Introduction
Homeostasis refers to the maintenance of the relatively constant physiological state of equilibrium in the internal environment of a living organism, despite fluctuations in the external environment. It is essential for maintaining optimal cellular function and ensuring reproduction and survival of the individual and the species. A change in the environment which an organism senses and (most likely) responds to is called a stimulus, and organisms must adjust both behavior and physiology to maintain homeostasis.1
System | Thermoregulation
An example of a system maintained by homeostasis is the thermoregulatory system. Thermoregulation is the body’s ability to maintain a blood temperature/core temperature (the variable in question) at which metabolic processes can be performed optimally.1 Animals that maintain a fairly constant internal body temperature (birds & mammals) are called endotherms, while all other living organisms which have varying body temperatures are called ectotherms. Humans are endotherms, otherwise referred to as “warm-blooded.”2 The ideal body temperature of a human lies within a reference range (the acceptable range of values for a variable, within which an organism remains healthy) of 36.5oC to 37.5oC, with a set point (ideal value for the variable) of 37oC. The temperature can fluctuate within the reference range, but extreme fluctuations beyond the reference range can result in severe side effects or death.1
The difference between endotherms and ectotherms is that endothermic animals rely on internal corrective mechanisms to maintain a constant body temperature, whereas ectotherms have no constant body temperature, but use behavioral mechanisms such as basking in the sun when cold or moving into the shade when hot to retain energy. Humans have a distinct advantage when it comes to thermoregulation, as we use both internal and behavioral mechanisms to maintain a constant body temperature.2
All homeostatic systems rely on a mechanism called negative feedback, by which the body responds by acting against the initial stimulus in order to restore the equilibrium and thus, maintain homeostasis. Negative feedback mechanisms responsible for this keep the body’s internal conditions within strict limits. This is because the enzymes in our bodies are very sensitive to change, and deviations from these limits can be harmful or fatal. An example of this is if a human’s internal body temperature exceeds around 42˚C, in which instance enzymes could denature. Denaturation occurs at higher than tolerable temperatures and is when the shape of an enzyme’s active site changes because the amino acid bonds that provide its structure are broken, and the enzyme unravels. The enzyme can then no longer fit with its specific substrate molecule to carry out its function.1
Negative Feedback Loop
Diagram via http://www.passbiology.co.nz/biology-level-3/homeostasis
Homeostasis is maintained by different systems and sensory components. A stimulus-response model is shown below.
Stimulus – the change in the external environment; e.g. the air temperature outside drops and therefore the core body temperature drops from the set reference point of 37˚C to 36˚C.
Receptor – the organ or component which analyses the change in an environmental factor; e.g. temperature-sensitive cells in the skin detect the change in air temperature. It then sends a message to the brain and the control center (hypothalamus).
Control Centre – Upon receiving information from the receptor, the controller decides if any actions need to be carried out; e.g. the hypothalamus acts as a thermostat and determines if the drop in air temperature needs to be reversed/corrected in order to regulate the core temperature. It outputs messages to the effector.
Effector – The effector receives the message from the control center and coordinates the appropriate response; e.g. if the hypothalamus detects that the core body temperature has dropped below the set reference point, it sends a message to the effector, in this case the muscles, to shiver.
Output/Response – The response which is carried out by the effector; i.e. when muscles continually contract and release (shivering), this generates friction and produces heat. This causes the blood temperature to rise, and as it travels around the body it will increase the core temperature back to the reference point of approximately to restore equilibrium and maintain homeostasis.1
Control Centre | Hypothalamus
Diagram via https://thrivecoach12.com/2016/03/23/the-pituitary-gland/
In most living organisms, the temperature regulation centre of the body is in the hypothalamus. The hypothalamus is located in the brain above the pituitary gland and is the control centre of many homeostatic mechanisms. In thermoregulation, the hypothalamus acts like a thermostat to detect changes in core body temperature. It also receives information about temperature change in the external environment via thermoreceptors in the skin. When the thermoreceptors detect a change in either core or external temperature, the hypothalamus then coordinates nervous and/or hormonal responses and outputs them to the appropriate control organs/components. These respond using negative feedback mechanisms in order to return the (human) body temperature to the set point of 37˚C.1
The hypothalamus plays an important role in linking the nervous system to the endocrine system via the pituitary gland. The pituitary gland is responsible for controlling involuntary nervous systems, which control things such as thirst, hunger, body temp., and heartbeat, by keeping hormone levels, energy, water and heat levels balanced within the body. It does this by synthesizing and secreting neurohormones from specialized neurosecretory cells which function as both nerve cells and endocrine cells. It is attached to the base of the hypothalamus and consists of two halves/lobes; the posterior pituitary and anterior pituitary. The posterior pituitary is basically an extension of the hypothalamus. Its neurosecretory cells release hormones into the blood in response to nerve impulses. The anterior pituitary is connected to the hypothalamus by blood vessels and receives neurohormones via capillaries.
