Excretory products and their elimination | ISC Class 11 Biology Notes

This note covers nitrogenous wastes and modes of excretion, kidney and nephron structure, urine formation, tubular functions, the counter current mechanism, osmoregulation, hormonal control, micturition, other excretory organs, kidney disorders, haemodialysis and transplantation.

What are excretory products, and how do animals eliminate nitrogenous wastes?

Excretion removes metabolic wastes and excess substances from the body. Metabolism means the chemical reactions occurring in living cells. Animals accumulate nitrogenous wastes, carbon dioxide, water and ions, electrically charged particles, through metabolism or other means, such as excess ingestion.

Nitrogenous wastes are waste substances containing nitrogen. Ammonia, urea and uric acid are their major forms. Their elimination is closely related to water availability: ammonia requires much water, whereas uric acid can be eliminated with minimum water loss.

How do ammonotelism, ureotelism and uricotelism differ?

Animals showing these modes are called ammonotelic, ureotelic and uricotelic, respectively. Readily soluble ammonia is generally eliminated by diffusion, the movement of a substance down its concentration gradient. Ammonium ions are the positively charged form in which ammonia is generally removed across these surfaces.

Kidneys do not play any significant role in ammonia removal in ammonotelic animals. In ureotelic animals, the liver converts ammonia produced by metabolism into urea. Blood carries this urea to the kidneys for elimination.

Some amount of urea may remain in the kidney matrix of some ureotelic animals to maintain a desired osmolarity, the concentration of dissolved particles that influence water movement. Thus, a waste substance can also contribute to maintaining the conditions needed for water balance.

Note: The words “many” and “mainly” matter. Many bony fishes are ammonotelic, while marine fishes are included among animals that mainly excrete urea. A mode of excretion should not be assigned to every fish without qualification.

How are the human urinary system and kidneys organised?

The human urinary system consists of two kidneys, two ureters, a urinary bladder and a urethra. The kidneys form urine; the ureters carry it to the urinary bladder, which stores it. The urethra provides the passage through which urine is released.

What is the external structure of a kidney?

Kidneys are reddish brown and bean shaped. They lie close to the inner back wall of the abdominal cavity, between the levels of the last thoracic vertebra and the third lumbar vertebra, the spinal bones of the chest and lower back regions.

Each adult kidney measures 10 to 12 centimetres in length, 5 to 7 centimetres in width and 2 to 3 centimetres in thickness. Its average weight is 120 to 170 grams. A tough outer covering forms the renal capsule; “renal” means relating to the kidney.

The hilum is the notch towards the centre of the kidney’s inner concave surface. It is associated with the passage of the ureter, blood vessels and nerves. Inside it lies the renal pelvis, a broad, funnel-shaped space with projections called calyces; one projection is a calyx.

What does a longitudinal section reveal?

A longitudinal section is a cut along the length of an organ. It reveals an outer cortex and an inner medulla. The medulla forms conical masses called medullary pyramids, which project into the calyces.

The cortex extends between these pyramids as renal columns, also called Columns of Bertini. Keep the regions distinct: the cortex and medulla are kidney tissue, while the renal pelvis is the funnel-shaped collecting space.

The drawing shows paired kidneys with ureters descending to the urinary bladder and a urethra leaving it. One kidney is opened to show its cortex, medulla and pelvis. Renal blood vessels are also labelled.

See Fig. 16.1 in your NCERT textbook

The section labels the renal capsule, cortex, renal column, medullary pyramid, calyx and renal pelvis. The renal artery, renal vein and ureter are shown at the inner side of the kidney.

See Fig. 16.2 in your NCERT textbook

How does a nephron connect filtration with tubular processing?

A nephron is the functional unit of the kidney. Each kidney contains nearly one million of these complex tubular structures. Each nephron has a glomerulus, a tuft of tiny blood vessels called capillaries, and a renal tubule, the tube that processes the filtered fluid.

The afferent arteriole, a small branch of the renal artery, brings blood into the glomerulus. The efferent arteriole carries blood away. The renal artery is the artery supplying the kidney; an arteriole is a small artery.

What is the route through the tubule?

