Download Document 10723877

Homeostasis Background Information:
In defining what it means to be alive, biologists often cite these characteristics: Living things are made
of cells and have organization. They utilize energy, grow, and reproduce. Finally, all living things,
whether unicellular or multi-cellular, practice homeostasis in order to stay alive.
Homeostasis can be defined as “A property of cells, tissues, and organisms that allows the maintenance
and regulation of the stability and constancy needed to function properly.” In simpler terms,
homeostasis is the set of processes living things use to maintain constant favorable internal conditions
in response to ever-changing external and internal conditions. Homeostasis, along with the relationship
between structure and function, is one of the overarching themes of biology. Examples of homeostasis
can be very useful to show the interrelationship among body systems in maintaining life.
Homeostatic responses can be observed on the organism level or the cellular level, and can be caused
by external changes or internal changes. The homeostatic response in humans may involve several body
systems, coordinated by the nervous system. On an organismal level, behavioral changes can also be
part of the homeostatic response. Because cells typically have internal conditions that differ from their
surrounding environments, homeostatic processes are occurring constantly at some level.
Example 1: Temperature control – Too cold
The human body needs to maintain a constant internal core temperature of 37o C. Homeostasis in the
body acts like a thermostat in a home. When the body is cold, a series of sensory nerves (e.g.
thermoreceptors in the peripheral nervous system, deeper nerves, and nerve endings in the
hypothalamus) register the information. The nervous system response takes several forms. A
behavioral response to this information is to put on another layer of clothing, turn up the thermostat, or
move to warmer location. Physiological responses also occur.
In one physiological response, the nervous system sends a signal to the circulatory system to restrict blood
flow to the extremities. Unlike the core, the extremities can withstand a temperature drop of 20 or more
degrees for short durations. Relegating blood to the core will maintain heat for the core and decrease
heat loss through the hands, feet, arms and legs which have a higher surface area to volume ratio and thus
contribute to greatest heat loss. This change in blood flow is why we lose some fine motor skills when we
are very cold – tying your shoes with cold fingers never works as well. The mechanism by which this
redirection of blood happens involves a nervous system response signaling hormones from the endocrine
system which act on the vessels of the circulatory system. Homeostatic responses themselves may change
internal conditions and require another response; as the change in blood location also changes blood
pressure, and the body responds to that with other homeostatic responses.
Another well known response to being cold is shivering. The involuntary muscle contractions of
shivering produce heat as a byproduct, helping the body counter the effects of cold. Hunger is
sometimes a corollary to being cold, as shivering to produce heat does use energy. The diets of
Antarctic researchers and extreme mountain climbers reflect this.
“Goose bumps” are another response to cold, although likely a vestigial one. Small muscle at the base
of each hair contract and cause the hairs to stand erect. On a fur-bearing mammal, this can increase the
insulatory properties of the fur. It is a vestigial response in humans as our hair provides little more
warmth when erect.
The above are immediate, short-term responses to cold. Organisms have long and short term responses
to changes in the environments. Extended cool conditions can elicit a response in thyroid gland
hormones which regulate temperature control and metabolism.
The consequences of failing to maintain homeostasis are great – usually death of the cell or organism.
Hypothermia happens if the core temperature falls by only two degrees, causing loss of consciousness
and irregular heartbeat. Death usually follows.
Example 2: Temperature control – Too hot
In an opposite temperature example, when the body is too warm, homeostatic processes work to cool
it. In the reverse process of Example 1, blood flow is increased to the extremities where heat can be lost
through conduction to the surrounding air. This heat loss is made more efficient by the surface area of
the extremities. This response is why some people may look red in the face when they are warm.
The body also produces sweat to release heat. Sweating is actually an energy transfer that involves
evaporation. As molecules of water evaporate from the skin, they are using energy to have a phase
change. The molecules of water “take their energy” with them when they leave. This is the same
reason behind the cold feeling that rubbing an alcohol swab on your arm can induce. Sweating becomes
less efficient in very humid external conditions because evaporation is inhibited, and the body must rely
more heavily on other homeostatic techniques.
