Excretory System

The excretory system carries 2 of the 40 Zoology questions. The paper builds them from three blocks: the organs and the nephron (what each part is and does), the formation of urine (filtration, reabsorption, secretion, the counter-current multiplier and the three hormones that set urine volume), and the disorders (nephritis, stones, failure, dialysis). Almost every question turns on a direction, a number or a permeability: which limb lets water through, which hormone makes urine small, which value is the renal threshold. The three traps that decide marks are the descending limb is permeable to water and the ascending limb is not, ADH increases water reabsorption so its deficiency gives a large dilute urine, and the posterior pituitary stores ADH but the hypothalamus makes it.

8.1 Excretory organs and the structure of the nephron

Excretion, egestion and secretion

Excretion is the removal of the waste products of metabolism from the body — substances that were made inside cells and can no longer be used. It is not the same as egestion, which is the removal of unabsorbed food from the gut, nor as secretion, which is the release of a useful substance such as a hormone or an enzyme.

The nitrogenous waste depends on the animal's habitat. Ammonotelic animals (bony fish, aquatic invertebrates, tadpoles) excrete ammonia, which is very toxic but dissolves easily in plentiful water. Ureotelic animals (man, mammals, adult frogs, cartilaginous fish) spend energy converting ammonia to urea, which is far less toxic and can be carried in a small volume of water. Uricotelic animals (birds, reptiles, insects) convert it to uric acid, which is almost insoluble and is passed as a paste — the best saving of water, at the greatest energy cost.

The organs

Organ	Structure	What it removes
Kidney (a pair)	bean-shaped, retroperitoneal, T12–L3	urea, uric acid, creatinine, excess salts, water, H⁺; also makes renin and erythropoietin
Ureter (a pair)	muscular tube, 25–30 cm, peristaltic	carries urine from the renal pelvis to the bladder
Urinary bladder	muscular sac with transitional epithelium and the detrusor muscle	stores urine (300–400 mL, up to 600–800 mL)
Urethra	female ≈ 4 cm, male ≈ 20 cm	carries urine out; in the male it also carries semen

Accessory excretory organs

• Skin — sweat glands remove water, sodium chloride, urea and lactic acid; sebaceous glands remove sebum. Sweat is not a major route for nitrogen in man, but it matters in fever and heavy work.
• Lungs — remove carbon dioxide and water vapour at every breath.
• Liver — deaminates surplus amino acids and hands the ammonia to the urea cycle (ornithine cycle), so urea is made here; it also breaks down haemoglobin and excretes bile pigments (bilirubin, biliverdin) into bile, and detoxifies drugs and alcohol.
• Large intestine — carries bile pigments and cholesterol out in faeces.

Position, coverings and blood supply

Each kidney is about 10–12 cm long, 5–7 cm wide and 3 cm thick, and weighs 130–150 g. It lies behind the peritoneum on the posterior abdominal wall. The right kidney is slightly lower than the left because the liver pushes it down. The medial border has a notch, the hilum, through which the renal artery enters and the renal vein and ureter leave. The hilum opens into a cavity, the renal sinus, filled with fat and the pelvis.

Three coverings protect it: the inner fibrous capsule, a middle perirenal fat cushion and the outer renal fascia.

The blood supply is a trap in itself:

Renal artery → segmental → interlobar → arcuate → interlobular → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries or vasa recta → interlobular vein → arcuate → interlobar → renal vein.

The kidney receives about 1.2 L of blood per minute, roughly a quarter of the cardiac output. The efferent arteriole is narrower than the afferent, so blood damps up in the glomerulus and the pressure there stays high — that is what drives filtration. The second capillary bed (peritubular and vasa recta) is at low pressure, which is what allows reabsorption.

The kidney in section

1Adrenal (suprarenal) gland2Renal pyramid3Renal hilum4Renal papilla5Renal pelvis6Renal cortex7Ureter

Region	Contents	Job
Cortex	renal corpuscles (glomerulus + Bowman's capsule), PCT and DCT, peritubular capillaries	filtration and most reabsorption
Medulla	8–18 renal pyramids, loops of Henle, collecting ducts, vasa recta	builds the osmotic gradient; concentrates urine
Renal columns (of Bertin)	cortical tissue between the pyramids	carry interlobar vessels
Papilla	the apex of each pyramid	drips urine into a minor calyx
Renal pelvis	funnel formed by major calyces	collects urine and passes it to the ureter

The nephron

A nephron is the structural and functional unit of the kidney; each kidney holds about one million. There are two kinds. Cortical nephrons (about 85 %) sit high in the cortex with short loops that barely enter the medulla. Juxtamedullary nephrons (about 15 %) lie near the medulla and have long loops running deep into it; only they can build the steep gradient needed for concentrated urine.

