During the day, the body goes through a series of reactions that allows for the integration of information necessary to allow for optimal performance of the body. This optimal performance is referred to as homeostasis and is usually kept within the very narrow range of extremes to allow for survival.
Survival is based on six (6) principle physiological functions that we will undergo throughout our lives: differentiation, growth, metabolism, movement, reproduction, and responsiveness. Differentiation is the ability for cells and tissues to change function for specificity of cellular and tissue functions for the organism. Growth is the simple act of increasing the volume or surface area of the cell, tissue, or organism through the addition of new materials and generation of additional cells through cellular mitotic division. Metabolism is the sum total of all biochemical processes in the cell, tissue, organ, and organism that leads to establishing, altering, or re-establishing homeostasis. Movement is the ability to change localization of chemicals, structures or the whole organism as to prevent injury or damage to tissue or to translocate position within space. Reproduction is the ability to produce a new generation of cell (via mitosis) or organism (via formation of gametes and sexual reproduction) from a previously established generation. Finally, there is responsiveness, which is the ability to interpret distinct stimuli and formulate integrated responses (either biochemical or kinematic) to that distinct stimulus. Responsiveness leads to a seventh function that we also will exhibit from time-to-time, adaptation.
When we study the physiology of the human body, we are studying the functions and reactions that the body goes through to maintain homeostasis. A concept that we generally speak about as developing and maintaining as a stable internal environment for the person within each and every cell, tissue, organ and organ system. Yet, it is important to remember that homeostasis is dynamic and fluctuating, not a static balance point that allows for optimal functioning.
Homeostasis is maintained through feedback. Feedback is the control and regulation of biochemical reactions and physiological processes of the cells, tissues, organs and organism by product formation that either inhibits earlier reactions (negative) or enhances earlier reactions (positive) in an effort to reestablish homeostasis. The two means for feedback will determine the means to either accentuate a response to the stimulus or minimize the overall reactivity that occurs in response to disruption. In the case of the positive feedback, there is an accentuation of the responses within each step of the overall response that occurs due to the stimulus and will continue until the stimulus itself is removed. This differs from the negative feedback, where the subsequent step in the response that occurs will attempt to limit the steps that preceded it. The attempt with the negative feedback is to have a “just right” response to the stimulus so to not have too much or too little of a response and is the mechanism that we examine in a majority of responses within our body.
It is positive feedback that leads to adaptations in our responsiveness. Adaptations that will lead to a new balance point, a new homeostasis. While negative feedback ensures that our current balance point does not get upset. Both will be engaged when seeing stresses, with positive feedback generally being the mechanism to a large enough response to get rid of the stress so we can survive and negative feedback trying to limit how large that response gets, so that we don’t face a new stress to our optimal level of performance. We see this balance of feedback at play when looking at our body temperature balance point (thermoregulation). Our body is constantly producing heat from the metabolism that goes on in every cell of the body, this heat is needed to keep our body at a stable temperature. Yet, we don’t want to get too hot and so we use our sudoriferous (sweat) glands to make sweat to help evaporate away some heat and the blood vessels in the dermis of the skin to radiate away some of that heat. When we are just living, most of us won’t even notice this balance of cooling and heating of the body, but as soon as we get faced with a stressful event our skin flushes as the blood vessels in dermis of the skin fill with blood due to a change in blood flow to the skin away from other areas of the body in an attempt to radiate away the anticipated extra heat. A switch from negative feedback, just the right amount of blood to the skin and the other organs of the body, to a positive feedback of shifting away from “just right” and moving blood to skin and not equally sharing. Now we start to exercise and what happens to that sweat rate, well it increases. We go from not seeing visible drops of sweat to seeing collections of water on skin and wetting of the clothing. Once again we shift from negative feedback, just the right amount of water entering into sweat, to a positive feedback shift away from just right and pulling more water into sweat away from the water found in the tissues of the body. If we cannot get the thermoregulation balance point returned to optimal, what can we get… dehydration.
Getting back to the idea of control versus regulation within the feedback mechanism. In this sense, control functions within the biochemical reactions on a cellular level based on the sequence of reactions within the metabolic process. While regulation is a means of control of several physiological processes on a systemic level through messenger molecules either enhancing or inhibiting metabolic processes. The regulation and control exhibited is based on either inhibition or excitation of the cell. In which inhibition leads to the reduction in the biochemical or physiological processes in response to a stimulus that calls for the reestablish homeostasis after a disruption. On the other hand, excitation acts to increase in the biochemical or physiological processes of a cell, tissue, or organ in response to reestablishing homeostasis after a disruption.