This MCQ module is based on: Regulation Micturition Disorders
TOPIC 74 OF 90
Regulation Micturition Disorders
🎓 Class 11
Biology
CBSE
Theory
Ch 16 – Excretory Products and their Elimination
⏱ ~14 min
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Regulation of Kidney Function, Micturition and Disorders
The kidney can now filter, reabsorb and concentrate. What it still needs is instruction — how much water to save today, how much salt, how much to raise the blood pressure. This part covers the three hormonal systems that give those instructions, the reflex that empties the bladder, the other organs that help with excretion, and what happens when the kidney fails.
16.5 Regulation of Kidney Function
The functioning of the kidneys is efficiently monitored and regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and to a certain extent, the heart.
1. The ADH mechanism
Osmoreceptors in the body are activated by changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid from the body can activate these receptors, which stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis.
ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis. An increase in body fluid volume can switch off the osmoreceptors and suppress the ADH release, to complete the feedback.
A second action of ADH. ADH can also affect the kidney function by its constrictory effects on blood vessels. This causes an increase in blood pressure. An increase in blood pressure can increase the glomerular blood flow and thereby the GFR.
Read the name and you have the function. Anti-diuretic means “against the production of large volumes of urine”. So ADH's job is to retain water, never to eliminate it — which makes urine more concentrated, not less. NCERT's Exercise 3(b) states the opposite deliberately, as a trap. Note also where ADH acts: on the latter parts of the tubule — the DCT and collecting duct, precisely the segments described in Part 3 as doing conditional reabsorption. The condition is ADH.
2. The renin-angiotensin mechanism
The JGA plays a complex regulatory role. Follow the chain in order:
- A fall in glomerular blood flow / glomerular blood pressure / GFR can activate the JG cells to release renin.
- Renin converts angiotensinogen in blood to angiotensin I and further to angiotensin II.
- Angiotensin II, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR.
- Angiotensin II also activates the adrenal cortex to release Aldosterone.
- Aldosterone causes reabsorption of Na⁺ and water from the distal parts of the tubule. This also leads to an increase in blood pressure and GFR.
This complex mechanism is generally known as the Renin-Angiotensin mechanism.
3. The ANF mechanism — the brake
An increase in blood flow to the atria of the heart can cause the release of Atrial Natriuretic Factor (ANF). ANF can cause vasodilation (dilation of blood vessels) and thereby decrease the blood pressure. ANF mechanism, therefore, acts as a check on the renin-angiotensin mechanism.
| ADH mechanism | Renin-angiotensin mechanism | ANF mechanism | |
|---|---|---|---|
| Trigger | Excessive loss of fluid, detected by osmoreceptors | Fall in glomerular blood flow, blood pressure or GFR | Increase in blood flow to the atria of the heart |
| Source | Hypothalamus → neurohypophysis | JG cells of the JGA; then adrenal cortex | Atria of the heart |
| Chemical(s) | ADH or vasopressin | Renin → angiotensin I → angiotensin II → aldosterone | Atrial Natriuretic Factor |
| Main effect | Water reabsorption from latter parts of the tubule; prevents diuresis | Vasoconstriction; Na⁺ and water reabsorption from distal parts of the tubule | Vasodilation |
| Effect on blood pressure | Increases (constrictory effect on vessels) | Increases | Decreases |
| Effect on GFR | Increases | Restores it to normal | Lowers it |
Why there must be a brake. Both the ADH and the renin-angiotensin systems act in the same direction — conserve fluid, constrict vessels, raise blood pressure. A control system with only accelerators would drive blood pressure steadily upward. ANF supplies the opposing signal, and the chapter says so explicitly: it acts as a check on the renin-angiotensin mechanism. Notice also who sends it — the heart, which is the organ best placed to notice that it is being asked to handle too much blood.
16.6 Micturition
Urine formed by the nephrons is ultimately carried to the urinary bladder, where it is stored till a voluntary signal is given by the central nervous system (CNS).
The reflex, step by step.
• This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
• In response, the stretch receptors on the walls of the bladder send signals to the CNS.
• The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, causing the release of urine.
• This signal is initiated by the stretching of the urinary bladder as it gets filled with urine.
• In response, the stretch receptors on the walls of the bladder send signals to the CNS.
• The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter, causing the release of urine.
