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Must-know facts
18 facts
- 1Ammonia: most toxic, needs most water. Uric acid: least toxic, least water. Urea sits between.
- 2Ammonotelic: many bony fishes, aquatic amphibians, aquatic insects. Ureotelic: mammals, many terrestrial amphibians, marine fishes. Uricotelic: reptiles, birds, land snails, insects.
- 3Urea is made from ammonia in the liver; kidneys play no significant role in removing ammonia.
- 4Protonephridia (flame cells): Planaria, rotifers, some annelids, Amphioxus. Nephridia: earthworm. Malpighian tubules: insects. Green (antennal) glands: prawns.
- 5Kidney: 10–12 cm × 5–7 cm × 2–3 cm, 120–170 g, between last thoracic and third lumbar vertebra.
- 6Columns of Bertini: cortex extending between medullary pyramids.
- 7Nearly one million nephrons per kidney; glomerulus + Bowman's capsule = Malpighian body (renal corpuscle).
- 8Corpuscle, PCT and DCT in cortex; loop of Henle dips into medulla; juxta medullary nephrons have long loops and a well-developed vasa recta.
- 9Kidneys filter 1100–1200 mL of blood per minute, about 1/5 of each ventricle's output.
- 10GFR ≈ 125 mL/min = 180 L/day; urine ≈ 1.5 L/day, so nearly 99% of filtrate is reabsorbed.
- 11Filtration barrier: glomerular endothelium, basement membrane, Bowman's capsule epithelium (podocytes with filtration slits); all plasma except proteins passes.
- 12PCT reabsorbs nearly all nutrients and 70–80% of electrolytes and water.
- 13Descending limb: permeable to water, almost impermeable to electrolytes. Ascending limb: impermeable to water, electrolytes leave.
- 14Medullary gradient 300 → about 1200 mOsmol L⁻¹, made mainly by NaCl and urea; urine up to about 4× as concentrated as filtrate.
- 15ADH (vasopressin), from neurohypophysis: more water reabsorbed, less diuresis. Renin → angiotensinogen → angiotensin I → angiotensin II → aldosterone. ANF: vasodilation, checks renin–angiotensin.
- 16Urine: 1–1.5 L/day, pH 6.0, 25–30 g urea/day. Glycosuria and ketonuria indicate diabetes mellitus.
- 17Lungs remove about 200 mL CO₂ per minute.
- 18Haemodialysis: heparin before, anti-heparin after; dialysing fluid = plasma minus nitrogenous wastes.
Common traps
Where marks are lost
Saying the ascending limb of Henle's loop is permeable to water.
Descending limb: water out, electrolytes stay. Ascending limb: electrolytes out, water stays. Down concentrates, up dilutes.
Calling marine fishes ammonotelic because they live in water.
Many bony fishes are ammonotelic, but NCERT lists marine fishes as ureotelic.
Thinking urea is made in the kidney.
The liver converts ammonia into urea; the kidneys filter it out of the blood.
Putting the loop of Henle in the cortex.
Malpighian corpuscle, PCT and DCT are cortical; the loop of Henle dips into the medulla.
Mixing up the kidney's blood flow and GFR.
1100–1200 mL of blood is filtered per minute, but only about 125 mL of filtrate forms per minute (GFR).
Thinking ADH makes more, dilute urine.
ADH increases water reabsorption and prevents diuresis, so urine becomes less and more concentrated.
Saying ANF works with the renin–angiotensin mechanism.
ANF causes vasodilation and lowers blood pressure; it is a check on the renin–angiotensin mechanism.
Believing the vasa recta is well developed in cortical nephrons.
Cortical nephrons have no vasa recta, or only a highly reduced one; it is well developed beside the long loops of juxta medullary nephrons.
Forgetting which drug goes in at which end of dialysis.
Heparin (anticoagulant) is added as blood leaves the artery; anti-heparin before it returns through a vein.
Key terms
22 terms
- Ammonotelism
- Excreting nitrogen mainly as ammonia, which needs plenty of water.
- Ureotelism
- Excreting nitrogen mainly as urea, made from ammonia in the liver.
- Uricotelism
- Excreting nitrogen as uric acid pellets or paste with little water loss.
- Protonephridia
- Flame-cell excretory structures, mainly for osmoregulation, as in Planaria.
- Hilum
- The notch on the kidney's concave side where ureter, vessels and nerves enter.
- Columns of Bertini
- Extensions of the renal cortex between the medullary pyramids.
- Nephron
- The functional unit of the kidney: a glomerulus plus a renal tubule.
- Malpighian body
- Glomerulus together with Bowman's capsule; also called the renal corpuscle.
- Vasa recta
- A U-shaped capillary running beside Henle's loop, from the peritubular network.
- Podocytes
- Bowman's capsule epithelial cells whose arrangement leaves filtration slits.
- Ultrafiltration
- Filtration so fine that everything in plasma except proteins passes.
- GFR
- Volume of filtrate formed by the kidneys per minute, about 125 mL.
- JGA
- Sensitive region where DCT and afferent arteriole meet; releases renin when GFR falls.
- Counter current mechanism
- Opposite flows in Henle's loop and vasa recta that keep the medullary osmotic gradient.
- ADH
- Antidiuretic hormone (vasopressin): increases water reabsorption in the later tubule.
- Aldosterone
- Adrenal cortex hormone, released under angiotensin II, that increases distal Na⁺ and water reabsorption.
- ANF
- Atrial Natriuretic Factor: dilates vessels and lowers blood pressure, checking renin–angiotensin.
- Micturition
- Release of urine from the bladder, driven by the micturition reflex.
- Uremia
- Build-up of urea in blood when the kidneys fail.
- Haemodialysis
- Clearing urea from blood through a cellophane membrane in an artificial kidney.
- Renal calculi
- Kidney stones of crystallised salts such as oxalates.
- Glomerulonephritis
- Inflammation of the glomeruli.