Ch 44: Osmoregulation and Excretion Study Guide

Osmosis

Diffusion of water across a selectively permeable membrane.

Osmolarity

Solute concentration expressed as molarity (moles of solute per liter).

Water movement and osmolarity

Water moves from hypoosmotic (lower solute) to hyperosmotic (higher solute) solutions.

Tonicity

Ability of a surrounding solution to cause a cell to gain or lose water.

Determining solution tonicity

Compare non-penetrating solute concentrations relative to the cell's cytoplasm.

Osmoregulation

Process by which animals control solute concentrations and balance water gain/loss.

Osmoconformers vs. Osmoregulators

Conformers are isosmotic with surroundings; regulators control internal osmolarity independently.

Stenohaline animals

Animals that cannot tolerate substantial changes in external osmolarity.

Euryhaline animals

Animals that can survive large fluctuations in external osmolarity.

Are euryhaline animals osmoconformers?

Not necessarily; many are osmoregulators capable of adjusting to varied salinities.

Marine animals: osmoregulatory challenge

They lose water by osmosis and gain salt by diffusion.

Marine animals: coping mechanism

Drink seawater, excrete salt via gills/glands, produce concentrated urine.

Freshwater animals: osmoregulatory challenge

They gain water by osmosis and lose salt by diffusion.

Freshwater animals: coping mechanism

Excrete dilute urine and actively take up ions via gills.

Active transport in aquatic excretory systems

Ions like Na⁺ and Cl⁻ are actively transported across gill membranes.

Terrestrial animals: main challenge

Dehydration due to evaporative water loss.

Terrestrial adaptations for water conservation

Waxy body coverings, nocturnal behavior, and concentrated excretory products.

Transport epithelia

Specialized epithelial cell layers moving specific solutes in controlled directions.

Marine bird salt glands function

Actively transport excess NaCl from blood to secretory tubules for excretion.

Countercurrent exchange in salt glands

Blood flows opposite to fluid, maintaining a gradient for continuous salt secretion.

Three main nitrogenous wastes

Ammonia, urea, and uric acid.

Ammonia characteristics

Most toxic, requires high water for dilution, low energy cost.

Urea characteristics

Low toxicity, moderate water loss, requires energy to synthesize.

Uric acid characteristics

Least toxic, insoluble paste (conserves water), highest energy cost.

Evolution and nitrogenous waste type

Habitat water availability and egg structure dictate waste type excreted.

Excretory system main functions

Dispose metabolic wastes, regulate solute concentration, and maintain water balance.

Four steps of urine formation

Filtration, reabsorption, secretion, and excretion.

Protonephridia (Flatworms)

Flame-bulb network filtering fluid primarily for osmoregulation; excrete dilute waste.

Metanephridia (Earthworms)

Tubules in each segment collecting coelomic fluid to excrete dilute urea urine.

Malpighian tubules (Insects)

Outpocketings of digestive tract secreting uric acid with minimal water loss.

Vertebrate excretory organs

Kidneys, ureters, urinary bladder, and urethra.

Nephron

Functional unit of the vertebrate kidney.

Two main types of nephrons

Cortical nephrons (short loops) and juxtamedullary nephrons (long loops into medulla).

Nephron anatomical components

Bowman's capsule, proximal tubule, Loop of Henle, distal tubule, collecting duct.

Nephron blood vessels

Afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, vasa recta.

Glomerular filtration process

Blood pressure forces water and small solutes through capillaries into Bowman's capsule.

Filtrate processing to urine

Valuable solutes/water are reabsorbed into blood; extra wastes are secreted.

Proximal tubule function

Reabsorbs ions, nutrients, water, and HCO₃⁻; secretes H⁺ and ammonia.

Descending limb of Loop of Henle

Permeable to water via aquaporins; impermeable to solutes.

Ascending limb of Loop of Henle

Permeable to NaCl (active transport in thick segment); impermeable to water.

Distal tubule function

Regulates K⁺ secretion, NaCl reabsorption, and pH balance via H⁺/HCO₃⁻ transport.

Collecting duct function

Final concentration of urine; reabsorbs NaCl, water, and controlled amounts of urea.

Nutrient reabsorption location

Occurs primarily in the proximal tubule via active transport.

K+ handling in nephron

Reabsorbed in proximal tubule; secreted in distal tubule.

Nephron pH control

Secretes H⁺ and reabsorbs HCO₃⁻ in proximal and distal tubules.

Ammonia and Urea transport mechanisms

Ammonia is passively secreted; urea is passively reabsorbed in inner medulla.

Urine concentration mechanism

Hyperosmotic renal medulla creates gradient, driving osmotic reabsorption of water.

Filtrate osmolarity pathway

Isosmotic in proximal → hyperosmotic at loop bend → hypoosmotic at distal tubule → hyperosmotic in urine.

Countercurrent multiplier system

Loop of Henle maintains high interstitial NaCl concentration using active transport and energy.

Desert mammal kidney adaptation

Long juxtamedullary loops of Henle to produce extremely concentrated urine.

Bird and reptile kidney adaptations

Short loops (birds) or cortical-only nephrons (reptiles); excrete uric acid.

Freshwater fish kidney adaptations

Abundant glomeruli excrete large volumes of dilute urine while conserving salt.

Marine fish kidney adaptations

Fewer, smaller glomeruli produce small volumes of urine to conserve water.

ADH release trigger

Increased blood osmolarity detected by hypothalamic osmoreceptors.

ADH targets and effect

Inserts aquaporins into collecting duct membranes, increasing water reabsorption.

RAAS trigger

Juxtaglomerular apparatus (JGA) releases renin in response to low blood pressure/volume.

Renin function

Cleaves angiotensinogen (secreted by liver) to form angiotensin I.

Angiotensin II functions

Constricts arterioles to raise BP and stimulates aldosterone release from adrenal cortex.

Aldosterone function

Causes distal tubules and collecting ducts to reabsorb more Na⁺ and water.

RAAS vs. ADH control difference

ADH responds to osmolarity changes; RAAS responds to volume/pressure drops without changing osmolarity.

Atrial Natriuretic Peptide (ANP)

Hormone released by stretched heart atria to lower blood volume and pressure.

ANP mechanism

Inhibits renin, aldosterone, and NaCl reabsorption, promoting Na⁺ and water excretion.