1Interlobular artery2Afferent arteriole3Juxtaglomerular apparatus4Renal corpuscle5Glomerulus6Bowman's capsule7Efferent arteriole8Proximal convoluted tubule9Loop of Henle10Distal convoluted tubule11Connecting tubule leading to the collecting duct12Interlobular vein

Segment	Epithelium	Main job
Bowman's capsule	outer parietal layer of squamous cells; inner visceral layer of podocytes	receives the filtrate
Proximal convoluted tubule (PCT)	cuboidal with a brush border of microvilli and many mitochondria	65–70 % of reabsorption: all glucose and amino acids, most Na⁺, water and HCO₃⁻
Descending limb	thin squamous	water leaves by osmosis; solutes stay
Ascending limb	thin then thick cuboidal	impermeable to water; Na⁺ and Cl⁻ pumped out (the "diluting segment")
Distal convoluted tubule (DCT)	cuboidal, no brush border	Na⁺ reabsorption under aldosterone, Ca²⁺ under PTH, K⁺ and H⁺ secretion
Collecting duct	principal cells and intercalated cells	water under ADH; final acid–base adjustment; urea recycling
Peritubular capillaries / vasa recta	endothelium	take back water and solutes; the vasa recta is a counter-current exchanger

The juxtaglomerular apparatus (JGA) sits where the DCT touches its own afferent arteriole. It has three parts: the macula densa (tall, crowded cells of the DCT that sense Na⁺ and Cl⁻ in the filtrate), the juxtaglomerular cells (smooth-muscle-like cells of the afferent arteriole that store renin) and the extraglomerular mesangial cells between them. This is the sensor that starts the renin–angiotensin system when blood pressure or sodium falls.

8.2 Formation of urine and its regulation

Urine is made in three processes, in this order: ultrafiltration in the renal corpuscle, selective reabsorption along the tubule, and tubular secretion from the blood into the tubule. A fourth process, concentration, happens in the collecting duct under ADH.

Step 1 — ultrafiltration

Blood pressure forces water and small solutes out of the glomerular capillaries into Bowman's capsule. The barrier has three layers:

Layer	Structure	What it holds back
Fenestrated endothelium	pores 70–100 nm	blood cells
Basement membrane	glycoproteins and collagen, negatively charged	large proteins; the charge repels albumin
Podocyte slit diaphragms	foot processes with slits about 4–7 nm	medium proteins

So the filtrate is almost protein-free: water, glucose, amino acids, urea, uric acid, creatinine, ions and small peptides pass; cells and plasma proteins do not. Albumin (69 kDa) is held back by both its size and its negative charge — a question that offers only "size" is incomplete when "size and charge" is available.

The pressures at the glomerulus are:

Pressure	Value
Glomerular capillary hydrostatic pressure	55 mmHg (high because the efferent arteriole is narrow)
Plasma oncotic pressure (proteins pull water back)	30 mmHg
Capsular hydrostatic pressure	15 mmHg
Net filtration pressure	10 mmHg

GFR (glomerular filtration rate) is about 125 mL min⁻¹, i.e. 180 L day⁻¹ — the whole plasma volume filtered about 60 times a day. The filtration fraction is GFR ÷ renal plasma flow = 125 ÷ 625 = 20 %.

Step 2 — selective reabsorption

Reabsorption is selective: the tubule takes back what the body needs and leaves the waste. About 99 % of the water and 99.5 % of the sodium are recovered, along with all the glucose and amino acids.

Substance	Main site	Mechanism	Reabsorbed
Glucose	PCT	secondary active transport with Na⁺ (SGLT), then facilitated diffusion out	100 % below the threshold
Amino acids	PCT	active transport with Na⁺	100 %
Na⁺	PCT (65 %), ascending limb (25 %), DCT/CD (aldosterone)	Na⁺/K⁺ ATPase on the basolateral side drives it	99.5 %
Water	PCT (65–70 %), descending limb, collecting duct (ADH)	osmosis	99 %
HCO₃⁻	PCT	indirectly, as H⁺ is secreted	about 90 %
Urea	PCT and collecting duct	diffusion (some facilitated)	40–50 %
Creatinine	—	not reabsorbed; some secreted	0 %

Glucose and the renal threshold. The carrier proteins can move only so much glucose per minute; their limit is the transport maximum (Tm), about 375 mg min⁻¹. As long as the filtered load stays below it, all glucose is recovered. The plasma level at which the load reaches the Tm is the renal threshold, about 180 mg/dL. Above that, the carriers saturate and glucose appears in the urine — glycosuria.