Example 3: Water regulation
Cells are primarily water-based and need to regulate this balance, particularly when faced with
hypertonic or hypotonic environments. Active transport, osmosis and other passive transports are
typically used to make sure cells maintain an appropriate water balance. In the case of the human
organism, multiple body systems work together to make sure that the whole organism has the
appropriate water content. Examples of systems involved are the nervous, endocrine/hormone,
excretory/urinary, blood/circulatory systems and musculatory system.
When the body has less than an appropriate amount of water, the hypothalamus in the brain senses
this. (This is done using a set of specialized neurons called osmoreceptors, which are stimulated by
changes in sodium and water concentration in the blood.) The hypothalamus signals the pituitary gland
to release the hormone ADH (anti-diuretic hormone, also known as vasopressin). This hormone acts on
the kidneys to reduce the amount of water in urine, thus keeping water in the body. (ADH works
specifically on the collecting duct of the nephron by increasing the permeability of water through it.)
The net result is water conservation for the body; what we visually observe is darker colored, more
concentrated urine. (Note: Vasopressin also has other functions on other tissues.)
Homeostatic processes extend to behavior as well as internal physiology. The thirst response is also
regulated by the hypothalamus, and is the reason we feel thirsty when our fluid balance is low.
As part of homeostatic response, the body is designed to not “overdo” a response, such as the response
to thirst. When the osmoreceptors in the hypothalamus sense that the water balance in the blood is
more appropriate (or when other receptors acknowledge that the organism has taken in fluids), the
hypothalamus signals the pituitary gland to release less ADH, and the process is stopped. This is
example of what is known as negative feedback. In any situation, when the body senses the response is
enough, it is the remediation actions are ceased. Negative feedback processes are a very important part
of homeostasis. They insure that the cure for an imbalance doesn’t cause an additional imbalance in the
other direction. (Positive feedback, more rare, is where the response to a signal creates a yet larger
response in the organism. It is less associated with homeostasis.)
Homeostasis is vital to survival. Failure to regulate water balance is fatal. Dehydration can result in
cramps, dizziness, rapid heartbeat and ultimately death. Likewise, too much water (or too much water
too quickly) can result in low blood sodium and heart and organ failure, potentially fatal conditions. (It
is very rare to have too much water, but it is possible in athletes who hydrate without electrolyte
rebalance, and other rare situations.) Overall, lifestyle choices can help homeostatic responses. Salt
and fluid intake, temperature, exercise, and illegal drugs such as Ecstasy (‘E’) can all influence how hard
our bodies must work to maintain water balance.
Example 4: Glucose/Insulin/Glucagon – Blood Sugar Regulation
Diabetes mellitus describes a set of disorders in which the body is unable to maintain homeostasis of
blood glucose levels for some reason. In a healthy individual (without diabetes mellitus), elevated blood
glucose levels will stimulate the  cells of the pancreas to release the hormone insulin. Insulin acts on
liver and muscle cells to store glucose, typically in the form of glycogen. When blood glucose levels are
too low, insulin is not released and glucose is allowed to remain in the blood stream. Too-high levels of
glucose cause the  cells of the pancreas release glucagon, which essentially does the reverse of insulin,
converting stored polymer glycogen back into glucose molecules.
In Type I diabetes, the pancreas does not produce insulin. Generally this is an autoimmune disorder
where the body has itself damaged the pancreatic cells. Type I diabetes is typically diagnosed at a young
age and requires constant monitoring and insulin therapy. In Type II diabetes, the body may not
produce insulin, may not produce enough insulin, or the target cells may not respond to insulin. Type II
diabetes is often brought on by lifestyles with poor diet and insufficient exercise but also has hereditary
factors. Historically, Type II diabetes is diagnosed in older people, however it is more and more
commonly found in young individuals. Doctors have correlated this change with increased occurrence of
obesity and sedentary lifestyles of recent decades.