The process of release of urine is called micturition, and the neural mechanism causing it is called the micturition reflex.
The characteristics of urine
- An adult human excretes, on an average, 1 to 1.5 litres of urine per day.
- The urine formed is a light yellow coloured watery fluid which is slightly acidic (pH 6.0) and has a characteristic odour.
- On an average, 25–30 gm of urea is excreted out per day.
Why a doctor asks for a urine sample. Various conditions can affect the characteristics of urine. Analysis of urine helps in clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney. For example, presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine are indicative of diabetes mellitus.
Two words in the definition to notice. The chapter calls micturition both a voluntary signal and a reflex — which sounds contradictory until you see how the two parts divide. The stretch receptors and the signal to the CNS are involuntary: you cannot choose not to feel a full bladder. The timing of the motor messages is under voluntary control, which is why an adult can postpone micturition and an infant, whose control over the urethral sphincter is not yet developed, cannot. NCERT's Exercise 3(a) — “micturition is carried out by a reflex” — is true.
16.7 Role of Other Organs in Excretion
Other than the kidneys, lungs, liver and skin also help in the elimination of excretory wastes.
| Organ | What it eliminates |
|---|---|
| Lungs | Remove large amounts of CO₂ (approximately 200 mL/minute) and also significant quantities of water every day |
| Liver | The largest gland in our body, it secretes bile-containing substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs. Most of these substances ultimately pass out along with digestive wastes |
| Sweat glands (skin) | Sweat is a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. Though the primary function of sweat is to facilitate a cooling effect on the body surface, it also helps in the removal of some of the wastes |
| Sebaceous glands (skin) | Eliminate certain substances like sterols, hydrocarbons and waxes through sebum. This secretion provides a protective oily covering for the skin |
| Saliva | Small amounts of nitrogenous wastes could be eliminated through saliva too |
A distinction worth making. For each of these organs, excretion is a by-product of its main job, not its purpose. The lungs exist for gas exchange; the liver for metabolism and bile secretion; sweat glands for cooling — the chapter is explicit that the primary function of sweat is to facilitate a cooling effect; sebaceous glands for protecting the skin. Only the kidney exists to excrete. That is why the kidney alone can regulate what it removes, while the others simply lose whatever happens to be in the fluid they secrete.
16.8 Disorders of the Excretory System
Uremia. Malfunctioning of kidneys can lead to accumulation of urea in blood, a condition called uremia, which is highly harmful and may lead to kidney failure.
Haemodialysis
In such patients, urea can be removed by a process called hemodialysis. The chapter describes the procedure in full:
- During the process of haemodialysis, the blood drained from a convenient artery is pumped into a dialysing unit called artificial kidney, after adding an anticoagulant like heparin.
- The unit contains a coiled cellophane tube surrounded by a fluid (dialysing fluid) having the same composition as that of plasma except the nitrogenous wastes.
- The porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient.
- As nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood.
- The cleared blood is pumped back to the body through a vein after adding anti-heparin to it.
This method is a boon for thousands of uremic patients all over the world.
The single most elegant point in the design. The dialysing fluid has the same composition as plasma except the nitrogenous wastes. Because it matches plasma in everything else, there is no concentration gradient for glucose, amino acids, Na⁺ or anything else useful, so none of them is lost. But for urea the gradient is maximal, so it freely moves out. Selectivity is achieved not by a clever membrane but by choosing the fluid on the other side of it — pure diffusion, no active transport at all.
Kidney transplantation and two more disorders
Kidney transplantation is the ultimate method in the correction of acute renal failures (kidney failure). A functioning kidney is used in transplantation from a donor, preferably a close relative, to minimise its chances of rejection by the immune system of the host. Modern clinical procedures have increased the success rate of such a complicated technique.
Renal calculi: Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within the kidney.
Glomerulonephritis: Inflammation of glomeruli of kidney.
Glomerulonephritis: Inflammation of glomeruli of kidney.
📐 Activity 16.4 — Model the dialysing membrane
What to do. Take a length of cellophane or dialysis tubing and knot one end. Fill it with a mixture of starch solution and glucose solution, knot the other end, rinse the outside, and suspend it in a beaker of plain distilled water. Set up a second, identical bag and suspend it in a beaker of glucose solution of the same strength as the bag's contents. After thirty minutes, test the water in both beakers with Benedict's solution for glucose and with iodine for starch.