Water and the counter-current multiplier. Water is not pumped; it follows solute by osmosis. The loop of Henle makes that possible by creating a gradient of osmolarity in the medulla, from 300 mOsm L⁻¹ at the corticomedullary junction to 1200 mOsm L⁻¹ (up to 1400) at the papilla. The mechanism has four steps:

1. The filtrate entering the descending limb is isotonic (300).
2. The descending limb is permeable to water but not to solute. As it runs down into the salty medulla, water leaves by osmosis and the filtrate becomes steadily more concentrated, reaching about 1200 at the tip.
3. The ascending limb is impermeable to water but actively pumps Na⁺ and Cl⁻ into the interstitium. Solute leaves, water cannot, so the filtrate becomes dilute — about 100 at the top.
4. The NaCl left behind in the medulla keeps the interstitium hypertonic, and the vasa recta carry away the water without washing the salt out. Because the two limbs carry fluid in opposite directions and have opposite permeabilities, the gradient is multiplied along the length of the loop.

Level	Interstitial osmolarity	Filtrate in the loop
Cortex / PCT	300	300 (isotonic)
Outer medulla	400–600	descending 400–600
Inner medulla	900	descending 900
Papilla (tip)	1200	1200 at the bend
Top of ascending limb	300	100 (dilute)

The length of the loop sets the steepness of the gradient. Desert mammals such as the kangaroo rat have very long loops and make urine of 5000 mOsm L⁻¹; birds have short loops and concentrate weakly; fish have no loop at all. The same logic explains why a long loop means a dry habitat.

The DCT and collecting duct. The DCT receives hypotonic fluid (about 100 mOsm L⁻¹). Here Na⁺ is reabsorbed under aldosterone and Ca²⁺ under parathyroid hormone. The collecting duct is where the final decision is made: if ADH is present, its cells insert aquaporin-2 channels and water leaves into the hypertonic medulla, so the urine becomes concentrated; if ADH is absent, the duct stays almost impermeable and a large volume of dilute urine is passed.

Step 3 — tubular secretion

Secretion moves substances from the blood of the peritubular capillaries into the tubule, the opposite direction to reabsorption. It is the tubule's way of getting rid of things that filtration alone cannot handle.

Substance	Where secreted	Why it matters
H⁺	PCT and intercalated cells of the collecting duct	regulates blood pH; can acidify urine to pH 4.5
K⁺	principal cells of the DCT and collecting duct	the main route for potassium loss; increased by aldosterone
NH₃	PCT	combines with H⁺ to form NH₄⁺, trapping acid in the urine
Creatinine	PCT	a waste that is also filtered
Drugs	PCT	penicillin, aspirin, morphine, diuretics — many are protein-bound and never filtered
Organic acids and bases	PCT	e.g. para-aminohippuric acid (PAH), used to measure renal blood flow

Secretion matters for two reasons. First, some wastes (K⁺, H⁺) are almost entirely removed this way. Second, drugs bound to plasma proteins are too large to filter, so secretion is their only exit.

Hormonal regulation

Three hormones set the final volume and concentration of urine.

ADH (vasopressin). Made in the supraoptic and paraventricular nuclei of the hypothalamus, carried down axons and stored in the posterior pituitary, from which it is released. Osmoreceptors in the hypothalamus detect a rise in plasma osmolarity (as little as 2 %), and ADH is released. It binds V₂ receptors on the collecting duct and inserts aquaporin-2 water channels, so water is reabsorbed and the urine becomes small and concentrated. A fall in osmolarity switches ADH off and a large dilute urine is produced.

Diabetes insipidus. A deficiency of ADH (or a failure of the kidney to respond to it) gives polyuria — up to 15–20 L a day of pale urine with a specific gravity of 1.001–1.005 — and intense polydipsia. The urine is dilute because the water is not reabsorbed, not because glucose is pulling water into the tubule.

Aldosterone. A steroid from the zona glomerulosa of the adrenal cortex. It increases Na⁺ reabsorption and K⁺ secretion in the DCT and collecting duct; water follows the sodium. The result is a small volume of urine with little sodium and more potassium, and a rise in blood pressure.

The renin–angiotensin–aldosterone system (RAAS). When blood pressure, blood volume or Na⁺ falls (or K⁺ rises), the juxtaglomerular cells release renin. Renin converts angiotensinogen (from the liver) to angiotensin I; angiotensin-converting enzyme (ACE), mostly in the lungs, converts it to angiotensin II. Angiotensin II is a strong vasoconstrictor and also stimulates aldosterone release, ADH release and thirst. The net effect is to retain sodium and water and raise blood pressure.

