Chapter 25.7 Homeostasis

This textbook is designed specifically for Kansas State's Biology 198 Class. The course is taught using the studio approach and based on active learning. The studio manual contains all of the learning objectives for each class period and is the record of all student activities. Hence, this textbook is more of a reference tool while the studio manual is the learning tool.
Authors: Robert Bear, David Rintoul, Bruce Snyder, Martha Smith-Caldas, Christopher Herren, and Eva Horne
Kansas State University Libraries
New Prairie Press
Bear, Robert; Rintoul, David; Snyder, Bruce; Smith-Caldas, Martha; Herren, Christopher; and Horne, Eva, "Principles of Biology" (2016). Open Access Textbooks. 1. https://newprairiepress.org/textbooks/1
The textbook was originally published and is also available to download at http://cnx.org/contents/[e-mail] is licensed under a Creative Commons Attribution License 4.0 license.

Episode Transcript

(00:00):
Welcome to Principles of Biology. Thisbook was written by the Open Alternative Textbook
Initiative at Kansas State University and isbeing released as a podcast and distributed under
the terms of the Creative Commons AttributionLicense. Today's episode is chapter twenty five
point seven Homeostasis. All hyperlinks,images and sources can be found at the

(00:22):
link to the book. In thedescription, the constant conditions which are maintained
in the body might be termed equilibria. That word, however, has come
to have a fairly exact meaning asapplied to relatively simple physico chemical states inclosed
systems where known forces are balanced.The coordinated physiological processes which maintain most of

(00:43):
the steady states in the organism areso complex and so peculiar to living beings,
involving as they may, the brainand nerves, the heart, lungs,
kidneys, and spleen, all workingcooperatively, that I have suggested a
special designation for these states, homeostasis. The word does not imply something set
and a mobile, a stagnation.It means a condition, a condition which

(01:04):
may vary, but which is relativelyconstant. Walter Cannon, The Wisdom of
the Body nineteen thirty two, pagetwenty four. Cannon was an American physiologist
who coined and popularized the concept ofhomeostasis to describe the observations that animals could
maintain stable internal body conditions even whenthe external conditions changed. Animal organs and

(01:26):
organ systems constantly adjust to internal andexternal changes through a process called homeostasis steady
state. These changes might be inthe level of glucose or calcium in blood,
or in external temperatures. Homeostasis meansto maintain dynamic equilibrium in the body.
It is dynamic because it is constantlyadjusting to the changes that the body

(01:49):
systems encounter. It is equilibrium becausebody functions are kept within specific ranges.
Even an animal that is apparently inactiveas painting, this homeostatic equilibrium homeostatic process.
The goal of homeostasis is the maintenanceof equilibrium around a point or value
called a set point. While thereare normal fluctuations from the set point,

(02:14):
the body systems will usually attempt togo back to this point. A change
in the internal or external environment iscalled a stimulus and is detected by a
receptor. The response of the systemis to adjust the parameter towards the set
point. For instance, if thebody becomes too warm, adjustments are made
to cool the animal. If bloodglucose concentration rises after a meal, adjustments

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are made to lower the blood glucoselevel, increasing uptake of glucose from blood
into various tissues where it can beconverted to storage products like glycogen or triglyceride.
Control of homeostasis, when a changeoccurs in an animal's environment, an
adjustment must be made. The receptorsenses the change in the environment, then
sends a signal to the control center, in most cases the brain, which

(03:00):
in turn generates a response that issignaled to an effector. The effector is
a muscle that contracts or relaxes,or a gland that secretes. Homeostasis is
maintained by negative feedback loops. Positivefeedback loops actually push the organism further out
of homeostasis, but may be necessaryfor life to occur. Homeostasis is controlled

(03:22):
by the nervous and endocrine system ofmammals, as described by Cannon in the
nineteen thirties, negative feedback mechanisms.Any homeostatic process that changes the direction of
the stimulus is a negative feedback loop. It can either cause an increase or
a decrease in the level of thestimulus that triggered the response. In all
cases, the response is in theopposite direction of the change in the stimulus.