1. Bowman’s capsule is the double-walled, cup-like beginning of the renal tubule. It surrounds the glomerulus. Together, the glomerulus and capsule form the renal corpuscle, also called the Malpighian body.
2. The capsule continues into the proximal convoluted tubule (PCT), the first highly coiled portion of the tubule.
3. The next region is Henle’s loop, a hairpin-shaped segment with a descending limb carrying fluid downwards and an ascending limb carrying it upwards.
4. The ascending limb continues into the distal convoluted tubule (DCT), another highly coiled region.
5. DCTs of many nephrons open into a collecting duct, a straight tube receiving their fluid. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.

How do cortical and juxtamedullary nephrons differ?

The renal corpuscle, PCT and DCT lie in the cortex, while Henle’s loop dips into the medulla. In the majority of nephrons, the loop is short and extends only very little into the medulla. These are cortical nephrons.

Some nephrons have very long loops extending deep into the medulla; these are juxtamedullary nephrons. The efferent arteriole forms peritubular capillaries, a fine blood-vessel network around the tubule. A minute vessel running parallel to Henle’s loop forms the U-shaped vasa recta.

The vasa recta is absent or highly reduced in cortical nephrons. Distinguish the two pathways: blood flows through the vessels, whereas filtrate, the fluid produced by filtration, passes through the capsule and tubule.

The drawing labels the glomerulus, Bowman’s capsule, afferent and efferent arterioles, proximal and distal convoluted tubules, both limbs of Henle’s loop, vasa recta and collecting duct. Blood vessels surround the tubular pathway.

See Fig. 16.3 in your NCERT textbook

How do filtration, reabsorption and secretion form urine?

Urine formation involves glomerular filtration, reabsorption and tubular secretion. Filtration moves fluid from glomerular blood into Bowman’s capsule. Reabsorption recovers substances from the filtrate. Secretion adds substances from tubular cells to the filtrate, helping control body-fluid composition.

Why is glomerular filtration called ultrafiltration?

On average, the kidneys filter 1,100 to 1,200 millilitres of blood per minute, roughly one-fifth of the blood pumped by each ventricle per minute. A ventricle is a lower pumping chamber of the heart. Glomerular capillary blood pressure drives filtration.

The filtration barrier has three layers: the endothelium, or inner lining of glomerular blood vessels; a supporting basement membrane; and the epithelium, or cellular lining, of Bowman’s capsule. The capsule’s specialised epithelial cells are called podocytes.

Podocytes leave minute gaps called filtration slits or slit pores. Filtration is so fine that almost all plasma constituents except proteins, molecules made of building units called amino acids, enter the capsule’s lumen, its inner space. Plasma is the fluid portion of blood. This process is therefore called ultrafiltration.

How much filtrate is recovered?

Glomerular filtration rate (GFR) is the volume of filtrate formed by the kidneys per minute. In a healthy individual it is approximately 125 millilitres per minute, or 180 litres per day. This is filtrate volume, not final urine volume.

Comparing 180 litres of daily filtrate with 1.5 litres of urine shows that nearly 99 per cent of filtrate is reabsorbed. Different tubule segments use active transport, which requires energy, or passive transport, which does not directly require metabolic energy.

Glucose, a simple sugar, amino acids, the building units of proteins, and sodium ions, written Na⁺, are examples of substances actively reabsorbed. Water is passively reabsorbed in the initial nephron segments. Nitrogenous wastes are absorbed by passive transport. The plus sign denotes a positive electrical charge on an ion, a charged particle.

During tubular secretion, cells add hydrogen ions (H⁺), potassium ions (K⁺) and ammonia to the filtrate. Active tubular secretion and selective reabsorption help maintain ionic balance, the appropriate proportions of ions, and acid-base balance, regulation of acidity and alkalinity.

Note: Blood flow, filtrate formation and urine output are different quantities. The figure of approximately 125 millilitres per minute describes filtrate formation; most of that fluid is subsequently reabsorbed.

How do the different tubular regions modify the filtrate?

Tubular regions differ in their permeability, meaning how readily substances can pass through their walls. They also differ in the substances they reabsorb or secrete. Consequently, the composition and concentration of filtrate change as it moves along the nephron.

Electrolytes are substances that form ions in solution. Essential nutrients are substances needed for the body’s functioning. Their recovery helps retain useful components that entered the filtrate.

Which functions belong to each segment?

The PCT has simple cuboidal brush-border epithelium: a single layer of cube-shaped lining cells with a surface border that increases the area available for reabsorption.