ANP (atrial natriuretic peptide). Stretch of the atria — as when blood volume rises — releases this 28-amino-acid peptide from atrial muscle cells. ANP inhibits Na⁺ reabsorption in the collecting duct, inhibits renin and aldosterone, dilates the afferent arteriole and constricts the efferent one, so GFR rises. The result is a large volume of sodium-rich, dilute urine and a fall in blood pressure. It is the physiological opponent of the RAAS.

Hormone	Source	Main action	Urine volume	Urine concentration
ADH	hypothalamus → posterior pituitary	↑ water permeability of DCT/CD (aquaporin-2)	↓	↑ (up to 1200 mOsm L⁻¹)
Aldosterone	adrenal cortex, zona glomerulosa	↑ Na⁺ reabsorption, ↑ K⁺ secretion	↓	↑ (water follows Na⁺)
ANP	atrial muscle	↓ Na⁺ reabsorption, ↓ renin, ↑ GFR	↑	↓ (dilute)
PTH	parathyroid	↑ Ca²⁺ reabsorption, ↓ phosphate reabsorption	little change	—

Composition of urine

Property	Normal value
Volume	1–2 L per day (1.5 L average)
Colour	pale yellow (urochrome)
pH	4.6–8.0, average about 6
Specific gravity	1.003–1.035 (average 1.020)
Osmolarity	300–1200 mOsm L⁻¹
Water	about 95 %
Urea	25–35 g per day — the most abundant solute
Uric acid	0.5–1 g per day
Creatinine	1–1.5 g per day
Others	Na⁺, K⁺, Cl⁻, sulphate, phosphate, ammonia, urobilinogen, hormones, vitamins

Abnormal constituents are the quickest route to a diagnosis, and the paper likes them as one-line matches.

Constituent	What it signals
Glucose	diabetes mellitus; also renal glycosuria (low threshold) and pregnancy
Protein (albumin)	glomerulonephritis, nephrotic syndrome, pre-eclampsia; transient after heavy exercise or fever
Ketones	starvation, uncontrolled type 1 diabetes (ketoacidosis), persistent vomiting
Blood / red cells	glomerulonephritis, stones, trauma, tumour, urinary infection
Casts (hyaline, granular, red-cell)	renal tubular damage; red-cell casts point to glomerulonephritis
Pus cells and nitrite	urinary tract infection (cystitis, pyelonephritis)
Bilirubin	obstructive jaundice, hepatitis
Crystals	oxalate or urate stones, gout
hCG	pregnancy (basis of the pregnancy test)

8.3 Renal disorders

Nephritis

Nephritis is inflammation of the kidney. The commonest form is glomerulonephritis, an immune-complex (type III) reaction: antibodies made against a β-haemolytic streptococcus (after a throat or skin infection) cross-react with the glomerular basement membrane, or antigen–antibody complexes lodge there. It usually appears 1–3 weeks after the infection.

Features: haematuria (smoky or cola-coloured urine), proteinuria, oedema (especially around the eyes in the morning), hypertension and oliguria (less urine). Most children recover; some progress to chronic renal failure. Nephrotic syndrome is a related picture with heavy protein loss, low blood albumin and generalised oedema.

Kidney stones (renal calculi)

Stones form when salts in concentrated urine crystallise. About 75–80 % are calcium oxalate; others are uric acid, cystine and struvite (the last is associated with infection). Risk factors are a low fluid intake, a hot climate, hypercalcaemia, a diet high in oxalate (spinach, rhubarb, nuts, tea), gout and repeated urinary infection.

A stone causes renal colic — severe pain from the loin to the groin as it moves down the ureter — with haematuria and sometimes obstruction and infection. Small stones pass with fluids and analgesia; larger ones are broken by extracorporeal shock-wave lithotripsy (ESWL) or removed through a ureteroscope.

Renal failure

Renal failure is a fall in GFR severe enough to retain waste.

	Acute renal failure	Chronic renal failure
Onset	hours to days	months to years
Common causes	shock, severe dehydration, blood loss, nephrotoxic drugs, obstruction	diabetes mellitus and hypertension (the two commonest), glomerulonephritis, polycystic kidney
Reversible?	often, if the cause is corrected	no; progressive
Features	oliguria or anuria, rising urea and creatinine, high K⁺	uraemia, anaemia (less erythropoietin), acidosis, oedema, bone disease

When the kidneys fail, urea and creatinine rise (uraemia), K⁺ rises (hyperkalaemia, dangerous for the heart), H⁺ accumulates (acidosis) and erythropoietin falls (anaemia). Treatment is by dialysis or transplantation.

Dialysis

Dialysis filters the blood through a semipermeable membrane against a dialysate whose concentration is set so that wastes diffuse out while useful substances stay.