Additional Components
Receptors – Skin – the body’s largest organ which plays a large role in heat transfer (explained in the response table below), CNS (central nervous system).
Diagram via http://www.passbiology.co.nz/biology-level-3/homeostasis
Effectors – Blood & blood vessels, skeletal muscles, sweat glands, hairs on the skin and the thyroid gland. There are also a range of behavioral adaptions (in humans and other ectotherms) that can increase or decrease body temperature as necessary. An effector-response table is shown below to demonstrate how a body temperature that is too warm or too cool can return to normal via components in the thermoregulatory system in order to restore homeostasis.
Effector Response to Low Temperature Response to High Temperature
Blood vessels Blood vessels in the skin contract, causing vasoconstriction. This means that less blood, (which is the main carrier of heat around the body) flows from the core to the surface of the skin, thus preventing heat loss and maintaining core temperature. This can cause the skin to go slightly blue in colour because there is less blood (and therefore heat) being carried around the body.2 Blood vessels in the skin dilate, called vasodilation. This allows more blood to flow from the core to the surface of the skin, where heat is “lost” by convection and radiation. This can cause the skin to go red because more the blood is close to the surface.2
Skeletal muscles Shivering; the repeated contract and release of muscles. This generates heat by friction.2 It also requires energy, so the body must respire more. Heat is a bi-product of respiration, so this also releases more heat into the blood so the core temperature increases.3 No shivering.
Sweat glands No sweat produced. Sweat glands secrete sweat onto the surface of the skin, which cools the body as it evaporates. The ability for sweat to evaporate from the skin can be impaired in high humidity or by wearing tight clothing.2
Hairs on the skin The erector pili muscles in the skin which are attached to the skin hairs contract, raising the hairs and trapping an insulating layer of still air next to the surface of the skin. (most effective in mammals with fur) 2 The erector pili muscles relax, lowering the hairs on the skins and allowing air to circulate all over, encouraging evaporation of sweat and convection of heat.2
Thyroid gland Secretes thyroxine (thyroid hormone) to increase the metabolic rate, thus generating heat.1 Stops secreting thyroxine to bring metabolic rate back to (approx.) normal.1
Adaptive behaviors – Putting on more clothes – creates an insulation layer around the skin to prevent heat loss by convection.
– Curling up – reduce surface area exposed to air which could give off heat.
– Finding shelter if in exposed environmental conditions.2 – Removing layers of clothing – allows for the convection of heat to cool the core temperature.
– Stretching out – promotes the circulation of air over the body and allows for easy evaporation of sweat.
– Finding shade from the sun.2
Heat Transfer
While physiology helps us to understand the way the body will respond to being exposed to different environments, physics helps us understand heat transfer in relation to thermoregulation. Heat can be transferred into and out of the body, altering the core temperature. While heat energy can never be lost, we use the term “lost” to signify when heat energy is transferred away from or out of something. In this context, we are referring to the human body. We can also generate or produce heat. There are several different ways in which heat transfer can happen, including conduction, convection, radiation, or evaporation.
