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Soon after a meal, your digestive system breaks
down the food you have eaten into a simple

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sugar called glucose. Glucose is absorbed
from the gut into the bloodstream, causing

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your blood sugar level to increase. In healthy
individuals, feedback mechanisms in the body

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bring blood sugar levels back to normal. In
some people, this process breaks down, resulting

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in diabetes.

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In this video, we'll take a closer look at
feedback loops, how they tie into the body's

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mechanism of internal regulation, and what
happens when these mechanisms fail.

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This video is part of the Information Flow
series. A system is shaped and changed by

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the nature and flow of information into, within,
and out of the system.

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Hi, my name is Leah Okumura and I am a Technical
Instructor in the Biology Department at MIT.

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Before watching this video, you should be
familiar with the concept that your body is

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a tightly regulated environment. After watching
this video, you will be able identify the

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general components of a feedback loop, examples
of negative and positive feedback loops in

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the body, and describe how feedback loops
are vital to healthy function and survival.

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Our bodies rely heavily on feedback loops
to control and regulate important biochemical

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and physiological functions.

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There are two types of feedback loops in biology:
negative feedback loops, and positive feedback

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loops. These act to either reduce or amplify
the changes that occur in a given system.

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Let's see how feedback loops can be applied
to thermoregulation. Imagine walking into

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a room that has an air conditioner on at full
blast. After a few minutes, you find yourself

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shivering from the cold. Does the shivering
serve a purpose? Yes! The rapid muscle contractions

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from shivering generate heat within the body
and warm you back up. Now imagine taking a

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walk outside on a very hot and sunny day.
After a few minutes under the hot sun, you

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are perspiring profusely. What purpose does
sweating serve? As sweat evaporates from your

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skin, it helps to cool you down.

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Thermoregulation serves to control our body's
temperature much like a thermostat regulating

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the temperature of a room. From this diagram,
you can see how thermoregulation follows a

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simple loop. Because the response in this
case is always to reverse a given change in

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body temperature, we call this a negative
feedback loop. Positive feedback loops are

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just the opposite. When a change occurs in
a system, a positive feedback loop acts to

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increase or exacerbate it. Labor preceding
childbirth is a classic example of a positive

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feedback loop. Contractions of the uterus
during childbirth stimulate the release of

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a hormone, oxytocin, which in turn induces
more uterine contractions. The self-amplifying

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nature of the positive feedback loop is repeated
over and over with increasing intensity until

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the baby is born. After birth, contractions
stop and the feedback loop ceases. Here are

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a few well-known physiological processes and
parameters that involve feedback loops. Pause

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the video here and determine which ones involve
negative feedback and which ones involve positive

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feedback.

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Negative feedback loops minimize deviations
within a system, keeping its parameters close

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to a desired set-point. The control of blood
glucose, blood pressure, and breathing rates

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rely heavily on negative feedback loops.

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Positive feedback loops, on the other hand,
tend to destabilize a system by amplifying

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a stimulus towards an extreme. This is important
in irreversible processes such as action potential

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generation, blood clotting, lactation, and
ovulation.

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There are many other examples of negative
and positive feedback loops in the human body.

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Now let's take a closer look at specific components
involved in a feedback loop.

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The receptor is the component that detects
and measures changes in a given parameter.

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The receptor then relays this information
to a control center. The control center compares

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the measured parameter to a desired set point.
Based on the extent of deviation, the control

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center decides on the appropriate response
and sends signals to an effector. Effectors

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can be muscles, organs, or any other component
that receives signals from the control center.

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In response to these signals, the effectors
can either enhance or reduce the deviation.

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If the deviation is enhanced, we have a positive
feedback loop. And if it is reduced, we have

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a negative feedback loop. Now let's go back
to our thermoregulation example in greater

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detail. Here, the parameter we are controlling
is internal body temperature. The receptors

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of temperature in our body are specialized
cells called thermoreceptors. Thermoreceptors

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continually monitor our temperature and pass
this information to the control center in

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the brain, called the hypothalamus. The hypothalamus
compares the measured temperatures to the

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set point of 37 ᵒC. It can distinguish temperature
differences as small as a hundredth of a degree!

