Campbell Biology Chapter 44 Review: Osmoregulation and Excretion

44.1 Balancing water and salt

pp. 953–958

On the AP exam? Yes

Topic 2.7 covers osmosis and osmoregulation, and Topics 2.5–2.8 cover the passive and active transport behind it. Words like osmoconformer, euryhaline and anhydrobiosis are background; the exam uses tonicity and water potential instead.

In the course: Topic 2.7 Tonicity and Osmoregulation, Topic 2.5 Membrane Transport, Topic 2.6 Facilitated Diffusion, Topic 2.8 Mechanisms of Transport, Topic 3.5 Cellular Respiration (notes, videos and more questions)

Key points

Osmoregulation is how an animal controls the water and dissolved particles in its body fluids. Animals mostly move solutes on purpose and let water follow by osmosis.
Osmolarity is the total concentration of dissolved particles, measured in milliosmoles per liter (mOsm/L). The ocean is about 1,000 mOsm/L and human blood about 300. Water moves toward the hyperosmotic side, the side with more solute and lower water potential.
Osmoconformers, which all live in the sea, let their body fluids match the water around them, though they still adjust individual ions. Osmoregulators hold their internal osmolarity steady, which lets them live in fresh water, on land, or in the sea with fluids unlike seawater.
Marine bony fishes are less salty than the ocean, so they lose water and gain salt. They drink seawater, pump salt out at their gills and make little urine. Freshwater fishes face the reverse: they hardly drink, make lots of dilute urine and pump salt in at their gills. Sharks avoid water loss by keeping urea in their blood.
Land animals lose water in urine, feces, sweat and breath. They save it with waterproof body coverings, nighttime activity and concentrated urine, and they replace it by drinking, eating moist food and making water through cellular respiration. A few tiny animals can dry out almost completely and revive later; sugars such as trehalose and special protective proteins keep their cells intact.
Keeping body fluids different from the surroundings costs ATP, because pumps must keep working against diffusion. The bigger the difference, the higher the cost, so freshwater animals tend to have more dilute body fluids than their marine relatives.
Transport epithelia do most of this work: sheets of cells with different pumps and channels on their two sides, so they move particular solutes in one direction. Salt glands in some seabirds and marine reptiles are an example that lets them drink seawater and still gain water.

Key terms (14)

osmoregulation: Controlling the amounts of water and dissolved solutes in your body fluids so they stay in a healthy range.
osmolarity: The total concentration of dissolved particles in a solution, often given in milliosmoles per liter (mOsm/L).
hyperosmotic: Having a higher total solute concentration than the solution it's being compared with. Water moves toward it by osmosis.
hypoosmotic: Having a lower total solute concentration than the solution it's being compared with. Water moves away from it.
isoosmotic: Having the same total solute concentration as another solution, so there's no net water movement between them.
osmoconformer: An animal whose body fluids match the osmolarity of the seawater around it instead of being held at a set level.
osmoregulator: An animal that keeps its internal osmolarity steady, even when it differs from the water or air around it.
stenohaline: Able to survive only a narrow range of salt concentrations in the surrounding water.
euryhaline: Able to survive a wide range of salt concentrations, like fish that move between rivers and the sea.
anhydrobiosis: A dormant state in which a few tiny animals survive losing almost all their body water, then revive when wet.
trehalose: A disaccharide some drought-surviving organisms make that protects proteins and membranes while they're dried out.
metabolic water: Water made inside cells as a product of cellular respiration. Some desert animals get most of their water this way.
transport epithelium: A layer of cells with different transport proteins on each side, so it moves particular solutes in one direction.
salt gland: A gland in some seabirds and marine reptiles that pumps out a fluid saltier than seawater, getting rid of extra salt.

Check yourself: 44.1 Balancing water and salt

4 questions on 44.1 Balancing water and salt. Pick an answer to see if you got it, and why.

Question 1 of 4

A shore crab was held in water of different salt concentrations until its hemolymph (body fluid) stopped changing. The results are shown below (invented data). Water osmolarity (mOsm/L) | Hemolymph osmolarity (mOsm/L) 1,000 | 1,010 800 | 820 600 | 700 400 | 640 200 | 600 Which statement best describes this crab?

Question 2 of 4

The same crabs used about 20% more oxygen per gram of body mass in 200 mOsm/L water than in 1,000 mOsm/L water. Which explanation best fits all of the data?

Question 3 of 4

A small estuary fish that can live in both seawater and fresh water is moved from the ocean into a freshwater stream. Which set of changes would best help it keep its body fluids stable?