	Haemodialysis	Peritoneal dialysis
Membrane	cellulose or synthetic membrane in a dialyser	the peritoneum
Blood flow	pumped from an arteriovenous fistula	the patient's own capillaries
Schedule	3–4 hours, 2–3 times a week, in hospital	4–6 hour dwells, about 4 exchanges a day, at home
Efficiency	high	lower
Main risk	clotting (heparin used), hypotension, infection of the access	peritonitis

In haemodialysis the blood and the dialysate flow in opposite directions (counter-current), which keeps a concentration gradient along the whole membrane so that diffusion does not stop halfway.

Kidney transplantation

A healthy kidney from a living related donor or a cadaver is placed in the iliac fossa and joined to the iliac vessels and the bladder. The donor and recipient are matched for ABO and HLA antigens. The recipient takes immunosuppressant drugs (ciclosporin, tacrolimus, steroids) for life to prevent rejection. A working transplant gives a better quality of life and longer survival than dialysis, but the supply of organs is far short of the need.

Formula and fact sheet

Organs and structure

Fact	Value
Kidneys	a pair, T12–L3, right slightly lower; 10–12 × 5–7 × 3 cm; 130–150 g each
Coverings	fibrous capsule, perirenal fat, renal fascia
Regions	cortex, medulla (8–18 pyramids), renal columns, pelvis, calyces
Nephron count	about 1 million per kidney
Cortical / juxtamedullary nephrons	85 % / 15 % (long loops)
Ureter	25–30 cm, peristaltic
Bladder	transitional epithelium, detrusor muscle, 300–400 mL
Urethra	female 4 cm, male 20 cm
Blood flow	1.2 L min⁻¹, about 25 % of cardiac output
Efferent vs afferent arteriole	efferent narrower → high glomerular pressure

Filtration

Fact	Value
Filtration barrier	fenestrated endothelium (70–100 nm), basement membrane (negative), slit diaphragms (4–7 nm)
Filtered	water, glucose, amino acids, urea, uric acid, creatinine, ions
Not filtered	cells, plasma proteins (albumin 69 kDa)
Pressures	55 − (30 + 15) = 10 mmHg net
GFR	125 mL min⁻¹ = 180 L day⁻¹
Filtration fraction	125/625 = 20 %
Reabsorbed	about 99 % of water, 99.5 % of Na⁺

Reabsorption and secretion

Fact	Value
PCT	65–70 % of reabsorption; brush border; glucose and amino acids 100 %
Glucose transport	secondary active (Na⁺ cotransport, SGLT)
Renal threshold for glucose	180 mg/dL
Transport maximum (Tm)	about 375 mg min⁻¹
Descending limb	permeable to water, not solute
Ascending limb	impermeable to water; pumps Na⁺/Cl⁻
Gradient	300 → 1200 mOsm L⁻¹ (max 1400)
Filtrate leaving the loop	about 100 mOsm L⁻¹ (dilute)
Secreted	H⁺, K⁺, NH₃, creatinine, drugs (penicillin, aspirin), PAH

Hormones

Hormone	Source	Effect on urine
ADH (vasopressin)	hypothalamus → posterior pituitary	↑ water reabsorption → small, concentrated
Aldosterone	adrenal cortex, zona glomerulosa	↑ Na⁺ reabsorption, ↑ K⁺ secretion → small volume
ANP	atrial muscle	↑ Na⁺ and water excretion → large, dilute
PTH	parathyroid	↑ Ca²⁺ reabsorption, ↑ phosphate excretion
Renin	JG cells	starts RAAS; converts angiotensinogen → angiotensin I
ACE	lung endothelium	angiotensin I → angiotensin II

Urine

Fact	Value
Volume	1–2 L day⁻¹
pH	4.6–8.0 (average 6)
Specific gravity	1.003–1.035
Water	95 %
Most abundant solute	urea (25–35 g day⁻¹)
Diabetes insipidus	15–20 L day⁻¹, SG 1.001–1.005, dilute
Diabetes mellitus	polyuria with glucose in urine, high SG

Disorders

Disorder	Key fact
Glomerulonephritis	immune complex after streptococcal infection; haematuria, proteinuria, oedema
Stones	calcium oxalate commonest; renal colic; lithotripsy
Acute failure	reversible; shock, dehydration, toxins
Chronic failure	diabetes and hypertension commonest; uraemia, anaemia, acidosis
Haemodialysis	dialyser, counter-current, 3–4 h, 2–3 times a week
Peritoneal dialysis	peritoneum as membrane, done at home, risk of peritonitis
Transplant	HLA matching, immunosuppression for life

