Excretory Products and their Elimination ISC Class 11: Kidney and Urine Formation

What Excretory Products and their Elimination is about

Weightage: Excretory Products and their Elimination belongs to the unit Human Physiology, which CISCE weights at 18 of the 70 marks in the ISC Class XI Biology Paper I (Theory). Class XI is examined by the school, so your school sets the paper within this weightage.Source: CISCE ISC Biology syllabus, examination year 2028, Class XI unit weightage

Key concepts

Modes of excretion: ammonotelism, ureotelism and uricotelism

Ammonotelism: excretion of nitrogen mainly as ammonia. Ammonia is highly toxic but very soluble, so it needs a large amount of water to remove; it is excreted as ammonium ions by simple diffusion across body surfaces or gills. Seen in bony fishes, aquatic amphibians (tadpoles) and aquatic insects. Ureotelism: ammonia is converted in the liver into urea, which is less toxic and can be held in the body in moderate concentration, so it needs less water. Seen in mammals, many terrestrial amphibians and marine fishes. Some urea is retained in the kidney medulla of mammals to help keep its osmolarity high. Uricotelism: nitrogen is excreted as uric acid, which is least toxic and almost insoluble, so it can be passed as a paste or pellet with minimum water loss. Seen in reptiles, birds, land snails and insects, and suits animals that lay shelled eggs, where wastes must be stored safely inside the egg.

Worked example

Why do birds excrete uric acid while fishes excrete ammonia?

Try it first, then show the solutionSolution

1. Fishes live in water and can afford to flush highly soluble, toxic ammonia away continuously
2. Birds must save water and weight for flight, and their embryos develop in shelled eggs; insoluble, non-toxic uric acid can be stored and released as a paste

Answer: The choice of waste depends on water availability: plenty for fishes, scarce for birds

Human excretory system and the structure of the kidney (L.S.)

Organs: a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. The kidneys are reddish-brown, bean-shaped organs lying between the levels of the last thoracic and third lumbar vertebrae, close to the dorsal inner wall of the abdominal cavity. Each is about 10–12 cm long, 5–7 cm wide, 2–3 cm thick and weighs about 120–170 g. On the inner concave side is a notch, the hilum, through which the ureter, blood vessels and nerves enter or leave. Inside the hilum is a broad, funnel-shaped space, the renal pelvis, with projections called calyces. Longitudinal section: the kidney is covered by a tough capsule. Two zones can be seen inside it: the cortex on the outside and the medulla within. The medulla is made of several cone-shaped medullary pyramids, which project into the calyces. The cortex extends in between the pyramids as renal columns (columns of Bertini). Each kidney has about one million nephrons, the functional units. Urine from the pelvis passes down the ureter to the bladder, where it is stored, and leaves through the urethra.

Worked example

In an L.S. of a kidney, a student labels the dark triangular areas as cortex. Correct the mistake.

Try it first, then show the solutionSolution

1. The triangular masses projecting into the calyces are the medullary pyramids, part of the medulla
2. The cortex is the outer zone, extending between the pyramids as renal columns

Answer: They are medullary pyramids; cortex is the outer layer and the renal columns

Structure of a nephron

Each nephron has two parts: the glomerulus and the renal tubule. The glomerulus is a knot of capillaries supplied by the afferent arteriole, a small branch arising from the renal artery; blood leaves it through the efferent arteriole, which is narrower, creating high pressure in the glomerulus. The renal tubule begins as a double-walled cup, Bowman's capsule, which encloses the glomerulus; together they form the Malpighian body (renal corpuscle). The tubule continues as the highly coiled proximal convoluted tubule (PCT), then a hairpin-shaped Henle's loop with a descending and an ascending limb, then the highly coiled distal convoluted tubule (DCT). The DCTs of many nephrons open into a straight collecting duct; collecting ducts converge and open into the renal pelvis through the medullary pyramids. Malpighian bodies, PCT and DCT lie in the cortex; Henle's loop dips into the medulla. In most nephrons (cortical nephrons) the loop is short and dips only a little; in juxtamedullary nephrons the loop is long and runs deep into the medulla. The efferent arteriole forms a network of peritubular capillaries around the tubule; a fine branch of it runs parallel to Henle's loop as a U-shaped vasa recta, which is absent or highly reduced in cortical nephrons.

