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Controlling Mechanisms

Applied Biological Science

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Aim of the lecture

� To identify the principles behind the physiological control mechanisms used to maintain body function at its optimum level.

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Learning Outcomes

At the end of the session the students will be able to:

� Recognise the need for physiological control systems

� Define homeostasis� Give common examples of homeostatic regulation� Explain negative and positive feedback

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Definition of homeostasis

� The automatic, self-regulating physiological processes necessary to maintain the normal, or standard state of the body’s internal environment.

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Need for homeostasis

Evolution has meant that the environment in which an organism lives is often:-

� Inconsistent� Unpredictable� Potentially dangerous

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Historical perspective

The importance of the concept of homeostasis was first discussed in the middle of the 19th century by Claude Bernard who stated:

“La fixite du milieu interieur est la condition de la vie libre”

“The consistency of the internal environment is the condition forfree life”

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Historical perspective

� Walter Bradford Cannon (1932) from the Greek homoios (same, like, resembling) and stasis (to stand, posture).

� Absolutely vital to any organism – failure leads to death

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Homeostasis

Homeostasis is arguably the most important concept within physiology. It represents the processes necessary for the maintenance of conditions under which cells, and hence the body, can function optimally; since even small changes in body fluid composition can disrupt biochemical activities within a cell or may even kill it.

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Dynamic Equilibrium

� Dynamic equilibrium refers to a condition in which the parts of a system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole does not change.

� Cell membrane is semi permeable therefore some substances e.g. ions (K, Cl, Na, Ca etc) continually leaching in and out

� Concentrations of solutes, water etc in cell must however remain constant.

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Homeostasis

An organism is said to be in homeostasis when its internal environment:-– contains the optimum concentration of gases,

nutrients, ions and water– has an optimal temperature– has an optimal pressure for the cells

These optimal levels can be considered as “set points”

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Control mechanisms

� All homeostatic control mechanisms have at least three interdependent components for the variable being regulated:

� Receptor� Control system� Effector

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Receptors

� The receptor is the sensing component that monitors and responds to changes in the environment. When the receptor senses a stimulus, it sends information to a controlcentre.– Chemoreceptors– Baroreceptors

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Control Centre

� The control centre is the component that sets the range at which a variable is maintained and determines an appropriate response to the stimulus. – Central Nervous Systen

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Effector

� The receptor thus stimulated then responds by either enhancing the stimulus with positive feedback or by depressing it with negative feedback .– Organ system, organ, tissue, or cellular level

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Positive feedback loop

� This is when a stimulus causes a deviation away from an original set point which in turn causes an increase in the stimulus causing further deviation away from the set point

� Induces change

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Negative feedback loop

This is when a stimulus causes a deviation away from an original set point which results in a decrease in the stimulus returning the deviation to the set point.

Maintains stability

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Maintaining Body Temperature

Maintenance of body temperature is achieved by the following mechanisms

� Auto regulation� Extrinsic regulation

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Autoregulation

� Occurs when cell, tissue, organ or organ system adjusts its activities automatically in response to an environmentally stimulus

� Example– Increased body temperature results in increased

sweating and flushing of the skin.

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Extrinsic regulation

� This results from the activities of the nervous or endocrine systems

� Example – Individual feeling too hot decides to take off some

warm clothing i.e. remove coat

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Negative feedback - thermoregulation

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What’s the connection?

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Summary

Physiological systems work together to maintain a stable internal environment, the condition of homeostasis. In doing so they monitor and adjust the volume and composition of body fluids and keep body temperature within normal limits.

Failure to do so results in death

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Any Questions?

Thank you

See you all next time