Physiological Homeostasis - Regulation of Biological Systems
Negative Feedback Control
In animals such as ourselves, the internal environment of our bodies must keep certain conditions within
tolerable limits to continue healthy functioning of the whole organism.
This is done by a process called negative feedback control, where various receptors and effectors bring about a
reaction to ensure that such conditions remain favourable. In this activity, we investigate the control of blood
sugar concentrations, water concentrations and temperature.
The principle of negative feedback control is illustrated by the diagram below
An example of this as a corrective mechanism in a person is:
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The level of glucose in the bloodstream drops
The person requires glucose in cells to meet the demand for ATP
The body detects this with a particular receptor designed for this function
These receptors release hormones, chemical messages that initiate the start of the feedback mechanism
The hormones travel to their target tissue and initiate a corrective response
In this case, the corrective response is the secretion of more glucose into the bloodstream
Because mammals are warm blooded, the enzymes that are part of their make-up as a warm blooded animal
require a certain temperature to operate optimally. Also, the water concentration of a cell and the concentration
of various chemicals must remain at a certain level to allow normal cellular processes to occur. Thus, a
feedback mechanism in warm blooded animals is essential in allowing the body to work in optimal conditions so any change in from the norm in temperature is corrected by the feedback mechanism.
Adaptive advantages of Homeostasis
Homeostasis has survival value because it means an animal can adapt to a changing environment. When
internal or external factors vary, the body will attempt to maintain a norm, the desired level of a factor to
achieve homeostasis. However, it can only work within tolerable limits, where extreme conditions can disable
the negative feedback mechanism. In these instances, death can result, unless medical treatment is executed to
bring about the natural occurrence of these feedback mechanisms
Use the following website to research one specific homeostatic mechanism from:
Water balance in humans– osmoregulation
Breathing rate and dissolved gas concentrations in humans
Blood glucose levels in humans
Student instructions for Expert Group formative task for Biology 3.4:
Demonstrate understanding of how an animal maintains a stable internal
environment
Introduction : This practice activity requires you to explain and discuss one homeostatic control system
that would be used by an athlete to maintain a stable internal environment despite fluctuating
environmental conditions. You must describe the purpose and components of the system, and explain
how the components work together. You will also consider how the system responds to changes in
external and/or internal conditions.
Conditions : You have 2 hours in your expert group to use the resources provided (and any others you
manage to locate in your own time) and produce a group report. Your report could include evidence in
written, or visual form or a constructed model. For example, a diagram or model could be produced.
Task : Use the information available to produce a report on the biological ideas related to the
homeostatic control system for a human to maintain a stable internal environment.
Include each of the following in your report:
o a description of the purpose and components of this homeostatic control system, which may
include annotated diagrams or models
o an explanation of the mechanism of this control system i.e how and why it responds to the normal
range of environmental fluctuations, the interaction and feedback mechanisms between parts of
the system
o an explanation of how balance is re-established following one specific disruption to this control
system by internal or external influences e.g. extreme environmental conditions, disease or
infection, drugs or toxins, genetic conditions or metabolic disorders
Aim to link biological ideas about maintaining a stable internal environment for any one of the following:
o a discussion of the significance of this control system in terms of its adaptive advantage
o an explanation of the biochemical and/or biophysical processes underpinning the mechanism of
this control system e.g. equilibrium reactions, changes in membrane permeability, metabolic
pathways
o an analysis of a specific example of how external and/or internal environmental influences result
in a breakdown of this control system
Resources available for expert groups:
Life Science (Blue text): Ch 19 Homeostasis pp170-179
Campbell and Reece text:
http://en.wikibooks.org/wiki/Alevel_Biology/Central_Concepts/Control,_coordination_and_homeostasis
http://tle.westone.wa.gov.au/content/file/ea6e15c5-fe5e-78a3-fd7983474fe5d808/1/hum_bio_Science_3a.zip/content/003_homeostasis/page_11.htm
Homeostasis of Organism Water Regulation = Osmoregulation
Ref: http://www.biology-online.org/4/2_water_homeostasis.htm
Osmoregulation is the regulation of water concentrations in the bloodstream, effectively controlling the amount
of water available for cells to absorb. The homeostatic control of water is as follows:
•A change in water concentration leads to active via negative feedback control
•Osmoreceptors that are capable of detecting water concentration are situated on the hypothalamus next to the
circulatory system
•The hypothalamus sends chemical messages to the pituitary gland next to it.