Worked example

Why is the efferent arteriole narrower than the afferent arteriole?

Try it first, then show the solutionSolution

1. Blood enters the glomerulus faster than it can leave through the narrow efferent vessel
2. This raises blood pressure in the glomerular capillaries, which drives ultrafiltration

Answer: To build up the high glomerular pressure needed for filtration

Urine formation: ultrafiltration, selective reabsorption, tubular secretion

(1) Glomerular filtration (ultrafiltration): blood pressure in the glomerulus forces fluid through three layers: the endothelium of the glomerular capillaries, the epithelium of Bowman's capsule (cells called podocytes, with tiny filtration slits) and the basement membrane between them. Almost all constituents of plasma except proteins (and cells) pass into the capsule; the filtrate is like plasma without proteins. The volume filtered per minute is the glomerular filtration rate (GFR): about 125 mL per minute, i.e. about 180 L per day. (2) Selective reabsorption: only about 1.5 L of urine is produced daily, so about 99% of the filtrate is reabsorbed by the tubular cells, actively (e.g. glucose, amino acids, Na⁺) or passively (e.g. nitrogenous wastes partly, water). PCT (lined by cuboidal epithelium with a brush border of microvilli): reabsorbs nearly all nutrients (glucose, amino acids) and 70–80% of electrolytes and water; it also secretes H⁺ and ammonia and reabsorbs HCO₃⁻, helping keep pH and ionic balance. Henle's loop: the descending limb is permeable to water but almost impermeable to electrolytes, so the filtrate becomes more concentrated as it moves down; the ascending limb is impermeable to water but transports electrolytes out, so the filtrate becomes more dilute as it moves up. The loop does little net reabsorption but keeps the medullary interstitium highly concentrated. DCT: conditional reabsorption of Na⁺ and water (under hormonal control) and of HCO₃⁻; secretes H⁺, K⁺ and NH₃. Collecting duct: can reabsorb large amounts of water to make concentrated urine; lets a little urea pass into the medullary interstitium to keep its osmolarity high; secretes H⁺ and K⁺. (3) Tubular secretion: tubular cells actively add H⁺, K⁺ and ammonia to the filtrate, helping maintain the ionic and acid–base balance of body fluids.

Worked example

If GFR is 125 mL per minute and a person passes 1.5 L of urine a day, what percentage of the filtrate is reabsorbed?

Try it first, then show the solutionSolution

1. Filtrate per day = 125 × 60 × 24 = 180 000 mL = 180 L
2. Reabsorbed = (180 − 1.5) ÷ 180 × 100 ≈ 99.2%

Answer: About 99% of the filtrate is reabsorbed

Counter current system and concentration of urine

Mammals can produce concentrated urine, mainly because of Henle's loop and vasa recta. In each, fluid flows in opposite directions in the two limbs, forming a counter current. The close arrangement of the loop and the vasa recta and the counter current flow maintain an increasing osmolarity in the medullary interstitium, from about 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla. The gradient is produced mainly by NaCl and urea. (1) NaCl pumped out of the ascending limb of Henle's loop is exchanged with the descending limb of the vasa recta, and NaCl is returned to the interstitium by the ascending portion of the vasa recta. (2) Small amounts of urea enter the thin ascending limb and are carried back to the interstitium by the collecting tubule. This special arrangement is the counter current mechanism. Because the interstitium around the collecting duct is so concentrated, water is drawn out of the duct (when ADH makes it permeable), and the urine can become up to about four times as concentrated as the original filtrate (1200 ÷ 300).

Worked example

Why can a desert rat, whose nephrons have very long loops of Henle, produce extremely concentrated urine?