Conduction is the process whereby heat is “lost” or transferred out of the body when the skin comes into physical contact with a substance which is a thermal conductor. In other words, it is the flow of internal energy from a region of higher temperature to an area of lower temperature. An example of this is sitting on a metal chair. The heat from the part of the body in contact with the chair will transfer to the cold chair.4 When sitting on a cold chair, only a small surface area of the body is in contact with the seat so not too much heat should be lost through conduction in this instance, meaning the core temperature of the body should stay relatively the same.
Convection is the transfer of internal heat into or out of the body (also from area of higher temp to area of lower temp) via circulating air or liquid surrounding the skin.4 Unlike conduction where there must be direct contact in order to transfer heat, convection relies on the circulating motion of molecules to take place. An example of this is using a fan to cool down if the core body temperature is slightly higher than the reference point of 37˚C. The amount of heat loss by convection is dependent on how fast or strong the air is blowing.5 Heat will be transferred out of the body by convection until the core temperature returns to around 37˚C, at which time a mechanism called vasoconstriction will kick in to prevent too much heat being “lost” from the body. This is when the blood vessels are narrowed, restricting the blood flow slightly so that not as much blood is flowing to the surface of the body at one time where it could lose heat. This prevents the blood/core temperature from dropping below the reference point which could cause the body to become too cold.12
Radiation is the absorption of heat from sources such as the sun, heaters or fire through infrared rays, where no physical contact is involved. For example, the body warms up when basking or sun-bathing. It absorbs the heat from the infrared rays.5 This increases the core body temperature, and in an increase above 37˚C may trigger sweating. Sweat glands perspire, releasing sweat which cools the skin as it evaporates.2
This brings us to the next method of heat loss, evaporation. Most commonly occurs when the body is sweating to reduce the core temperature. This is an endothermic reaction which absorbs heat from the surroundings. Fluid from the sweat glands evaporates (turns to gas/vapor) off the skin and heat is released from the body through convection or radiation, lowering the core body temperature. When returned to normal, the hypothalamus heat-loss center shuts off and the body temperature is restored. However, this is a slightly less efficient process as it requires energy to take place, but this is usually in the form of heat energy which comes from the skin.12
Heat production is when heat is generated [by the body] via metabolic reactions, for example respiration. Respiration is the process whereby the body releases energy in the form of Adenosine Triphosphate (ATP), using glucose and oxygen.1
C6H12O6 + 6O2 6CO2 + 6H2O + ATP
Glucose + Oxygen Carbon + Water + Adenosine
Triphosphate
(28-38)
Heat is a bi-product of respiration, which increases the core temperature when respiration is taking place. Many living organisms, such as humans, want a core body temperature which is higher than that of the surrounding external environment, as this is one of the most effective methods of maintaining a temperature within the reference range (i.e. 36-37˚C).1
Scenario
Tongariro National Park is New Zealand’s oldest national park, renowned for its cultural identity and beautiful scenery. The Tongariro Alpine Crossing is a 19.4km track that takes 5-8 hours to complete, peaking at around 1800 meters above sea level.6 Moderate fitness is required because of the tracks steep inclines, rough volcanic debris, loose rock, boggy sections, and the potential for strong winds which can be tiring.7
According to Student Resource B, “A group of senior students are completing the Tongariro Crossing in June 4th, 2016, on a guided tour. They arrive at the car park at 8.30am with all their back packs and gear. They plan to complete the entire crossing by 3pm.” According to the weather forecast provided, the coldest area of the crossing is at the highest point Red Crater, in the morning, with a Southeasterly wind temperature of -8˚C.7 Because this section is approximately 3 hours into the hike, the group should reach it around midday when the temperature is expected to be closer to -1˚C. It is also wise that they aim to finish by 3pm, as the temperature plummets again in the evening.
Amongst the group of students to complete the crossing is Cartia from Northland, New Zealand. 18-year-old Cartia is a fit young adult who regularly exercises at the gym and previously played sports. She is challenging herself to complete the Tongariro Crossing ahead of further mountaineering adventures she plans to undertake overseas.
