Unit 13: Thermoregulation and Osmoregulation — Short Questions

Osmoconformers are animals that allow the osmotic concentration of their body fluids to match (remain isotonic to) their surrounding medium, since they cannot actively maintain their internal water/solute concentrations; examples include marine invertebrates, hagfishes, sharks and rays. Osmoregulators are animals that can actively regulate and maintain constant internal water and solute concentrations despite changes in the external environment, using mechanisms such as active transport of salts, salt accumulation and water conservation; most aquatic and terrestrial vertebrates are osmoregulators.

Thermoregulation is the homeostatic process by which organisms maintain their internal body temperature within a tolerable range, regardless of the temperature of the external environment. It is needed because enzymes have an optimal temperature for maximum efficiency; below it, enzyme-substrate collisions decrease and metabolic rate drops, while well above it, enzymes denature and permanently lose function. Thermoregulation is also needed to protect the lipid bilayer of cell membranes, which loses selective permeability at high temperatures and becomes rigid and brittle at low temperatures.

On the basis of the ability to thermoregulate, animals are classified as poikilotherms and homeotherms. Poikilotherms cannot maintain their body temperature within narrow limits, and it varies considerably, usually matching the environment; all invertebrates and the vertebrates fishes, amphibians and reptiles are poikilotherms. Homeotherms maintain a relatively constant internal body temperature regardless of environmental fluctuations, using a hypothalamic "thermostat"; mammals and birds, including humans (about 37°C), are homeotherms.

Osmoregulators actively maintain a constant internal concentration of water and solutes despite changes in the external medium, using mechanisms like active salt transport and water conservation (e.g., most aquatic and terrestrial vertebrates). Osmoconformers cannot regulate their internal osmotic concentration; it changes with and remains isotonic to the surrounding medium, and they are typically found in environments with stable solute concentrations such as the sea (e.g., marine invertebrates, hagfishes, sharks, rays).

Ectotherms gain most of their body heat from external sources, so their body temperature fluctuates with the environment; they have low metabolic rates and rely on behavioural thermoregulation such as basking in sun or sitting on warm rocks (most invertebrates and the vertebrates fishes, amphibians, reptiles). Endotherms generate the majority of their body heat through internal metabolism and maintain a relatively constant, often higher, body temperature using responses like shivering, burning brown adipose tissue, and insulation such as hair, feathers or fat (birds and mammals).

Vasodilation is the expansion (widening) of blood capillaries just beneath the skin, which increases blood flow to the skin so that more heat is lost from the blood; it occurs in response to a rise in body temperature (hyperthermia) and is often accompanied by sweating. Vasoconstriction is the narrowing of blood vessels, which reduces blood flow to the skin and so reduces heat loss; it occurs in response to a fall in body temperature (hypothermia) and is usually accompanied by shivering.

Homeotherms maintain a relatively constant internal body temperature regardless of environmental fluctuations, using a hypothalamic "thermostat" to trigger heating or cooling responses; examples are mammals and birds (humans maintain about 37°C). Poikilotherms are unable to maintain their body temperature within narrow limits; it varies considerably, usually matching the temperature of the environment, as in all invertebrates and the vertebrates fishes, amphibians and reptiles.

Ammonotelic animals excrete ammonia (NH₃) as their major nitrogenous waste; ammonia is highly toxic and requires about 500 mL of water per gram to dilute it to a nontoxic level, so this strategy is used by freshwater and marine animals with plenty of water available, e.g. most fishes, protozoans, sponges, coelenterates. Ureotelic animals excrete urea (CO(NH₂)₂) as their major nitrogenous waste; urea is less toxic than ammonia and needs only about 50 mL of water per gram, allowing terrestrial animals, which convert ammonia to urea in the liver, to conserve water.

Ureotelic animals excrete urea as their major nitrogenous waste; urea is moderately toxic and needs about 50 mL of water per gram for safe excretion. Uricotelic animals (insects, desert animals, birds, most reptiles) convert ammonia into uric acid (C₅H₄N₄O₃) using a large amount of energy; uric acid is even less toxic than urea, is poorly soluble in water and forms precipitates, so it needs only about 1 mL of water per gram, making it an adaptation to extreme water conservation.

Positive feedback amplifies the original stimulus rather than switching it off, so unless it is properly regulated, the response keeps escalating instead of returning the body to its set point. For example, in severe inflammation or infection, immune cells release cytokines that further stimulate the immune response; if this positive feedback loop is not properly controlled, it leads to excessive inflammation and tissue damage, causing severe health problems.