Try it first, then show the solutionSolution

1. Longer loops and vasa recta build a steeper osmolarity gradient in the medulla
2. Collecting ducts passing through this very concentrated region lose more water by osmosis, concentrating the urine

Answer: Long loops create a higher medullary concentration, which pulls more water from the collecting ducts

Regulation of kidney function: ADH, renin–angiotensin–aldosterone, ANF, erythropoietin

Kidney function is monitored and regulated by hormonal feedback involving the hypothalamus, the juxtaglomerular apparatus (JGA) and, to some extent, the heart. ADH: osmoreceptors respond to changes in blood volume, body fluid volume and ion concentration. When much fluid is lost (e.g. sweating, dehydration), they stimulate the hypothalamus to release antidiuretic hormone (ADH, vasopressin) from the posterior pituitary (neurohypophysis). ADH increases water reabsorption from the DCT and collecting duct, preventing excess water loss (diuresis). When body fluid volume rises, the osmoreceptors switch off and ADH release is suppressed, so more dilute urine is passed. ADH also constricts blood vessels, raising blood pressure and GFR. Renin–angiotensin–aldosterone system (RAAS): the JGA is a special region where the DCT touches the afferent arteriole. A fall in glomerular blood flow, glomerular blood pressure or GFR stimulates the JG cells, which secrete the enzyme renin. Renin acts on a plasma protein, angiotensinogen, producing angiotensin I, which is then changed into angiotensin II. Angiotensin II is a powerful vasoconstrictor, raising glomerular blood pressure and GFR; it also stimulates the adrenal cortex to release aldosterone, which increases reabsorption of Na⁺ and water from the distal parts of the tubule, again raising blood pressure and GFR. Atrial natriuretic factor (ANF): when blood flow to the atria of the heart increases (high blood volume or pressure), the atrial walls release ANF, which causes vasodilation and lowers blood pressure; it acts as a check on the renin–angiotensin mechanism. Erythropoietin: the JG cells of the kidney release erythropoietin, a hormone that stimulates formation of red blood cells (erythropoiesis) in the bone marrow, especially when blood O₂ is low.

Worked example

A person loses a lot of blood in an accident. How does the kidney respond to restore blood pressure?

Try it first, then show the solutionSolution

1. Lower blood pressure reduces glomerular blood flow and GFR, so JG cells release renin
2. Renin leads to angiotensin II (vasoconstriction) and aldosterone (Na⁺ and water reabsorption), while low volume also triggers ADH; together these raise blood volume and pressure

Answer: Through the renin–angiotensin–aldosterone system, supported by ADH

Micturition, role of other organs and homeostasis

Micturition: urine made by the nephrons is carried by the ureters to the urinary bladder and stored there until a voluntary signal is given by the central nervous system. As the bladder fills and stretches, stretch receptors in its wall send signals to the CNS; the CNS sends motor messages that make the smooth muscle of the bladder contract and the urethral sphincter relax at the same time, releasing urine. This neural mechanism is the micturition reflex. An adult passes about 1–1.5 L of light yellow, slightly acidic (pH about 6.0) urine per day, containing about 25–30 g of urea. Glucose (glycosuria) or ketone bodies (ketonuria) in urine indicate diabetes mellitus. Role of other organs: lungs remove large amounts of CO₂ (about 200 mL per minute) and water vapour; the liver makes urea from ammonia and secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which pass out with faeces; the skin's sweat glands remove water, NaCl, small amounts of urea and lactic acid (sweat mainly cools the body), and sebaceous glands remove sterols, hydrocarbons and waxes in sebum, which forms a protective oily layer. Small amounts of nitrogenous waste also leave in saliva. Homeostasis: by regulating water, ions, pH and waste levels, the kidneys and these organs keep the internal environment stable despite changes outside.

Worked example

Why does a person feel the urge to pass urine more often on a cold day when sweating is low?

Try it first, then show the solutionSolution

1. Less water is lost as sweat, so body fluid volume rises
2. Osmoreceptors reduce ADH release, less water is reabsorbed, and more dilute urine fills the bladder faster, triggering the micturition reflex

