The three nitrogenous wastes
3 rows| Waste | Toxicity | Solubility | Water cost | Energy cost | Who uses it |
|---|---|---|---|---|---|
| Ammonia (NH₃) | Very toxic | Highly soluble | Highest — needs lots of water | Lowest — no conversion | Bony fishes, amphibian tadpoles, aquatic invertebrates |
| Urea | Much less toxic | Water-soluble | Moderate | Moderate | Mammals, adult amphibians, cartilaginous fish |
| Uric acid | Least toxic | Insoluble — precipitates | Lowest — saves the most water | Highest | Reptiles, birds, insects |
Osmoregulation strategies by group
9 rows| Group | Relative to environment | Drink? | Ion handling | Urine produced |
|---|---|---|---|---|
| Marine invertebrates | Isotonic (osmoconformers) | n/a | Little regulation needed | — |
| Hagfish | Isotonic — the only strict vertebrate osmoconformer | n/a | Matches seawater | — |
| Freshwater teleost | Hypertonic — water floods in | No | Actively absorb Na⁺/Cl⁻ at gills; reabsorb ions in tubules | Large volume, very dilute (hypotonic) |
| Marine teleost | Hypotonic — ~1/3 seawater; water lost | Yes, seawater | Actively secrete Na⁺/Cl⁻ at gills; secrete Mg²⁺/SO₄²⁻ into tubules | Isotonic, small volume |
| Shark / ray (elasmobranch) | Isotonic via retained urea (~100× mammal levels) | No need | Reabsorb urea in the nephron | Isotonic |
| Amphibian (freshwater) | Hypertonic | No | Pump Na⁺ inward across the skin | Dilute |
| Marine reptile / marine bird | Hypotonic | Yes, seawater | Salt glands (nasal, orbital, or near the eyes) | Isotonic (reptile) / up to 2× (bird) |
| Terrestrial reptile | Loses water to air | Fresh water | Reabsorb most salt and water; extra water pulled back in cloaca | Never above plasma |
| Mammal / bird | Loses water to air | Fresh water | Loop of Henle builds a medullary gradient | Hypertonic — only these two groups |
What each nephron segment does
7 rows| Segment | Location | Main job | Permeability / key detail |
|---|---|---|---|
| Glomerulus | Cortex | Filtration under pressure | Afferent arteriole larger than efferent; proteins and cells stay in blood |
| Bowman's capsule | Cortex | Collects the filtrate | Slit openings admit filtrate; filtrate is isotonic (300 mOsm) |
| Proximal convoluted tubule | Cortex | Reabsorb all nutrients + 2/3 of NaCl and water; reabsorb K⁺ | Na⁺ pumped actively, Cl⁻ follows electrically, water follows osmotically; fluid stays isotonic |
| Descending limb | Medulla | Water leaves by osmosis | Permeable to water, NOT to NaCl; fluid gets concentrated toward the bend |
| Ascending limb | Medulla | NaCl leaves | Impermeable to water; thick part pumps Na⁺ actively, thin part lets NaCl diffuse |
| Distal convoluted tubule | Cortex | Fine-tune Na⁺ (aldosterone), secrete K⁺ and H⁺, reabsorb HCO₃⁻ | Fluid arrives hypotonic at 100 mOsm |
| Collecting duct | Cortex → medulla | Final adjustable water reabsorption | Permeability set by ADH inserting aquaporins; also leaks urea into the medulla |
Numbers to memorize
15 rows| Quantity | Value |
|---|---|
| Blood through the kidneys per day | ~2000 L |
| Glomerular filtrate per day | 180 L |
| Urine per day | 1–2 L (as low as 600 mL at maximal ADH) |
| Total blood volume | ~5 L |
| Nephrons per human kidney | ~1 million |
| Filtrate entering the loop of Henle | 300 mOsm |
| Bottom of the longest loops / inner medulla | >1200 mOsm |
| Fluid entering the distal tubule | 100 mOsm (hypotonic) |
| Medullary gradient | 300 cortex → 600 outer medulla → 1200 inner medulla |
| Fraction of NaCl and water reabsorbed in the proximal tubule | Two-thirds (180 L → 60 L) |
| Renal glucose carrier saturation | ~180 mg/100 mL blood |
| Urine pH / blood pH | 5–7 / 7.35–7.45 |
| Max urine concentration: human / camel / gerbil / pocket mouse | 4.2× / 8× / 14× / 22× plasma |
