Excretory Products and their Elimination — NCERT Solutions

Given / Concept: GFR refers to the volume of filtrate formed by both kidneys per minute.

Definition: Glomerular Filtration Rate (GFR) is defined as the amount of filtrate formed by the glomeruli of both kidneys per minute.

Key value: In a normal healthy adult, the GFR is approximately 125 mL per minute, which means about 180 litres of filtrate is produced per day.

This filtration is driven by the glomerular capillary blood pressure and is a non-selective process — all small molecules (water, glucose, amino acids, urea, ions) pass through, while large proteins and blood cells are retained.

Concept Used: The Juxta Glomerular Apparatus (JGA) is responsible for the autoregulation of GFR.

Step-by-step explanation:

Step 1 – Role of JGA:
The JGA is a specialised tissue located at the junction of the afferent arteriole and the distal convoluted tubule (DCT). It consists of juxta glomerular (JG) cells (modified smooth muscle cells of the afferent arteriole) and the macula densa (specialised cells of DCT).

Step 2 – When GFR falls:

• A fall in GFR activates the JG cells to release renin.
• Renin converts angiotensinogen (in blood) → Angiotensin I → Angiotensin II.
• Angiotensin II is a powerful vasoconstrictor; it increases glomerular blood pressure and thereby restores GFR.
• Angiotensin II also activates the adrenal cortex to release aldosterone.
• Aldosterone causes reabsorption of Na⁺ and water from the DCT, increasing blood volume and pressure, which further helps restore GFR.

Step 3 – When GFR rises:

• Increased stretch of the afferent arteriole wall causes it to constrict, reducing blood flow into the glomerulus and bringing GFR back to normal.

Conclusion: This entire mechanism — involving renin, angiotensin, and aldosterone — is called the Renin-Angiotensin-Aldosterone System (RAAS) and it maintains GFR within a narrow, normal range.

(a) Micturition is carried out by a reflex.
TRUE.
Micturition (urination) is initiated by a stretch reflex. When the urinary bladder fills and its wall is stretched, stretch receptors send signals to the CNS, which triggers a reflex contraction of the detrusor muscle and relaxation of the urethral sphincter, resulting in urination. However, it is also under voluntary control from higher brain centres.

(b) ADH helps in water elimination, making the urine hypotonic.
FALSE.
ADH (Anti-Diuretic Hormone) promotes reabsorption of water from the distal convoluted tubule (DCT) and collecting duct back into the blood. This makes the urine hypertonic (concentrated), not hypotonic. ADH reduces water elimination, not increases it.

(c) Protein-free fluid is filtered from blood plasma into the Bowman's capsule.
TRUE.
Glomerular filtration is a non-selective process based on size. Large plasma proteins (e.g., albumin, globulin) cannot pass through the glomerular filtration membrane. Therefore, the filtrate (ultrafiltrate) collected in the Bowman's capsule is essentially protein-free.

(d) Henle's loop plays an important role in concentrating the urine.
TRUE.
Henle's loop (along with vasa recta) establishes and maintains the osmolar gradient in the kidney medulla (300 mOsmol L⁻¹ in the cortex to 1200 mOsmol L⁻¹ in the inner medulla) through the counter current mechanism. This gradient is essential for concentrating the urine in the collecting duct.

(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
TRUE.
Glucose is completely reabsorbed in the PCT by active transport (requiring energy/ATP) against the concentration gradient. Under normal conditions, no glucose appears in the final urine.

Concept: The counter current mechanism involves the loop of Henle and the vasa recta working together to concentrate urine by maintaining an osmolar gradient in the renal medulla.

Components involved:

1. Loop of Henle (descending and ascending limbs)
2. Vasa recta (capillary network running parallel to the loop of Henle)

Step-by-step account:

Step 1 – Osmolar gradient in the medulla:
The medullary interstitium maintains an increasing osmolarity from the cortex (300 mOsmol L⁻¹) to the inner medulla (1200 mOsmol L⁻¹). This gradient is created and maintained by the counter current mechanism.

Step 2 – Descending limb of Henle's loop:

• The descending limb is permeable to water but impermeable to solutes.
• As the filtrate moves down into the increasingly hypertonic medullary interstitium, water moves out by osmosis.
• The filtrate becomes progressively concentrated (hypertonic) as it descends.