Answer: Low ADH leads to more urine, which stretches the bladder sooner

Disorders, dialysis and kidney transplant

Uraemia: accumulation of urea (and other wastes) in the blood because the kidneys are not working properly; it is very harmful and may lead to kidney failure. Renal failure: the kidneys can no longer filter blood adequately; it may be acute (sudden) or chronic (gradual), and wastes and excess fluid accumulate. Renal calculi (kidney stones): stones or insoluble masses of crystallised salts (such as calcium oxalate) formed within the kidney; they can block urine flow and cause severe pain. Nephritis (glomerulonephritis): inflammation of the glomeruli of the kidney, reducing filtration and sometimes letting proteins and blood into urine. Haemodialysis (artificial kidney): blood is drawn from a convenient artery, mixed with an anticoagulant such as heparin, and pumped through a dialysing unit: a coiled cellophane tube surrounded by dialysing fluid that has the same composition as plasma except for nitrogenous wastes. The porous cellophane lets molecules pass according to their concentration gradient, so urea and other wastes diffuse out of the blood into the fluid while useful substances stay. The cleaned blood, after adding anti-heparin, is returned to the body through a vein. Kidney transplant: the ultimate treatment for acute renal failure; a functioning kidney from a donor, preferably a close relative to reduce the chance of rejection by the immune system, is placed in the patient, who then takes immunosuppressant drugs.

Worked example

Why must dialysing fluid contain glucose and salts at the same concentration as normal plasma?

Try it first, then show the solutionSolution

1. Substances move across the cellophane according to their concentration gradient
2. If glucose and salts were absent, they would diffuse out of the blood; matching their levels keeps them in, while urea (absent from the fluid) diffuses out

Answer: So that only wastes leave the blood and useful substances are retained

Excretory Products and their Elimination in real life

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

Topics in this chapter

• Nephron Structure and Urine Formation
• Regulation of Kidney Function, Micturition and Disorders

Excretory Products and their Elimination: key facts

Glomerular filtration

$GFR\approx 125{\text{mL min}}^{-1}=180{\text{L day}}^{-1};\text{urine}\approx 1.5{\text{L day}}^{-1}$

So about 99% of filtrate is reabsorbed.

Percentage reabsorbed

$\%\text{reabsorbed}=\frac{\text{filtrate}-\text{urine}}{\text{filtrate}}\times 100$

Use the same units for filtrate and urine.

Medullary gradient

$300{\text{mOsmol L}}^{-1}(\text{cortex})\to 1200{\text{mOsmol L}}^{-1}(\text{inner medulla});\frac{1200}{300}=4$

Urine up to about four times as concentrated as the filtrate.

Renin–angiotensin–aldosterone

$\text{angiotensinogen}\stackrel{\text{renin}}{\to }\text{angiotensin I}\to \text{angiotensin II}\to \text{aldosterone}$

Raises blood pressure and GFR; ANF opposes it.

Path of urine

$\text{glomerulus}\to \text{Bowman’s capsule}\to PCT\to \text{Henle’s loop}\to DCT\to \text{collecting duct}\to \text{pelvis}\to \text{ureter}\to \text{bladder}\to \text{urethra}$

Learn as one chain.

Excretory Products and their Elimination: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Excretory Products and their Elimination

The question patterns to expect, and the method that works for each.

Tricks to remember Excretory Products and their Elimination

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

• A-U-U: Aqua, Usual, Utmost dry

  Ammonia for aquatic animals, Urea for usual land mammals, Uric acid for the driest (birds, reptiles, insects).

  Use it when: Modes of excretion.

• Every Bowman Prefers Hot Dal Curry

  Efferent/afferent glomerulus, Bowman's capsule, PCT, Henle's loop, DCT, Collecting duct.

  Use it when: Parts of the nephron in order.

• Down for water, up for salt

  Descending limb lets water out; ascending limb pumps salt out.

  Use it when: Henle's loop.

• Low pressure, Renin rises

  Fall in GFR → renin → angiotensin II → aldosterone → BP up. ANF from the atria brings it down.

  Use it when: Regulation.

• Ultra, Select, Secrete

  Three steps of urine formation in order.

  Use it when: Urine formation.

Excretory Products and their Elimination: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section D style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Excretory Products and their Elimination

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

• ✗Mistake: Saying filtration removes only wastes

  Filtration is thought to be selective.

  Wrong working

  The glomerulus filters out only urea and salts.

  Right working

  Everything in plasma except proteins is filtered; useful substances are then reabsorbed.

  ✓Do this: Filtration is non-selective; reabsorption is selective.

  Usually costs: 1 mark.

• ✗Mistake: Reversing the permeability of Henle's loop

  The two limbs are confused.

  Wrong working

  The ascending limb is permeable to water.

  Right working

  Descending limb: permeable to water; ascending limb: impermeable to water but transports electrolytes.

  ✓Do this: Down for water, up for salt.