Question 4 of 4

Sea stars are osmoconformers that tolerate only small changes in salinity. After a heavy rain, a tide pool's water drops to 70% of normal seawater osmolarity. What most likely happens to a sea star stranded in the pool?

0 of 4 answered

44.2 Ammonia, urea and uric acid

pp. 958–959

On the AP exam? Background

The current course doesn't cover nitrogen wastes, but it does test that nitrogen comes from proteins and nucleic acids (Topic 1.2), that building molecules costs energy (Topic 3.3), and that traits fit an organism's environment through natural selection (Topic 7.2).

In the course: Topic 1.2 Elements of Life, Topic 3.3 Cellular Energy, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

When animals break down amino acids or nucleic acid bases, the nitrogen comes off as ammonia (NH₃). Ammonia is toxic even at low levels, so it has to be flushed out fast or turned into something safer first.
Ammonia dissolves easily and slips across membranes, so many water-dwelling animals simply let it diffuse out across their gills or body surface. That only works when lots of water is on hand to carry it off.
Urea is much less toxic, so it can travel in the blood and be stored until it's excreted in fairly concentrated urine. Mammals, most adult amphibians and sharks rely on it. The liver builds it from ammonia and CO₂, which costs ATP.
Uric acid barely dissolves, so it can leave the body as a whitish paste with very little water. Insects, land snails, birds and other reptiles use it. It saves the most water but is the most expensive to make.
The waste an animal uses reflects both its evolutionary history and its habitat, especially how much water it has. Embryos in shelled eggs that let gases through but not liquids benefit from uric acid, which drops out of solution as a harmless solid instead of building up.
How much nitrogen waste an animal makes depends on how much it eats and what it eats. Endotherms take in more food than ectotherms, and animals that burn a lot of protein for fuel make more nitrogen waste than those living mainly on fats and carbohydrates.

Key terms (9)

nitrogenous waste: The nitrogen-containing waste left after an animal breaks down proteins and nucleic acids: ammonia, urea or uric acid.
ammonia (NH₃): A small, very soluble, toxic molecule released when amino groups are removed. It must be diluted in lots of water to be excreted safely.
ammonium (NH₄⁺): The ion ammonia forms when it picks up an H⁺. At body pH, most of the ammonia in body fluids is in this form.
urea: A low-toxicity nitrogen waste made in the liver from ammonia and CO₂. It costs energy to make but saves water.
uric acid: A nitrogen waste that barely dissolves in water, so it leaves the body as a thick paste. It saves the most water but costs the most energy to make.
excretion: Getting rid of metabolic wastes, such as nitrogen wastes, from the body.
amino group: The –NH₂ part of an amino acid. Removing it to use the rest of the molecule for energy is what frees nitrogen as ammonia.
purine: A type of two-ring nitrogen base found in nucleic acids (adenine and guanine). Breaking down purines produces uric acid in many animals.
shelled egg: An egg wrapped in a shell that lets gases in and out but holds liquids in, as in birds and other reptiles.

Check yourself: 44.2 Ammonia, urea and uric acid

4 questions on 44.2 Ammonia, urea and uric acid. Pick an answer to see if you got it, and why.

Question 1 of 4

Question 2 of 4

Gulf toadfish usually excrete most of their nitrogen as ammonia. Researchers hypothesize that toadfish kept crowded in small tanks switch to excreting mostly urea. Which result would best support this hypothesis?

Question 3 of 4

Four groups of the same animal species eat diets with equal calories. Which diet would most likely make them excrete the most nitrogen waste?

Question 4 of 4

A lizard embryo develops for two months inside a shell that lets gases through but not liquids. Suppose a mutation made the embryo excrete its nitrogen as urea instead of uric acid. What would most likely happen?

0 of 4 answered

44.3 Excretory systems built from tubes

pp. 960–963

On the AP exam? Background

You won't be asked about flame bulbs, metanephridia, Malpighian tubules or kidney anatomy. Filtration, reabsorption and secretion are applications of membrane permeability (Topic 2.4), passive and active transport (Topics 2.5 and 2.8) and osmosis (Topic 2.7).

