Osmoregulation and Excretion

Chapter 44: Osmoregulation and Excretion

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint® Lecture Presentations for Biology Eighth Edition
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp

Overview: A Balancing Act

• The physiological systems of animals operate in a fluid environment.

• Relative concentrations of water and solutes must be maintained within fairly narrow limits.

• Osmoregulation: Regulates solute concentrations and balances the gain and loss of water.

Adaptations in Different Environments

• Freshwater animals exhibit adaptations that reduce water uptake and conserve solutes.

• Desert and marine animals confront desiccating environments that can quickly deplete body water.

• Excretion: The process of eliminating nitrogenous metabolites and other waste products.

Osmoregulation Defined

• Osmoregulation involves the controlled movement of solutes between internal fluids and the external environment.

• Cells require a balance between osmotic gain and loss of water.

• Osmolarity: The solute concentration of a solution, which determines the movement of water across a selectively permeable membrane.

  • If two solutions are isoosmotic, the movement of water occurs equally in both directions.

  • If two solutions differ in osmolarity, the net flow of water moves from the hypoosmotic solution to the hyperosmotic solution.

Osmotic Challenges

• Osmoconformers:

  • Consist only of some marine animals that are isoosmotic with their surroundings and do not regulate their osmolarity.

• Osmoregulators:

  • These animals expend energy to control water uptake in hypoosmotic environments and loss in hyperosmotic environments.

• Most animals are stenohaline; they cannot tolerate substantial changes in external osmolarity.

• Euryhaline animals can survive large fluctuations in external osmolarity.

• Example: Sockeye salmon, known as euryhaline osmoregulators.

Osmoregulation in Marine Animals

• Most marine invertebrates are osmoconformers.

• Marine vertebrates and some invertebrates are osmoregulators.

• Marine Bony Fishes:

  • They are hypoosmotic to seawater, losing water by osmosis and gaining salt by diffusion and food intake.

  • They balance water loss by drinking seawater and excreting salt.

  • Osmoregulation in marine bony fishes involves:

  • Drinking seawater

  • Excreting salt ions from gills

  • Osmotic water loss through gills and body surface

  • Excretion of scanty, concentrated urine from kidneys.

• Contrast with Freshwater Animals:

  • Freshwater animals constantly take in water by osmosis from their hypoosmotic environment, lose salts by diffusion, and maintain water balance by excreting large amounts of dilute urine.

Excretion in Freshwater Animals

• Freshwater animals maintain water balance by:

  • Taking in water by osmosis

  • Losing salts through diffusion

  • Replacing lost salts through food and uptake across the gills.

Adaptations of Animals in Temporary Waters

• Some aquatic invertebrates living in temporary ponds can lose almost all their body water and survive in a dormant state called anhydrobiosis.

Water Balance in Terrestrial Mammals

• Land Animals:

  • Manage water budgets by drinking, eating moist foods, and using metabolic water.

  • Desert Animals:

  • Get major water savings through anatomical features and behaviors, such as a nocturnal lifestyle.

Water Loss and Gains Comparisons

Energetics of Osmoregulation

• Osmoregulators must expend energy to maintain osmotic gradients.

• They regulate the composition of body fluid bathing their cells.

• Transport Epithelia:

  • Specialized epithelial cells that regulate solute movement; essential for osmotic regulation and metabolic waste disposal.

  • Found in complex tubular networks, example: salt glands of marine birds remove excess sodium chloride.

Salt Glands in Seabirds

• Seabirds possess salt glands near the nostrils, which concentrate brine and can be "sneezed" out.

• This adaptation enables them to drink seawater and remove excess salt from food, as their kidneys cannot handle the concentration.

Countercurrent Exchange Mechanisms

• Two countercurrent exchange mechanisms assist seawater birds in removing salt: a. A salt extraction system using a countercurrent multiplication mechanism:

  • Salt is actively pumped from blood venules into gland tubules.

  • Even though the fluid has a higher salt concentration than the blood, a countercurrent exchange ensures a minimal salt gradient.
    b. The blood supply to the gland utilizes a countercurrent exchange loop to keep a high salt concentration within the gland.

Diagram of Countercurrent Exchange

• Shows how salt is transported through secretory cells and capillaries in the nasal glands of seabirds.

Nitrogenous Wastes and Habitat Reflection

• The type and quantity of nitrogenous waste products can significantly affect an animal’s water balance.

• Major wastes include toxic ammonia (NH_3), which some animals convert into less toxic compounds before excretion.

Types of Nitrogenous Wastes

Nitrogenous Waste Pathways

• Animals excrete wastes differently based on habitat:

  • Ammonotelic animals: Release ammonia across the body surface or through gills.

  • Ureotelic animals: Convert ammonia to urea via liver processing; requires energy and water but is less toxic.

  • Uricotelic animals: Convert ammonia to uric acid; excretion requires minimal water.

Summary Review

• You should be able to distinguish the following terms:

  • Isoosmotic, hyperosmotic, hypoosmotic

  • Osmoregulators and osmoconformers

  • Stenohaline and euryhaline animals

• Define the terms:

  • Osmoregulation, excretion, and anhydrobiosis

• Compare osmoregulatory challenges of freshwater vs marine animals.

• Describe factors affecting the energetic cost of osmoregulation.