Exercises
Define Glomerular Filtration Rate (GFR).
Answer
Glomerular Filtration Rate (GFR) is the volume of the filtrate (primary urine) formed by both kidneys per minute. In a healthy adult, GFR is approximately 125 mL/min (about 180 litres per day). It is determined by the net filtration pressure (hydrostatic pressure minus colloidal osmotic pressure in glomerular capillaries), the permeability of the filtration membrane, and the surface area available for filtration.
Explain the autoregulatory mechanism of GFR.
Answer
GFR is autoregulated primarily by the juxtaglomerular apparatus (JGA), a specialised structure at the junction of the afferent arteriole and the distal convoluted tubule. When GFR drops (e.g., due to low blood pressure), the JGA cells (juxtaglomerular cells of the afferent arteriole) sense the reduced stretch and secrete renin. Renin converts angiotensinogen (from liver) to angiotensin I, which is converted to angiotensin II by ACE (angiotensin-converting enzyme). Angiotensin II: (i) constricts the efferent arteriole, raising glomerular pressure and GFR; (ii) stimulates ADH secretion (water reabsorption); (iii) stimulates aldosterone secretion (Na+ reabsorption). Additionally, a myogenic response causes afferent arterioles to dilate/constrict in response to changes in blood pressure to maintain stable GFR.
Q3. Indicate whether the following statements are true or false
(a) Micturition is carried out by a reflex.
Ans. True. Micturition (urination) is initiated by a spinal reflex triggered when the bladder is sufficiently full, though it is subject to voluntary control from higher brain centres.
(b) ADH helps in water elimination, making the urine hypotonic.
Ans. False. ADH (antidiuretic hormone) promotes water reabsorption from the distal convoluted tubule and collecting duct, making the urine concentrated (hypertonic), not hypotonic. It reduces water elimination.
(c) Protein-free fluid is filtered from blood plasma into the Bowman's capsule.
Ans. True. The glomerular filtration membrane allows water, ions, glucose, urea, and other small molecules to pass, but proteins (large molecules) are retained in blood. The filtrate in Bowman's capsule is protein-free.
(d) Henle's loop plays an important role in concentrating the urine.
Ans. True. Henle's loop (descending limb permeable to water; ascending limb pumps out NaCl) generates the hypertonic medullary interstitium through the countercurrent multiplier mechanism, which is essential for producing concentrated urine.
(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
Ans. True. Glucose is completely reabsorbed from the filtrate in the proximal convoluted tubule (PCT) by active transport (sodium-glucose co-transporters, SGLT), so normal urine is glucose-free.
Give a brief account of the counter current mechanism.
Answer
The countercurrent mechanism in the kidney enables the production of concentrated urine. It involves two components:
Countercurrent multiplier (Henle's loop): The descending limb of Henle's loop is permeable to water but not to solutes; water moves out by osmosis into the increasingly hypertonic medullary interstitium, concentrating the tubular fluid. The ascending limb is impermeable to water but actively pumps NaCl out into the interstitium. This creates a high osmotic gradient in the medulla (up to 1200 mOsm/L at the inner medulla, versus ~300 mOsm/L in cortex).
Countercurrent exchanger (vasa recta): The capillary network (vasa recta) runs parallel and counter to Henle's loop. As blood flows down in the descending limb of vasa recta, solutes diffuse in and water diffuses out; in the ascending limb, the reverse occurs. This exchange prevents washout of the medullary gradient. Urea also contributes to the inner medullary gradient.
This high interstitial osmolarity allows the collecting duct (under ADH influence) to absorb water from the dilute tubular fluid, producing concentrated (hypertonic) urine.
Describe the role of liver, lungs and skin in excretion.
Answer
Liver: Converts ammonia (toxic) to urea (less toxic) via the urea cycle; excretes bile pigments (bilirubin, biliverdin — from haemoglobin breakdown) into bile
⇒ intestine
⇒ faeces; detoxifies drugs, hormones, and foreign substances; eliminates cholesterol and steroid hormones via bile.
Lungs: Excrete CO2 (a major excretory product of cellular respiration) and water vapour during exhalation. They also expel volatile substances (alcohol, some drugs) to a small extent.
Skin: Sweat glands excrete water, NaCl, urea, lactic acid, and traces of other waste products in sweat. Sebaceous glands secrete sebum (containing waxes, sterols, and fatty acids). The skin thus has a minor excretory function alongside its thermoregulatory role.
Explain micturition.
Answer
Micturition (urination) is the process of expelling urine from the urinary bladder. As the bladder fills (to about 300–400 mL), stretch receptors in its wall are activated and send signals to the micturition centre in the sacral spinal cord. This triggers the micturition reflex: the detrusor muscle (bladder wall) contracts and the internal urethral sphincter (smooth muscle) relaxes reflexly. The external urethral sphincter (skeletal muscle) is under voluntary control from higher brain centres (frontal lobe), allowing urination to be consciously initiated or suppressed. When voluntary control is exerted, the external sphincter relaxes and urine is expelled.