The PCT selectively secretes hydrogen ions and ammonia, and reabsorbs bicarbonate ions (HCO₃⁻). The minus sign denotes a negative electrical charge. These movements help regulate pH, the measure of acidity or alkalinity, as well as the ionic balance of body fluids.

How do the later regions adjust the fluid?

The DCT can also reabsorb bicarbonate and selectively secrete hydrogen ions, potassium ions and ammonia. Its sodium and water reabsorption is conditional, meaning that it is adjusted according to the body’s requirements rather than remaining fixed.

The collecting duct extends from the cortex into the inner medulla. Along with water and urea movements, selective secretion of hydrogen and potassium ions helps it maintain blood pH and ionic balance.

Henle’s loop has a different central role: it helps maintain high osmolarity in the medullary interstitial fluid, the fluid in the spaces between structures of the medulla. The descending limb concentrates filtrate through water loss; the ascending limb dilutes it through electrolyte loss.

How does the counter current mechanism conserve water?

A counter current consists of flows in opposite directions in adjacent limbs. Filtrate moves in opposite directions in the descending and ascending limbs of Henle’s loop. Blood also flows in opposite directions through the two limbs of the vasa recta.

The close arrangement of Henle’s loop and vasa recta helps maintain an increasing osmolarity towards the inner medulla. A concentration gradient is a difference in concentration between regions. Here the gradient rises from 300 to about 1,200 milliosmoles per litre.

A milliosmole per litre, written mOsmol L⁻¹, represents one-thousandth of an osmole of dissolved particles per litre; an osmole measures the amount of osmotically active particles, which influence water movement. It is a unit of osmolarity; L⁻¹ means “per litre”. The value of 300 is associated with the cortex and about 1,200 with the inner medulla. The gradient is mainly produced by sodium chloride and urea.

How is the medullary gradient maintained and used?

1. Sodium chloride (NaCl), a salt composed of sodium and chloride ions, is transported from the ascending limb of Henle’s loop and exchanged with the descending limb of the vasa recta.
2. The ascending portion of the vasa recta returns sodium chloride to the interstitium, the tissue spaces between the kidney’s tubular and vascular structures.
3. Small amounts of urea enter the thin segment of Henle’s ascending limb. Urea is transported back into the interstitium by the collecting tubule.
4. The arrangement and transport together maintain the medullary concentration gradient. This is the counter current mechanism.
5. The gradient allows water to pass readily from the collecting tubule into the surrounding region. Removal of water concentrates the remaining filtrate, producing concentrated urine.

Human kidneys can produce urine nearly four times as concentrated as the initial filtrate. This ability conserves water while allowing nitrogenous wastes to be eliminated. The opposite flows and the transport of sodium chloride and urea are both essential parts of the explanation.

The nephron and vasa recta are drawn alongside one another, with arrows showing flow and substance movement. The surrounding levels are marked 300, 600, 900 and 1,200 milliosmoles per litre, from the cortex towards the inner medulla.

See Fig. 16.6 in your NCERT textbook

How do homeostasis and antidiuretic hormone regulate water balance?

Definition: Homeostasis is the maintenance of a relatively stable internal environment despite changes within or outside the body. Osmoregulation is the regulation of body-fluid volume and ionic concentration.

Kidneys contribute to homeostasis by eliminating wastes while adjusting water recovery, ion movements and acid-base balance. Osmoregulation is one aspect of this wider regulation. Producing urine therefore involves controlling what is retained as well as what leaves the body.

What is the source and effect of antidiuretic hormone?

Hormones are non-nutrient chemicals produced in small amounts that act as messengers between cells. Antidiuretic hormone (ADH), also called vasopressin, is synthesised by the hypothalamus, a region of the forebrain. It is stored and released by the neurohypophysis, or posterior pituitary.

Osmoreceptors are receptors activated by changes in blood volume, body-fluid volume and ionic concentration. Excessive fluid loss can activate them. They stimulate the hypothalamus, leading to ADH release from the neurohypophysis.

1. Excessive loss of body fluid can activate osmoreceptors that monitor changes in body fluids.
2. The resulting signals stimulate the hypothalamus and the release of ADH from the posterior pituitary.
3. ADH facilitates water reabsorption from the later parts of the tubule, reducing water loss through urine, called diuresis.
4. An increase in body-fluid volume can switch off the osmoreceptors and suppress further ADH release, completing the feedback response.

