ANS322: Environmental Physiology HOMEOSTASIS

Homeostasis  

Homeostasis - maintenance of a stable internal environment Assignment: Read about Claude Bernard (father of physiology)   

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term describes physical and chemical parameters that an organism must maintain to allow proper functioning of its component cells, tissues, organs, and organ systems. Recall that  

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write one page summary of your undestanding of his work and contribution to the current understanding of physiology Font: Arial, Font size: 12, line spacing: 1.5 Due date 16th April 2015

enzymes function best when within a certain range of temperature and pH, cells must strive to maintain a balance between having too much or too little water in relation to their external environment.

Both situations demonstrate homeostasis.

Homeostasis

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Certain temperature range (or comfort zone) to function Also a range of environmental (internal as well as external) parameters within which to work best.

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There are basically 2 levels at which regulation to maintain homeostasis can occur:

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1) at the body integument (outer surface) and

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2) at the cell membrane.

The extracellular fluids of most animals vary greatly in ion concentrations as well as in the total concentration of solutes, the osmotic concentration

Homeostasis   

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Organisms must accomplish their homeostasis within the single cells Single-celled organisms are surrounded by their external environment. Move materials into and out of the cell by regulation of the cell membrane and its functioning Most multicellular organisms have most of their cells protected from the external environment, surrounded by an aqueous internal environment. internal environment must be maintained in such a state as to allow optimum efficiency

Homeostasis Difference between homeostasis in single celled organisms and multi-cellular organisms based on basic organizational plan  single cell can dump wastes outside the cell and just be done with it  multi-cellular organisms also dump wastes outside those cells, but those wastes must be cleared  Clearing wastes accomplished by the circulatory system in conjunction with the excretory system 

Homeostasis 

ultimate control of homeostasis accomplished by the nervous system 

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and the endocrine system 

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for rapid responses e.g. reflexes to avoid picking up a hot pot off the stove for longer-term responses, e.g. maintaining the body levels of calcium, etc.

Homeostatic control in form of feedback loops

Biological feedback 

Two types of biological feedback: 

positive and negative

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Negative feedback turns off the stimulus that caused it in the first place

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Positive feedback causes an amplification of the stimulus by the reaction

The internal environment 

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comprise of two types of extracellular fluids in animals: 

the extracellular fluid that surrounds and bathes cells

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plasma, the liquid component of the blood.

Internal components of homeostasis: 

Concentration of oxygen and carbon dioxide

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pH of the internal environment

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Concentration of nutrients and waste products

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Concentration of salt and other electrolytes

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Volume and pressure of extracellular fluid

Control mechanism All homeostatic control mechanisms have at least three interdependent components for the variable being regulated:  Receptor: sensing component that monitors and responds to changes in the environment  Control center: where the receptor sends information - the component that sets the range at which a variable is maintained  Determines an appropriate response to the stimulus  In most homeostatic mechanisms the control center is the brain 

Control mechanism Effector: receives signals from control center  can be muscles, organs or other structures that receive signals from the control center  Signal received leads to some change to correct deviation  Through 

enhancing (positive feedback) or  depressing (negative feedback) 

Control mechanisms Most physiological systems use feedback to maintain internal environment  Most homeostatic systems are extrinsic - controlled from outside  Endocrine and nervous systems are the major control systems in higher animals  nervous system depends on sensors 

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to receive stimuli and transmit message to spinal cord or brain (CNS)

CNS processes sensory input Sends signals to effector system, e.g. muscles or glands, that effects response to stimulus

Control mechanisms Endocrine system – 2nd extrinsic control  Involves chemical communication  Sensors detect change and send message to endocrine effector e.g. parathyroid gland which makes PTH  PTH released when blood calcium levels are low  PTH stimulates bone to release calcium into circulatory system 

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levels and shut down PTH Raises blood calcium production

Control mechanisms  

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Some reflexes have a combination of nervous and endocrine response Thyroid gland secretes thyroxin into CS Thyroxin controls metabolic rate Falling levels of thyroxin stimulate receptors in the brain Signals hypothalamus to release a hormone that acts on the pituitary gland – TSH-RH PG releases thyroid-stimulating hormone (TSH) into the blood TSH acts on the thyroid, causing it to increase production of thyroxin