In order to be prepared for the track, the appropriate supplies should be carried. These include food, plenty of water, extra layers of light but warm clothing, a first aid kit and wet weather gear. Because of the tracks location and the points of altitude, it can be a cold environment at any time of the year. However, this guided tour is taking place in June at the beginning of winter, meaning low temperatures are certain. Hikers should wear warm clothing, such as polypropylene thermals and fleece as these are the best thermal insulators, reducing the chance of heat loss via radiation or evaporation. A wind-proof jacket and trousers are also ideal as these prevent heat-loss via convection, especially in windy conditions.
Cartia is wearing thermals under a windproof jacket, as well as a beanie, gloves, thick socks and sturdy boots. She wears thermal leggings underneath her nylon hiking trousers, however these are not windproof.
What Cartia doesn’t know is that she suffers from hypothyroidism; an internal disruption in her Tongariro Crossing journey. Hypothyroidism or ‘underactive thyroid’ is a condition where the thyroid gland doesn’t produce sufficient levels of important hormones which can upset chemical reactions in the body. It is most common in women older than 60 but can occur at any stage in life.8
The group sets off just before 9am and make good time, reaching South Crater in 2 hours. Thermoreceptors in the skin send messages to the brain which tell Cartia that the external environment is very cold, and even though she thinks she should be warm from the exercise, she puts this down to the fact that she is not used to temperatures this cold in Northland winters. However, a tell-tale symptom of hypothyroidism is a sensitivity to cold.8 Although fatigued, Cartia is feeling okay and continues on the crossing with the group.
3 hours into the crossing, the group is heading towards Red Crater, the highest point of the track. This area is more exposed, and because they are climbing from 1650m-1886m, the temperature cooler than expected, at -2˚C, providing a slight external disruption. Even though this is one of the more physically difficult sections, so she is exerting herself, Cartia is feeling even colder and has started to shiver. This could be promoted by sweat evaporating off Cartia’s skin, resulting in further heat loss via radiation. One of the other hikers has noticed that Cartia has dropped to the back of the group and is unusually pale in the face. The group stops on the side of the track for a break and they sit Cartia on a rock. She has a drink of water and the tour guide gives her another coat, but he notices that her breathing has become shallower and labored, she is now shivering more rapidly and is easily confused by the questions he is asking her. He suspects that her body temperature has dropped below 35˚C (below reference range and well below set point) and she has developed mild-moderate hypothermia. Hypothermia can result when the body is exposed to very low temperatures for a short period of time or, in this case, moderately low temperatures for a longer time. This hinders the body’s ability to thermoregulate effectively, so metabolic reactions slow causing mental fatigue, loss of coordination and difficulty moving. The treatment for hypothermia is to rewarm the body gradually with extreme care so as not to shock it. A sudden increase in external temperature may cause the body to attempt to remove excess heat, causing the core temperature to decrease further which could be fatal.12
With a body temperature of 35-32˚C, mild hypothermia is apparent. The control centre, the hypothalamus, acts as a thermostat, detecting the drop in internal temperature and sends a message to an effector, the muscles. The effector coordinates the shivering mechanism as a response, attempting to produce heat via friction. This process requires energy to take place which is released during respiration (where heat is a by-product), however because the internal conditions are not favorable, respiration is not as efficient so only so much energy can be released. At this stage, Cartia’s muscles and blood vessels are also working together in vasoconstriction to reduce blood flow to extremities as an attempt to reduce heat loss. This explains the lack of pinkness in her face and slightly blue lips because less blood is being carried to the surface of the body where it could be lost by convection.2 The cure for this stage of hypothermia is “passive rewarming,” using methods such as curling up, adding warm clothing (insulating layers), and moving to a warm and dry environment. The tour guide is concerned for how cold Cartia appears to be despite the physical intensity of the hike. He makes the call to ring the park to get some help, and a rescue helicopter is called to airlift Cartia to hospital. While they are waiting for help to arrive, the group wrap Cartia in a foil or “space blanket” from the first aid kit. This consists of vaporized aluminum which is deposited onto a very thin plastic film, creating a thin, flexible and thermal-reflective material. The aluminum helps to redirect infrared energy and keep heat in to prevent further core temperature decrease.10
