Homeostasis (thermoregulation)

These notes were made in May, 2011, as preparation for mid year exams.

Homeostasis is the maintenance of the internal environment according to the external environment. It goes around a set point (dynamic equilibrium). It has two types of ‘feedback’ systems – in negative feedback, the stimulus is reduced, in positive, the stimulus is increased.

Thermoregulation

Heat gain must equal heat loss, as a stable temperature is needed for optimum enzyme activity. When core temperatures are above 42°C (hyperthermia), enzymes denature and death can result. When core temperature is less than 35°C (hypothermia), enzymes become inactive, and eventually death can also result.

Metabolic rate is the rate at which energy is released by food breakdown. Basal metabolic rate is the resting metabolic rate. Energy is released as ATP and heat. Metabolic rate can increase up to 40× when exercising. Stress also increases it (sympathetic nervous system) and a 1°C rise in core temperature can increase it by 10%.

Thermoreceptors, which send information to the hypothalamus, are found in the skin and mucous membrane, and a central thermoreceptor is found in the hypothalamus and detects core temperature.

Losing heat/reducing heat gain - Skin is very effective at losing heat- it loses heat via conduction, convection, radiation and evaporation. Vasodilation is controlled by the parasympathetic nervous system. It causes a rise in skin temperature but heat is lost by conduction, convection, and radiation.

Sweat is released from the exocrine system (sweat glands), controlled by the sympathetic nervous system, and evaporates heat off skin. It contains H20, NaCl, urea, lactic acid, etc.

Evaporation also occurs in hot air from the lungs. It is only effective if the air is dry and hotter than the sweat. It is the only way the body can lose heat in temperatures higher than core temperature.

Metabolic rate is decreased by reducing thyroxine release. Less metabolic reactions produces less heat. We also have certain behaviours when hot – we turn on a fan, or take off hot clothing.

Producing heat/reducing heat loss - Vasoconstriction is controlled by the sympathetic nervous system, and keeps the blood near internal organs, and skin temperature drops. The sympathetic nervous system also stimulates the adrenal medulla to release adrenalin and noradrenalin.

Piloerection (erect hairs) traps hot air like insulation. Shivering is forced muscle contractions (10-20/s) that increase muscle tone in rhythmic contractions.

The hypothalamus releases thyroid releasing hormone, which stimulates the anterior pituitary to produce more thyroid stimulating hormone. This causes the thyroid to produce more thyroxine to increase metabolic rate, therefore increasing heat production.

Heat stroke is when core temperatures continue to rise as heat loss mechanisms become ineffective and cease as the metabolic rate becomes too high to counteract. Heat exhaustion is fluid loss and vasodilation, causing a low blood pressure and fainting. Body temperature is still regulated, and unaffected.

Hyperthyroidism/Hypothyroidism - Hyperthyroidism is increased activity of the thyroid and so the overproduction of thyroxine, and an increased metabolic rate. Symptoms include an enlarged thyroid, rapid heart rate, increased appetite but weight loss, fatigue, sweating and anxiety.

Treatment is surgery to remove all or part of the thyroid, drugs to block thyroid production, or radioactive iodine which is absorbed and kills thyroxine producing cells.

Hypothyroidism is usually caused by a lack of iodine, and is becoming more common. Babies with iodine deficiencies are born with cretinism; they are usually mentally and physically retarded. Symptoms include a goitre, increased fatigue, weight gain, lethargy, and cold intolerance.

Osmoregulation

Definition: The control of water levels and salt levels in the body. The correct water balance between cells and the surrounding fluids must be maintained to prevent problems with osmosis

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The ADH (antidiuretic) hormone is involves in controlling water levels by reabsorption

More ADH = more water absorbed

When a cell body becomes crenated (water moved out down a water potential gradient) then an action potential is triggered which causes vesicles to release ADH into the blood by fusing with the membrane and releasing the hormone by exocytosis.

ADH binds to complementary receptors on the surface of cells and initiates a messenger and a series of enzyme controlled reactions to occur.

Vesicles containing aquaporin in the membrane are created

(aquaporins are channels which allows water to move across the cells surface membrane)

The vesicles fuse with the cell surface membrane. As a result there are now more aquaporins incorporated into the membrane.

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Unlike other shark species that stick to either fresh or saltwater, bull sharks (Carcharhinus leucas) manage to thrive in both. They’ve specialized their ability of osmoregulation, or the ability of an organism to maintain its body. Since all fish, fresh and saltwater alike, have to osmoregulate, bull sharks have figured out how to adjust theirs to all salinity levels.

An external and internal environment is separated by a membrane where substances can move. For fish, this is skin. If the internal has more water than the external, and vice versa, water will move from the highest concentration area to the lowest. So marine animals must prevent dehydration, whereas freshwater animals must reserve their salt levels.Bull sharks have managed to balance these processes out.

Urea and other salty substances normally stabilize based on the environment, which is controlled by the kidneys. By regulating the amount of salt released from the kidneys, the bull shark can adjust and balance by releasing less salt and more urea based on the water. Since people often spend time in estuaries and rivers where bull sharks lurk, encounters with them are often surprised and unexpected; Especially when they were found 2,000 miles upstream in the Amazon River!

Photo credit: wirodive

I thought it was only in cartoons that a place would have a shark-infested lake in a random place like a golf course, but apparently it is reality in Australia. After a flood several years ago, a handful of bull sharks found themselves stranded in a lake on a golf course. Bull sharks are able to survive in fresh water and rather than this lake posing an issue for survival, the six sharks have thrived — and even started breeding.

Wow!! Amazing adaptation of their osmoregulation to go from seawater to freshwater. This link explains how :) (Also that’s one way to make golf interesting!)

(Credit to Cuttlefish-are-my-pals for telling me about this)

I want to see an actress go super-science on an interviewer when they get one of those sexist questions god I can just see it 

Interviewer: So what kind of diet did you go on to fit into this costume?
Actress: Well, if you consider the average basal metabolic rate is 6279 kJ/day and remember that aerobic exercise doesn’t actually affect the resting power of metabolism to effectively predict your fat-free mass its actually quite complicated. Not to mention you use most of that energy on thermogenesis and osmoregulation alone…..

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