Predict: in which beaker will glucose appear outside the bag? Will starch appear in either? And which beaker corresponds to a real dialysing unit?
Observations. In the first beaker (distilled water) glucose appears outside the bag — Benedict's test is positive. In the second beaker (glucose solution of equal strength) essentially no net movement of glucose occurs. Starch appears in neither beaker; iodine gives no colour in the surrounding fluid in either case.
What each result models. The starch result models the membrane's role: the porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient, but only if they are small enough. Starch, like the plasma proteins in real blood, is too large. So the membrane supplies the size selectivity.
The two beakers model the choice of dialysing fluid, and this is the important part. Glucose left the bag in beaker one only because there was a gradient. In beaker two the gradient was abolished and glucose stayed. A real dialysing unit is deliberately set up like beaker two for everything useful: the fluid has the same composition as that of plasma except the nitrogenous wastes, so glucose, amino acids and Na⁺ have no gradient and are not lost. It is set up like beaker one for urea alone: as nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood.
The design insight. Selectivity in dialysis comes from what you put on the far side of the membrane, not from any cleverness in the membrane itself. Beaker one shows what would happen if plain water were used — the patient would be stripped of nutrients and salts along with the urea. This is also why heparin must be added on the way in and anti-heparin on the way out: the blood must not clot inside the coil, but must retain its ability to clot once returned.
🎯 Interactive: Which mechanism responds?
Mechanism activated: ADH mechanism
An excessive loss of fluid activates the osmoreceptors, which stimulate the hypothalamus to release ADH, or vasopressin, from the neurohypophysis. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis, and its constrictory effect on blood vessels raises blood pressure and hence GFR.
🎯 Competency-Based Questions
Scenario: Four patients are seen. Patient P has a tumour of the neurohypophysis and passes 15 litres of very dilute urine a day. Patient Q has a narrowed renal artery and persistently high blood pressure. Patient R is in kidney failure with blood urea far above normal. Patient S, a child, cannot yet postpone micturition.
Q1. Explain patient P's enormous urine output. L4 Analyse
The neurohypophysis is the source of ADH: osmoreceptors stimulate the hypothalamus to release antidiuretic hormone (ADH) or vasopressin from the neurohypophysis. Damage there means little or no ADH is released. Since ADH facilitates water reabsorption from latter parts of the tubule, thereby preventing diuresis, its absence means water is not reabsorbed in the DCT and collecting duct — recall from Part 3 that reabsorption there is conditional, and ADH is the condition. The water therefore leaves as urine, giving a very large volume of very dilute urine, which is diuresis. Note what is not wrong: filtration is normal, the medullary gradient is intact, and the loop of Henle is working — the kidney has simply lost permission to use them.
Q2. Why does a narrowed renal artery in patient Q raise the blood pressure of the whole body? L4 Analyse
Because the kidney mistakes a local obstruction for a body-wide fall in pressure. The narrowing reduces glomerular blood flow and glomerular blood pressure, and a fall in glomerular blood flow / glomerular blood pressure / GFR can activate the JG cells to release renin. Renin then converts angiotensinogen in blood to angiotensin I and further to angiotensin II, which, being a powerful vasoconstrictor, increases the glomerular blood pressure and thereby GFR — but it constricts vessels everywhere, not just in the kidney. Angiotensin II also activates the adrenal cortex to release Aldosterone, and aldosterone causes reabsorption of Na⁺ and water from the distal parts of the tubule, adding volume to the circulation. Both effects lead to an increase in blood pressure. The renin-angiotensin mechanism is working exactly as designed; the trouble is that its sensor is downstream of the blockage, so it never registers success and keeps signalling.
Q3. Fill in the blanks: An adult excretes ______ litres of urine a day, of pH ______, containing on average ______ gm of urea. Lungs remove about ______ mL of CO₂ per minute. Presence of glucose in urine is called ______ and of ketone bodies ______. L1 Remember
1 to 1.5; 6.0 (slightly acidic); 25–30; 200; Glycosuria; Ketonuria.
