Lecture 20, Regulating the Internal
Environment: I. Thermoregulation
•
•
•
•
•
Heat Gain or Loss is controlled by:
Conduction: direct transfer of heat between molecules of the
environment and those of body surface
Convection:transfer of heat by movement of air or water past body
surface
Radiation: emission of electromagnetic radiation
Evaporation: loss of heat from the surface of a liquid that is losing some
of its molecules as gas
• Ectotherms: (poikilotherms) derive heat mainly from their
surroundings
• Endotherms: (homeotherms) derive heat mainly from their
metabolism
Endothermy: Advantages &
Disadvantages
• Advantages:
– Terrestrial environments
are characterized by large
temperature fluctuations
– Endothermy liberates an
animal from dependence
on ambient temperature
• Disadvantages:
– Energetically very
expensive:
• Resting Homo sapiens:
at 200C, consumes 1300
- 1800 kcal per day
• Resting alligator of
equivalent body mass,
consumes 60 kcal per
day
1
Thermoregulation: Mechanisms
1
1. Adjusting Rate of Heat
Exchange Between an
Animal & Its Environment
• Body insulation: hair,
feathers, fat
• Vasodilation: enhances
transfer of heat to
environment
• Vasoconstriction: reduces
transfer of heat to
environment
• Counter-current heat
exchanger
Thermoregulation: Mechanisms 2
2. Cooling by Evaporative
Heat Loss
• Phase change from liquid
water to water vapor
releases a large amount of
heat
• Very effective when
relative humidity is low to
moderate
• Mammals: sweat or pant
• Birds: gular fluttering
• Behavioral adaptations:
– e.g., Bathing ---->
Thermoregulation: Mechanisms 3
3. Behavioral
Responses:
• Particularly
important
in
ectotherms
2
Endothermic Invertebrates
• Pre-flight warm-up in large moths, beetles and bees
• Contract all flight muscles in synchrony in order to heat up
• Make use of countercurrent heat exchanger to maintain
high thoracic temperature
Endothermic Fish
Endothermic Fish: Rete mirable
(e.g., Bluefin tuna, swordfish, great white shark)
3
Torpor
• State of reduced metabolic activity in which the
heart rate and respiratory system slow down
• Used by many endotherms as a mechanism to
reduce energy consumption
• Short-term torpor: exhibited on daily basis in
small mammals (e.g., shrews) & birds (e.g.,
hummingbirds)
• Long-term torpor:
– Hibernation: response to cold (triggered by
photoperiod)
– Estivation: response to heat or drought
“Ectothermic” Birds
“Ectothermic” Mammals: Hibernation
4
Feedback Mechanisms in Thermoregulation
Heat-shock Proteins
• High body temperature can be particularly
dangerous because many biologicallyimportant proteins begin to denature at ~420C.
• In response to high temps, cells release heat
shock proteins which help buffer other proteins
against being denatured
• Lee Weber, UNR, expert on heat shock
proteins:
– using cut-throat trout heat shock titers as an index
of environmental quality
Regulating Internal Environment II:
Water Balance & Waste Disposal
• Water balance & excretion are controlled by
transport epithelia: a layer or layers of of
specialized epithelial cells that regulate solute
movements
• Transport epithelia:
– Either face the external environment
– Or, a tubular channel that leads to the outside through
an opening on the body surface
– Usually arranged in tubular networks with extensive
surface area
5
Waste Products of Metabolism
Nitrogenous Wastes
• Ammonia, the direct product of metabolism (via
deamination of proteins & nucleic acids), is very toxic
stuff (good for cleaning windows, however)
• Ammonia, however, is very soluble in H2 O and passes
easily through biological membranes
• Because of its toxicity, ammonia can only be tolerated
in very low concentrations
Form of Nitrogenous Waste Disposal
• Form of nitrogenous waste
is associated with an
animal’s phylogeny and
habitat
• Ammonia: restricted to
aquatic organisms (FW fish,
tadpoles); Requires lots of
H2O
• Urea: Mammals, adult
amphibians, marine fish &
marine turtles. 100,000x
less toxic than NH3
• Uric acid: Birds, reptiles,
land snails, insects.
Relatively non-toxic and
very insoluble in H2O;
excreted as a paste.
6
Urea vs. Uric Acid: Importance of
Reproductive Mode
• Urea: Excreted in H2O as urine. In adult amphibians &
mammals, produced by liver in reaction of NH3 &
CO2. Energetically expensive but conserves H2O by
reducing toxicity.
• Uric Acid: Excreted as paste. Energetically expensive
but very effective at conserving H2O.
• What about embryos?
– Amphibian eggs: urea (urine) can diffuse out of shell-less
egg;
– Mammalian fetus: urea can diffuse out into mother’s
bloodstream;
– Bird & insect eggs: shelled eggs are impermeable to liquids
(urine); however, uric acid precipitates out of solution and
stored in egg as a solid
Terminology of Osmoregulation
Marine fish: hypotonic osmoregulator: constantly loses
water through osmosis and therefore has to drink
constantly; Gills actively transport Cl- out
7
Freshwater Fish: hypertonic osmoregulator: constantly gains
water through osmosis; produces copious quantities of dilute
urine; gills actively transport Cl- in
Osmotic Balance on Land
• Desiccation: a major
threat in terrestrial
environments
• Major sources of H2O
loss:
– Gas exchange organs
– Urine
• Mammals very greatly in
their ability to minimize
water loss via urine
• Kangaroo rat: recovers
90% of water loss through
metabolic H2O.
• KR: 22.5% lost in urine
• Human: 60% lost in urine
Waste Products of Metabolism
8