Feedback means that the outcome of a response influences the signals controlling that response. Here, improved fluid volume can reduce the stimulus for additional ADH release. This links water conservation to the condition of the body fluids.

ADH can also affect kidney function by narrowing blood vessels. This can raise blood pressure; an increase in pressure can increase glomerular blood flow and GFR. Its water-conserving effect and its effect on blood vessels should be distinguished.

How do renin, atrial natriuretic factor and erythropoietin act?

The juxtaglomerular apparatus (JGA) is a sensitive region formed by modified cells of the DCT and afferent arteriole where they contact one another. Juxtaglomerular cells, abbreviated JG cells, can release renin, which initiates the renin-angiotensin regulatory mechanism.

What happens when glomerular flow or pressure falls?

1. A fall in glomerular blood flow, glomerular blood pressure or GFR can activate JG cells to release renin.
2. Renin initiates conversion of angiotensinogen, a precursor in blood, into angiotensin I, an intermediate, and subsequently angiotensin II, the active regulator in this pathway.
3. Angiotensin II is a powerful vasoconstrictor, a substance that narrows blood vessels. It increases glomerular blood pressure and thereby GFR.
4. Angiotensin II also stimulates the adrenal cortex, the outer part of the adrenal gland above the kidney, to release aldosterone, a hormone regulating salt and water balance.
5. Aldosterone promotes sodium and water reabsorption from distal parts of the tubule. This also increases blood pressure and GFR, contributing to restoration of filtration.

This sequence is the renin-angiotensin-aldosterone system. Its linked effects concern both blood vessels and tubular reabsorption. A complete account should connect the initial fall in flow or pressure to the responses that increase pressure and filtration.

How does the heart provide a check?

Atrial natriuretic factor (ANF) is a hormone released from the heart’s atrial walls. The atria are the upper chambers of the heart. Increased blood flow to the atria can cause ANF release.

ANF can cause vasodilation, widening of blood vessels, thereby decreasing blood pressure. Its mechanism therefore checks the renin-angiotensin mechanism. It should not be described as another pressure-raising step in the same sequence.

What is the role of erythropoietin?

Erythropoietin is a peptide hormone produced by juxtaglomerular cells of the kidney. A peptide consists of linked amino acids. Erythropoietin stimulates erythropoiesis, the formation of red blood cells. This gives the kidney a hormonal role in addition to its roles in urine formation and fluid regulation.

How is urine released, and what are its normal characteristics?

Micturition is the release of urine. Urine formed by nephrons reaches the urinary bladder and remains stored until a voluntary signal is given by the central nervous system (CNS), which consists of the brain and spinal cord.

What is the micturition reflex?

1. Urine accumulates in the bladder and stretches its wall.
2. Stretch receptors, receptors responding to stretching in the bladder wall, send signals to the CNS.
3. The CNS sends motor messages, nerve signals controlling muscle action, to the urinary structures.
4. The bladder’s smooth muscles contract while the urethral sphincter, the muscle controlling the outlet, relaxes simultaneously.
5. Urine is released. The neural mechanism producing this response is called the micturition reflex.

The reflex and voluntary control belong in the same explanation. Stretching supplies the initial signal, while coordinated muscle contraction and sphincter relaxation allow release. The bladder stores urine; it does not perform glomerular filtration.

Which urine characteristics should be remembered?

An adult human excretes, on average, 1 to 1.5 litres of urine daily. Urine is a light yellow, watery fluid with a characteristic odour. It is slightly acidic, with a pH of 6.0. Average daily urea excretion is 25 to 30 grams.

These are average characteristics, and various conditions can alter them. They must not be treated as fixed quantities for every person on every day. Urine analysis can help identify metabolic disorders and malfunctioning kidneys.

Glycosuria means glucose in urine; ketonuria means ketone bodies in urine. Ketone bodies are substances associated with fat metabolism. Their presence, together with glucose, is indicative of diabetes mellitus, a disorder associated with prolonged high blood glucose. “Indicative” should not be replaced with an absolute diagnosis.

How do the lungs, liver and skin contribute to excretion?

Kidneys are not the only organs involved in excretion. The lungs, liver and skin eliminate different substances through different routes. Their contributions show why excretion should not be equated with urine production alone.

What do the lungs and liver remove?

The lungs remove large amounts of carbon dioxide (CO₂), approximately 200 millilitres per minute, and significant quantities of water each day. Carbon dioxide elimination is therefore both a respiratory and an excretory function.