When the paramedics arrive, they confirm that Cartia has developed moderate hypothermia. Her body temperature is now at 31˚C so it is essential that she is moved to a sheltered place out of the cold. Because of the warm layers Cartia is wearing, they are concerned as to why only she has developed hypothermia while the rest of the group are alright. The trousers she is wearing allow for heat loss by convection, via the circulating air around the skin at the slightly windier Red Crater. While this is not a major external disruption, it has not aided the natural restoration of Cartia’s thermoregulatory system. However, they suspect that there must be an underlying factor in Cartia’s hypothermia. On the way to hospital they have Cartia under several warm blankets and heat packs to gently warm the body and keep her condition stable, all while closely monitoring her breathing, pulse and temperature. When they get to the hospital, Cartia’s temperature has risen slightly to 33.5˚C and she is more alert, but she is still moderately hypothermic. She is given a mask to inhale warm, moist air (vaporized water) at 40-42˚C to warm internally. Her lungs receive this warmer air and the temperature translates to her core temperature, which rises up to 35˚C in the first 10 minutes. Because of this response, the hospital determine that Cartia does not require intravenous delivery of warm fluids as her temperature is rising at a good rate. She is offered warm fluids such as herbal teas that don’t contain caffeine to help warming but not reduce heat product (caffeine may provide another disruption to the thermoregulatory system in this instance)1. A blood test finds that Cartia has a low level of the thyroid hormone thyroxine, but a high level of thyroid-stimulating hormone (TSH). The doctor explains that she has an underactive thyroid, so the high TSH levels indicate that her pituitary gland is producing more TSH in an effort to stimulate the thyroid gland. Thus, she is diagnosed with hypothyroidism.9 The findings explain other symptoms Cartia noticed, including her increased sensitivity to the cold, fatigue, dry skin and occasional muscle aches.8 The doctors prescribe Cartia with the oral medication Levothyroxine to use daily, which will restore adequate hormone levels and stop symptoms of hypothyroidism. She is checked on every so often but soon her temperature remains constant at 37˚C, so the doctors are confident that her thermoregulatory system has restored itself completely and she is free to go home.
The quick intervention of Cartia’s team ensured her survival and the restoration of her thermoregulatory system. She was very lucky that she was able to receive the appropriate treatment quickly, and also that she didn’t go any longer without being able to carry out essential life processes. Luckily hypothermia does not result in the immediate denaturation of enzymes like, for example, hyperthermia would (the opposite of hypothermia where the body becomes too hot). Her bodily functions were temporarily inhibited and even though it could have been life threatening, she could make a full recovery.
Failure to Restore Homeostasis
In the instance that Cartia was not taken to hospital quickly, her hypothermia could have become much more severe and the thermoregulatory system may have failed to restore homeostasis without medical intervention, putting her in a life-threatening situation. If at any time people had reacted too quickly and tried to warm the body suddenly, e.g. if she was in a place to get into a hot bath, the side effects could have been just as severe. Sudden warmth can cause sudden vasodilation as the body attempts to remove “excess” heat but in fact causes the core temperature to plummet. 12
32-28˚C – Moderate hypothermia (Cartia came close to this, showing some symptoms) – muscle coordination becomes difficult with movements slow and labored. There is further constriction of the blood vessels in the ears, nose, fingers and toes, turning them blue in colour. Mental confusion sets in. Cure – “active external rewarming,” involving warming devices like hot water bottles or warm water baths (not too warm as this could cause shock). 12
28˚C and below – Severe – speech fails as mental processes become irrational and distorted. Cure – “active internal” or “core warming.” This requires intravenous delivery of warm fluids, inhalation of warm vaporized water, or warming the blood by using a heart-lung machine. This is the most dangerous stage of hypothermia because it is life threatening.