•The pituitary gland secretes anti-diuretic hormone (ADH), which targets the kidney responsible for
maintaining water levels.
•When the hormone reaches its target tissue, it alters the tubules of the kidney to become more / less permeable
to water
•If more water is required in the blood stream, high concentrations of ADH make the tubules more permeable.
•If less water is required in the blood stream, low concentrations of ADH make the tubules less permeable.
This is illustrated by the flow
chart :
Fluid Replacement for Athletes:
You might be surprised to learn that the most important part of any athlete’s diet is fluids. While humans can
survive for about month without food, they can only survive a few days without water. Athletes need to drink
extra fluids to replace body water lost while practicing or competing.
One of the most important functions of water is to cool the body. If the athlete does not replace water lost as
sweat by drinking more fluids, the body’s water balance will be upset and the body may overheat.
All athletes must drink water before, during and after exercise. Dehydration can start when an athlete loses as
little as 1 percent of body weight. Plain cold water is the best and most economical source of fluid. The body
absorbs cold fluids faster than warmer ones, and drinking water is the easiest way to replace body fluids.
Athletes also can use sports drinks, especially during activities lasting more than 90 minutes. These drinks
should contain no more than 15 to 18 grams of carbohydrates per cup. Fruit juice may only be used as a fluid
replacement if it is diluted at least twofold: 1 cup water for every 1 cup of juice. Carbonated beverages, highsugar drinks and undiluted fruit juice are too high in carbohydrates and may cause stomach cramps, nausea and
diarrhea. Caffeinated beverages, such as tea, coffee and cola beverages, will dehydrate the body even more.
Athletes can also replace their body fluids with foods containing a lot of water, such as oranges, watermelon,
apples, grapes, lettuce, and tomatoes, along with water. These foods provide water and carbohydrates and they
are good for replacing lost water and energy after exercise.
Ref: Susan Mills-Gray, Mills-GrayS@missouri.edu
Regional Specialist, Nutrition and Health Education
University of Missouri Extension
Level of dissolved gases (carbon dioxide and oxygen) in the
bloodstream/cells
Oxygen, carbon dioxide and hydrogen ions are carried in the blood and their concentrations all have an effect
on your breathing rate, which in turn determines the level of these chemicals, via homeostatic feedback
mechanisms.
Two key structures involved in the homeostatic mechanism of breathing
rate are the respiratory centre (located within the medulla located at the
base of the brain) and the carbon dioxide receptors located in the aorta of
the heart.
Hydrogen ions : When carbon dioxide dissolves in water it forms carbonic acid (H2CO3) which then breaks
down to form hydrogen ions (H+) and bicarbonate ions (HCO3-). Increased H+ will cause a decrease in blood
pH and this causes an increase in breathing rate.
Carbon dioxide: A slight increase in carbon dioxide concentration leads to a marked increase in breathing rate.
As mentioned above, an increase in H+ leads to an increased rate of breathing. Together CO2 and H+ stimulate
receptors located in the respiratory centre and in major arteries - the carotid and the aorta. Receptors that are
stimulated by chemicals like CO2 or H+ are called chemoreceptors .
Oxygen: As oxygen is being continually used by cells, so its concentration in the blood decreases. In contrast to
carbon dioxide, oxygen concentration needs to fall significantly before any increase in breathing rate results.
Chemoreceptors that detect oxygen concentrations are located in the walls of two major arteries, the carotid and
the aorta. These cells are known as the aortic and carotid bodies. When these cells detect a decrease in
concentration, nerve impulses are sent to the respiratory centre. From here, impulses are sent to the respiratory
muscles to stimulate the breathing process.
A slightly more detailed version of how breathing rate is controlled is:
Carbon dioxide levels regulate breathing via the chemoreceptors in your brain, carotid arteries, and aorta. As
Carbon dioxide levels go up, the pH of the cerebrospinal fluid goes down (becomes acidic) and triggers a
reaction by the chemoreceptors in your brain to cause you to breath. Specifically, those receptors are located in
the floor of the fourth ventricle (in your brain stem for all intents and purposes). Expelling CO2 by breathing
brings your cerebrospinal fluid's pH back down to acceptable levels. The chemoreceptors in your carotid
arteries and aorta respond to the partial pressures of CO2 as well as the partial pressure of oxygen. Basically, it
also tells you to breath when your CO2 levels get too high. Ultimately, when you hold your breath and "run out
of air", you don't actually run out of oxygen, but you accumulate too much CO2 (after all, we can make ATP
without oxygen, it just makes nasty by-products -Remember muscles and lactic acid from Year 12BIO?)