  Usually costs: 1 mark.

• ✗Mistake: Saying ADH comes from the hypothalamus directly into blood

  Source and release site are mixed up.

  Wrong working

  ADH is released by the anterior pituitary.

  Right working

  ADH is made in the hypothalamus and released from the posterior pituitary (neurohypophysis).

  ✓Do this: ADH: hypothalamus makes, posterior pituitary releases.

  Usually costs: 1 mark.

• ✗Mistake: Mixing up ANF and RAAS effects

  Both deal with blood pressure.

  Wrong working

  ANF raises blood pressure.

  Right working

  ANF causes vasodilation and lowers blood pressure, acting as a check on RAAS, which raises it.

  ✓Do this: Atria say 'Never Forget' to lower pressure.

  Usually costs: 1 mark.

• ✗Mistake: Labelling the pyramids as cortex

  Contrast in diagrams misleads.

  Wrong working

  Pyramids labelled cortex in the L.S.

  Right working

  Pyramids are medulla; cortex is outside and between them as columns of Bertini.

  ✓Do this: Columns of cortex, pyramids of medulla.

  Usually costs: 1-2 marks in a diagram.

Practice questions on Excretory Products and their Elimination

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

1. Q1 · 1 mark · MCQExam level

   The functional unit of the kidney is the:
   (a) neuron
   (b) nephron
   (c) glomerulus
   (d) ureter

   Hint

   About a million per kidney.

   Answer

   (b)

2. Q2 · 1 mark · MCQExam level

   The hormone released by the atrial walls that lowers blood pressure is:
   (a) ADH
   (b) renin
   (c) ANF
   (d) aldosterone

   Hint

   Natriuretic.

   Answer

   (c)

3. Q3 · 1 mark · Very short answerExam level

   Name the anticoagulant used during haemodialysis.

   Hint

   Also from basophils.

   Answer

   Heparin

4. Q4 · 1 mark · Very short answerExam level

   What are renal calculi?

   Hint

   Stones.

   Answer

   Stones or insoluble masses of crystallised salts (such as oxalates) formed within the kidney.

5. Q5 · 2 marks · Short answerExam level

   State two functions of skin in excretion.

   Hint

   Two glands.

   Answer

   Sweat glands remove water, NaCl, small amounts of urea and lactic acid; sebaceous glands remove sterols, hydrocarbons and waxes through sebum.

6. Q6 · 2 marks · Short answerHarder exam level

   Distinguish between cortical and juxtamedullary nephrons.

   Hint

   Loop length.

   Answer

   Cortical nephrons (the majority) have short loops of Henle that dip only slightly into the medulla, with little or no vasa recta. Juxtamedullary nephrons have long loops running deep into the medulla with a well-developed vasa recta; they help concentrate urine.

7. Q7 · 2 marks · Short answerHarder exam level

   What is erythropoietin? Where is it made and what does it do?

   Hint

   RBCs.

   Answer

   A hormone released by the juxtaglomerular cells of the kidney that stimulates the formation of red blood cells in the bone marrow, especially when blood oxygen is low.

8. Q8 · 3 marks · Short answerHarder exam level

   Explain the micturition reflex.

   Hint

   Stretch receptors.

   Answer

   As the bladder fills, stretch receptors in its wall send signals to the CNS. The CNS sends motor messages that contract the smooth muscle of the bladder and relax the urethral sphincter at the same time, releasing urine. Release is under voluntary control in adults.

9. Q9 · 3 marks · Short answerHarder exam level

   Describe the role of the liver and lungs in excretion.

   Hint

   Urea, bile; CO₂.

   Answer

   Liver: converts ammonia to urea; excretes bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs in bile, which leave with faeces. Lungs: remove large amounts of CO₂ (about 200 mL per minute) and some water vapour.

10. Q10 · 3 marks · Short answerHarder exam level

    Distinguish between uraemia, nephritis and renal failure.

    Hint

    Blood, glomeruli, kidney as a whole.

    Answer

    Uraemia: accumulation of urea in blood due to poor kidney function. Nephritis: inflammation of the glomeruli, reducing filtration. Renal failure: kidneys unable to filter blood adequately, acute or chronic, leading to accumulation of wastes and fluid.

