Chapter 40 - Animal Water Balance

Importance of Water and Electrolyte Balance

• All biochemical reactions occur in aqueous environments

• Electrolytes are required for nerve signaling, muscle contraction, and enzyme function

  • Na+, K+, Cl-, Ca2+

• Imbalances can disrupt critical physiological functions; maintenance of homeostasis ensures proper function

• Water balance relates to excretion (wastes need to be dissolved in water)

Osmolarity

• Concentration of solutes in a solution that determines movement of water across membranes via osmosis

• Measured in osmoles per liter

• Higher osmolarity = lower concentration of free water molecules

• When separated by selectively permeable membrane: water moves from low → high regions of osmolarity

  • Lower osmolarity = lower solute concentration

  • Higher osmolarity = higher solute concentration (more opportunity for interaction)

Hyperosmotic, Hypoosmotic, Isosmotic

Determine direction of water movement across membranes

• Hyperosmotic: a solution with a higher solute concentration relative to another solution

• Hypoosmotic: a solution with a lower solute concentration relative to another solution

• Isosmotic: two solutions have equal solute concentrations

Osmosis

• Passive movement of water across a selectively permeable membrane

• Moves from regions of low osmolarity (lots of free water) → high osmolarity (little free water/lots of solutes)

• Movement occurs because solutes bind water molecules, reducing free water availability

• Water is not attracted to solutes; movement is driven by differences in free water concentration

Passive v. Active Transport

• Passive transport moves solutes down their concentration or electrochemical gradient without energy input

  • Simple diffusion: small, nonpolar molecules (O2 and CO2)

  • Facilitated diffusion: uses membrane proteins to move larger/charged molecules

• Active transport moves solutes against their gradient using energy (ATP or ion gradients); either primary or secondary

Primary vs. Secondary Active Transport

• Primary: uses ATP directly to move ions against their gradient (e.g., Na+/K+ ATPase)

  • Creates electrochemical gradients across membranes

• Secondary = uses these gradients to move other solutes

  • Efficient transport of multiple substances with minimal ATP use

  • Symporters move solutes in the same direction; antiporters move them in opposite directions

Osmoconformers vs. Osmoregulators

• Osmoconformers maintain internal osmolarity equal to their environment

  • Minimizes osmotic stress

  • Typically marine invertebrates

• Osmoregulators actively control internal osmolarity, keeping it different from the environment

  • Requires energy but allows survival in variable environments

  • Typically fishes and terrestrial animals

Marine Bony Fish: Osmoregulator

• Seawater is hyperosmotic relative to fish body fluids

  • Body loses water by osmosis (high free water → low) across the gill epithelium

  • Gain salts by diffusion (higher [ ] in seawater → lower [ ] in the body)

• Drink/ingest seawater to replace lost water and salts

• Excess salts are actively excreted through specialized chloride cells in gills

Chloride Cells in Marine Bony Fish

Pumping excess solutes OUT of the body

• Uses Na+/K+/ATPase to create an electrochemical gradient

  • 3 Na+ out, 2 K+ in

• Na+ gradient (made by sodium potassium pump) allows Na+ to passively move down its gradient into the cell

  • K+ and Cl- move back into the cell via secondary active transport using the Na+/K+/Cl- cotransporter located b/w cell and interstitial fluid

  • K+ moves passively back to the interstitial fluid from the chloride cell

  • Cl- moves passively out of the chloride cell into the surrounding seawater

• Saves energy by not needing to actively pump every solute

Freshwater Fish: Osmoregulator

• Freshwater is hypoosmotic relative to fish body fluids

  • Body gains water by osmosis (high free water → low) across gill epithelium)

  • Loses salts by diffusion (higher [ ] in the body → lower [ ] in the freshwater)

• Dealing with too much incoming water

• Salts are actively transported into the body through chloride cells

• Excrete large volumes of dilute urine to remove excess water

Chloride Cells in Freshwater Bony Fishes

Same mechanism as in marine bony fishes; but the location of the cotransporter differs because we’re actively pumping salts INTO the body

• Uses Na+/K+/ATPase to create an electrochemical gradient

  • 3 Na+ out, 2 K+ in

• Na+ gradient (made by sodium potassium pump) allows Na+ to passively move down its gradient into the cell

  • K+ and Cl- move back into the cell via secondary active transport using the Na+/K+/Cl- cotransporter located b/w the cell and pond water

  • K+ and Cl- move passively to the interstitial fluid from the chloride cell

Sharks (Cartilaginous Fish): Osmoconformer

• Body fluids nearly isosmotic to seawater

• Maintain high urea concentrations to increase osmolarity without

  • B/c they’re relatively isosmotic to sea water, there’s not a high concentration of salt anywhere to drive osmosis

