Biology and Functions of Homeostasis and Thermoregulation

Homeostasis means “to stay the same” and describes the physiological consistency of an organism’s body while being exposed to changing external conditions (Courses.lumenlearning.com, 2019). Feedback regulation is important for homeostasis to occur as this feedback regulation allows for an organism to sense external changes, allowing for the organism to then regulate and respond in the necessary manner via internal systems (Shepherd and Heeney, 2013).

Homeostatic control systems require the components of a receptor (detects changes), the control centre (the brain) and an effector (reacts in a necessary manner eg. muscles). This report will explain homeostasis and how this process occurs in terms of regulation required by an athlete completing the Tongariro Crossing and whom therefore experiences external changes in temperature from hot to cold.

When beginning the Tongariro crossing, the athlete was said to be dressed in “shorts, lizard shirt and camelback” and was walking pretty fast or running until she reaches the “small plateau” located up from some stairs. This shows that the athlete at the beginning of the crossing was feeling quite warm. Thermoreceptors monitored her temperature and as her temperature rises above 37.5°C, responses occur in order to accelerate heat loss. Thermoregulation occurs through negative feedback systems so to cool down the athlete, the hot temperature disrupts her base temperature of 36.5°C which receptors in the athlete's dermis layer and core can detect. The brain receives input about the temperature being too warm from thermoreceptors and proceeds to send output to effectors (such as her sweat glands to produce sweat for heat loss via evaporation) in order to bring about change or response to alter her temperature so she does not get too overheated which could result in hyperthermia.

Because the athlete’s receptors sense it is too hot, negative feedback systems resulted in a behavioral response of taking off layers to allow for air to come into contact with as much skin as possible to evaporate sweat produced by sweat glands which was another response to cool her down as the sweat from running/walking fast along with “strong winds” will evaporate quicker and easier with less clothing/layers, thus cooling her down.

During the Tongariro crossing when the athlete reaches the stage beyond the plateau as she reaches zones on the crossing which are higher altitude, she begins to show signs of being too cold. She begins to put more clothing on “I took out my thermal top and put it on” and also slows to a walk rather than running as she did earlier on the crossing. ‘At this point, my hat also blew off my head and over the ledge” which indicates that the wind where she is now is stronger and because she is at a higher point on the track there is more exposure to wind and the cold. Thermoreceptors monitor her temperature and as it drops below 35.8°C, responses occur in order to retain and create more heat. Thermoregulation occurs in order to increase external and core temperature to allow her temperature to shift back to near the base temperature of 36.7°C. The brain will receive input from thermoreceptors when the temperature is too cold for her which will allow for responses which correct this. Because the athlete is cold, there is the behavioural response of putting on more layers to prevent heat exchange via evaporation of sweat and/or convection. She also begins to walk rather than run which will decrease the amount of sweat produced and also conserve more energy.

Thermoregulation is important because organisms will perform their best at certain temperatures and will often experience consequences if the body is not consistently at its base temperature eg. could experience hypothermia or hyperthermia as the body can not function correctly while attempting to function outside of its maximum temperature capacity (Xavier, 2019). Thermoregulation is described as regulation of an organism's body temperature via the thermoregulatory centre located within the hypothalamus. The hypothalamus is located in the centre of the brain in between both the thalamus and pituitary glands and is vital for the production of hormones along with aiding an organism undergoing processes such as thermoregulation (Johnson, 2018). Input is received from thermoreceptors from both the hypothalamus, which monitors blood temperature by blood flowing through an organism’s brain (which is core temperature), along with receptors located in the skin (these receptors monitor the body's external temperature). Impulses are sent from the thermoregulatory centre to effectors in order for them to adjust the bodies temperature accordingly (A2 Homeostasis.pdf).

The hypothalamus in a human has a base point of around 36.7°C, this is the temperature which is prefered by humans so when thermoregulation occurs, the hypothalamus will either produce or retain heat as a response to changes in temperature externally or internally. Hormonal and nervous system responses will then occur as a response in order for the body to counteract any external changes for the body to regulate itself back to its base temperature. The hypothalamus will detect change back to a normal state and will stop any corrective mechanisms as there is nothing to correct anymore. The systems involved come under the autonomic nervous system as this is split into two sections which are parasympathetic and sympathetic. Thermoregulation in regards to counteracting heat loss occurs mainly via responses in the sympathetic nervous system. Whereas thermoregulation in terms of counteracting heat gain occurs mainly through responses in parasympathetic nerves of the autonomic nervous system. These work together to maintain temperature balance as these are internal systems which allow for changes to take place in correspondence to external changes in temperature which raise or lower body temperature below certain points.

