Day 2 animal form & function - Osmoregulation

Osmoregulation includes regulation of

1. total water content (ECF volume)

2. overall osmolarity of ECF

3. concentrations of specific solutes in ECF 

4. elimination of nitrogenous wastes

(ammonia, urea, or uric acid)

Important ions/electrolytes 

Na⁺, K⁺, Ca²⁺, Cl^-

Excretion

movement of unwanted substances out of ECF

Secretion

more general term for moving anything out of the ECF

Absorption

movement of substances into ECF

Reabsorption

applies when it was previously in ECF

Filtration

process of forcing solution through biological sieve

Osmolarity units

osmoles (Osm)

1.0 Osm solution equals

1.0 moles dissolved particles per liter

Nitrogenous wastes

the end products of protein metabolism

Nitrogenous wastes in envirnments

different wastes are best in different environments

Solubility - High

Toxicity - High

Energy loss - Low

ammonia

Solubility - Medium

Toxicity - Medium

Energy loss - Medium

urea

Solubility - Very low

Toxicity - Low

Energy loss - High

uric acid

Osmoregulation in aquatic organisms

animals that have osmotic concentrations different than their environment must osmoregulate

Aquatic organisms that must osmoregulate

1. all freshwater organisms

2. for marine species, mainly bony fish

Osmoconformers

1. body is isomotic (isotonic) with seawater

2. typically show some ion regulation

What are osmoconformers

many marine invertebrates and Chondrichthyes

Marine Shark ECF with seawater

Sharks are isotonic

ECF = 1050 mOsm

Seawater = 1050 mOsm

Na⁺ between shark and seawater

Na⁺_{shark ECF} = 300mM

Na⁺ _seawater = 500mM

Electrochemical gradient in marine shark

favors Na⁺ (and Cl^-) influx

Osmotic gradient in marine shark

is about zero, so NO net osmosis of water

Other major osmolytes

urea and TMAO

Marine shark water flux

1. limited influx of water through food and metabolism

2. water is excreted (efflux) in urine made in kidneys

Marine shark ion flux

1. main route of NaCl influx (absorption) is gills, some NaCl and other ions in food

2. main route of NaCl efflux (secretion) is rectal gland, other ions through urine from kidneys

Allows most NaCl influx at the gills

gills have thin walls and a large SA for O₂ diffusion

(also allows water and solute diffusion)

Sharks don’t actively drink

they don’t actively drink since they need no additional water

Vertebrate and shark urine concentration

most vertebrates (including sharks) cannot make urine more concentrated than their ECF (isomotic urine)

Ray-Finned fish ECF compared to freshwater and seawater

Not Isosmotic

ECF _Ray-finned = 300mOsm

Freshwater = < 150 mOsm

Seawater = 1050 mOsm

Challenge for freshwater fish

challenged by water influx and ion efflux at gills

Freshwater fish combat high water influx

1. make large amounts of dilute urine to eliminate water

2. avoid drinking

Freshwater fish combat ion efflux

1. rely on food and active gill uptake for ions

Challenges for marine fish

challenged by water efflux and ion influx at gills

Marine fish combat water efflux

1. drink to gain water

2. make small amounts of urine to conserve water

Marine fish combat ion influx

1. rely on active excretion of ions at gills

2. urine removes larger ions

Nitrogenous wastes in most aquatic organisms are in the form of

ammonia

Why elimination of ammonia is easy in aquatic organism

1. can diffuse down its gradient from ECF into surrounding water

2. diffuses mainly via gills

Differences between being in water and being in air(land)

1. ions cannot be gained or lost by diffusion

2. water cannot be gained or lost by osmosis

• water can be lost by evaporation

• a few can absorb water from humid air

Challenges in terrestrial organisms

1. reduce water loss - especially in hot, dry habitats

2. need to take in sufficient ions

3. need to eliminate nitrogenous wastes

Reduce water loss

respiratory surfaces are most susceptible to water loss

take in sufficient ions

herbivores in particular may lack NaCl

Eliminate nitrogenous wastes

need to avoid toxic effects while conserving water

General terrestrial adaptations

1. internal respiratory surfaces rather than gills

2. integument resistant to desiccation

3. formation of moderate to low urine volumes for ions and waste excretion

General terrestrial adaptations in the tortoise

1. lungs - internal sac-like structures

2. scales containing keratin

3. formation of uric acid - precipitates out of solution in the cloaca, so urine can hold other solutes

What is keratin

a protein that creates a highly water-resistant barrier