BiologyISC Class 11 · Excretory Products and their Elimination

Regulation of Kidney Function, Micturition, Other Excretory Organs and Disorders

Updated 17 October 2026 · Fact-checked

Kidney function is regulated by osmoreceptors and ADH, the juxtaglomerular apparatus (renin, angiotensin II, aldosterone) and the atrial natriuretic factor (ANF), which oppose each other in controlling blood volume and pressure. Micturition is a reflex. Lungs, skin and liver also excrete wastes; kidney failure is treated by haemodialysis or transplantation.

Notice the two opposing pathways: renin, angiotensin II and aldosterone raise blood pressure, while ANF from the atrium lowers it.

Illustration drawn for QuestionBuddy.

Understand Regulation of Kidney Function, Micturition and Disorders

ADH. The osmoreceptors in the hypothalamus detect a rise in body fluid osmolarity or a fall in volume; the hypothalamus makes and the posterior pituitary releases antidiuretic hormone (vasopressin), which increases water reabsorption in the DCT and collecting duct, so a small volume of concentrated urine is formed. When the fluid volume rises, ADH release is suppressed and diuresis occurs.

Juxtaglomerular apparatus (JGA). Specialised cells in the afferent arteriole next to the DCT. A fall in glomerular blood flow, pressure or GFR makes the JG cells release renin, which converts angiotensinogen to angiotensin I and then to angiotensin II (a vasoconstrictor that raises the GFR and BP). Angiotensin II stimulates the adrenal cortex to release aldosterone, which increases reabsorption of Na⁺ and water in the DCT and so raises blood pressure.

ANF. The atrial wall secretes atrial natriuretic factor when the blood pressure rises; it causes vasodilation and lowers the blood pressure, opposing the renin-angiotensin mechanism.

Micturition. Urine in the bladder stretches its wall, sending signals to the CNS; motor messages cause contraction of the smooth muscle of the bladder and relaxation of the urethral sphincter, releasing urine through the urethra (a reflex under voluntary control).

Other excretory organs. Lungs remove CO₂ (about 200 mL per minute) and water vapour; the liver secretes bile pigments (bilirubin, biliverdin), cholesterol and drugs; the skin removes sweat (NaCl, urea, lactic acid) from sweat glands and sebum from sebaceous glands.

Urine. Normally 1 to 1.5 L per day, pale yellow, slightly acidic (pH about 6.0), with about 25 to 30 g of urea per day. Glycosuria (glucose in urine) and ketonuria (ketone bodies) indicate diabetes mellitus.

Disorders. Uremia: high blood urea due to kidney failure. Renal calculi: stones of crystallised calcium oxalate. Glomerulonephritis: inflammation of the glomeruli. In haemodialysis, the blood is passed through cellophane tubes in a dialysing fluid (same composition as plasma without nitrogenous wastes), with heparin to prevent clotting, and returned after the removal of wastes. Kidney transplant from a closely matching donor is the permanent solution.

Regulation of Kidney Function, Micturition and Disorders: key facts

Hormones: ADH water reabsorption
Aldosterone Na⁺ reabsorption
Renin angiotensin pathway
ANF lowers BP; ADH from the posterior pituitary.
Renin pathway: 1.Angiotensinogen —(renin)→ angiotensin I
2.angiotensin II
3.aldosterone; Raises blood pressure.
Urine facts: 1 to 1.5 L per day
pH about 6.0
urea 25 to 30 g per day; Normal values.
Dialysis: Cellophane tube
Dialysing fluid without wastes
Heparin; Haemodialysis.

ADH	water reabsorption
Aldosterone	Na⁺ reabsorption
Renin	angiotensin pathway
ANF	lowers BP

Regulation of Kidney Function, Micturition and Disorders in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

A hot day and little water

Suppose a person works in the sun without drinking water for several hours.

Explain the hormonal changes and effect on urine.

Try it first, then show the solutionSolution

Body fluid osmolarity rises, stimulating osmoreceptors in the hypothalamus.
ADH release from the posterior pituitary increases, and water reabsorption in the DCT and collecting duct rises.
The urine is small in volume and concentrated; water is conserved and the osmolarity falls back.

Answer: More ADH, more water reabsorption, small volume of concentrated urine.

Memory hook: ADH means water saved.

Tricks to remember Regulation of Kidney Function, Micturition and Disorders

Short memory aids. Say each one aloud once, then use it on a question the same day.

ADH: Anti-Diuretic
ADH stops diuresis by retaining water.
Use it when: Meaning of the name.
Renin, aldosterone raise; ANF lowers
The renin-angiotensin-aldosterone axis raises BP; ANF reduces it.
Use it when: Opposing mechanisms.

Regulation of Kidney Function, Micturition and Disorders: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Hormone for water reabsorption · 1 mark · MCQBoard level

The hormone which increases the reabsorption of water in the distal convoluted tubule is:
(a) aldosterone
(b) ADH
(c) renin
(d) ANF

Try it first, then show the solutionSolution

ADH makes the DCT and collecting duct permeable to water.

Answer: (b) ADH

Example 2: Renin–angiotensin mechanism · 3 marks · Short answerBoard level

Describe how a fall in blood pressure is corrected by the JGA.

Try it first, then show the solutionSolution

A fall in glomerular blood flow or pressure stimulates the JG cells to release renin. (1 mark)
Renin converts angiotensinogen to angiotensin I and then to angiotensin II, which constricts blood vessels and raises BP. (1 mark)
Angiotensin II makes the adrenal cortex release aldosterone, which promotes Na⁺ and water reabsorption. (1 mark)

Answer: Renin → angiotensin II → aldosterone → higher BP.

