The internal environment of our bodies is constantly kept within strict limits. Take for example temperature. It must be a challenge keeping our fleshy selves at 37°C while the outside fluctuates wildly! Or how about blood glucose concentration? We’ve all heard of diabetes – if it goes too high our organs sustain damage, if it goes too low a coma may be induced or even death.
Welcome to homeostasis – the maintenance of physiological parameters within optimal range. Other parameters include blood pressure and pH.
Negative feedback and thermoregulation
The operation of an oven is an easy example of negative feedback acting both ways, which is how it usually acts. That means that there are deviations in two opposing directions. If an oven is set at 220°C, both a decrease and an increase in temperature is a deviation. So if the temperature drops or rises, a sensor picks that up and commands the heater to turn on or off.
This is negative feedback because it returns the system to its original state.
Sometimes completely separate sensors control a rise or fall in temperature. This separation gives a high level of control.
Not all organisms maintain their body’s core temperature the same way. Some control it internally like we do. We’re endothermic. Reptiles for example are not. They’re ectothermic. Two pennies for who guesses how these fine specimens maintain their temperature. Do they:
A) Run for heat?
B) Shiver for heat?
C) Sit where the sun shines?
They sit where the sun shines, ladies and gentlemen, they just sit there until it gets too hot. And what do they do when it gets too hot? Well, funny you should ask. They move their scaly little selves to a shady place. Why didn’t our monkey-faced ancestors think of that? (They did, long story.)
Needless to say, a lot of ectothermic organisms live in extremely stable regions where temperature doesn’t fluctuate wildly.
Fun fact: that bit in the brain, the hypothalamus, is responsible for temperature homeostasis. Thermoreceptors pick up increases and decreases in optimal body temperature and send signals via nerves to effectors to act. This is what happens:
Sweat cools down the body by producing water that upon evaporation removes heat energy. Vasodilation is the dilation of blood vessels. It results in the distance between them and the skin shortening so that excess heat can dissipate quicker. Flat hairs ensure that no air is trapped close to the skin. Air is a poor heat conductor so effectively this removes any potential insulation.
No sweat occurs. Vasoconstriction has the opposite effect to vasodilation, as the vessels tighten up instead of dilate to prevent heat loss. Raised hairs create a layer of air trapped between hairs and skin, minimising heat loss to the environment. Shivering causes muscles to do work, thus releasing more heat. Hormones like adrenaline may be released to increase metabolism.
Why, though, is it so important to keep a strict temperature and pH? From a physiological viewpoint, think of enzyme activity. Its sensitivity to both temperature and pH means that the only conditions of optimal function will be strictly defined. Lots of enzymes denature past 40°C and are inactivated below 35°C.
Similarly, extreme pH is detrimental to optimal enzyme function, so blood pH must be tightly regulated.
Temperature affects all chemistry so that higher temperature increases kinetic energy of molecules and therefore their probability of interaction as they move more. This has an effect on one of the main movement types in cells – diffusion – as it allows fast enough propagation of chemicals to undergo reactions and support metabolic function.
Thyroxine is a key hormone secreted by the thyroid gland, which is regulated via a negative feedback loop between the brain’s hypothalamus and pituitary gland. These secrete other hormones which stimulate the secretion of thyroxine in the thyroid gland, and cease the production of those hormones in the presence of enough thyroxine in the blood.
Thyroxine plays key roles in metabolism by setting the basal metabolic rate, controlling the metabolism of fats, protein and carbohydrates, maintaining body temperature, regulating digestion and other functions.
Excessive thyroxine causes symptoms of heat intolerance, weight loss and increased appetite, while low thyroxine causes intolerance to cold, low heart rate, weight gain and reduced appetite. Totally backwards with the weight changes and appetite changes (but true)!
Measuring body temperature, hyperthermia and hypothermia
Getting accurate core temperature readings from adults and children alike is key to diagnosing various health conditions, including fever and hypothermia. Fever is associated with infections, and tiny temperature changes of just one degree Celsius can bring on severe complications.
Therefore, multiple methods of measurement exist: oral, tympanic (ear), axillary (underarm) and rectal.
Rectal measurements are the most accurate, while axillary measurements are the least accurate. Body temperature may change up to 0.6 °C throughout the day, depending on activity and wakefulness.
A rectal or ear measurement is 0.3 – 0.6 °C higher than an oral measurement, while an axillary measurement is 0.3 – 0.6 °C lower than an oral measurement.