(03:46):
In other words, if a levelis too high, the body does
something to bring it down, andconversely, if a level is too low,
the body does something to make itgo up, hence the term negative
feedback. An example is animal maintenanceof blood glucose levels, as mentioned above.
When an animal has eaten, bloodglucose levels rise. This is sensed

(04:10):
by the nervous system. Specialized cellsin the pancreas sense this, and the
hormone insulin is released by the endocrinesystem. Insulin causes blood glucose levels to
decrease, as would be expected ina negative feedback system, as illustrated in
figure. However, if an animalhas not eaten in blood glucose levels decrease.
This is sensed in another group ofcells in the pancreas, and the

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hormone glucagon is released, causing glucoselevels to increase. This is still a
negative feedback loop, which is definedas a situation where a change in one
direction is countered by a response inthe opposite direction. Negative feedback loops are
the predominant mechanism used in homeostasis.Illustration shows the response to consuming a meal.

(04:56):
When food is consumed and digested,blood glucose levels rise. In response
to the higher concentration of glucose.The pancreas secretes insulin into the blood.
In response to the higher insulin levelsin the blood, glucose is transported into
many body cells. Liver cells storeglucose as glycogen. As a result,

(05:17):
blood sugar levels drop. In responseto the lower concentration of glucose. The
pancreas stops secreting insulin. Blood sugarlevels are controlled by a negative feedback loop
credit modification of work by John Sullivanpositive feedback loop. A positive feedback loop
maintains the direction of the stimulus,possibly accelerating it. Few examples of positive

(05:43):
feedback loops exist in animal bodies,but one is found in the cascade of
chemical reactions that result in blood clottingor coagulation. As one clotting factor is
activated, it activates the next factorin sequence until a fibrine clot is achieved.
The direction is made maintained, notchanged, so this is positive feedback.
Another example of positive feedback is uterinecontractions during childbirth. As illustrated in

(06:09):
figure. The pituitary hormone oxytocin stimulatesthe contraction of the uterus. This produces
pain, which is sensed by thenervous system. Instead of lowering the oxytocin
and causing the pain to subside,the nervous system causes the pituitary to secrete
more oxytocin, stimulating stronger contractions untilthe contractions are powerful enough to produce childbirth.

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Prior to birth, the baby pushesagainst the cervix, causing it to
stretch. Stretching of the cervix causesnerve impulses to be sent to the brain.
As a result, the brain stimulatesthe pituitary to release oxytocin. Oxytocin
causes the uterus to contract. Asa result, the baby pushes against the

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cervix in a positive feedback loop.The birth of a human infant is the
result of positive feedback set point.It is possible to adjust a system set
point i e. The level aroundwhich the parameter of interest fluctuates. When
this happens, the feedback loop worksto maintain the new setting An example of

(07:15):
this is blood pressure. Over time, the normal or set point for blood
pressure can increase. As a resultof continued increases in blood pressure, the
body no longer recognizes the elevation asabnormal and no attempt is made to return
to the lower set point. Theresult is the maintenance of an elevated blood
pressure that can have harmful effects onthe body. Medication can lower blood pressure

(07:38):
and lower the set point in thesystem to a more healthy level. Changes
can be made in a group ofbody organ systems in order to maintain a
set point in another system. Thisis called a climatization. This occurs,
for instance, when an animal migratesto a higher altitude than it is accustomed
to. In order to adjust tothe lower oxygen level at the new altitude,

(08:01):
the body increases the number of redblood cells circulating in the blood to
ensure adequate oxygen delivery to the tissues. Another example of acclimatization is animals that
have seasonal changes in their coats.A heavier coat in the winter ensure's adequate
heat retention, and a light coatin summer assists in keeping body temperature from
rising to harmful levels. Homeostasis thermoregulation. Body temperature affects body activities. Generally,

