1. Why bother to regulate?
An organism may be defined as a physic-chemical system existing in a steady state with its external environment. It is this ability to maintain a steady state within a constantly changing environment that contributes to the success of living systems. In order to maintain this condition, organisms, ranging from the morphologically simplest to the most complex, have developed a variety of anatomical, physiological and behavioural mechanisms designed to preserve a constant internal environment.
How does maintenance of a steady state contribute to the success of living organisms? All metabolic systems operate most effectively if maintained within narrow limits on either side of optimal conditions (a.k.a. norm). When the body cannot maintain physiological parameters within its norm, a disease condition results. It is the role of homeostatic organs and systems to operate both separately and together in order to buffer against fluctuations form the norm, which are caused by variations in the internal and external environments.
Man is a more efficient in maintaining a steady state than an amoeba. Why? This is because man is multicellular and can be organized into homeostatic systems that buffer against environmental changes. An amoeba is unicellular and is directly exposed to environmental changes.
2. What exactly is Homeostasis?
Claude Bernard (1857) defined it as follows. External environment is the environment in which an organism lives. The internal environment is the environment in which individual cells of the organism live. In mammals, it is the tissue or extracellular or interstitial fluid plus the plasma, lymph and cerebrospinal fluids (the ultrafiltrate of plasma but lacks many plasma proteins).
“The constancy of the internal environment is the condition of free life.”
Walter Cannon (1932) defined it as follows. Homeostasis is the maintenance of a relatively constant internal environment, providing the organism with a degree of independence of the environment. Coordination is the ability to make appropriate response, both qualitatively and quantitatively, to a particular stimulus. This maintains a steady state i.e. interplay of external forces that tend to change the internal environment and internal control mechanisms that oppose such changes. In steady state, a particular parameter / variable of the system (e.g. temperature) is not changing, but energy (e.g. heat) must be added continuously to keep the variable constant. It involves the coordination of responses to a stimulus. This self-regulating mechanism has a receptor / detector that receives signals, a control center / regulator that coordinates responses to the stimulus and sends messages to the target organ, which is the effector.
Homeostasis is the regulation, by an organism, of the chemical composition of its blood and body fluids and aspects of its internal environment within tolerable limits so that physiological processes can proceed at optimum rates.
What variables of the internal environment are regulated? They are body temperature, fluid composition e.g. pH, blood, sugar level, oxygen, carbon dioxide and water potential, and blood volume and pressure.
3. Principles and Mechanisms of Homeostatic Control
The efficiency of the internal control / self-regulatory mechanism is measured in terms of how little displacement from the reference point (optimal level) occurs and the speed with which the reference point is restored.
Homeostatic mechanisms must be free to fluctuate, as it is the fluctuations of the parameters / variables that activate the control systems and return the parameters to the optimum level.
Such control systems rely upon their components being linked together, so that the output can be regulated in terms of the input. This process is known as feedback. Feedback requires the action of the system to be referred back to a reference point or set point or norm with the optimal level of the parameter, so that subsequent action can be modified to restore the set-point. There are positive and negative feedback mechanisms existing.
Reference point is the set level at which the system operates. A receptor / detector signals the extent of any deviation from the reference point. A regulator / control centre / integrating centre coordinates the information from various detectors and sends out instructions that will correct the deviation. The effector brings about the changes (responses) necessary to return the system to the reference point. The feedback loop informs the detector of any changes in the system as a result of action by the effector.
3.1 Negative Feedback
It is associated with increasing stability of systems. Disturbance or error in a system sets in motion a sequence of events that counteract the disturbance. It tends to restore the system to its original state. For example, a decrease in body temperature leads to responses that tend to increase the body temperature back to the norm. This control is orchestrated by the hypothalamus found at the base of the brain.
Output of the feedback loop is used to attenuate / counteract the input. In the case of heat loss to the environment, the blood vessels supplying the skin narrow, restricting the amount of warm blood flowing to the skin and thus reducing heat loss. Also, shivering occurs and chemical reactions associated with muscle contraction produce large quantities of heat to replace the heat lost, thus restoring body temperature.
This is the most common homeostatic mechanism in living organisms.
3.2 Positive Feedback
It is not homeostatic because it does not favour stability. It displaces a system from its steady state operating point. An initial disturbance in a system sets off a train of events that increases the disturbance even further.
Output of the feedback loop is used to enhance the input. Action of effectors reinforces the changes that stimulate the effector. For example, in the propagation of nerve impulses, depolarization of the neuronal membrane produces an increase in sodium ion permeability (i.e. response). As sodium ions pass into the axon through the membrane, they cause a depolarization, which leads to even more ions entering. The rate at which sodium ions enter thus increases rapidly and this produces an action potential.
During
labour, oxytocin stimulates muscular contractions of the uterus, which in turn
stimulate the release of more.