Q4. In patient R's dialysis, why is the dialysing fluid not simply distilled water, which would remove urea fastest? L4 Analyse
Because the porous cellophane membrane of the tube allows the passage of molecules based on concentration gradient — and it cannot distinguish a waste from a nutrient. Against distilled water there would be a steep gradient for everything small enough to cross: glucose, amino acids, Na⁺, K⁺, Ca⁺⁺, bicarbonate, all of it. The patient would be cleared of urea and simultaneously stripped of the substances the body needs, which could be fatal.
The actual fluid therefore has the same composition as that of plasma except the nitrogenous wastes. That single design choice creates zero gradient for everything useful and a maximal gradient for urea, so that as nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. The selectivity of dialysis lies in the composition of the fluid, not in the membrane. Note two further details of the procedure that show the same care: heparin is added before the blood enters, to stop it clotting in the coil, and anti-heparin is added before the cleared blood is pumped back to the body through a vein, so that the patient's clotting ability is restored.
The actual fluid therefore has the same composition as that of plasma except the nitrogenous wastes. That single design choice creates zero gradient for everything useful and a maximal gradient for urea, so that as nitrogenous wastes are absent in the dialysing fluid, these substances freely move out, thereby clearing the blood. The selectivity of dialysis lies in the composition of the fluid, not in the membrane. Note two further details of the procedure that show the same care: heparin is added before the blood enters, to stop it clotting in the coil, and anti-heparin is added before the cleared blood is pumped back to the body through a vein, so that the patient's clotting ability is restored.
Q5. “Micturition cannot be both a reflex and voluntary — the chapter contradicts itself.” Evaluate, using patient S. L5 Evaluate
There is no contradiction; the two words describe different halves of the same arc.
What is involuntary. Urine is stored till a voluntary signal is given by the central nervous system, but the sequence begins without any choice: this signal is initiated by the stretching of the urinary bladder as it gets filled with urine, and the stretch receptors on the walls of the bladder send signals to the CNS. You cannot decide not to sense a full bladder, and you cannot decide how strongly the receptors fire. The whole afferent limb is automatic — which is why the chapter calls the neural mechanism the micturition reflex, and why NCERT's own Exercise 3(a), “micturition is carried out by a reflex”, is marked true.
What is voluntary. The efferent limb can be gated. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter — and the timing of those messages is what an adult controls. Hence the word voluntary signal.
Patient S settles the matter. A young child has the reflex fully working — the bladder fills, the receptors fire, the CNS responds — but has not yet acquired control over the urethral sphincter, so micturition happens as soon as the reflex is triggered. Learning bladder control in early childhood is precisely the acquisition of the voluntary gate on top of an already functioning reflex. The same point is made in reverse by spinal injury, where the voluntary gate is lost and the reflex alone remains.
The accurate formulation: micturition is a reflex whose motor output is under voluntary control — a common design in the body, as with breathing, which continues automatically yet can be held.
What is involuntary. Urine is stored till a voluntary signal is given by the central nervous system, but the sequence begins without any choice: this signal is initiated by the stretching of the urinary bladder as it gets filled with urine, and the stretch receptors on the walls of the bladder send signals to the CNS. You cannot decide not to sense a full bladder, and you cannot decide how strongly the receptors fire. The whole afferent limb is automatic — which is why the chapter calls the neural mechanism the micturition reflex, and why NCERT's own Exercise 3(a), “micturition is carried out by a reflex”, is marked true.
What is voluntary. The efferent limb can be gated. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter — and the timing of those messages is what an adult controls. Hence the word voluntary signal.
Patient S settles the matter. A young child has the reflex fully working — the bladder fills, the receptors fire, the CNS responds — but has not yet acquired control over the urethral sphincter, so micturition happens as soon as the reflex is triggered. Learning bladder control in early childhood is precisely the acquisition of the voluntary gate on top of an already functioning reflex. The same point is made in reverse by spinal injury, where the voluntary gate is lost and the reflex alone remains.
The accurate formulation: micturition is a reflex whose motor output is under voluntary control — a common design in the body, as with breathing, which continues automatically yet can be held.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): ADH makes the urine more concentrated.
Reason (R): ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis.
Answer: A. Both are true and the reason is the correct explanation. Water returns to the blood while the solutes remain in the tubule, so the urine leaving is smaller in volume and more concentrated.
Assertion (A): ANF and the renin-angiotensin mechanism act in the same direction on blood pressure.
Reason (R): ANF causes vasodilation and thereby decreases the blood pressure.