(08:28):
as body temperature rises, enzyme activityrises as well. For every ten
degree centigrade rise in temperature, enzymeactivity doubles up to a point. Body
proteins, including enzymes, begin todenature and lose their function. At even
higher temperatures. As you learned ina previous module, enzyme activity will also

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decrease by half for every ten degreecentigrade drop in temperature to the point of
freezing. With a few exceptions,some fish can withstand freezing solid and return
to normal with thawing, and onemammal, the Arctic ground squirrel urstallusperii,
can lower its body temperature to minusthree degrees celsius during its winter hibernation.
Endotherms and ectotherms. Animals can bedivided into two groups. Some maintain a

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constant body temperature in the face ofdiffering environmental temperatures, while others have a
body temperature that is the same astheir environment and thus varies with the environment.
Animals that do not control their bodytemperature are ectotherms. This group has
been called cold blooded, but theterm may not apply to an animal in
the desert with a very warm bodytemperature. In contrast to ectotherms, which

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rely on external temperatures to set theirbody temperatures, quikilotherms are animals with constantly
varying internal temperatures. An animal thatmaintains a constant body temperature in the face
of environmental changes is called an endotherm. Endotherms are animals that rely on internal
sources for body temperature, but whichcan exhibit extremes in tempera. These animals

(10:01):
are able to maintain a level ofactivity at cooler temperature, whereas an ectotherm
cannot. Heat can be exchanged betweenan animal and its environment through four mechanisms
radiation, evaporation, convection, andconduction. Figure Radiation is the emission of
electromagnetic heat waves. Heat comes fromthe sun in this manner and radiates from

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dry skin the same way heat canbe removed with liquid from a surface during
evaporation. This occurs when a mammalsweats. Convection currents of air remove heat
from the surface of dry skin asthe air passes over it. Heat will
be conducted from one surface to another. During direct contact with the surfaces,
such as an animal resting on awarm rock. PHOTOA shows the sun.

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Photobeat shows a sweaty person. PhotoSAT shows a lion with its main blowing
in the wind. Photode shows aperson holding a steaming hot drink. Heat
can be exchanged by four mechanisms.A radiation, B evaporation, C convection
or D conduction. Credit B modificationof work by colors slash flicker, credit

(11:13):
C modification of work by Chad Rosenthal. Credit D modification of work by Stacy
dot D slash flicker. Neural controlof thermoregulation. The nervous system is important
to thermoregulation, as illustrated in figure. The processes of homeostasis and temperature control
are centered in the hypothalamus of theadvanced animal. Brain. Flow chart shows

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how normal body temperature is maintained.If the body temperature rises, blood vessels
dilate, resulting in loss of heatto the environment. Sweat glands secrete fluid.
As this fluid evaporates, heat islost form the body. As a
result, the body temperature falls tonormal body temperature. If body temperature falls,

(12:00):
blood vessels constrict so that heat isconserved. Sweat glands do not secrete
fluid, shivering involuntary contraction of musclesreleases heat, which warms the body.
Heat is retained and body temperature increasesto normal. The body is able to
regulate temperature in response to signals fromthe nervous system. The hypothalamus maintains the

(12:24):
set point for body temperature through reflexesthat cause vasodilation and sweating when the body
is too warm, or vasoconstriction andshivering when the body is too cold.
It responds to chemicals from the body. When a bacterium is destroyed by phacacidic
leucocytes, chemicals called endogenous pyrogens pyrequals fire and genie and equals to produce

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are released into the blood. Thesechemicals circulate to the hypothalamus and reset the
thermostat. This allows the body's temperatureto increase in what is commonly called a
fever. An increase in body temperaturecauses iron to be conserved, inhibiting bacterial
division, since iron is an essentialnutrient for bacteria. An increase in body

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heat also increases the activity of theanimals, enzymes, and protective cells,
while inhibiting the enzymes and activity ofthe invading microorganisms. Finally, heat itself
may also kill the pathogen Thus,a fever that was once thought to be
a complication of an infection is nowunderstood to be a normal defense mechanism.

(13:30):
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