See also: http://www.livestrong.com/article/372001-mechanisms-that-regulate-breathing-rates/
Blood Sugar Homeostasis
Ref: http://www.biology-online.org/4/3_blood_sugar.htm
The body requires volumes of glucose in order to create ATP. The amount of ATP demanded will fluctuate, and
therefore the body regulates the availability of glucose to maximise its energy making potential.
Two hormones are responsible for controlling the concentration of glucose in the blood. These are insulin and
glucagon. The diagram illustrates the principle of negative feedback control in action involving blood/sugar
levels.
Pancreas Receptors
The receptors of the pancreas are responsible for monitoring glucose levels in the blood, since it is important in
every cell for respiration. Two types of cell release two different hormones from the pancreas, insulin and
glucagon. These hormones target the liver, one or the other depending on the glucose concentration
•In cases where glucose levels increase, less glucagon and more insulin is released by the pancreas and targets
the liver
•In cases where glucose levels decrease, less insulin and more glucagon is released by the pancreas and targets
the liver
The Liver
The liver acts as a storehouse for glycogen, the storage form of glucose. When either of the above hormones
target the liver, the following occurs
•Insulin - Insulin is released as a result of an increase in glucose levels, and therefore promotes the conversion
of glucose into glycogen, where the excess glucose can be stored for a later date in the liver
•Glucagon - Glucagon is released as a result of an decrease in glucose levels, and therefore promotes the
conversion of glycogen into glucose, where the lack glucose can be compensated for by the new supply of
glucose brought about from glycogen
Diabetes
Diabetes insipidus is a condition where excess urine is excreted caused by the sufferers inability to produce
ADH and promote the retention of water.
Diabetes Mellitus is another form of diabetes where the sufferer does not have the ability to produce sufficient
insulin, meaning that glucose cannot be converted into glycogen. Anyone who has this condition usually has to
take injections of insulin after meals and snacks to maintain their storage of glucose needed in emergencies.
Fight or Flight
In emergencies, adrenaline is released by the body to override the homeostatic control of glucose. This is done
to promote the breakdown of glycogen into glucose to be used in the emergency. These emergencies are often
known as 'fight or flight reactions'.
Adrenaline is secreted by the adrenal glands. The secretion of it leads to increased metabolism, breathing and
heart rate. Once the emergency is over, and adrenaline levels drop, the homeostatic controls are once again back
in place
Temperature Regulation in Animals
Ref: http://www.biology-online.org/4/4_temperature_regulation.htm
Animals capable of temperature regulation within a given range are deemed homeotherms (alternatively
homiotherms or homotherms). They have the ability to regulate temperature via negative feedback control.
Temperature is controlled in a variety of ways in these animals.
The hypothalamus once again acts as a receptor in regulation, by detecting fluctuations in temperature. These
receptors are better known as thermoreceptors.
Skin also possesses thermoreceptors which can detect the temperature of the external environment. This
information is relayed to the hypothalamus which can in turn transmit nerve pulses for corrective mechanisms
to occur
Corrective Mechanisms in Temperature Control
Increased sweating is a corrective response aimed to reduce the temperature of the organism.
Vasodilation is a corrective response where the blood vessels close to the skin surface become more dilated,
meaning their is a larger surface area for heat to be lost of the external environment from the blood vessel
carrying over-heated blood.
Vasoconstriction is the opposite of this and occurs when temperatures in an organism drop. The blood vessels
become constricted so that minimal heat loss occurs.
The hairs on your body also play an important role in temperature regulation. A corrective response can occur
where the hairs 'stand on end', and trap a layer of air between the hair and the skin. This insulation of warmer
air next to the skin reduces heat lost, while a thin layer of insulation would increase heat loss.
Other corrective mechanisms are involved, such as a drop in metabolic rate and shivering when temperatures
drop.