The liver, the body’s largest gland, secretes bile, a secretion containing substances that can be eliminated through the digestive tract. These include bilirubin and biliverdin, bile pigments, together with cholesterol, degraded steroid hormones, vitamins and drugs.

Cholesterol is a sterol, a type of lipid; steroid hormones are hormones derived from cholesterol. Most of the substances listed in bile ultimately pass out with digestive wastes. The liver also forms urea from ammonia, linking waste conversion to its eventual removal by the kidneys.

How do skin secretions carry wastes?

Sweat glands produce sweat, a watery secretion containing sodium chloride, small amounts of urea, lactic acid and other substances. Lactic acid is a metabolic product. Sweat’s primary function is to cool the body surface, but it also removes some wastes.

Sebaceous glands release sebum, an oily secretion that provides a protective covering for the skin. They eliminate certain substances, including sterols, hydrocarbons and waxes. Hydrocarbons contain hydrogen and carbon; waxes are lipid substances.

Sweat and sebum should be distinguished by both their composition and their main effects: sweat is watery and supports cooling, whereas sebum provides an oily protective covering. Their excretory contributions accompany these other functions.

Small amounts of nitrogenous wastes could also be eliminated through saliva. This is a limited contribution, so it should not be presented as equivalent to the kidneys’ role in removing nitrogenous wastes and regulating body fluids.

What are the main kidney disorders, and how do dialysis and transplantation help?

Uraemia is the accumulation of urea in blood due to kidney malfunction. It is highly harmful and may lead to renal failure, failure of kidney function. “May lead” preserves the distinction between uraemia and an inevitable progression to kidney failure.

Renal calculi are stones or insoluble masses of crystallised salts, such as oxalates, formed within the kidney. Oxalates are salts of oxalic acid. Nephritis means kidney inflammation; glomerulonephritis specifically means inflammation of the glomeruli. Inflammation is a tissue response to injury or irritation.

How does haemodialysis remove nitrogenous wastes?

Haemodialysis removes nitrogenous wastes from blood using a dialysing unit, or artificial kidney. The unit contains a coiled cellophane tube surrounded by dialysing fluid, a solution with the same composition as plasma except for nitrogenous wastes.

1. Blood drained from a convenient artery receives heparin, an anticoagulant, meaning a substance that prevents blood clotting.
2. The blood is pumped into the artificial kidney’s coiled cellophane tube. Cellophane forms the porous membrane separating blood from the surrounding fluid.
3. The membrane allows molecules to pass according to their concentration gradient. Nitrogenous wastes are absent from the dialysing fluid, so they move out of the blood.
4. The cleared blood receives anti-heparin, which counteracts heparin, and is pumped back into the body through a vein.

The concentration difference explains waste removal. Describing only a tube and a fluid leaves out the mechanism: nitrogenous wastes move across the membrane because their concentration differs on its two sides.

What is a kidney transplant?

Kidney transplantation places a functioning kidney from a donor into a person with kidney failure. A donor is the person supplying the organ. A close relative is preferred to minimise the chance of rejection by the recipient’s immune system, the body’s defence system.

Transplantation provides a functioning organ, whereas haemodialysis clears nitrogenous wastes from blood through an external unit. A close relationship reduces the chances of immune rejection; it does not guarantee that rejection cannot occur.

Glossary

• Ammonotelism — Excretion of nitrogenous waste as ammonia, requiring a large amount of water for elimination.
• Ureotelism — Excretion mainly of urea, formed from ammonia in the liver and eliminated by kidneys.
• Uricotelism — Elimination of nitrogenous wastes as uric acid in the form of a pellet or paste with minimum water loss.
• Nephron — Functional unit of the kidney, consisting of a glomerulus and a renal tubule.
• Renal corpuscle — Structure formed by the glomerulus together with the surrounding double-walled Bowman’s capsule.
• Glomerular filtration rate — Volume of filtrate formed by the kidneys per minute, approximately 125 millilitres in a healthy individual.
• Reabsorption — Recovery of substances from the filtrate by different segments of the renal tubule.
• Tubular secretion — Addition of substances such as hydrogen ions, potassium ions and ammonia to the filtrate by tubular cells.
• Vasa recta — U-shaped blood vessel running parallel to Henle’s loop and contributing to the counter current mechanism.
• Osmoregulation — Regulation of ionic concentration and fluid volume to maintain appropriate conditions within the body.
• Homeostasis — Maintenance of a relatively stable internal environment despite changes within or outside the body.
• Micturition — Release of stored urine through coordinated bladder muscle contraction and relaxation of the urethral sphincter.
• Erythropoietin — Peptide hormone produced by the kidney’s juxtaglomerular cells that stimulates formation of red blood cells.
• Uraemia — Harmful accumulation of urea in blood due to kidney malfunction, which may lead to kidney failure.
• Haemodialysis — Removal of nitrogenous wastes from blood across a porous membrane into surrounding dialysing fluid.

