The body and its constituents Box 1.1 Examples of physiological variable Interstitial Temperature Water and electrolyte concentrations pH(acidity or alkalinity) of body fluids brane Blood glucose levels Blood and tissue oxygen and carbon dioxide levels ,a Small particles able to pass through pores in the cellmembrane A Large particles outside-cannot pass into the cell system.The thermostat(temperature detector) is sensitive a Large particles inside- cannot pass out of the cell to changes in room temperature(variable factor). The ther- Large particles inside and outside-cannot pass through cell membrane mostat is connected to the boiler control unit(control cen- tre), which controls the boiler (effector). The thermostat constantly compares the information from the detector with the preset ten ments are made to alter the room temperature. When the thermostat detects the room temperature is low it sends an Detector input to the boiler control unit, switching it on. The result hemostat the preset temperature is reached, the system is reversed Input The thermostat detects the higher room temperature and sends an input to the boiler control unit, turning it off.The Control centre output of heat from the boiler stops and the room slowly (boiler control unit cools as heat is lost. This series of events is a negative feed Turns off Output urns on back mechanism and it enables continuous self-regulation or control of a variable factor within a narrow range Effector Body temperature is a physiological variable con trolled by negative feedback(Fig. 1. 4). When body tem- perature falls below the preset level, this is detected by specialised temperature sensitive nerve endings. They .Room temperature transmit this information as an input to groups of cells in the hypothalamus of the brain which form the control centre. The output from the control centre activates Gradual heat loss from room mechanisms that raise body temperature (effectors) stimulation of skeletal muscles causing shivering Room temperature narrowing of the blood vessels in the skin reducing the blood flow to, and heat loss from, the peripheries Figure 1.3 Example of a negative feedback mechanism: control of behavioural changes, e.g. we put on more clothes or room temperature by a domestic boiler When body temperature rises to within the normal Negative feedback mechanisms range, the temperature sensitive nerve endings no longer stimulate the cells of the control centre and therefore the In systems controlled by negative feedback the effector output of this centre to the effectors ceases response decreases or negates the effect of the original Most of the homeostatic controls in the body use nega- stimulus, restoring homeostasis(thus the term negative tive feedback mechanisms to prevent sudden and serious feedback). Control of body temperature is similar to the changes in the internal environment. Many more of these non-physiological example of a domestic central heating are explained in the following chaptersThe body and its constituents Box 1.1 Examples of physiological variables ^PlWiP _6 Figure 1.3 Example of a negative feedback mechanism: control of room temperature by a domestic boiler. Negative feedback mechanisms In systems controlled by negative feedback the effector response decreases or negates the effect of the original stimulus, restoring homeostasis (thus the term negative feedback). Control of body temperature is similar to the non-physiological example of a domestic central heating system. The thermostat (temperature detector) is sensitive to changes in room temperature (variable factor). The ther￾mostat is connected to the boiler control unit (control cen￾tre), which controls the boiler (effector). The thermostat constantly compares the information from the detector with the preset temperature and, when necessary, adjust￾ments are made to alter the room temperature. When the thermostat detects the room temperature is low it sends an input to the boiler control unit, switching it on. The result is output of heat by the boiler, warming the room. When the preset temperature is reached, the system is reversed. The thermostat detects the higher room temperature and sends an input to the boiler control unit, turning it off. The output of heat from the boiler stops and the room slowly cools as heat is lost. This series of events is a negative feed￾back mechanism and it enables continuous self-regulation or control of a variable factor within a narrow range. Body temperature is a physiological variable con￾trolled by negative feedback (Fig. 1.4). When body tem￾perature falls below the preset level, this is detected by specialised temperature sensitive nerve endings. They transmit this information as an input to groups of cells in the hypothalamus of the brain which form the control centre. The output from the control centre activates mechanisms that raise body temperature (effectors). These include: • stimulation of skeletal muscles causing shivering • narrowing of the blood vessels in the skin reducing the blood flow to, and heat loss from, the peripheries • behavioural changes, e.g. we put on more clothes or curl up. When body temperature rises to within the normal range, the temperature sensitive nerve endings no longer stimulate the cells of the control centre and therefore the output of this centre to the effectors ceases. Most of the homeostatic controls in the body use nega￾tive feedback mechanisms to prevent sudden and serious changes in the internal environment. Many more of these are explained in the following chapters. Temperature Water and electrolyte concentrations pH (acidity or alkalinity of body fluids Blood glucose levels Blood and tissue oxygen and carbon dioxide levels Blood pressure