Control mechanisms    

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Homeostatic systems can also be intrinsic controlled from within Local (intrinsic) controls usually involve only one organ or tissue E.g. when muscles use more oxygen, and also produce more carbon dioxide, Intrinsic controls cause dilation of blood vessels allowing more blood into those active areas of the muscles Eventually the vessels will return to "normal"

Negative feedback Mechanism reduces output/activity of any organ or system to normal range of functioning  Initiated to maintain or regulate physiological functions within a set and narrow range  e.g.1 regulating BP  Blood vessels can sense resistance of blood flow against the walls when blood pressure increases  Blood vessels act as receptors, relay message to brain  Brain sends message to heart and blood vessels (effectors) 

Negative feedback Heart rate decreases as blood vessels increase in diameter - vasodilation  This change results in BP falling back to normal range  Opposite happens if blood pressure decreases – vasoconstriction 

Negative feedback  

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E.g. 2 food deprivation Body resets metabolic rate to lower than normal value Allows body to continue to function, at a slower rate, even though the body is starving Hence depriving food while trying to lose weight, sheds weight initially and much harder to lose more after Due to body readjusting itself to a lower metabolic set point to allow the body to survive with its low supply of energy Exercise can change this effect by increasing the metabolic demand.

Negative feedback E.g 3 temperature control.  Hypothalamus monitors body temperature,  Capable of determining slight variation of normal body temperature  Response to variation could be 

stimulation of glands that produces sweat to reduce the temperature or  signaling various muscles to shiver to increase body temperature 

Both feedbacks are equally important for the healthy functioning of ones body  Complications can arise if any of the two feedbacks are affected or altered in any way. 

Negative feedback

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Used by most body systems

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Called negative because the information caused by the feedback causes a reverse of the response

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TSH as an example: blood levels of TSH serve as feedback for production of TSH

Positive feedback 

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mechanisms designed to accelerate/enhance output created by a stimulus that has already been activated Input increases or accelerates the response mechanisms are designed to push levels out of normal ranges Series of events initiates a cascading process that builds to increase effect of stimulus Process can be beneficial but is rarely used due to acceleration risks becoming uncontrollable

Positive feedback

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Examples 

blood platelet accumulation - blood clotting in response to a break/tear in lining of blood vessels

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release of oxytocin to intensify the contractions that take place during parturition. 

Oxytocin causes an increase in frequency and strength of uterine contractions

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This in turn causes further production of oxytocin

Positive feedback Positive feedback can also be harmful  example 

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When fever causes a positive feedback within homeostasis that pushes the temperature continually higher At extreme body temperature (45°C) cellular proteins denature,  causing the active site in proteins to change,  thus causing metabolism to stop, resulting in death 

Feedback Systems in Homeostasis

Conclusion 

Homeostasis 

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depends on action and interaction several body systems to

maintain a range of conditions within which the body can best operate has survival value - animal can adapt to a changing environment

The body will attempt to maintain a norm, the desired level of a factor to achieve homeostasis  Only works within tolerable limits, 

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extreme conditions can disable the negative feedback mechanism Resulting in death unless medical treatment to bring back natural occurrence of feedback mechanisms

Enantiostasis ability to stabilize and conserve function in spite of an unstable environment  Typical of estuarine organisms in order to survive constantly changing salt concentrations  defined as "the maintenance of metabolic and physiological functions in response to variations in the environment"  Enantiostasis is not a form of homeostasis  Involves maintaining only functionality in spite of external fluctuations 

Tutorial

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Work in threes

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Identify and discuss two studies/experiments that demonstrate homeostasis 

Target function/procesess

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Materials and methods

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Results

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Take home message

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Choose one estuarine species as an example

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Discuss the importance of enantiostasis in estuarine organisms in maintaining appropriate salt concentrations

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10 minutes presentation from each team

Read and discuss interaction between homestasis homeorhesis