Chapter summary

• The excretory organs are the kidneys, ureters, bladder and urethra; the skin, lungs and liver are accessory routes, and the liver makes urea while the kidney excretes it.
• Each kidney has about one million nephrons, in a cortex (corpuscles, convoluted tubules) and a medulla (pyramids, loops, collecting ducts) that drains through the pelvis into the ureter.
• Ultrafiltration at the glomerulus passes water and small solutes but holds back cells and protein by pore size and negative charge; net pressure ≈ 10 mmHg, GFR ≈ 125 mL min⁻¹ (180 L day⁻¹).
• Reabsorption recovers about 99 % of the filtrate: glucose and amino acids by active transport in the PCT (threshold 180 mg/dL, Tm 375 mg min⁻¹), water by osmosis.
• The counter-current multiplier works because the descending limb is permeable to water and the ascending limb is not but pumps salt; the medulla is graded 300 → 1200 mOsm L⁻¹, and the vasa recta preserve the gradient.
• Secretion adds H⁺, K⁺, NH₃, creatinine and drugs to the tubule and is the main route for acid removal.
• ADH (made in the hypothalamus, stored in the posterior pituitary) concentrates urine; aldosterone retains Na⁺ and loses K⁺; ANP does the opposite. Diabetes insipidus = ADH problem = large dilute urine; diabetes mellitus = insulin problem = large glucose-containing urine.
• Abnormal constituents: glucose (diabetes), protein (nephritis), ketones (starvation, ketoacidosis), blood (nephritis, stones), casts (tubular damage).
• Disorders: nephritis, calcium oxalate stones, acute and chronic failure; haemodialysis uses a dialyser, peritoneal dialysis uses the peritoneum, and transplantation needs HLA matching and lifelong immunosuppression.

Practice questions

Set A — Recall (24 questions)

1. The structural and functional unit of the kidney is theaneuronbrenal corpusclecnephrondmalpighian tubule
2. Each human kidney contains abouta10 000 nephronsb1 million nephronsc100 million nephronsd1000 nephrons
3. The glomerulus is enclosed byathe loop of Henlebthe collecting ductcthe renal pelvisdBowman's capsule
4. Which part of the nephron is freely permeable to water but not to solutes?adescending limb of the loop of Henlebascending limb of the loop of Henlecdistal convoluted tubuledcollecting duct
5. The greatest part of the reabsorption of the filtrate takes place in theaBowman's capsulebloop of Henlecproximal convoluted tubuledcollecting duct
6. Glucose is reabsorbed from the filtrate byasimple diffusionbactive transport in the PCTcosmosis in the loopdfiltration
7. Glucose first appears in the urine when its plasma concentration exceeds abouta100 mg/dLb80 mg/dLc300 mg/dLd180 mg/dL
8. Antidiuretic hormone is released from theaposterior pituitarybanterior pituitarycadrenal cortexdthyroid
9. ADH makes the collecting duct more permeable toaglucosebsodiumcwaterdproteins
10. A person who passes 15–20 litres of dilute urine a day most probably lacksainsulinbADHcaldosteronedthyroxine
11. Aldosterone is secreted by theaadrenal medullabposterior pituitarycjuxtaglomerular cellsdadrenal cortex
12. Renin is secreted by theajuxtaglomerular cellsbmacula densacliverdadrenal cortex
13. Angiotensin I is converted to angiotensin II mainly in theakidneybliverclungsdheart
14. Atrial natriuretic peptide is secreted byathe adrenal medullabstretched atrial muscle cellscthe juxtaglomerular cellsdthe posterior pituitary
15. The normal glomerular filtration rate in an adult is abouta125 mL/minb1250 mL/minc25 mL/mind125 mL/hour
16. The normal daily volume of urine in an adult is abouta15 Lb0.1 Lc5 Ld1–2 L
17. The most abundant nitrogenous waste in urine isauric acidbcreatininecureadammonia
18. The specific gravity of normal urine lies betweena1.000 and 1.001b1.003 and 1.035c1.040 and 1.060d0.900 and 0.950
19. Which of the following is NOT an excretory organ?askinblungscliverdspleen
20. The urinary bladder is lined byatransitional epitheliumbsimple squamous epitheliumcstratified keratinised epitheliumdciliated columnar epithelium
21. Which substance is almost completely reabsorbed from the filtrate?aureabcreatininecglucoseduric acid
22. The vasa recta arealymph vessels of the cortexbbranches of the renal vein in the cortexcthe afferent arteriolesdcapillary loops running beside the loop of Henle
23. The commonest type of kidney stone is made ofauric acidbcalcium oxalateccystinedstruvite
24. In peritoneal dialysis the membrane used is theaperitoneumbglomerular membraneccellulose tubing of a dialyserdbladder wall

Answers and explanations — Set A

1. (c) — the nephron is the functional unit; the renal corpuscle (b) is only its first part, the glomerulus plus Bowman's capsule.