Rectal measurements are good for patients who cannot provide an oral measurement due to having to breathe through the mouth, inability to cooperate, children, etc. They are performed with a special thermometer which is lubricated and inserted 0.5 – 1 inch into the rectum. Readings are ready when the thermometer beeps, around 3 minutes.
Oral measurements are carried out on adults and children who are able to breathe through their nose and cooperate with holding the thermometer under their tongue and to one side of their mouth for around 3 minutes.
Axillary measurements are done by holding a thermometer under the arm, tightly for 5 minutes.
Tympanic measurements are carried out with a special thermometer which is inserted in the ear. The ear lobe is pulled up and back for those 1-year old or older, while for those younger it is pulled down and back.
Fever is not necessarily the same thing as hyperthermia, although it does involve an elevated temperature. Fever is an inherent reaction the body can have to various conditions, notably infection. It is seen with temperature measurements above 38 °C. Fever is not an illness in itself, so the underlying cause must be determined and treated, as appropriate.
Hyperthermia is an out of control rise in bodily temperature. It requires immediate medical intervention to prevent damage to the body or death. It arises due to failed thermoregulation, for example in heatwaves. Other causes include exertion, drugs and wearing personal protective equipment.
Symptoms manifest through heat stroke via confusion, dizziness and dry skin. Depending on severity, hyperthermia can be treated passively by drinking more water, resting in the shade and in colder places, or actively by using water or a bath to quickly cool down.
Climate change is creating more areas around the globe with frequent, more elevated temperatures, and more heat waves. Those susceptible to illness from hyperthermia, such as the elderly, can and do lose their lives during these times.
On the other end of the spectrum, hypothermia refers to a below-optimal body temperature, defined as less than 35 °C. It can be caused by falling into cold water, wearing wet clothes, living in cold conditions or being tired and in cold conditions.
Symptoms of mild hypothermia (above 32 °C) include shivering, rapid breathing, tiredness, confusion, cold skin and slurred speech. More severe hypothermia can result in a lack of shivering and losing consciousness.
Hypothermia must be treated with removal of the cold conditions or clothes, protection with blankets, and hydration and nutrition with non-alcoholic, hot beverages and high-energy food (if the person is able to swallow).
In the UK, fuel poverty refers to the ability to pay for heating a home. The elderly can be eligible for fuel payments form the government to help maintain warm homes and decrease the incidence of illness or death caused by hypothermia.
Just as for hyperthermia, climate change is creating extreme weather conditions that can make hypothermia more likely.
In contrast with negative feedback, positive feedback drives a system further away from its original state, often starting out with small disturbances which cause a factor which further stimulates the disturbance. For example, a panicking sheep will cause more sheep around to get panicked, potentially causing a stampede.
Unlike the sharp control available in negative control loops, positive feedback may end up spiralling itself out of control and self-destructing.
An example of positive feedback is the release of oxytocin during labour. Oxytocin stimulates uterine contraction which in turn enhances the secretion of more oxytocin via prostaglandin release by the placenta.
Contractions result in birth.
Control of heart rate
There are just 2 scenarios involved here. Either the heart rate must be increased or decreased.
1. The heart rate must go up because the blood pressure is low, there’s a shortage of oxygen, an excess of CO2 or the pH is too low – some of these go together e.g. O2, CO2 and pH interaction during exercise, stress, etc.
The hormone that increases heart rate is noradrenaline. This can be released by the brain’s medulla via the sympathetic neurons when aortic and carotidbaroreceptors which sense low blood pressure or chemoreceptors which sense excess or a lack of certain chemicals send signals via sensory neurons. The aorta is the principal vessel carrying oxygenated blood from the heart to the rest of the body, while the carotid arteries carry it on to the brain via the neck.
Noradrenaline then binds to the heart’s sino-atrial node which results in increased contractions.
2. The heart rate must go down because the blood pressure is high, there’s excess O2, low CO2 or the pH is too high
The sensing process is identical – baroreceptors and chemoreceptors do their jobs. The only difference? Of course, the released product must be different – the medulla orders acetylcholine instead of noradrenaline, and this signal follows the parasympathetic pathway.
Acetylcholine makes the SA node slow contractions.
The autonomic nervous system is hence central to the increase of heart rate as part of the sympathetic stress or flight/fight response. Upon perception of any stimulus that causes this response, the system directs the release of adrenaline to increase heart rate, alongside other outcomes such as increased blood pressure, inhibition of digestion, etc.