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“It the fixity of the Milieu interview, which is the condition of free and independent life, and all the vital mechanisms, however varied they may be, have only one object, that of preserving constant the condition of life in the internal environment.”
The above statement by Claude Bernard emphasises that the stability of an internal environment within the human body enables free and independent life, the survival of an organism through external changes. Also stating that all organs and organ systems within the body have only one aim and function that it to maintain the composition of the internal environment in a state compatible with the survival of all living cells within the body. In order for a cell to obtain energy, organic nutrients must be broken down by intracellular processes, this is the utilization of oxygen and the production of carbon dioxide. If the cell was an amoeba then the nutrients would be obtained from the external environment, the fluid surrounding the cell and the wastes would also be excreted into the external environment. In humans the situation is different they possess an internal environment as their cell bodies cannot be in direct contact with the air. This internal environment in anatomical terms is extracellular fluid, which provides the cells with an appropriate physical conditions for all cells to survive, nutrients would be recieved and wastes would be excreted via this extracellular fluid. ( Figure.1)
Exchanges of matter occuring between the external and internal environment.
It is now known that a multicellular organism can only survive in order that the composition of internal environment is maintained in a state compatible with the survival of cells. The concept that the composition of the internal environment is maintained relatively constant is homeostasis. The magnitudes of any changes that do occur are only small and are kept within narrow limits, such stability can only be achieved by carefully coordinated biological processes. The body components and systems which function to maintain the physical and chemical property of this internal environment relatively stable are homeostatic control systems. ( Figure 1.2)
To achieve a relatively constant internal environment the functions of the body components and systems must be controlled and regulated. Control meaning the ability to change the rate of reaction and regulation meaning the ability to maintain a variable within specific levels or limits. Controlled and regulated physiological variables with systems within the body include: blood pressure; the heart and blood vessels control the amount of blood being pumped around the body, glucose levels, pH levels, respiratory gases, salt/water regulation and temperature.
Begin ROOM TEMPERATURE
HEAT LOSS FROM BODY
CONSTRICTION OF BLOOD VESSELS
HEAT LOSS (body’s responses)
RETURN OF BODY
TOWARD ORIGINAL VALUE
Figure 1.2 A homeostatic control system
To describe the homeostatic control systems temperature regulation will be used as an example. The body temperature of a human would be 370c and the external temperature is 200c, there would be heat loss to the external environment but chemical reactions occurring within the cells result in heat production and therefore the rate of heat loss is balanced by the rate of heat production. The body heat remains constant and the system is said to be in a steady state. The temperature maintained is known as the set point of the thermoregulatory system. If the external temperature was at a much lower than the internal temperature, 50c for example, then the rate of heat loss would be faster than the rate of heat production and therefore become unbalanced. Such an imbalance would activate homeostatic responses in prevention of further heat loss.
Constriction of blood vessels would reduce the flow of warm blood through the skin and ‘curling up’ would reduce surface area of skin available to heat loss. Complete elimination of heat loss still does not occur and although at a slower rate still continues.
To balance the heat input and output shivering occurs and heat production stabilizes the body temperature at a new steady state. The thermoregulatory system described is negative feedback, where a change in the variable regulated causes responses to react
in the opposite direction of the original change thus bringing the variable to a steady state. Negative feedback systems
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Cybernetics, Heat transfer, Animal physiology, Physiology, Homeostasis, Thermoregulation, Negative feedback, Blood, Heat, Suprachiasmatic nucleus, Endotherm
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