Common errors and misconceptions

• Misconception: Urea is formed in the kidneys. Correct: Ammonia is converted into urea in the liver; blood carries urea to the kidneys for elimination.
• Misconception: The ureter and urethra are the same passage. Correct: Ureters carry urine to the bladder; the urethra carries urine from the bladder during release.
• Misconception: All glomerular filtrate becomes urine. Correct: Nearly 99 per cent of filtrate is reabsorbed, so daily urine volume is much smaller than filtrate volume.
• Misconception: Both limbs of Henle’s loop freely allow water through. Correct: The descending limb is permeable to water; the ascending limb is impermeable to water.
• Misconception: ADH increases water loss in urine. Correct: ADH facilitates water reabsorption from later tubular regions, reducing water loss.
• Misconception: ANF reinforces the pressure-raising action of angiotensin II. Correct: ANF can cause vasodilation and lower blood pressure, checking the renin-angiotensin mechanism.
• Misconception: Sweat’s primary function is nitrogenous waste removal. Correct: Sweat primarily cools the body surface, while also eliminating some wastes.
• Misconception: Dialysing fluid initially contains the same nitrogenous wastes as blood. Correct: These wastes are absent from the fluid, allowing their movement out of blood down a concentration gradient.

Exam-style questions with model answers

Q1. Define glomerular filtration rate and distinguish it from urine output. [2 marks]

1. Glomerular filtration rate is the volume of filtrate formed by the kidneys per minute.
2. Urine output is the fluid finally excreted after tubular reabsorption and secretion; it is not the same as the initial volume filtered.

Q2. Compare ammonotelism, ureotelism and uricotelism by naming the main nitrogenous waste and giving one animal group for each. [3 marks]

1. Ammonotelism is excretion of ammonia. Many bony fishes show this mode, which requires a large amount of water for elimination.
2. Ureotelism is excretion mainly of urea. Mammals show this mode; ammonia is converted into urea in the liver before renal elimination.
3. Uricotelism is excretion of nitrogenous wastes as uric acid. Birds show this mode, eliminating it as a pellet or paste with minimum water loss.

Q3. In a daily-volume comparison, kidneys form 180 litres of filtrate and release 1.5 litres of urine. Calculate the volume reabsorbed, then use that volume to calculate the percentage reabsorbed. Give the percentage to one decimal place. [2 marks]

1. Volume reabsorbed = filtrate volume minus urine volume = 180 minus 1.5 = 178.5 litres per day.
2. Percentage reabsorbed = 178.5 divided by 180, multiplied by 100 = 99.2 per cent to one decimal place, consistent with nearly 99 per cent reabsorption.

Q4. Explain urine formation in four points: filtration pressure, the filtration barrier, reabsorption and tubular secretion. [4 marks]

1. Glomerular capillary blood pressure drives filtration from the blood into Bowman’s capsule, beginning the process of urine formation.
2. The barrier includes vascular endothelium, a basement membrane and capsular epithelium with podocyte slit pores. Almost all plasma constituents except proteins enter the capsule.
3. Renal tubules reabsorb useful substances and water from the filtrate by active or passive mechanisms, recovering nearly 99 per cent of its volume.
4. Tubular cells secrete substances such as hydrogen ions, potassium ions and ammonia into the filtrate, helping maintain ionic and acid-base balance.

Q5. Explain the counter current mechanism in five points, covering flow directions, sodium chloride transport, urea movement, the medullary gradient and water recovery. [5 marks]

1. Filtrate flows in opposite directions through the two limbs of Henle’s loop. Blood also flows in opposite directions in the closely associated vasa recta.
2. Sodium chloride transported by Henle’s ascending limb is exchanged with the descending vasa recta. The ascending vasa recta returns it to the interstitium.
3. Small amounts of urea enter the thin ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.
4. These movements and the arrangement of the vessels and tubules maintain increasing osmolarity towards the inner medulla, mainly through sodium chloride and urea.
5. The medullary gradient favours water passage out of the collecting tubule. Water recovery concentrates the filtrate and helps the kidneys conserve water.