In the course: Topic 2.5 Membrane Transport, Topic 2.8 Mechanisms of Transport, Topic 2.4 Membrane Permeability, Topic 2.7 Tonicity and Osmoregulation (notes, videos and more questions)

Key points

Most excretory systems work in four steps. Pressure pushes body fluid through a barrier that holds back cells and proteins (filtration). Useful substances are pulled back (reabsorption), unwanted ones are added (secretion), and what's left leaves the body (excretion).
Filtration isn't picky: everything small enough passes through, nutrients and wastes alike. The body then takes back what it wants, so it can get rid of unfamiliar toxins without needing a special carrier for each one.
Excretory organs are built from networks of tubes, which give a huge surface area for exchanging water and solutes.
Flatworms have protonephridia: branching dead-end tubes capped by flame bulbs, whose beating cilia draw fluid in. In freshwater flatworms they mainly get rid of extra water as dilute urine.
Earthworms have a pair of metanephridia in each segment. Each one collects fluid from the body cavity through a ciliated funnel, reclaims most of its solutes into nearby capillaries and releases dilute urine.
Insects use Malpighian tubules, which skip filtration. Their walls secrete wastes and salts into the tubule, water follows, and the rectum later takes back most water and salts, leaving nearly dry uric acid. This water saving is one reason insects do so well on land.
Vertebrates have kidneys packed with tubules called nephrons. In humans, each nephron has a glomerulus (a ball of capillaries) inside Bowman's capsule, then a proximal tubule, a loop of Henle and a distal tubule that drains into a shared collecting duct. Urine leaves each kidney through a ureter to the bladder and exits through the urethra.

Key terms (14)

filtration: The first step of making urine: pressure pushes water and small solutes out of the blood or body fluid, while cells and large proteins stay behind.
filtrate: The fluid formed by filtration. It holds water and small solutes at about the same concentrations as blood plasma.
reabsorption: Moving useful substances, such as glucose, salts and water, from the filtrate back into the body fluids.
secretion: Moving substances, such as toxins or extra ions, from body fluids into the filtrate.
protonephridium: A flatworm's excretory organ: a branching network of dead-end tubes that mainly removes extra water.
flame bulb: The cap at the end of a protonephridium tube. Its beating tuft of cilia draws body fluid into the tube.
metanephridium: An earthworm's excretory organ, one pair per segment, which collects fluid from the body cavity through a ciliated funnel.
Malpighian tubule: An insect excretory tube that opens into the gut. Its walls secrete wastes from the hemolymph into it instead of filtering them.
kidney: The vertebrate organ that filters blood and adjusts the filtrate to control water, salt and waste levels.
nephron: One of the tiny tubules that do the kidney's work. A human kidney holds about a million of them.
glomerulus: A ball of capillaries at the start of a nephron, where fluid is pushed out of the blood.
Bowman's capsule: The cup-shaped start of a nephron that wraps around the glomerulus and collects the filtrate.
ureter: The tube that carries urine from a kidney to the urinary bladder.
urethra: The tube that carries urine from the bladder out of the body.

Check yourself: 44.3 Excretory systems built from tubes

4 questions on 44.3 Excretory systems built from tubes. Pick an answer to see if you got it, and why.

Question 1 of 4

Question 2 of 4

A freshwater rotifer's protonephridia empty into a small bladder that contracts to push fluid out. Researchers counted bladder contractions in water of different solute concentrations (invented data). Medium osmolarity (mOsm/L) | Bladder contractions per minute 5 | 12 40 | 8 80 | 4 120 | 1 Which explanation best fits these results?

Question 3 of 4

The goosefish, a marine bony fish, has kidneys with no glomeruli at all, so no fluid is filtered from its blood. It still produces a small amount of urine rich in Mg²⁺ and SO₄²⁻. How is this urine most likely formed?

Question 4 of 4

A mutant fruit fly makes nonfunctional ion pumps in the cells lining its rectum. Its Malpighian tubules work normally. Compared with normal flies, the mutants produce much wetter droppings and die sooner in dry air. Which step of excretion has failed?

0 of 4 answered

44.4 Inside the nephron: from filtrate to urine

pp. 963–968

On the AP exam? Background

Nephron regions aren't tested by name, but the kidney is a favorite setting for tested ideas: aquaporins and facilitated diffusion (Topic 2.6), osmosis and water potential (Topic 2.7), active transport (Topic 2.8) and structure shaped by natural selection (Topic 7.2).

In the course: Topic 2.6 Facilitated Diffusion, Topic 2.7 Tonicity and Osmoregulation, Topic 2.8 Mechanisms of Transport, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