  • Helps reduce water loss by osmosis

• Actively excrete excess salts from the rectal gland to maintain ion balance

• Must produce proteins to protect cells from urea toxicity

Cartilaginous Fish (Shark): Rectal Gland

Process occurs in the epithelial cell of the rectal gland. Apical membrane = near lumen (empties into environment); basolateral membrane = near interstitial fluid

1. Na+/K+/ATPase pumps Na+ out of cell into interstitial fluid and K+ into cell from the interstitial fluid, building an electrochemical gradient

2. Na+/Cl-/K+ transporter that’s power by the Na+ gradient actively moves all three ions from interstitial fluid into the cell (secondary active transport)

3. Cl- diffuses into the lumen via chloride channel; K+ diffuses to interstitial fluid thru K+ channel

4. Na+ diffuses into the lumen along its electrochemical gradient

5. Now salt (Na+ and Cl-) are in the lumen of the rectal gland ready to be excreted

Nitrogenous Wastes: Types and Trade-Offs

• Ammonia: highly toxic, requires little energy to produce but large water loss for excretion

• Urea: toxic and requires moderate water loss and energy to synthesize

• Uric acid: least toxic and conserves water but requires high energy to produce

• Different organisms use different waste forms based on environment and water availability

Mammalian Kidney: Structure

• Kidneys filter blood and regulate water and electrolyte balance (filtration, reabsorption, excretion)

• Blood enters via renal artery and exits via renal vein

• Urine flows from kidney → ureter → bladder → urethra → out

• Nephrons are functional units of the kidney

Nephron Structure

• Renal corpuscle (glomerulus + Bowman's capsule) filters blood to form the pre-urine (filtrate)

• Proximal tubule uses Na+/K+-ATPase and cotransporters to reabsorb nutrients, ions, and water into the blood

• Loop of Henle establishes osmotic gradient in the surrounding interstitial fluid

• Distal tubule reabsorbs ions and water from the filtrate based on body needs

  • Hormone (aldosterone) regulated transport

• Collecting duct regulates water reabsorption to maintain homeostasis; influences urine concentration

Filtration in Renal Corpuscle

• Blood enters glomerulus under pressure

• Small molecules (water, ions, glucose, urea) pass through pores and filtration slits into Bowman's capsule

• Large molecules (proteins, cells) remain in bloodstream

• Creates initial filtrate/pre-urine that leaves Bowman’s capsule

Proximal Tubule: Reabsorption Mechanisms

Epithelial cells contain microvilli to increase surface area for transport

1. Na+/K+ ATPase creates an electrochemical gradient that favors Na+ entry in from the lumen

2. Na+-dependent cotransporters in the apical membrane use that Na+ gradient to remove ions and nutrients (Cl-, glucose, vitamins) from the filtrate that’s in the lumen

3. Those solutes (glucose, Cl-, vitamins) move from the cell → interstitial fluid → nearby blood vessels

4. Water moves into those blood vessels via osmosis (following the movement of solutes)

• Recovers valuable nutrients and prevents loss

Loop of Henle: Countercurrent

• Descending limb is permeable to water only; there’s is an OUTFLOW of water across that epithelium by osmosis

• Ascending limb is nearly impermeable to water; it moves Na+ and Cl- out to the interstitial fluid

  • Thin ascending limb = passive

  • Thick ascending limb = active

    • Movement of NaCl from ascending limb raises interstitial osmolarity (there’s more solutes in the interstitial fluid, which helps pull water out from the descending limb)

    • Helps form concentrated filtrate

• Countercurrent flow enhances gradient efficiency

Distal Tubule: Regulation of Ions

• Reabsorbs Na+ and Cl- based on body needs

• Aldosterone increases Na+ reabsorption and K+ secretion

• Helps regulate blood pressure and electrolyte balance

• Also plays a role in pH regulation via ion exchange

• Adjusts composition of filtrate before final processing

Collecting Duct: Final Water Balance Control

• Water reabsorption depends on presence of ADH

• When permeable, water exits filtrate into high osmolarity (lots of solutes) medulla

• Produces concentrated urine when dehydrated

• Low ADH leads to dilute urine

ADH: Role in Water Balance

• ADH is released in response to dehydration or increased blood osmolarity

  • High ADH → small volume of concentrated urine (b/c you’re dehydrated and need to reabsorb water)

  • Low ADH → large volume of dilute urine

• It triggers insertion of aquaporins in collecting duct cells, increasing water permeability for reabsorption

• ADH also increases urea permeability, strengthening osmotic gradient

Negative Feedback in Osmoregulation

• Sensor detects change in osmolarity or hydration status

• Integrator (brain) compares to set point

• Effector (kidney, hormones) adjusts water/ion balance

• Response restores conditions toward set point

• Maintains stable internal environment

ADH Blocking

• If ADH is blocked, aquaporins are not inserted

• Collecting duct remains impermeable to water

• Less water is reabsorbed

• Results in large volume of dilute urine

• Explains effects of alcohol on urination