Thermoregulation via skin creates a negative feedback loop. Feedback systems require something which stimulates sections of the body, which proceeds to invoke a reaction from another section of the body, this section will then change the stimulus. Negative feedback loops maintain setpoints (such as temperature) and mean that if there is a change, the body will respond by using a corrective mechanism which will allow for the reversal of whatever change occurred in order to bring the organism’s systems back into the desired setpoint. Although the body will never be able to perfectly and consistently maintain the exact setpoint (eg. 36.7°C), systems allow for the body to maintain a condition close to the setpoint and having a homeostatic system that is efficient will, in turn, reduce and minimize the fluctuation of condition. An organism's body systems are maintained via feedback systems which can be either positive or negative.

Negative feedback systems (process shown in the diagram to the right) are a system which forms a loop which allows for an organism to restore their bodies condition to its preferred and balanced state. A stimulus will disrupt an organism's homeostatic state (temperature rise or fall) within a controlled condition (temperature base point of 36.7°C) which is monitored by receptors (thermoreceptors located within the dermis layer and hypothalamus) which send input to the control centre (brain), the brain then receives this input and produces an output for effectors (eg. muscle cells) that result in a response (eg. shivering if it is too cold) which will change the controlled condition (temperature). This response’s purpose is to allow the body to return to a homeostatic state. Positive feedback loops result in amplification of response to a stimulus eg. breastfeeding whereas negative feedback loops result in a return to a level state eg. thermoregulation.

Response to conditions which are hot or cold is voluntary, if a human feels too hot their reaction to this could be taking off clothes or shifting to shadier areas whereas if it’s perceived to be too cold then their reaction could be putting more clothing layers on or setting their heater to a higher temperature. If these responses do not provide the needed temperature regulation then the thermoregulatory system will then begin working, which is connected to the autonomic nervous system meaning that the following responses are involuntary. If a human is too hot then the hypothalamus will activate the heat loss section and if it is too cold then the hypothalamus will activate the heat conservation section. Thermogenesis can occur which are responses to too low temperatures used to create heat, whereas other responses are merely to conserve the bodies heat. Some responses will allow for cooling down and others will reduce the production of heat or help transfer the heat to the body surface.

As the diagram above shows (Quizlet, 2019), if homeostasis is imbalanced by a stimulus (change in temperature) then the change will be detected by thermoreceptors (receptors in skin and hypothalamus), the change will be noted and input will be sent to the control centre (brain) and then in order to reverse this, output will be sent via pathways (hormonal/chemical or nervous system) to an effector (muscles, sweat glands, erector pili muscles, skeletal muscles, adrenal thyroid glands and behaviour) which will respond to the initial change in order to correct the homeostatic imbalance to return the organism to a homeostatic condition (A2 Homeostasis.pdf).

Hormonal/chemical and nervous system pathways are two categories of thermoregulatory responses. Nervous system response to low temperature begins with the effector being muscles within arterioles in the skin, these muscles will contract and lead to vasoconstriction. This limits heat flow from the bodies core out towards the surface allowing for core temperature maintenance. Far points of the body such as fingers can change colour and turn blue and feel cold due to lack of blood flow which can cause damage if left for too long otherwise known as frostbite. Whereas a nervous system response using the same effector but for a response to high temperatures would result in the muscles relaxing resulting in vasodilation rather than vasoconstriction. This means heat in the body can flow from the bodies core outwards towards the bodies surface where heat will be lost due to convection (transfer of heat to surrounding air through skin exposure) and radiation (heat transfer through infrared waves) and skin will turn red in colour as blood flows to the surface (Opentextbc.ca, 2019).

Another nervous system response to low temperature is the lack to sweat produced by sweat glands whereas the nervous system response to high temperature is the production of sweat by sweat glands as when sweat evaporates it has a cooling effect. When humidity is high and when a person wears many layers of clothing, sweat evaporation becomes difficult and does not allow for cooling down.

Another nervous system response to low temperature is erector pili muscles located in skin (which is attached to hair on the skin), these muscles begin to contract as a response to the cold and can lead to the rise of hairs on the skin which contains air that is still warm near the skin to insulate and keep the body warm (though in humans this is said to not be super effective as it just causes goosebumps in most people).In terms of erector pili muscles and their response to high temperatures, the muscles will relax and allow hair to lie flat which means that the air can access and circulate around skin easier which can help to encourage both convection (transfer of heat to surrounding air through skin exposure) and evaporation (heat transfer via water/sweat evaporation).

A different nervous system response to low temperature is skeletal muscles resulting in shivering. Shivering when cold is done by muscles contracting quickly which allows for the generation of heat through both metabolic pathways and by friction whereas, in response to high temperatures, the body will not proceed to shiver.

A chemical/hormonal system response to low temperature is through adrenal thyroid glands. When cold the body will react by secreting both adrenaline and thyroxine from the adrenal thyroid glands. Adrenaline and thyroxine both speed up the metabolic rate which in turn results in burning more fuel (food/glucose which is involved in respiration) which then produces energy in the form of ATP which in turn results in the production of heat energy which is important for thermoregulation, namely thermoregulation in terms of heating up in cold temperatures. ATP (adenosine phosphate) is a body chemical which allows for storage and usage of energy which in turn allows for many bodily functions to occur.