2. (b) — about one million per kidney, giving roughly two million for the pair.

3. (d) — Bowman's capsule is the cup-shaped beginning of the tubule; the glomerulus is the capillary tuft inside it.

4. (a) — the descending limb has aquaporins but no solute carriers, so only water leaves. The ascending limb (b) is the opposite.

5. (c) — the PCT recovers 65–70 % of the filtrate because of its brush border and dense mitochondria.

6. (b) — glucose is moved by secondary active transport with Na⁺ (SGLT) in the PCT; it is never reabsorbed by diffusion alone.

7. (d) — the renal threshold is about 180 mg/dL; above it the carriers are saturated and glucose spills into the urine.

8. (a) — ADH is made in the hypothalamus but released from the posterior pituitary, which stores it.

9. (c) — ADH inserts aquaporin-2 channels, so water is reabsorbed and the urine becomes concentrated.

10. (b) — 15–20 L of dilute urine is the classic picture of diabetes insipidus, an ADH deficiency.

11. (d) — aldosterone comes from the zona glomerulosa of the adrenal cortex; the medulla (a) makes adrenaline.

12. (a) — the juxtaglomerular cells of the afferent arteriole store and release renin.

13. (c) — angiotensin-converting enzyme sits on the endothelium of the lung capillaries.

14. (b) — ANP is released by atrial muscle cells when they are stretched by a rise in blood volume.

15. (a) — 125 mL/min, which is 180 L a day. Option (b) is ten times too high and (d) mixes minutes with hours.

16. (d) — 1–2 L a day, about 1 % of the filtrate; 15 L (a) suggests diabetes insipidus.

17. (c) — urea, 25–35 g a day, is the most abundant solute; uric acid and creatinine are much smaller in amount.

18. (b) — 1.003–1.035; a fixed low value suggests diabetes insipidus, a fixed high one diabetes mellitus.

19. (d) — the spleen destroys old red cells but does not excrete waste; skin, lungs and liver all do.

20. (a) — transitional epithelium stretches as the bladder fills and is impermeable to urine.

21. (c) — glucose is completely reabsorbed below the threshold; creatinine (b) is not reabsorbed at all.

22. (d) — the vasa recta are the long capillary loops of the juxtamedullary nephrons that run beside the loop of Henle.

23. (b) — calcium oxalate accounts for about 75–80 % of stones.

24. (a) — the peritoneum is the dialysing membrane in peritoneal dialysis; the dialyser membrane (c) is used in haemodialysis.

Set B — Understanding (14 questions)

1. Plasma proteins are not filtered at the glomerulus mainly because ofatheir small sizebtheir rapid reabsorptionctheir size and negative chargedtheir absence from plasma
2. The efferent arteriole is narrower than the afferent arteriole, whichakeeps glomerular pressure highblowers glomerular pressurecstops filtrationdincreases glucose reabsorption
3. If the ascending limb of the loop of Henle became permeable to water, the medullary gradient wouldadoublebstay the samecriseddisappear
4. ADH reduces urine volume because itaincreases the GFRbincreases water reabsorption in the collecting ductcincreases sodium excretiondconstricts the ureter
5. Diabetes insipidus and diabetes mellitus both cause polyuria, but in diabetes insipidus the urine isarich in glucosebrich in proteinclarge in volume and dilutedsmall in volume and concentrated
6. Aldosterone raises blood pressure mainly byaincreasing Na⁺ and water reabsorptionbincreasing glucose reabsorptioncdilating the afferent arterioledinhibiting renin
7. ANP lowers blood pressure byaincreasing renin releasebpromoting sodium and water excretioncconstricting the afferent arterioledincreasing ADH release
8. Tubular secretion of H⁺ is important because itafilters proteinbreabsorbs glucosecconcentrates urinedhelps regulate blood pH
9. The counter-current mechanism works because the two limbs of the loopahave opposite permeabilities and opposite flowsbcarry fluid in the same directioncboth absorb waterdboth secrete glucose
10. Glucose appears in the urine in uncontrolled diabetes mellitus becauseathe PCT stops workingbthe GFR risescthe filtered load exceeds the transport maximumdADH falls
11. A patient with glomerulonephritis usually showsaglucose and ketones in urinebprotein and red cells in urinecbile and urobilinogendno abnormality at all
12. Compared with haemodialysis, peritoneal dialysisais fasterbneeds a dialysercneeds heparinduses the peritoneum and can be done at home
13. The renal cortex containsaglomeruli and convoluted tubulesbonly collecting ductscthe renal pelvisdthe renal pyramids
14. For urine to be concentrated in the collecting duct, the medullary interstitium must beahypotonicbisotonicchypertonicdfree of sodium

Answers and explanations — Set B

1. (c) — the basement membrane is negatively charged and the slits are narrow, so albumin is repelled as well as blocked. Size alone (a) is the tempting half-answer.