Eventually if left untreated, hypothermia could result in a breakdown of the thermoregulatory control system and homeostasis could not be restored. The body’s major organs would begin to shut down, possibly result in cardiac arrest and then death. 12
Thermoregulatory Adaptive Advantage
For all living organisms (endotherms in particular), the thermoregulatory system has a huge significance and provides massive adaptive advantages that ultimately contribute to the survival of an individual and the species, in terms of reproductive success. This often involves hostile external environments that are otherwise threatening to organisms. Therefore, they can efficiently conserve energy that can be used for interaction, survival and reproduction.1
One of the main adaptive advantages provided by thermoregulation is that essential life/metabolic processes can still be carried out in unfavorable or rapidly fluctuating external environments, such as in the Arctic where it is extremely cold. This is because, within the temperature reference range of the internal environment, enzyme activity can still function. Not only will the temp be kept within a healthy and functioning range, but it is optimized so that metabolic processes can be the most efficient possible. These important processes include; movement, respiration, DNA replication, sensitivity, growth, protein synthesis, reproduction, and excretion. In the long term, organisms with this adaptive advantage are able to inhibit a much wider range of habitats and reproduce successfully in order to maintain their population.11
Metabolic processes require energy which is released by the process of respiration, and thermoregulation honors this process by helping to conserve energy that can be used for life processes. For example, thermoregulation means that the body doesn’t spend all its time and energy trying to stay warm or stay cool. This is another adaptive advantage, because the body avoids extreme stresses of things such as hypo- or hyperthermia which would use up vital energy and put an organism in a very vulnerable situation.11 In Cartia’s situation, without the intervention of her hiking group, she may not have survived the Tongariro Crossing.
For other animals other than human, adaptive advantages in the wild include;
– The ability to adapt to the changing environments and external factors (i.e. weather/climate, temperature) and therefore occupy new or different niches, reducing competition and predation and increasing reproductive success.
– Withstanding hot and cold temperatures due to thermoregulation means that some species do not have to migrate in order to survive. When they are not susceptible to the environment, they don’t lose energy migrating and risking predation, so have more energy to survive and reproduce.11
Bibliography
1. Crossland, I. (2018). Personal Notes. 13Biology KKHS.
2. BiologyMad. (Year Unknown). Homeostasis. Retrieved from Biology Mad: http://www.biologymad.com/resources/A2%20Homeostasis.pdf
3. Wilson, M. (Year Unknown). Biology L3 > Homeostasis. Retrieved from Pass Biology: http://www.passbiology.co.nz/biology-level-3/homeostasis#TOC-The-hypothalamus
4. Elert, G. (1998-2018). Conduction. Retrieved from The Physics Hypertextbook: https://physics.info/conduction/
5. SoftSchools.com. (2003). Convection Examples. Retrieved from SoftSchools.com: http://www.softschools.com/examples/science/convection_examples/8/
6. NZ_Government. (2018). Tongariro Alpine Crossing. Retrieved from DoC New Zealand: https://www.doc.govt.nz/tongariroalpinecrossing
7. NZQA. (2018). Biology Achievement Standard 3.4 (91604) V2 . Thermoregulation Scenario Information; Student Resource B.
8. Mayo_Clinic. (2016). Hypothyroidism (underactive thyroid) – Diagnosis. Retrieved from Mayo Clinic: https://www.mayoclinic.org/diseases-conditions/hypothyroidism/diagnosis-treatment/drc-20350289
9. Mayo_Clinic. (2016). Hypothyroidism (underactive thyroid). Retrieved from Mayo Clinic: https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284
10. Ronca, D. (2016). How Space Blankets Work. Retrieved from Adventure | How Stuff Works: https://adventure.howstuffworks.com/survival/gear/space-blanket1.htm
11. Emaze. (Year Unknown). Thermoregulation. Retrieved from Emaze: https://www.emaze.com/@AITZOOI/thermoregulation
12. BioZone. (2018). 3.4 Homeostasis. In BioZone, NCEA Level 3 Biology Internals. BioZone.
Please also refer to individual sources cited below each diagram.
Citation Order
1. (Crossland, 2018)
2. (BiologyMad, Year Unknown)
3. (Wilson, Year Unknown)
4. (Elert, 1998-2018)
5. (SoftSchools.com, 2003)
6. (NZ_Government, 2018)
7. (NZQA, 2018)
8. (Mayo_Clinic, 2016)
9. (Mayo_Clinic, Hypothyroidism (underactive thyroid) – Diagnosis, 2016)
10. (Ronca, 2016)
11. (Emaze, Year Unknown)
12. (BioZone, 2018)