Q6. A fall in glomerular blood flow activates juxtaglomerular cells. Explain the renin-angiotensin-aldosterone response in four points. [4 marks]

1. Activated juxtaglomerular cells release renin, initiating the regulatory response to the fall in glomerular blood flow.
2. Renin initiates conversion of blood angiotensinogen into angiotensin I and subsequently angiotensin II, which is a powerful vasoconstrictor.
3. Angiotensin II raises glomerular blood pressure through vasoconstriction and thereby increases glomerular filtration rate. It also stimulates the adrenal cortex.
4. The adrenal cortex releases aldosterone, promoting sodium and water reabsorption from distal tubular regions. This also increases blood pressure and glomerular filtration rate.

Q7. Describe haemodialysis in five points, covering anticoagulation, the artificial kidney, dialysing-fluid composition, waste movement and blood return. [5 marks]

1. Blood is drained from a convenient artery and an anticoagulant such as heparin is added to prevent clotting during the process.
2. The blood is pumped into a dialysing unit called an artificial kidney. It contains a coiled cellophane tube surrounded by dialysing fluid.
3. The dialysing fluid has the same composition as plasma except that nitrogenous wastes are absent, establishing a concentration difference for those wastes.
4. The porous cellophane membrane permits movement according to the concentration gradient. Nitrogenous wastes move from blood into the surrounding fluid, clearing the blood.
5. Anti-heparin is added to the cleared blood, which is then pumped back into the body through a vein, completing the described sequence.

Q8. State one excretory contribution each of the lungs, liver and sweat glands, naming the substances involved. [3 marks]

1. The lungs eliminate carbon dioxide and significant quantities of water, so the removal of respiratory waste also contributes to excretion.
2. The liver secretes bile containing substances such as bilirubin, biliverdin and cholesterol. Most of these substances ultimately leave along with digestive wastes.
3. Sweat glands eliminate sodium chloride, small amounts of urea and lactic acid in watery sweat, although sweat’s primary function is cooling the body surface.

Key takeaways

• Ammonia requires much water for elimination, while formation of urea and uric acid helps conserve water in terrestrial adaptation.
• The nephron combines glomerular filtration with tubular reabsorption and secretion to regulate waste elimination and body-fluid composition.
• The PCT recovers nearly all essential nutrients and 70 to 80 per cent of electrolytes and water.
• Opposite flows in Henle’s loop and vasa recta help maintain the medullary gradient needed to concentrate urine.
• ADH promotes water recovery, while the renin-angiotensin-aldosterone system links blood-pressure regulation with sodium and water reabsorption.
• ANF can lower blood pressure and check the renin-angiotensin mechanism; erythropoietin stimulates red blood cell formation.
• Micturition coordinates bladder contraction with sphincter relaxation, combining a neural reflex mechanism with voluntary control.
• Haemodialysis removes nitrogenous wastes through a membrane, while transplantation supplies a functioning kidney from a donor.

Test yourself

Where is urea formed, and how does it reach the kidneys?

Urea is formed from ammonia in the liver and released into blood, which carries it to the kidneys.

What structures together form the renal corpuscle?

The glomerulus and its surrounding Bowman’s capsule together form the renal corpuscle, also called the Malpighian body.

Why does filtrate become more concentrated in Henle’s descending limb?

The descending limb is permeable to water but almost impermeable to electrolytes, so water loss concentrates the filtrate.

What happens to water permeability in Henle’s ascending limb?

The ascending limb is impermeable to water, while electrolytes can leave actively or passively, diluting the filtrate.

What can suppress ADH release after fluid conservation?

An increase in body-fluid volume can switch off osmoreceptors and suppress ADH release, completing the feedback response.

What does erythropoietin stimulate?

Erythropoietin stimulates erythropoiesis, meaning the formation of red blood cells.

How does uraemia differ from renal calculi?

Uraemia is accumulation of urea in blood; renal calculi are stones or insoluble masses of crystallised salts within the kidney.

Why is a close relative preferred as a kidney donor?

A close relative is preferred to minimise the chances of rejection by the recipient’s immune system.