Match the items of column I with those of column II
Answer
(a) Ammonotelism — (iii) Bony fish (and aquatic invertebrates excrete ammonia directly)
(b) Bowman's capsule — (v) Renal tubule (the Bowman's capsule is the cup-shaped beginning of the renal tubule)
(c) Micturition — (iv) Urinary bladder (micturition involves emptying the bladder)
(d) Uricotelism — (i) Birds (birds excrete uric acid to conserve water)
(d) ADH — (ii) Water reabsorption
What is meant by the term osmoregulation?
Answer
Osmoregulation is the homeostatic mechanism by which organisms maintain a stable osmotic concentration of their body fluids (balance of water and solutes) despite changes in the external environment or intake. In humans, the kidneys are the primary osmoregulatory organs, adjusting water and ion excretion under the control of hormones (ADH regulates water reabsorption; aldosterone regulates Na+/K+ balance) to keep blood osmolarity constant at ~300 mOsm/L.
Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic. Why?
Answer
Ammonia (the product of ammonotelism) is highly toxic and very soluble, requiring large amounts of water for dilution and excretion. Aquatic animals can excrete ammonia directly into surrounding water. Terrestrial animals have limited water availability, so they must conserve water. Therefore, they convert ammonia to either urea (ureotelic) or uric acid (uricotelic), which are far less toxic: urea can be excreted in concentrated urine requiring much less water; uric acid is almost insoluble and can be excreted as a semi-solid paste with minimal water loss (advantage for birds, reptiles, insects).
What is the significance of juxtaglomerular apparatus (JGA) in kidney function?
Answer
The JGA regulates GFR and blood pressure through the renin-angiotensin-aldosterone system (RAAS): when GFR or blood pressure falls, JGA cells secrete renin, triggering a cascade that leads to vasoconstriction and increased water/Na+ reabsorption, restoring blood pressure and GFR. It also detects NaCl concentration in the distal tubule fluid at the macula densa, adjusting afferent arteriole tone to maintain stable filtration. Thus, the JGA is a critical feedback mechanism linking blood pressure, GFR, and electrolyte balance.
Q11. Name the following
(a) A chordate animal having flame cells as excretory structures
Ans. Amphioxus (Branchiostoma/lancelet) — a cephalochordate with protonephridia (flame cells).
(b) Cortical portions projecting between the medullary pyramids in the human kidney
Ans. Columns of Bertini (renal columns).
(c) A loop of capillary running parallel to the Henle's loop
Ans. Vasa recta.
Q12. 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 / vasopressin).
(c) Dialysis fluid contains all the constituents as in plasma except nitrogenous wastes (urea, creatinine).
(d) A healthy adult human excretes (on an average) 25–30 gm of urea/day.
Important Extra Questions
What is renal threshold?
Answer
Renal threshold is the plasma concentration of a substance above which it begins to appear in urine because the tubular reabsorption capacity (transport maximum, Tm) is exceeded. For glucose, the threshold is ~180 mg/dL. Above this level (as in diabetes mellitus), glucose appears in urine (glucosuria) because transporters in the PCT are saturated.
Distinguish between ultrafiltration and tubular reabsorption.
Answer
Ultrafiltration (glomerular filtration) is a non-selective, pressure-driven process in the glomerulus that forces water, ions, glucose, urea, and small molecules from blood into the Bowman's capsule, retaining proteins and cells. Tubular reabsorption is selective transport of useful substances (glucose, amino acids, Na+, water) from the filtrate back into peritubular capillaries, occurring throughout the renal tubule.
What is the role of aldosterone in kidney function?
Answer
Aldosterone (secreted by the adrenal cortex) acts on the distal convoluted tubule and collecting duct to increase Na+ reabsorption (and K+ secretion). Na+ reabsorption draws water by osmosis, increasing blood volume and pressure. It is the main long-term regulator of blood Na+ and K+ balance and is part of the RAAS.
What is the difference between ammonotelism, ureotelism, and uricotelism?
Answer
Ammonotelic animals (bony fish, aquatic invertebrates) excrete ammonia — highly toxic, requires abundant water. Ureotelic animals (mammals, most adult amphibians) excrete urea — less toxic, moderately soluble, requires less water. Uricotelic animals (birds, reptiles, insects) excrete uric acid — least toxic, almost insoluble, excreted as semi-solid with minimal water loss, ideal for water conservation and for embryos in eggs.
What is dialysis (haemodialysis) and when is it used?
Answer
Haemodialysis is a procedure used to remove metabolic wastes (urea, creatinine, excess ions) from blood when the kidneys fail. Blood is passed through a dialysis machine containing a selectively permeable (dialysis) membrane immersed in dialysate fluid. Waste products diffuse from blood (high concentration) across the membrane into the dialysate (low/zero concentration), while blood cells and proteins are retained. It is used in acute or chronic renal failure until kidney transplantation is possible.