Introduction to homeostasis

Homeostasis is the maintenance of stable conditions in the internal environment. The internal environment is constantly disturbed by stress (stimuli that imbalances the internal environment). This stress can be external (heat, cold, lack of oxygen) or internal (pain, tumours). Control systems are in place to keep the physical and chemical state of the aqueous internal environment constant. Organ homeostatic control systems can be intrinsic/local (e.g autoregulation of tissue blood flow) or extrinsic/distant (e.g ANS, hormonal signalling). A breakdown in homeostasis is a feature of many disease states.

Consider the balance of an organism as a see-saw and stresses as destabilising elements. In general, phyisological control systems will use receptors/sensor to detect an imbalance of a variable, send information down an afferent pathway to the control center which will process this signal and sent efferent  signals to the effector that will feedback to influence the magnitude of stimulus and return the variable back to homeostasis (Figure 1 and 2) .

Figure 1

Figure 2

Definitions: When discussing the facets of homeostasis it is important to use the correct terminology.
  • Homeostasis: maintenance of a relatively constant internal environment
  • Control systems: Defined physiological processes that use feedback or feedforward to maintain homeostasis. These systems can be local, neural and endocrine
  • Set point: The desired output of the controlled variable i.e where the see saw wants to rest
  • Negative feedback: When deviation of output from the set point initialtes a response that negates the deviation i.e action that negates destabilisation of homeostasis and moves the system back towards the set point
  • Positive feedback: Deviation of output from the setpoint initiates a response that increase the deviation i.e action by the system that worsens instability
  • Feed forward: A control system that contains all the instructions required for a given operation which will be carried out in the absence of feedback although it may be modified by feedback. i.e feed forward can be thought of the opposite of feedback. Feedback is a response to change but feedforward can be thought of the system that acts to produce the status quo when there is no influence from feedback e.g feed forward can be the system that regulates a regular heartbeat at rest whereas feedback are the systems put into action when exertion requires an increase in heartbeat. Feed forward refers to systems that are programmed to respond to external stresses before there is a change in variable Feed forward regulation anticipates changes in a regulated variable, improves the speed of the body's homeostatic responses and minimises fluctuation in the level of the variable being regulated. e.g temperature receptors on the skin detect external temperature changes which trigger a reflex that produce homeostatic measures before the external temperature can change the internal temperature. 
  • Error signal: The deviation from the original value. Negative feedback systems seeks to tip the see-saw back towards balance but balance of the variable cannot be achieved if the stressor remains. Error signal refers to the residual difference between the new steady state and the original state (Figure 3). This signal is necessary to maintain the responses necessary for the negative feedback. I.e if a negative feedback system was to successfully return the variable back to the original condition, the feedback would disappear and the system would move back towards imbalance.
Figure 3
  • Gain=Amount of correction of abnormality/ Amount of abnormality still remaining (Figure 3). Gain is a ratio that signifies the how much ground the system has "gained" towards homeostasis therefore the amount of correction is the numerator. Higher ratios will mean that the feedback system is very effective as the amount of correction is large and the error signal is small. Gain of different systems is variable e.g temperature regulation is 50, baroreceptor signal for blood pressure control is 2.
Homeostatic mechanisms aim for balance and this is also reflected in input vs output of nutients and resources. There is a net gain of resources to the body that enters the pool of substance (the body's readily available quantity usually the amount available in the EC fluid). These substances can be excreted resulting in a net loss, stored in the body or reversibly incorporated into other molecules Figure 4). 

Figure 4


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