Example 3: Opposing mechanisms · 4 marks · Short answerHarder board level

Compare the effects of ANF and the renin–angiotensin system. Describe the micturition reflex.

Try it first, then show the solutionSolution

ANF is released from the atrium when BP rises; it causes vasodilation and lowers the BP. (1.3 marks)
The renin–angiotensin–aldosterone system raises BP by vasoconstriction and Na⁺ and water retention. (1.3 marks)
Micturition: stretching of the bladder sends signals to the CNS; the bladder contracts and the sphincter relaxes. (1.4 marks)

Answer: ANF lowers and renin–angiotensin raises BP; micturition is a stretch reflex.

Example 4: Dialysis · 4 marks · Short answerHarder board level

Explain how haemodialysis works. Why is heparin added and why is a dialysing fluid without nitrogenous wastes used?

Try it first, then show the solutionSolution

The patient's blood is pumped through a cellophane tube (semipermeable) surrounded by dialysing fluid. (1 mark)
Wastes such as urea, creatinine and excess salts diffuse out along the concentration gradient. (1 mark)
The dialysing fluid has the same composition as plasma but lacks nitrogenous wastes, so the wastes diffuse out and useful substances are not lost. (1 mark)
Heparin prevents clotting; the cleaned blood is returned to the vein. (1 mark)

Answer: Diffusion of wastes through cellophane into a waste-free fluid; heparin stops clotting.

Example 5: Urine analysis · 5 marks · Long answerHOTS / competency

The urine of a patient contains glucose and ketone bodies. (i) Name the two conditions. (ii) Name the disease. (iii) Why does the glucose appear in the urine? (iv) Name two other excretory organs and a waste each removes. If the urine volume is 1.5 L and urea is 20 g L⁻¹ in a normal person, compute the urea per day.

Try it first, then show the solutionSolution

(i) Glycosuria and ketonuria. (0.75 mark)
(ii) Diabetes mellitus. (0.75 mark)
(iii) The glucose in the filtrate exceeds the reabsorption capacity of the PCT. (1 mark)
(iv) Lungs: CO₂; skin: salts, urea and water in sweat; liver: bile pigments. (1 mark)
Urea per day = 1.5 × 20 = 30 g. (1.5 marks)

Answer: Glycosuria and ketonuria in diabetes mellitus; 30 g of urea per day.

Where students lose marks in Regulation of Kidney Function, Micturition and Disorders

The slips examiners see most often, and what to write instead.

Mistake: Reversing the effects of aldosterone and ANF
Saying ANF raises BP.
Do this: ANF lowers BP; aldosterone raises it indirectly.
Usually costs: 1 to 2 marks.
Mistake: Naming the wrong gland for ADH
Saying ADH is secreted by the adrenal gland.
Do this: ADH is made in the hypothalamus and released by the posterior pituitary.
Usually costs: 1 mark.
Mistake: Not mentioning heparin in dialysis
Leaving out the anticoagulant.
Do this: Heparin prevents clotting in the apparatus.
Usually costs: 1 mark.

Exam tips

Draw a flow chart of the renin–angiotensin mechanism.
Compare the actions of ADH, aldosterone and ANF in a table.
Describe dialysis with a labelled diagram.
Give the normal values of urine.

Practice questions on Regulation of Kidney Function, Micturition and Disorders

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The adrenal hormone that increases Na⁺ reabsorption is:
(a) cortisol
(b) aldosterone
(c) adrenaline
(d) ADH
Hint
It acts on the DCT.
Answer
(b) aldosterone
Q2 · 2 marks · Short answerBoard level
What is diuresis and what causes it?
Hint
Think about ADH.
Answer
An increase in the urine volume, caused when ADH release falls (as after drinking lots of water) or by diuretics such as alcohol.
Q3 · 3 marks · Short answerHarder board level
Explain why the urine of a healthy person does not contain glucose.
Hint
PCT reabsorbs it completely.
Answer
All the filtered glucose is actively reabsorbed in the PCT back into the blood; it appears in the urine only if the blood glucose is very high.
Q4 · 3 marks · Short answerHarder board level
What is uremia? How is it treated?
Hint
Think about kidney failure.
Answer
A condition with a high level of urea in the blood due to kidney failure. It is treated by haemodialysis, and permanently by a kidney transplant.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): ANF decreases blood pressure. Reason (R): It causes dilation of blood vessels. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Vasodilation lowers BP.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a patient with kidney failure undergoes dialysis three times a week.
(i) Name the procedure. (1 mark)
(ii) Name the anticoagulant. (1 mark)
(iii) Name one permanent solution. (1 mark)
(iv) State one reason for matching the donor. (1 mark)
Hint
Recall the procedure.
Answer
(i) Haemodialysis. (ii) Heparin. (iii) A kidney transplant. (iv) To avoid rejection by the immune system.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Source of ADH release?Show answer
Posterior pituitary.
2.Enzyme released by JG cells?Show answer
Renin.
3.Hormone of the atrium?Show answer
ANF.
4.Anticoagulant in dialysis?Show answer
Heparin.

Regulation of Kidney Function, Micturition and Disorders: frequently asked questions

What is the juxtaglomerular apparatus?

A group of specialised cells at the contact of the afferent arteriole and the DCT that senses blood flow and releases renin.

Why does alcohol increase urination?

It inhibits the release of ADH, so less water is reabsorbed.

How do kidney stones form?

Crystallised calcium oxalate and other salts deposit in the pelvis of the kidney forming stones.