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The hypothalamus then sends out signals to
effectors in the body to initiate corrective

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mechanisms when our temperatures are too high
or too low. The effectors react accordingly

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to adjust our body temperature. When we are
too hot, signals are sent to activate our

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sweat glands. Blood vessels in the skin are
also stimulated to dilate. These changes increase

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heat loss from the skin. When we are too cold,
signals are sent to the tissues to increase

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metabolism, and to the muscles to induce shivering.
These mechanisms generate heat. Signals are

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also sent to constrict blood vessels and to
raise skin hairs to minimize heat loss.

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Both negative and positive feedback loops
are equally important for the healthy functioning

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of one's body. Complications can arise from
failure of these mechanisms.

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Let's look at blood glucose regulation, and
see what happens if the feedback mechanisms

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do not work as they should. Glucose is the
primary source of energy for the body's cells.

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Thus, in a healthy individual, glucose levels
are tightly regulated to maintain a fairly

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constant and optimal supply in the bloodstream.
Too little or too much blood glucose could

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be damaging to the body. Circulating levels
of glucose are monitored by specialized beta

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cells in islets of Langerhans in the pancreas.
In response to high glucose levels, for example

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after a large meal, these same beta cells
release a hormone called insulin into the

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bloodstream. Insulin is transported to the
rest of the body, whereupon it stimulates

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the cells to take up and adsorb glucose. Insulin
also promotes the storage of glucose in muscle

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and liver tissues. These actions act to remove
glucose from the blood, thus lowering blood

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glucose levels to the normal set-point. Blood
glucose regulation is a classic example of

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negative feedback. Pause the video and identify
the receptor, control center, and effectors

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in this particular feedback system.

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The receptors are the beta cells in the pancreas.
These same cells also act as the control center

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and send signals to the effectors in the form
of insulin. The effectors are the cells of

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the body that increase their uptake and storage
of glucose in response to insulin. Now let's

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see how failure of the body to regulate blood
glucose levels results in a disease known

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as diabetes. There are two common forms of
the disease.

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In Type I diabetes, also known as early-onset
diabetes, the insulin-producing beta cells

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of the pancreas are destroyed due to an autoimmune
reaction. In this form of diabetes, the body

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is unable to produce insulin. The lack of
insulin means that the cells of the body do

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not take up glucose in response to increasing
glucose levels. Hence, glucose levels in the

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blood continue to rise. In Type II diabetes,
or late-onset diabetes, insulin is still produced

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but the body's cells no longer respond to
it. This is often due to years of insulin

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over-production, caused by a high-sugar diet.
The inability of cells to use insulin properly

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means that high glucose levels in the blood
persist with time. Type I and Type II diabetes

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have very similar symptoms but they arise
from different causes. Pause the video and

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identify the parts of the feedback loop that
break down in each.

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In Type I diabetes, there is a lack of signaling
between the control center and the effectors.

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In Type II diabetes, the effectors fail to
respond to the signals from the control center.

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The end result is the same in both forms of
the disease - glucose accumulates to toxic

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levels in the blood. If left unchecked, diabetes
can lead to kidney failure, blindness, and

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heart disease. Feedback loop breakdown leads
to the development of an unstable internal

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environment. This tips the scales towards
imbalance, increasing the risk of illness

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and progressive damage to the body. Many other
diseases and pathological conditions follow

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a similar progression.

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In this video, you learned about negative
and positive feedback loops, their general

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components, and how they regulate the flow
of information. We identified feedback loops

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in the body and examined their role in childbirth,
thermoregulation, and blood glucose regulation.

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Our body relies on a finely-tuned system of
checks and balances to function smoothly.

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Feedback loops, like the ones described in
this video, provide this balance. Failure

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of these mechanisms, as in the case of diabetes,
can be detrimental. We hope that you will

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apply your knowledge of feedback loops and
the consequences of their failure in the study

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of other biological processes and diseases.