Cram sheet
30 points- Osmolarity = osmotically active moles of solute per liter. A 1 M sucrose solution is 1 Osm, but a 1 M NaCl solution is 2 Osm because NaCl dissociates into two particles. This is the classic calculation trick. Watch Osmosis and Water Potential (Updated) on YouTube, from Amoeba Sisters
- Water always moves from lower solute concentration toward higher solute concentration. Hypertonic solution → cell shrinks. Hypotonic → cell swells. Isotonic → no net movement. Watch Osmosis and Water Potential (Updated) on YouTube, from Amoeba Sisters
- Osmoconformers match their environment and sit in osmotic equilibrium — most marine invertebrates, and among vertebrates only the hagfish is a strict osmoconformer. Watch Osmoregulation: Osmoconformers & Osmoregulators on YouTube, from Biology Professor
- Osmoregulators hold blood osmolarity constant no matter what the environment does. All vertebrates except hagfish are osmoregulators. Watch Osmoregulation: Osmoconformers & Osmoregulators on YouTube, from Biology Professor
- Sharks and rays (Chondrichthyes) are isotonic to seawater by a trick: their blood NaCl is lower than seawater, but they retain urea at roughly 100× mammalian levels to make up the difference.
- Freshwater vertebrates are hypertonic to the water — water floods in and ions leak out. Fix: do not drink, excrete large volumes of very dilute urine, and actively pump ions IN across the gills. Watch Osmoregulation on YouTube, from Bozeman Science
- Marine bony fish (teleosts) are hypotonic — about one-third the osmolarity of seawater — so they lose water. Fix: drink seawater, actively pump Na⁺ and Cl⁻ OUT across the gills, secrete Mg²⁺ and SO₄²⁻ into the tubules, and produce isotonic urine. Watch Osmoregulation on YouTube, from Bozeman Science
- Nitrogen waste starts as ammonia (NH₃) from deamination of amino acids and nucleic acids in the liver. Ammonia is extremely toxic and needs lots of water. Watch Ammonotelic vs Ureotelic vs Uricotelic (with Examples) | NEET Class 11 | Bharti Ma'am on YouTube, from NCERT NEET Adda247
- Who excretes what: ammonia → bony fish and amphibian tadpoles (diffuses out the gills). Urea → mammals, adult amphibians, and cartilaginous fish. Uric acid → reptiles, birds, and insects. Watch Ammonotelic vs Ureotelic vs Uricotelic (with Examples) | NEET Class 11 | Bharti Ma'am on YouTube, from NCERT NEET Adda247
- The trade-off across the top of Figure 49.2: converting ammonia → urea → uric acid costs energy but saves water. Uric acid is the most expensive to make and the most water-thrifty. Watch Ammonotelic vs Ureotelic vs Uricotelic (with Examples) | NEET Class 11 | Bharti Ma'am on YouTube, from NCERT NEET Adda247
- Uric acid is insoluble, so it precipitates as a solid — that is why it works inside a shelled egg (a solid crystal cannot poison the embryo) and why bird droppings have white guano.
- Most mammals have uricase, which converts uric acid to soluble allantoin. Humans, apes, and Dalmatian dogs lack it — uric acid buildup in joints causes gout.
- Insects do NOT filter. Malpighian tubules secrete K⁺ and wastes into the tubule, water follows osmotically, and most water and K⁺ are then reabsorbed across the hindgut. Extremely water-efficient. Watch Protonephridia/Flame Cells, Nephridia, Malpighian Tubules, Green Glands, Kidneys: Excretory Organs on YouTube, from Examrace (UPSC, NET, NCERT, ICSE ...)
- Vertebrate kidneys work the opposite way: filter almost everything first, then selectively reabsorb what is needed. Wasteful-looking but flexible — the body never has to recognize every possible waste. Watch Renal | Filtration, Reabsorption, and Secretion: Overview on YouTube, from Ninja Nerd
- Reabsorption = out of the tubule, back to the blood. Secretion = out of the blood, into the tubule. Memorize the direction, not the word. Watch Renal | Filtration, Reabsorption, and Secretion: Overview on YouTube, from Ninja Nerd
- Nephron order: glomerulus → Bowman’s capsule → proximal convoluted tubule → descending limb of loop of Henle → ascending limb → distal convoluted tubule → collecting duct → renal pelvis → ureter. Watch Excretory System and the Nephron on YouTube, from Amoeba Sisters Watch Urinary System, Part 1: Crash Course Anatomy & Physiology #38 on YouTube, from CrashCourse
- The afferent arteriole is much larger than the efferent arteriole. That size mismatch is what creates the pressure that drives filtration. Watch Urinary System, Part 1: Crash Course Anatomy & Physiology #38 on YouTube, from CrashCourse
- Numbers: ~2000 L of blood pass through the kidneys per day, 180 L of filtrate is made, but only 1–2 L of urine leaves. Total blood volume is only ~5 L, so every liter is filtered many times a day.
