Water and Electrolyte Balance in Animals (Chapter 40 Study Notes)

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Water and Electrolyte Balance in Animals

Introduction

All life-sustaining chemical reactions occur in aqueous solutions, making the regulation of water and solute concentrations essential for survival. Disruption of this balance can halt vital biochemical processes. This chapter explores how animals maintain water and electrolyte balance across different environments.

Electrolyte: A compound that dissociates into ions when dissolved in water, such as Na+, K+, and Ca2+.
Cells require precise concentrations of electrolytes to function normally.

Osmoregulation and Excretion

Overview

Water and electrolyte balance is closely linked to excretion, as animals lose water when they excrete waste (urine). Animals must maintain this balance in three main environments: freshwater, marine, and terrestrial.

Excretion leads to water loss.
Animals must adapt to their specific environments to maintain homeostasis.

Mechanisms of Movement

Diffusion: Movement of uncharged substances down their concentration gradients.
Osmosis: Movement of water down its concentration gradient across a semipermeable membrane.
Osmolarity: The concentration of solutes in solution, measured in osmoles per liter.

Osmotic Stress and Osmoregulation

Osmotic Stress

Osmotic stress occurs when the concentration of dissolved substances in a cell or tissue deviates from normal set points, potentially disrupting cellular function.

Osmoregulation is the process by which organisms control the concentration of water and solutes within their bodies.
Key ions involved: sodium (Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), chloride (Cl-), carbonate (CO32-), bicarbonate (HCO3-), phosphate (PO43-).

Osmoconformers vs. Osmoregulators

Osmoconformers: Organisms (e.g., sponges, jellyfish) that do not actively regulate osmolarity because their internal environment is isosmotic with seawater.
Osmoregulators: Most marine and freshwater bony fishes and terrestrial animals actively regulate internal osmolarity to achieve homeostasis.

isomotic: solute concentrations inside and outside the animal are equal.

Hyperosmotic: Environment has higher solute concentration than the animal's tissues.

Hyposmotic: Environment has lower solute concentration than the animal's tissues.

Osmotic Stress in Different Environments

Marine Environments

Marine bony fishes live in a hyperosmotic environment (seawater has higher solute concentration than their tissues). They lose water by osmosis and gain electrolytes by diffusion.

To compensate, they drink seawater and actively excrete excess electrolytes through gill epithelium and produce small amounts of concentrated urine.

Freshwater Environments

Freshwater fishes live in a hyposmotic environment (freshwater has lower solute concentration than their tissues). They gain water by osmosis and lose electrolytes by diffusion.

They excrete large amounts of dilute urine and do not drink water.
Electrolytes are replaced by eating and active transport into the body.

Terrestrial Environments

Terrestrial animals lose water primarily by evaporation and through excretion. They must replace lost water by drinking, eating, or metabolic processes.

Water loss also occurs through moist respiratory surfaces and production of urine and feces.

Nitrogenous Wastes and Excretion

Types of Nitrogenous Wastes

Animals metabolize amino acids and nucleic acids, producing nitrogenous wastes that must be excreted to avoid toxicity.

Ammonia (NH3): Highly toxic, raises pH, excreted directly by some aquatic animals.
Other animals convert ammonia into less toxic compounds (e.g., urea, uric acid) before excretion.

Water and Electrolyte Balance in Terrestrial Vertebrates

Role of the Kidney

In terrestrial vertebrates, the kidney is the primary organ responsible for water and electrolyte balance and excretion of nitrogenous wastes.

Water is replaced mainly by drinking.
Electrolytes are replaced by eating.
Osmoregulation occurs primarily in the kidney.

Structure and Function of the Mammalian Kidney

Anatomy

Kidneys are paired organs located in the back of the body.
The renal artery brings blood with nitrogenous wastes to the kidney; the renal vein carries filtered blood away.
Urine formed in the kidney travels via the ureter to the bladder for storage, then exits the body through the urethra.

Nephron Structure

The nephron is the functional unit of the kidney, responsible for water and electrolyte balance.
Most nephrons are in the outer region (cortex); some extend into the inner region (medulla).

Nephron Function: Overview

Water cannot be transported actively; it moves by osmosis.
Cells set up strong osmotic gradients in the interstitial fluid to control water and electrolyte retention.
Nephrons have four major regions, each associated with a collecting duct and blood vessels.

Major Regions of the Nephron

Renal corpuscle: Filters blood, forming a filtrate of ions, nutrients, wastes, and water.
Proximal tubule: Reabsorbs nutrients, ions, and water from the filtrate back into the blood.
Loop of Henle: Establishes a strong osmotic gradient; osmolarity increases as the loop descends into the medulla.
Distal tubule: Reabsorbs ions and water to help maintain balance.
Collecting duct: May reabsorb more water to maintain homeostasis; urea moves from urine to interstitial fluid at the base, contributing to the osmotic gradient.

Table: Structure and Function of the Nephron and Collecting Duct

Region	Function
Renal corpuscle	Size-selective filtration: forms filtrate from blood (water and other substances enter nephron)
Proximal tubule	Reabsorbs electrolytes (active transport), nutrients, water
Loop of Henle (descending limb)	Permeable to water (passive transport out of filtrate)
Loop of Henle (thin ascending limb)	Permeable to Na+, Cl- (passive transport out of filtrate)
Loop of Henle (thick ascending limb)	Active transport of Na+, Cl- out of filtrate
Distal tubule	Aldosterone present: reabsorbs Na+; no aldosterone: does not reabsorb Na+
Collecting duct	Regulates water retention; ADH present: water leaves filtrate, producing small volume of urine hyperosmotic to blood; no ADH: water stays in filtrate, producing large volume of urine hyposmotic to blood
Inner medulla (collecting duct)	Urea leaks out by passive transport, establishing/maintaining high osmolarity of inner medulla

Problems with Kidney Function

Early signs of nephron damage include presence of albumin, white or red blood cells in urine.
Severe damage leads to accumulation of urea and other wastes, and retention of water and salts, which is life-threatening.
Kidney transplants are possible; only one functioning kidney is needed, but there is a risk of rejection.
Hemodialysis: Patient's blood is passed through a membrane in contact with dialysate; substances diffuse to remove harmful wastes and retain beneficial substances. Treatment is typically weekly.

Key Equations

Osmolarity:

Summary Table: Osmoregulation Strategies

Environment	Water Movement	Electrolyte Movement	Adaptation
Marine (hyperosmotic)	Water loss by osmosis	Electrolyte gain by diffusion	Drink seawater, excrete ions via gills, concentrated urine
Freshwater (hyposmotic)	Water gain by osmosis	Electrolyte loss by diffusion	Excrete dilute urine, active uptake of ions, do not drink
Terrestrial	Water loss by evaporation	Electrolyte loss in urine/sweat	Drink water, eat food, produce concentrated urine

Example: Marine bony fishes must drink seawater and actively excrete excess salts to maintain internal homeostasis, while freshwater fishes excrete large volumes of dilute urine to rid themselves of excess water.