Filtrate starts out with the same small solutes as blood plasma at the same concentrations, including salts, glucose, amino acids, vitamins and urea, but no cells and almost no protein. Humans make about 180 L of it a day and keep all but about 1.5 L.
The proximal tubule does the bulk recovery. It pumps out Na⁺, with Cl⁻ and water following, and reclaims nearly all the glucose and amino acids and most of the bicarbonate. It also secretes H⁺, ammonia and many drugs into the filtrate. The volume drops a lot, but the osmolarity stays near 300 mOsm/L.
The loop of Henle's descending limb is full of aquaporins but nearly closed to salt. As it dips into the ever saltier medulla, water leaves by osmosis and the filtrate gets more concentrated.
The ascending limb is the reverse: salt can leave but water can't. NaCl moves out passively in the thin part and is pumped out in the thick part, so the filtrate is more dilute than blood when it reaches the distal tubule. The distal tubule then fine-tunes K⁺, NaCl and pH.
The loop acts as a countercurrent multiplier. By spending ATP to push salt out of the ascending limb, it builds a gradient from about 300 mOsm/L in the cortex to about 1,200 deep in a human medulla. Urea leaking from the bottom of the collecting duct adds to it. The vasa recta, capillaries that loop down and back up, supply the medulla without washing the gradient away.
The collecting duct runs back down through that gradient. When its walls have aquaporins, water leaves by osmosis and urine can get as concentrated as the deep medulla, up to about four times the osmolarity of blood in humans. Without them, dilute urine flows out.
Kidneys fit habitat. Desert mammals have very long loops and make extremely concentrated urine, while mammals with plenty of fresh water have short loops. Birds have shorter loops than mammals and save water mostly by excreting uric acid. Freshwater fishes filter a lot and make dilute urine. Marine bony fishes make little urine, mainly to get rid of ions like Mg²⁺ and SO₄²⁻.

Key terms (13)

proximal tubule: The first stretch of the nephron after Bowman's capsule, where most salt and water and nearly all the useful nutrients are reclaimed.
loop of Henle: The hairpin section of the nephron that dips into the medulla and builds its salt gradient.
descending limb: The downward side of Henle's loop. It lets water out, but salt mostly can't leave.
ascending limb: The upward side of the loop of Henle. Salt leaves it, but water can't, so the filtrate becomes dilute.
distal tubule: The part of the nephron after the loop of Henle that fine-tunes K⁺, NaCl and pH before the filtrate enters a collecting duct.
collecting duct: The tube that gathers filtrate from many nephrons and carries it through the medulla, where its water permeability sets how concentrated urine becomes.
aquaporin: A channel protein that lets water cross a membrane quickly by facilitated diffusion.
renal cortex: The outer layer of the kidney, where glomeruli and the proximal and distal tubules sit.
renal medulla: The inner part of the kidney, where loops of Henle and collecting ducts run through a steep salt gradient.
countercurrent multiplier: A system of two side-by-side tubes with fluid flowing in opposite directions that uses energy to build up a concentration gradient, like the loop of Henle.
vasa recta: Hairpin-shaped capillaries running alongside the loops of Henle that bring blood to the medulla without washing out its salt gradient.
juxtamedullary nephron: A nephron with a long loop that reaches deep into the medulla. These nephrons let mammals make concentrated urine.
cortical nephron: A nephron with a short loop that barely reaches the medulla. Most human nephrons are this type.

Check yourself: 44.4 Inside the nephron: from filtrate to urine

4 questions on 44.4 Inside the nephron: from filtrate to urine. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A dog's kidneys filter about 60 L of fluid from its blood each day, and it excretes about 0.9 L of urine per day. About what percentage of the filtered water is reabsorbed?

Question 2 of 4

A class of diabetes drugs blocks the Na⁺–glucose cotransporter that reclaims most glucose from the filtrate in the proximal tubule. Which effect would you expect in a person taking one of these drugs?

Question 3 of 4

The table gives the osmolarity of the filtrate at several points along one nephron of a water-deprived mammal (approximate values). Point in the nephron | Filtrate osmolarity (mOsm/L) 1. End of proximal tubule | 300 2. Bend of the loop of Henle | 1,200 3. Start of distal tubule | 100 4. End of collecting duct | 1,200 Between which two points does the filtrate lose solute while its walls hold the water in?

Question 4 of 4

Suppose a mutation put aquaporins into the membranes of the thick part of the loop of Henle's ascending limb, where they're normally missing. What would most likely happen?

0 of 4 answered

44.5 Hormones that tune the kidney

pp. 968–971

On the AP exam? Background

ADH, renin, angiotensin, aldosterone and ANP aren't in the current course, so you won't be asked to name them or recall their details. This section is still great practice for tested ideas: hormones acting only on cells with the right receptor (Topic 4.1), second messengers like cAMP (Topics 4.2 and 4.3) and negative feedback (Topic 4.4).