11. Q11 · 5 marks · Long answerHarder exam level

    (a) Draw a labelled diagram of a nephron with its blood supply. [3]
    (b) State the function of the vasa recta and of the collecting duct. [2]

    Hint

    Counter current.

    Answer

    (a) Afferent and efferent arterioles, glomerulus, Bowman's capsule, PCT, descending and ascending limbs, DCT, collecting duct, peritubular capillaries, vasa recta. (b) Vasa recta: maintains the medullary osmolarity gradient by counter current exchange of NaCl. Collecting duct: reabsorbs large amounts of water (under ADH) to concentrate urine; lets urea into the medulla; secretes H⁺ and K⁺.

12. Q12 · 5 marks · Long answerHarder exam level

    (a) Explain how ADH and the renin–angiotensin–aldosterone system regulate kidney function. [4]
    (b) How does ANF act as a check on this system? [1]

    Hint

    Osmoreceptors and JGA.

    Answer

    (a) Water loss is sensed by osmoreceptors → hypothalamus → ADH from posterior pituitary → more water reabsorbed from DCT and collecting duct; less urine; ADH also constricts vessels. Fall in GFR/BP → JG cells release renin → angiotensinogen → angiotensin I → angiotensin II (vasoconstriction, raises GFR) → adrenal cortex releases aldosterone → Na⁺ and water reabsorption → blood pressure and GFR rise. (b) Increased blood flow to the atria releases ANF, which causes vasodilation and lowers blood pressure, opposing RAAS.

13. Q13 · 2 marks · Short answerHarder exam level

    If GFR falls from 125 to 50 mL per minute, how many litres of plasma are filtered per day, and why might wastes build up?

    Hint

    × 1440.

    Answer

    50 × 1440 = 72 000 mL = 72 L per day (instead of 180 L). Less plasma is filtered, so urea and other wastes are not removed fast enough and accumulate in blood.

14. Q14 · 3 marks · Short answerHOTS / competency

    A kidney stone completely blocks the left ureter. Predict what happens to (a) the pressure inside the Bowman's capsules of the left kidney, (b) filtration in that kidney, and (c) the patient's blood urea if the right kidney is healthy. Give reasons.

    Hint

    Filtration depends on the pressure difference between the glomerulus and the capsule.

    Answer

    (a) Urine cannot drain, so it backs up through the pelvis, collecting ducts and tubules, and the fluid pressure in the Bowman's capsules rises. (b) Filtration is driven by glomerular blood pressure pushing fluid into the capsule; the raised capsular pressure opposes it, so GFR in the left kidney falls sharply and the kidney may be damaged if the block persists. (c) Blood urea stays near normal, because the healthy right kidney can filter the whole blood supply and remove urea; uraemia develops only when both kidneys fail.

15. Q15 · 3 marks · Short answerHOTS / competency

    Alcohol inhibits ADH release. Predict the effect of drinking alcohol on urine volume and on body water balance.

    Hint

    Diuresis.

    Answer

    With less ADH, the DCT and collecting duct reabsorb less water, so a large volume of dilute urine is produced (diuresis). The body loses more water than usual and can become dehydrated, causing thirst.

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. 1.Waste of uricotelic animals?Show answer

   Uric acid.

2. 2.Notch on the kidney?Show answer

   Hilum.

3. 3.Cortex between pyramids?Show answer

   Columns of Bertini.

4. 4.Normal GFR?Show answer

   About 125 mL per minute (180 L per day).

5. 5.Main site of reabsorption?Show answer

   PCT.

6. 6.Inner medulla osmolarity?Show answer

   About 1200 mOsmol per litre.

7. 7.Enzyme released by JG cells?Show answer

   Renin.

8. 8.Normal urine pH?Show answer

   About 6.0 (slightly acidic).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you compare ammonia, urea and uric acid?
• ☐Can you label an L.S. of the kidney?
• ☐Can you name every part of a nephron in order?
• ☐Can you name the three filtration layers?
• ☐Can you state what each part of the tubule reabsorbs or secretes?
• ☐Can you explain the counter current mechanism with numbers?
• ☐Can you trace the renin–angiotensin–aldosterone pathway?
• ☐Can you describe haemodialysis?

Excretory Products and their Elimination: frequently asked questions