Answer: D. The assertion is false — they act in opposite directions, and the ANF mechanism acts as a check on the renin-angiotensin mechanism. The reason is true and is exactly why: angiotensin II is a vasoconstrictor, ANF a vasodilator.
Assertion (A): The dialysing fluid in haemodialysis does not contain nitrogenous wastes.
Reason (R): The porous cellophane membrane allows the passage of molecules based on concentration gradient, so wastes move out freely while substances present in equal concentration on both sides do not.
Answer: A. Both are true and the reason explains the assertion. The absence of wastes in the fluid creates the gradient that clears them, while matching plasma in everything else protects the nutrients.
Coming next. Part 5 is the exercise part: the chapter summary followed by full worked solutions to all twelve NCERT exercise questions of Chapter 16, including the true-or-false statements, the Column I–Column II matching, and the fill-in-the-gaps question.
Frequently Asked Questions - Regulation of Kidney Function, Micturition and Disorders
How is kidney function regulated?
By hormonal feedback mechanisms involving the hypothalamus, the JGA and, to a certain extent, the heart. The three systems are the ADH mechanism, the renin-angiotensin mechanism and the ANF mechanism.
What does ADH do?
Osmoreceptors activated by an excessive loss of fluid stimulate the hypothalamus to release ADH, or vasopressin, from the neurohypophysis. ADH facilitates water reabsorption from the latter parts of the tubule, thereby preventing diuresis. It also has a constrictory effect on blood vessels, raising blood pressure and hence glomerular blood flow and GFR.
Explain the renin-angiotensin mechanism.
A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin, which converts angiotensinogen in blood to angiotensin I and further to angiotensin II. Angiotensin II is a powerful vasoconstrictor that increases glomerular blood pressure and GFR, and it also activates the adrenal cortex to release aldosterone, which causes reabsorption of Na+ and water from the distal parts of the tubule.
What is the role of Atrial Natriuretic Factor?
An increase in blood flow to the atria of the heart causes release of ANF, which brings about vasodilation and thereby decreases the blood pressure. The ANF mechanism therefore acts as a check on the renin-angiotensin mechanism.
What is micturition?
Micturition is the process of release of urine. Stretching of the urinary bladder as it fills activates stretch receptors on its walls, which signal the CNS; the CNS then sends motor messages causing contraction of the bladder's smooth muscles and simultaneous relaxation of the urethral sphincter. The neural mechanism causing it is called the micturition reflex.
What are the characteristics of normal urine?
An adult human excretes on average 1 to 1.5 litres of urine per day. It is a light yellow coloured watery fluid, slightly acidic at pH about 6.0, with a characteristic odour, and contains on average 25 to 30 g of urea per day.
Why is urine analysis useful in diagnosis?
Because various conditions affect the characteristics of urine, so its analysis helps in the clinical diagnosis of many metabolic disorders as well as malfunctioning of the kidney. For example the presence of glucose, called glycosuria, and of ketone bodies, called ketonuria, is indicative of diabetes mellitus.
What is the role of the lungs, liver and skin in excretion?
The lungs remove large amounts of CO2, about 200 mL per minute, and significant quantities of water. The liver secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, most of which pass out with digestive wastes. Sweat glands eliminate a watery fluid with NaCl, small amounts of urea and lactic acid, while sebaceous glands eliminate sterols, hydrocarbons and waxes through sebum.
What is uremia and how is it treated?
Uremia is the accumulation of urea in blood caused by malfunctioning of the kidneys; it is highly harmful and may lead to kidney failure. Urea can be removed by haemodialysis, and the ultimate method in the correction of acute renal failure is kidney transplantation.
How does haemodialysis work?
Blood drained from a convenient artery is pumped, after adding heparin, into a dialysing unit called an artificial kidney. The unit contains a coiled cellophane tube surrounded by dialysing fluid having the same composition as plasma except the nitrogenous wastes. The porous membrane allows passage of molecules based on concentration gradient, so as the wastes are absent from the fluid they move out freely. The cleared blood is returned through a vein after adding anti-heparin.
What are renal calculi and glomerulonephritis?
Renal calculi are stones or insoluble masses of crystallised salts such as oxalates formed within the kidney. Glomerulonephritis is inflammation of the glomeruli of the kidney.
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