Lastly, behaviour will change depending on the temperature; if the temperature is low then a person may curl up into a ball, huddle together with others, find shelter, put on more clothes, etc all as measures to conserve body heat whereas if the temperature is high then a person may instead spread their body out, find shade to get out of the sun’s rays, go swimming as the water will cool their body through conduction (heat transfer through objects which are within direct contact), etc all as a means to cool down.

Extreme external and/or internal environmental influences can result in a breakdown or disruption of this control system (hypothermia or hypothermia) as the body attempts to operate outside of its natural capacity. The system could fail during the hike as temperature rises above the temperature boundaries or drops below the boundary which can in time lead to hypothermia and/or hyperthermia which can, in turn, result in death. Heat is produced via metabolic processes which maintains body organs and functions which are both vital for survival (Alana Biggers, 2019). As temperature falls, blood flows from the outer sections of the body into the centre in order to protect vital internal organs along with prevent heat loss via convection. If a person’s temperature becomes too high, the structure of body proteins can be permanently disrupted as enzymes are very sensitive to temperature changes and can result in death. If the body temperature rises too much, the body’s cells will begin to shut down and stop functioning, also resulting in death. Temperature can rise due to a number of reasons such as; fever due to illness/infection and prolonged exposure to harsh factors such as heat resulting in heatstroke or sunburn. For a human to experience hyperthermia, the core temperature must reach around 44°C.

This and hypothermia are why it is an adaptive advantage for humans as it allows for the body to maintain a temperature which is most suited for enzymes to function and work efficiently at in order to keep a person alive and functioning. Ectothermic animals do not have this adaptive advantage which means that in order for them to regulate their temperature they must rely on external factors such as sunlight (which is why reptiles are often located basking in the sunlight) which means that ectothermic animals are likely far less active during cold seasons/days and more active during warmer seasons/days whereas humans and other endothermic animals can be rather active in either warm or cold temperatures, endothermic animals such as humans, don’t have as much of a restraint when it comes to activity in different seasons/temperatures (Biozone.co.nz, 2019). Ectothermic animals also do not have the advantage of behavioural responses such as putting on or taking off clothes to regulate temperature, thus giving endothermic animals an adaptive advantage.

Enzymes/proteins functions can be disrupted with cold temperatures too, resulting in a person not being able to function properly. Hypothermia can lead to the death of body cells and cell function will cease. Frostbite can occur as (explained earlier) blood flows to the bodies core vital organs to protect them in an attempt to keep the body functioning however this leaves parts of the body vulnerable to the cold such as fingers as there is lack of blood flow to these areas when the body is in such a low-temperature environment. If the person's temperature maintains this low temperature or drops even further for even just a few hours, it is likely the person will die. Thermoregulation systems are disrupted when either a person is unable to produce/retain enough heat to the ratio of heat loss (hypothermia) or a person is unable to cool themselves down in an overheating environment (hyperthermia).

The athlete crossing the Tongariro crossing did not experience either hyperthermia nor hypothermia because she was able to regulate her temperature to keep her either warm enough or cool enough. However, her thermoregulatory systems would have experienced a breakdown if she were faced with extreme internal or external conditions. For example, if the athlete were to have sat still, without her thermal on, at a rather high point in the crossing where she exposed to harsh winds or rain, hypothermia could have occurred. The athlete will have had responses due to her body trying to regulate temperature through negative feedback loops resulting in her shivering, blood flowing to her core, huddling up into a ball, etc however if these responses were not enough to either retain or create heat then her temperature would continue to stay at a dangerously low level.

The low temperature (below 35°C) will lead to hypothermia (condition ranges in severity depending on how low the body’s temperature is) which can result in things such as; confusion, difficulty speaking, short term memory loss, failure of organs and death if left untreated. The athlete for example could have also experienced hyperthermia on the crossing if she were to have gone on the crossing; during a hot summer month, worn multiple warm layers, not had many liquids, etc as these would all raise her temperature and if she chose not to take layers off, drink water, shift into the shade, etc then she would more than likely overheat and with heat from the sun and hot season temperature, she could experience a breakdown in her thermoregulatory systems and experience hyperthermia. Both of these scenarios could ultimately result in death for the athlete.

Thermoregulation systems aid in maintaining a homeostatic state and provides an adaptive advantage to humans as it allows for us to regulate our temperature through a receptor, control centre and effector to maintain homeostasis. Applying these systems to the scenario of an athlete crossing the Tongariro Crossing and discussing that if there were also a disruption to these systems then hypothermia and/or hyperthermia could occur and result in death for the athlete.