Medical interventions for both forms of diabetes may range from regulation of diet and exercise, drug
therapies, to daily insulin injections. Failure to regulate diabetes can result in severe complications,
including death.
Example 5: Immunity
Homeostasis can also be disrupted by the presence of a pathogen or by disease. A severe example of
this is the disease cholera, which is caused by the bacteria Vibrio cholera.
The toxins produced by this water-borne bacteria cause a total disruption in fluid balance in the body.
Without prompt treatment, dehydration will kill the victim in a matter of days.
Other diseases, infections and pathogens are not always as obvious in their disruption of homeostasis,
but still require a response to prevent disruption. The body’s immune response is multi-layered to
prevent and contain infection in the most efficient way. Skin is the first line of defense against many
pathogens. The digestive system is also a highly-guarded entry point. Once a breach does occur, various
proteins and blood cells work in concert to stall and stop the infection.
In general, immune cells are named for their jobs or the location in the body where they mature. B cells
mature in the bone marrow, and are generally associated with antibody production. T cells mature in
the thymus gland. These are most associated with helping B cells and killing infected cells. There are
additional (not B, not T) immune system cells such as phagocytes (“cell eaters”). Some cells are
generalists and will attack any foreign invader. Other cells specifically target certain cells or pathogens
based on antigens they present. Each cell type has subtypes, again, typically named for job that they do.
Many of the cells required interaction with one another in order to begin functioning. As in negative
feedback, there are additional cells which stop the immune response when it is no longer needed.
A typical response to a breach, such as a sliver or paper cut, will first involve generalists such as
granulocytes which engulf foreign bodies through endocytosis. Other phagocytes will also arrive to the
cut location through the blood stream, and will in turn signal additional cells. All cells have identifying
markers on their cell membranes called antigens. These markers are used by the body cells in a number
of ways—identifying invader cells, communicating to other body cells what invaders are present, and
letting body cells know when a body cell itself is infected and needs to be destroyed. Antigens are also
used to make memory cells – useful to start a quick response should the body be reinfected by the same
pathogen. The immune response has a diversity of tactics. Some cells, like the granulocytes are firstresponders to the site of injury. Killer T cells are later responders, and instead of destroying pathogens
themselves, Killer T cells destroy body cells which are infected. The structure of the antigens, antibodies
and the immune cells and even the invaders are diverse for their diverse functions. They demonstrate
well the role of simple compounds (proteins, carbohydrates) in the elaborate living sphere. Electron
microphotographs of the cells in action can interest students.
Like an acute invasion, prolonged stressful situations can also cause the body’s normal balance to be
disrupted, requiring homeostatic measures to be taken, or causing them to fail. Other infections, such
as HIV, can ultimately impair the body’s ability to fend off infections and ultimately impairs the body’s
ability to maintain homeostasis. HIV targets T-cells of the immune system specifically. Not only does
this hijack important immune cells, but the T-cells are no longer able to activate B cells, thus opening the
door to secondary infections.
It is important to note that all organisms are homeostatic. Certain bacteria, such as acidophiles and
thermophiles, which live in extremely harsh environments by human standards (e.g. hot springs, ocean
trenches), still utilize homeostasis to maintain constant internal conditions. The only difference is the
conditions considered by those organisms to be favorable for survival. Likewise, organisms in different
biomes have different homeostatic adaptations for their different environments.
Behavioral response
Disease agents
Equilibrium
Homeostasis
Hormone
Neuron
pH
Physiological change
Regulatory response
Acquired immunity
Antibody
Antigen
Bacteria
Cellular respiration
Diabetes
Glucagon
Glucose
Hypothalamus
Immune response
Infection
Insulin
Negative feedback
Pancreas
Vaccine
Vestigial
Virus