposite directions in its descending and ascending limbs, creating a counter-current exchange that preserves the medullary osmotic gradient.
• Because of these counter-current arrangements, osmolarity of the renal interstitium increases from about 300 mOsm L^-1 in the cortex to about 1,200 mOsm L^-1 in the inner medulla. This gradient, maintained mainly by the movement of NaCl and urea, enables passive reabsorption of water from the collecting ducts when ADH is present.

Fig: counter current mechanism

Overall, the counter-current system concentrates solutes in the medulla and allows fine regulation of urine concentration according to the body's hydration state.

Q5: Describe the role of liver, lungs and skin in excretion.
Ans: Apart from the kidneys, the liver, lungs and skin contribute to excretion by eliminating specific waste substances:

Role of the liver:

• The liver detoxifies and helps excrete substances such as steroid hormones, drugs, vitamins and excess cholesterol via bile.
• It converts highly toxic ammonia (from protein breakdown) into the less toxic compound urea through the ornithine (urea) cycle; urea is then transported to the kidneys for excretion.
• The liver also degrades haemoglobin and converts its pigment into bile pigments such as bilirubin and biliverdin, which are removed in bile.

Fig: liver

Role of the lungs: The lungs remove gaseous wastes, chiefly carbon dioxide (CO₂), produced by cellular respiration. Exhalation eliminates CO₂ and helps regulate blood pH.

Fig: Role of the lungs

Role of the skin:

• The skin eliminates small amounts of metabolic wastes through sweat. Sweat glands excrete water, salts (NaCl) and trace amounts of urea and other solutes, aiding thermoregulation and salt balance.
• Sebaceous glands secrete sebum, an oily substance that protects the skin; sebaceous secretions are not major excretory routes for metabolic wastes.

Q6: Explain micturition.
Ans: Micturition is the process of voiding urine from the urinary bladder. The sequence is:

• As the bladder fills, its walls stretch and sensory stretch receptors send impulses to the spinal cord, initiating a reflex (the micturition reflex).
• This reflex produces contraction of the detrusor muscle (smooth muscle of the bladder) and relaxation of the internal urethral sphincter (involuntary control).
• Voluntary control is exerted via the external urethral sphincter (striated muscle) under somatic control. When socially appropriate, the brain permits relaxation of the external sphincter, allowing urine to be expelled.
• Thus, micturition involves both an involuntary reflex and voluntary control centres; in adults these are coordinated to allow conscious control of urination.
• An average healthy adult excretes about 1-1.5 litres of urine per day.

Fig: Micturition

Ques 7:  Match the items of column I with those of column II:

Ans:

 
Q8: What is meant by the term osmoregulation?
Ans:

Fig: osmoregulation

Osmoregulation is the physiological process that maintains the balance of water and dissolved ions (electrolytes) in the body fluids. It ensures that:

• the concentrations of water and ions in body fluids remain within ranges necessary for normal cellular function,
• excess water and salts are excreted while necessary amounts are retained, and
• the internal environment (homeostasis) is kept stable despite changes in external conditions.

Osmoregulation is achieved mainly by the kidneys, with support from hormonal control (for example, ADH/vasopressin and aldosterone).

Q9: Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why? 
Ans: Terrestrial animals are usually ureotelic or uricotelic rather than ammonotelic for two main reasons:

• Toxicity: Ammonia is highly toxic and must be converted into less toxic forms (urea or uric acid) before safe storage or excretion.
• Water conservation: Ammonia is highly soluble in water and requires large amounts of water to be safely excreted. Terrestrial animals, which risk water loss, convert nitrogenous wastes into urea or uric acid, which are less toxic and can be excreted with much less water (uric acid can even be excreted as a semi-solid paste in some animals).

Fig: Excretion in animals

Q10: What is the significance of juxtaglomerular apparatus (JGA) in kidney function?
Ans: The juxtaglomerular apparatus (JGA) is a specialised structure formed where the afferent arteriole enters the renal corpuscle and the distal convoluted tubule comes into close contact with the vascular pole. Its significance includes:

• It contains juxtaglomerular (JG) cells that synthesise and release renin in response to low blood pressure, reduced renal blood flow, or signals from the macula densa.
• Renin converts angiotensinogen to angiotensin I, which is then converted to angiotensin II; angiotensin II raises blood pressure by vasoconstriction and increases GFR by altering glomerular haemodynamics.
• Angiotensin II also stimulates the adrenal cortex to secrete aldosterone, which increases sodium and water reabsorption in the distal nephron and collecting ducts - thereby increasing blood volume and pressure.
• Through these actions (the renin-angiotensin mechanism), the JGA plays a central role in regulating blood pressure, blood volume and glomerular filtration rate.

Fig: JGA

Q11: 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.

Ans: 

​(a) Amphioxus - a chordate in which flame cells act as excretory and osmoregulatory structures.

(b) The cortical projections between medullary pyramids are called the columns of Bertini (renal columns).

(c) The loop of capillary running parallel to the loop of Henle is the vasa recta, which helps maintain the medullary concentration gradient.

Q12: 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 contains all the constituents as in plasma except ________.
(d) A healthy adult human excretes (on an average) _______ gm of urea/day.
Ans:
(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 vasopressin (ADH).
(c) Dialysis fluid contains all the constituents of plasma except the nitrogenous wastes (for example, urea and creatinine), which diffuse out from the blood into the dialysis fluid.
(d) A healthy adult human excretes (on average) 25-30 g of urea per day.