| Max urine concentration: bird | about 2× blood |
| Shark blood urea | ~100× mammalian levels |
The three osmoregulatory hormones
3 rows| Hormone | Source | Trigger | Target | Action | Net effect |
|---|---|---|---|---|---|
| ADH (antidiuretic hormone) | Made in hypothalamus, released by posterior pituitary | Rising plasma osmolarity (dehydration, salty food) | Distal convoluted tubule + collecting duct | Inserts aquaporins → more water permeability | Water retained; small volume of concentrated urine; thirst too |
| Aldosterone | Adrenal cortex | Low blood Na⁺ / volume / pressure, via renin → angiotensin II | Distal convoluted tubule + collecting duct | Reabsorb Na⁺ (Cl⁻ and water follow); secrete K⁺ | Salt and water retained; blood volume and pressure rise, blood K⁺ falls |
| ANP (atrial natriuretic peptide) | Right atrium of the heart | Stretch from high blood volume | Kidney; also suppresses aldosterone | Cuts Na⁺ and Cl⁻ reabsorption | Salt and water excreted; blood volume falls |
Exam traps
11 pairsQuestions get built out of near-misses. If you can state each difference in one sentence, you will not lose those points.
*Reabsorption*vs*Secretion*
Reabsorption goes OUT of the tubule and back into the blood (keeps things). Secretion goes INTO the tubule from the blood (throws things away). Say the direction out loud before answering.
*Renal cortex*vs*Renal medulla*
Cortex is the OUTER layer and holds glomeruli, Bowman’s capsules, and both convoluted tubules — osmolarity 300. Medulla is INNER, holds the loops of Henle and collecting ducts, and is where osmolarity climbs to 1200.
*ADH*vs*Aldosterone*
ADH moves WATER only, by inserting aquaporins. Aldosterone moves SALT (Na⁺ in, K⁺ out) and water follows the salt. Both act on the distal tubule and collecting duct, which is why they get confused.
*Aldosterone*vs*ANP*
They are exact opposites. Aldosterone = keep salt and water, raise blood volume. ANP = dump salt and water, lower blood volume. ANP also directly suppresses aldosterone.
*Freshwater fish*vs*Marine bony fish*
Freshwater fish are HYPERTONIC: water floods in, so do NOT drink, make lots of dilute urine, pump ions IN at the gills. Marine fish are HYPOTONIC: water leaks out, so DRINK seawater, make isotonic urine, pump ions OUT at the gills. Everything reverses.
*Marine bony fish (teleost)*vs*Shark (elasmobranch)*
Both live in seawater. The teleost is hypotonic and fights it constantly by drinking and pumping ions. The shark cheats — it RETAINS UREA until its blood is isotonic to seawater, so there is no gradient to fight.
*Ascending limb*vs*Descending limb*
Ascending = impermeable to WATER, loses SALT. Descending = permeable to WATER, keeps SALT. Mnemonic: A for Against water, D for Dehydrating.
*Countercurrent multiplier* (loop of Henle)vs*Countercurrent exchange* (vasa recta)
The loop BUILDS the gradient using energy. The vasa recta merely PRESERVES it while delivering blood. Multiplier makes, exchanger protects.
*Diabetes mellitus*vs*Diabetes insipidus*
Mellitus = sugar problem; blood glucose exceeds ~180 mg/100 mL and glucose spills into the urine. Insipidus = ADH problem; no aquaporins, so a flood of dilute, tasteless urine. Both cause thirst and high urine volume, for completely different reasons.
*Malpighian tubules* (insect)vs*Vertebrate kidney*
Insects SECRETE first (K⁺ and wastes in, water follows) and never filter — there is no pressure gradient. Vertebrates FILTER first, then selectively reabsorb. Opposite order.
*Uric acid* excretersvs*Urea* excreters
Uric acid: reptiles, birds, insects — insoluble, precipitates, saves the most water, works inside a shelled egg, costs the most energy. Urea: mammals, adult amphibians, sharks — soluble, cheaper to make, needs more water.