- Proximal convoluted tubule: reabsorbs virtually all nutrients plus about two-thirds of the NaCl and water. Active transport of Na⁺ leads; Cl⁻ follows electrically, water follows osmotically. Filtrate here stays isotonic. Watch Renal | Filtration, Reabsorption, and Secretion: Overview on YouTube, from Ninja Nerd
- The loop of Henle is found only in mammals and birds — that is exactly why only mammals and birds can make urine hypertonic to their blood. Watch Renal | Loop of Henle on YouTube, from Ninja Nerd
- Countercurrent multiplier: the ascending limb is impermeable to water and pumps NaCl out; the descending limb is permeable to water but not NaCl, so water leaves. Filtrate goes from 300 mOsm entering the loop to over 1200 mOsm at the bottom of the longest loops. Watch Countercurrent multiplication in the kidney | Renal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine Watch Renal | Loop of Henle on YouTube, from Ninja Nerd
- Medullary gradient to memorize: cortex 300 → outer medulla 600 → inner medulla 1200 mOsm. Urea leaking from the descending limb and collecting duct also contributes. Watch Countercurrent multiplication in the kidney | Renal system physiology | NCLEX-RN | Khan Academy on YouTube, from khanacademymedicine
- Fluid arriving at the distal convoluted tubule is hypotonic — only 100 mOsm — because NaCl was pumped out of the ascending limb without water following. Watch Renal | Loop of Henle on YouTube, from Ninja Nerd
- The longer the loop of Henle, the more concentrated the urine. Humans reach 4.2× plasma; camels 8×; gerbils 14×; pocket mice 22×. Kangaroo rats never drink — metabolic water alone is enough.
- ADH (made in the hypothalamus, released from the posterior pituitary) inserts aquaporins into the distal convoluted tubule and collecting duct → more water reabsorbed → small volume of concentrated urine (as little as 600 mL/day at maximum ADH). Trigger: rising plasma osmolarity. Watch Antidiuretic Hormone (ADH) and Aquaporin Channel Proteins on YouTube, from 5MinuteSchool Watch Urinary System, Part 2: Crash Course Anatomy & Physiology #39 on YouTube, from CrashCourse
- No ADH = diabetes insipidus = constant flood of dilute urine. Do not confuse it with diabetes mellitus, which is glucose in the urine once blood glucose passes ~180 mg/100 mL and saturates the tubule carriers. Watch Antidiuretic Hormone (ADH) and Aquaporin Channel Proteins on YouTube, from 5MinuteSchool
- Aldosterone (from the adrenal cortex) acts on the same distal tubule and collecting duct but for salt: reabsorb Na⁺ (Cl⁻ and water follow) and secrete K⁺. Net effect: blood volume and pressure go up, blood K⁺ goes down. Watch Renin-Angiotensin-Aldosterone System Overview on YouTube, from Armando Hasudungan Watch Urinary System, Part 2: Crash Course Anatomy & Physiology #39 on YouTube, from CrashCourse
- RAAS: low blood flow past the juxtaglomerular apparatus → renin → angiotensinogen becomes angiotensin I → converted to angiotensin II → vessels constrict AND adrenal cortex releases aldosterone. Watch Renin-Angiotensin-Aldosterone System Overview on YouTube, from Armando Hasudungan
- Atrial natriuretic peptide (ANP) is the opposite of aldosterone: released by the stretched right atrium when blood volume is high, it suppresses aldosterone and drives salt and water excretion. Watch Release & Functions of Atrial Natriuretic Peptide [ANP] on YouTube, from Catalyst University
- Alcohol and caffeine suppress ADH — extra urine, then dehydration. This is a favorite applied question.