2. (a) — a narrow outlet holds blood back, so glomerular pressure stays around 55 mmHg and filtration continues.

3. (d) — water would follow the salt out of the ascending limb, the salt would be diluted, and no gradient could be maintained.

4. (b) — ADH inserts water channels in the collecting duct; it does not change GFR or sodium handling.

5. (c) — no ADH means no water reabsorption, so the urine is dilute; glucose is the marker of diabetes mellitus (a).

6. (a) — sodium is retained and water follows, expanding blood volume; aldosterone does not affect glucose.

7. (b) — ANP is a natriuretic hormone: it increases sodium and water loss and inhibits renin and aldosterone.

8. (d) — secreting H⁺ removes acid from the blood; the urine can reach pH 4.5.

9. (a) — water leaves the descending limb and salt leaves the ascending limb, while the flows run in opposite directions.

10. (c) — above 180 mg/dL the filtered load passes the Tm of about 375 mg/min, so the excess is excreted.

11. (b) — glomerular damage lets protein and red cells through; glucose (a) points to diabetes.

12. (d) — the peritoneum is the membrane, so no dialyser or heparin is needed and the patient can treat himself at home.

13. (a) — the cortex holds the renal corpuscles and the convoluted tubules; the pyramids (d) are medullary.

14. (c) — water can leave the duct only if the surrounding medulla is saltier than the filtrate.

Set C — Application (10 questions)

1. A person has a GFR of 125 mL/min. The volume of filtrate formed in 24 hours is abouta18 Lb180 Lc1.8 Ld1800 L
2. Of the 180 L of filtrate formed daily, about 1.5 L is voided. The percentage reabsorbed is abouta50 %b75 %c90 %d99 %
3. Plasma glucose is 300 mg/dL and GFR is 125 mL/min. The filtered load of glucose per minute isa375 mgb37.5 mgc300 mgd2400 mg
4. Plasma glucose is 400 mg/dL, GFR 125 mL/min and the Tm for glucose 375 mg/min. Glucose excreted per minute is abouta0 mgb500 mgc125 mgd375 mg
5. Urine creatinine is 100 mg/dL, urine flow 1.5 L/day and plasma creatinine 1.0 mg/dL. Creatinine clearance is abouta15 mL/minb104 mL/minc150 mL/mind1.5 mL/min
6. A patient passes 18 L of urine a day with an osmolarity of 60 mOsm/L and no glucose. The likely diagnosis isadiabetes insipidusbdiabetes mellituscglomerulonephritisda kidney stone
7. In a water-deprivation test the urine stays dilute even after ADH is injected. This shows thatathe pituitary makes too much ADHbthe patient has diabetes mellituscthe kidneys are normaldthe kidney does not respond to ADH
8. A man on a very low salt diet has high renin and high aldosterone. His urine is likely to bearich in sodium and largebrich in glucoseclow in sodium and smallddilute and large
9. In metabolic acidosis the urine becomes more acidic because the tubules secrete moreaK⁺bH⁺cglucosedprotein
10. In haemodialysis the blood and the dialysate flow in opposite directions so thataa concentration gradient is kept along the whole membranebthe blood is warmedcglucose can be addeddclotting is prevented

Answers and explanations — Set C

1. (b) — 125 \times 60 \times 24 = 180\,000 mL = 180 L. Option (a) forgets the 60.

2. (d) — (180 - 1.5)/180 = 0.992, about 99 %.

3. (a) — 300 mg/dL = 3 mg/mL; 3 \times 125 = 375 mg/min. This equals the Tm, so glucose just begins to spill.

4. (c) — load = 4 \times 125 = 500 mg/min; excreted = 500 - 375 = 125 mg/min. Option (b) is the load, not the loss.

5. (b) — clearance = (100 \times 1500)/1.0 = 150\,000 mL/day = 104 mL/min, close to the normal GFR.

6. (a) — a large volume of very dilute urine without glucose is diabetes insipidus; in diabetes mellitus the urine contains glucose and is concentrated.

7. (d) — nephrogenic diabetes insipidus: the hormone is present but the collecting duct does not respond, so the urine stays dilute.

8. (c) — low salt raises aldosterone, which reclaims sodium; water follows, so the volume is small.

9. (b) — the tubule secretes H⁺ to remove acid; potassium secretion (a) is increased in alkalosis, not acidosis.

10. (a) — counter-current flow keeps the concentration difference across the membrane all the way along, so diffusion continues.