Chapter 40 - Animal Water Balance

Importance of Water and Electrolyte Balance

• All biochemical reactions occur in aqueous environments, so disruptions in water balance can halt metabolism

• Electrolytes (Na+, K+, Cl-, Ca2+) are required for nerve signaling, muscle contraction, and enzyme function

• Imbalances can disrupt membrane potentials and cellular function, leading to severe physiological consequences

• Water balance is tightly linked to excretion because wastes must be dissolved in water to be eliminated

• Maintaining stable internal conditions (homeostasis) ensures proper cellular and organismal function

Osmolarity

• Osmolarity is the concentration of solutes in a solution, measured in osmoles per liter

• It determines the movement of water across membranes via osmosis

• Higher osmolarity = lower concentration of free water molecules

• Water moves toward regions with higher osmolarity because there is more opportunity for interaction with solutes

• Osmolarity differences create osmotic stress that organisms must regulate

Hyperosmotic, Hypoosmotic, Isosmotic

• 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

• These are relative terms that depend on comparison between solutions

• These concepts determine direction of water movement across membranes

Osmosis

• Passive movement of water across a selectively permeable membrane

• Water moves from regions of high free water concentration (low osmolarity) to low free water concentration (high osmolarity)

• 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

• Aquaporins facilitate rapid water movement across membranes

Passive v. Active Transport

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

• Simple diffusion occurs for small, nonpolar molecules like O2 and CO2

• Facilitated diffusion uses membrane proteins to move larger or charged molecules

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

• Primary active transport directly uses ATP; secondary active transport uses gradients created by primary pumps

Primary vs. Secondary Active Transport

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

• This creates electrochemical gradients across membranes

• Secondary active transport uses these gradients to move other solutes

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

• This mechanism allows efficient transport of multiple substances with minimal ATP use

Osmoconformers vs. Osmoregulators

• Osmoconformers maintain internal osmolarity equal to their environment, minimizing osmotic stress

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

• Osmoregulation requires energy but allows survival in variable environments

• Osmoconformers are typically marine invertebrates; osmoregulators include fishes and terrestrial animals
  Represents a trade-off between energy cost and environmental flexibility

Marine Bony Fish: Osmoregulator

• Seawater is hyperosmotic relative to fish body fluids, causing water loss by osmosis

• Fish lose water across gill epithelium and gain salts by diffusion

• They drink/ingest seawater to replace lost water and salts

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

• Uses Na+/Na+/ATPase to create an electrochemical gradient (requires energy/ATP)

• Uses Na+/K+/Cl- cotransporters (b/w cell and interstitial fluid) and secondary activate transport to move K+ and Cl- into the chloride cell

Freshwater Fish: Osmoregulator

• Freshwater is hypoosmotic relative to fish body fluids, causing water gain by osmosis (dealing with too much incoming water)

• Fish gain water across gill epithelium and lose salts by diffusion

• They excrete large volumes of dilute urine to remove excess water

• Salts are actively transported into the body through gill cells

• Some ingestion but not to the same extent as bony fishes

• Same method/mechanism for chloride cells but in a different location

  • Na+/K+/Cl- cotransporter sits b/w the environment (pond water) and the chloride cell

Sharks (Cartilaginous Fish): Osmoconformer

• Body fluids nearly isosmotic to seawater

• Maintain high urea concentrations to increase osmolarity without high salt levels, reduces water loss by osmosis

• However, they must produce proteins to protect cells from urea toxicity

• They still actively excrete excess salts to maintain ion balance

Cartilaginous Fish: Rectal Gland

1. Na+ is pumped out while K+ is pumped in

• This builds a concentration gradient

• Investing a little bit of energy to save the need of spending energy later

2. Na+, Cl-, and K+ transported in

• When Na+ moves it pulls Cl- and K+ against their gradients (no need to spend ATP to actively pump them in → secondary active transport)

3. Cl- diffuses into the lumen, K+ passively diffuses to extracellular fluid

4. Na+ diffuses into the lumen (moves along its concentration gradient between cells to areas where its less abundant)

Transport Mechanisms Across Osmoregulatory Systems

• All systems rely on electrochemical gradients established by primary active transport

  • Created by Na+/K+ ATPase pumps Na+ out of epithelial cells

• Secondary active transport (cotransporters) uses Na+ gradients to move ions like Cl- against gradients

• Facilitated diffusion allows ions to move down gradients through channels

• Osmosis moves water based on free water concentration differences via aquaporins

• Marine fish gills excrete salts; freshwater fish gills import salts; shark rectal gland secretes concentrated NaCl

• Human proximal tubule reabsorbs solutes via Na+-dependent cotransport and water follows via osmosis

Nitrogenous Wastes: Types and Trade-Offs

• Ammonia is highly toxic but requires little energy to produce; requires large water loss for excretion

• Urea is less toxic and requires moderate water; costs energy to synthesize

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

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

• Represents a trade-off between energy cost and water conservation

Mammalian Kidney: Structure

• Kidneys filter blood and regulate water and electrolyte balance

• Blood enters via renal artery and exits via renal vein

• Urine flows from kidney through ureter to bladder and exits via urethra

• Nephrons are functional units of the kidney

• Structure allows filtration, reabsorption, and excretion

Nephron Structure

• Renal corpuscle (glomerulus + Bowman's capsule) filters blood

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

• Loop of Henle establishes osmotic gradient

• Distal tubule adjusts ion balance via hormone (aldosterone) regulated transport

• Collecting duct regulates water reabsorption and urine concentration

Filtration in Renal Corpuscle

• Blood enters glomerulus under pressure

• Small molecules (water, ions, glucose, urea) pass into Bowman's capsule

• Large molecules (proteins, cells) remain in blood

• Filtration is size-selective due to pores and filtration slits

• Creates initial filtrate for urine formation

Proximal Tubule: Reabsorption Mechanisms

• Epithelial cells contain microvilli to increase surface area for transport

• Na+/K+ ATPase creates sodium gradient driving reabsorption

• Cotransporters reabsorb glucose, amino acids, and ions

• Water follows solutes via osmosis into bloodstream

• This step recovers valuable nutrients and prevents loss

Loop of Henle: Countercurrent

• Descending limb is permeable to water but not solutes, causing water to leave

• Ascending limb is impermeable to water but transports NaCl out

• Creates osmotic gradient in medulla with increasing osmolarity deeper in kidney

• This gradient allows concentration of urine

• Countercurrent flow enhances gradient efficiency

Maintenance of Medullary Osmotic Gradient

• Active transport of NaCl in ascending limb increases interstitial osmolarity

• Water leaves descending limb due to osmotic gradient

• This increases concentration of filtrate entering ascending limb

• Cycle reinforces gradient through countercurrent multiplication

• Vasa recta removes excess water and maintains gradient

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 medulla

• Produces concentrated urine when dehydrated

• Low ADH leads to dilute urine

• Final step in urine concentration

ADH: Role in Water Balance

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

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

• Water leaves filtrate via osmosis into high-osmolarity medulla and returns to bloodstream

• ADH also increases urea permeability, strengthening osmotic gradient

• High ADH allows for → small volume of concentrated urine; low ADH → large volume of dilute urine

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

Water Movement in the Collecting Duct

• Water moves out of filtrate because surrounding interstitial fluid has higher osmolarity

• This means lower free water concentration outside the duct

• Water follows osmotic gradient into interstitial fluid

• From there, it is returned to bloodstream

• Explains concentration of urine

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