In the course: Topic 4.1 Cell Communication, Topic 4.2 Introduction to Signal Transduction, Topic 4.3 Signal Transduction Pathways, Topic 4.4 Feedback, Topic 2.6 Facilitated Diffusion (notes, videos and more questions)

Key points

A mammal's kidneys can switch between a little very concentrated urine, when water is short or salt intake is high, and lots of very dilute urine, when there's extra water to lose. Hormones decide which.
Antidiuretic hormone (ADH), which also goes by vasopressin, is made in the hypothalamus and released from the posterior pituitary. When sensors in the hypothalamus detect blood osmolarity rising above its set point (roughly 290–300 mOsm/L), the posterior pituitary puts out more ADH and you feel thirsty. A large drop in blood volume also triggers ADH.
ADH binds a receptor on collecting duct cells and raises cAMP inside them. The signal moves vesicles carrying aquaporins into the membrane facing the filtrate, so more water is reabsorbed and the urine gets concentrated. When ADH falls, the aquaporins are pulled back in and dilute urine flows.
This is negative feedback: holding onto water pulls osmolarity down to normal again, and that cuts ADH release. Only drinking fully makes up the loss. If ADH signaling fails, because the hormone, its receptor or the aquaporins don't work, a person makes huge amounts of dilute urine (diabetes insipidus, now often called vasopressin deficiency or resistance).
The renin–angiotensin–aldosterone system (RAAS) responds to low blood pressure or volume. Cells near the glomerulus release the enzyme renin, which starts a chain turning the blood protein angiotensinogen into angiotensin II. Angiotensin II narrows arterioles and makes the adrenal glands release aldosterone, which makes the distal tubules and collecting ducts reabsorb more Na⁺, with water following.
ADH and the RAAS overlap but do different jobs. ADH mainly fixes high osmolarity by adding back water. The RAAS fixes low volume or pressure by adding back salt and water together, which keeps osmolarity steady. As pressure recovers, renin release falls, another negative feedback loop.
Atrial natriuretic peptide (ANP), released when extra blood volume stretches the heart's atria, works against the RAAS. It reduces renin and aldosterone and salt reabsorption, so more salt and water leave in the urine.

Key terms (12)

antidiuretic hormone (ADH): A hormone, also called vasopressin, that makes the collecting ducts hold onto more water, giving smaller amounts of more concentrated urine.
osmoreceptor: A sensory cell, in the hypothalamus for example, that detects changes in the osmolarity of the blood.
posterior pituitary: The back part of the pituitary gland, which stores and releases ADH made by the hypothalamus.
diuresis: Making a large volume of urine. ADH works against it, which is where its name comes from.
diabetes insipidus: A condition in which ADH is missing or the kidney can't respond to it, so a person makes huge amounts of dilute urine. Doctors now often call it vasopressin deficiency or resistance.
set point: The target value that a feedback system keeps a variable close to, like a blood osmolarity of about 290–300 mOsm/L.
renin: An enzyme released by cells near the glomerulus when blood pressure or volume falls. It starts the chain that makes angiotensin II.
angiotensin II: A hormone made from a blood protein after renin release. It narrows arterioles and triggers aldosterone release.
aldosterone: A steroid hormone from the adrenal glands that makes the last parts of the nephron and the collecting ducts reclaim more Na⁺, with water following.
renin–angiotensin–aldosterone system (RAAS): The hormone chain that raises blood pressure and volume when they fall.
juxtaglomerular apparatus (JGA): A group of cells near the glomerulus's incoming arteriole that senses falling blood pressure and releases renin.
atrial natriuretic peptide (ANP): A hormone from the heart's atria, released when blood volume is high. It makes the kidneys excrete more salt and water.

Check yourself: 44.5 Hormones that tune the kidney

4 questions on 44.5 Hormones that tune the kidney. Pick an answer to see if you got it, and why.

Question 1 of 4

In collecting duct cells, ADH binding to its receptor raises cAMP, which leads to more aquaporins in the membrane. An enzyme called phosphodiesterase breaks cAMP down. If a drug blocked phosphodiesterase while ADH levels stayed low and steady, what would most likely happen?

Question 2 of 4

A healthy adult drinks 1 L of water quickly. The table shows measurements over the next three hours (invented data). Time (min) | Plasma osmolarity (mOsm/L) | Plasma ADH (pg/mL) | Urine flow (mL/min) 0 | 290 | 2.0 | 1.0 30 | 284 | 0.8 | 6.5 60 | 283 | 0.5 | 10.0 120 | 288 | 1.4 | 3.0 180 | 290 | 1.9 | 1.2 Which interpretation is best supported?

Question 3 of 4

A healthy person donates about 500 mL of blood. Plasma osmolarity is essentially unchanged, but blood volume and blood pressure drop slightly. Which response is most likely?

Question 4 of 4

A patient quickly receives 2 L of saline solution with the same osmolarity as blood. Which set of hormone changes is most likely?

0 of 4 answered