Step 3 – Ascending limb of Henle's loop:

• The ascending limb is impermeable to water but actively transports NaCl out into the interstitium.
• As the filtrate moves up, it becomes progressively dilute (hypotonic).
• The NaCl pumped out adds to the medullary osmolarity.

Step 4 – Role of urea:
The collecting duct is permeable to urea. As water is reabsorbed from the collecting duct, urea concentration increases and urea diffuses into the medullary interstitium, further contributing to the high osmolarity of the inner medulla.

Step 5 – Role of Vasa recta:

• The vasa recta runs parallel and in opposite direction to the loop of Henle, forming a counter current exchanger.
• As blood flows down the descending vasa recta, it gains solutes and loses water (becoming concentrated).
• As blood flows up the ascending vasa recta, it loses solutes and gains water.
• This prevents the washout of the medullary osmolar gradient while supplying nutrients to the medulla.

Significance:
The counter current mechanism allows the DCT and collecting duct to concentrate the filtrate from 300 mOsmol L⁻¹ to about 1200 mOsmol L⁻¹ (approximately 4 times), producing concentrated urine and conserving body water.

Apart from the kidneys, several other organs assist in the process of excretion:

1. Liver:

• The liver is the chief site for amino acid catabolism and produces urea from ammonia via the ornithine cycle (urea cycle). Urea is then excreted by the kidneys.
• It converts haemoglobin (from worn-out RBCs) into bile pigments — bilirubin and biliverdin — which are excreted through bile into the intestine and eliminated with faeces.
• The liver excretes cholesterol, steroid hormones, certain vitamins, and drugs through bile.
• It detoxifies many harmful substances (e.g., alcohol, drugs) and converts them into less toxic or water-soluble forms for excretion.

2. Lungs:

• The lungs are the primary organs for excretion of CO₂ (a major metabolic waste product of cellular respiration) and water vapour.
• During exhalation, approximately 200 mL of CO₂ per minute is expelled.
• Some volatile substances (e.g., alcohol vapours, certain anaesthetic gases) are also eliminated through the lungs.

3. Skin:

• The skin excretes waste through sweat glands.
• Sweat contains water, NaCl, small amounts of urea, lactic acid, and traces of other metabolic wastes.
• The primary function of sweating is thermoregulation (cooling the body), but it also serves as a minor excretory route.
• Sebaceous glands of the skin secrete sebum, which contains waxes, sterols, hydrocarbons, and fatty acids — these are also eliminated from the body.

Conclusion: While the kidneys are the principal excretory organs, the liver, lungs, and skin collectively assist in maintaining the internal environment by eliminating various metabolic wastes.

Definition: Micturition is the process of voiding (releasing) urine from the urinary bladder through the urethra. It is also called urination.

Step-by-step explanation:

Step 1 – Urine storage:
Urine formed by the kidneys is transported through the ureters to the urinary bladder, where it is stored. The urinary bladder can hold approximately 700–800 mL of urine, but the urge to urinate is felt when it contains about 200–400 mL.

Step 2 – Stretch receptors activated:
As urine accumulates, the wall of the urinary bladder stretches. This activates stretch receptors in the bladder wall.

Step 3 – Nerve signals to CNS:
The stretch receptors send afferent nerve impulses to the micturition centre in the spinal cord (and also to higher brain centres).

Step 4 – Reflex response:
The CNS sends efferent signals back to the bladder:

• The detrusor muscle (smooth muscle of the bladder wall) contracts.
• The internal urethral sphincter (involuntary) relaxes.
• The external urethral sphincter (voluntary, under conscious control) also relaxes.

Step 5 – Urine expelled:
The combined contraction of the detrusor muscle and relaxation of the sphincters forces urine out through the urethra.

Voluntary control:
Micturition is a reflex act but is also under voluntary control from higher brain centres (cerebral cortex). A person can consciously delay or initiate urination by controlling the external urethral sphincter.

Conclusion: Micturition is a coordinated reflex involving both the autonomic nervous system and voluntary control, ensuring timely and controlled elimination of urine from the body.