Systems Homeostasis Learning Outcomes

systems homeostasis Learning Outcomes • Describe the principles of homeostasis and how the internal environment is maintained essentially constant through negative feedback control • Recognise that homeostasis requires detectors to inform the system of the current state, a controller to compare the current state with the required state (set-point) and effectors to bring the current state to the set-point • Recognise that many physiological systems rely on negative feedback to maintain a constant internal environment, but that set points can be changed to accommodate different physiological requirements Homeostasis “home” = same “stasis”= stand Maintaining a constant level, a steady state The term homeostasis was coined by American physiologist Walter Bradford Cannon (18711945), but the foundations of the principles were laid earlier by French psychologist Claude Bernard Features of homeostasis 1. Constancy in an open system, such as require certain mechanisms that act maintain this constancy. This was based on insights into the ways steady states are regulated. 2. Steady states conditions require that any tendency towards a change automatically meets with resistance and factors that result in that resistance 3. The regulatory system that determines the homeostasis state consists of number of cooperating mechanisms that act simultaneously. (control of sugar) 4. Does not occur by chance. It is a result of an organised self-government in the body. French physiologist, Claude Bernard • • • • Le mileu interieur (the internal environment) The French physiologist Claude Bernard (1813-1878) stated that: “the living body, though it has need of the surrounding environment, is nevertheless relatively independent of it. This independence… derives from the fact that… the tissues are withdrawn from direct external influences and are protected by a veritable internal environment which is constituted, in particular, by the fluids circulating in the body.” “The constancy of the internal environment is the condition for free and independent life: the mechanism that makes it possible is that which assured the maintenance, within the internal environment, of all the conditions necessary for the life of the elements.” “The constancy of the environment presupposes a perfection of the organism such that external variations are at every instant compensated and brought into balance. In consequence, far from being indifferent to the external world, the higher animal is on the contrary in a close and wise relation with it, so that its equilibrium results from a • • continuous and delicate compensation established as if the most sensitive of balances.” At the time Claude Bernard was formulating his ideas on maintaining the internal environment constant, there was rapid development of machines that increased productivity during the so-called Industrial Revolution, steam engines were the first to use negative feedback systems. Inventions like steam engines required control systems to maintain a constant output: Scottish engineer James Watt developed the centrifugal governor to allow a constant speed of rotation Fermentation and homeostasis • • • • • • Production of fermentation relies on homeostasis Fermentation yeast consumes sugar to produce energy, CO2 and alcohol. This is an exothermic reaction The issue in this is that temperature can rise uncontrollably If we keep fermentation within strict the yeast production of undesirable is very much minimized To control temperature in fermentation is to place the fermentation vessel in the fridge. The broower monitors the temperature because it sensors them temperature with temperature probe being placed inside the fermentation vessel. Of the temperature goes up too much the cooling output turns on to bring it down to a predetermined level. (temperature controller) Homeostasis • • • • What do we need to maintain a steady state? We need to know what the current state is We need to know what the current state should be We need to bring the current state to the correct state Homeostatic control: • • • • • Detector(s) Controller(s) Effector(s) Control is brought about through “negative feedback” An error signal is detected and corrected by the controller Examples of physiological control processes involved in homeostasis: 1. Control of blood pressure 2. Control of body temperature 3. Control of food intake 4. Control of osmolality 5. Control of blood volume Control of blood pressure: • Arterial pressure is controlled continuously (beat-beat): • • • Sympathetic vasoconstriction: the autonomic nervous system innovates the smooth muscle on the vasculature in arterioles and arteries to squeeze and bring the pressure back up. Sympathetic and parasympathetic (via the vagus never for the parasympathetic and sympathetic via the paravertebral ganglia down the side if the spinal cord and going out to the heart) (cardiac vagal) effects on heart rate and stroke volume, how fast the heart beats, how much it is feeling capacity and ot is force of rejection is). The control system (the baroreflex) is a negative feedback loop: an increase in arterial pressure brings about a decrease in arterial pressure (and vice versa), maintaining a constant (set) level The baroreflex is a negative feedback loop • Control centre recognizes that it is not where it should be it is way too high, it tells the vessels stop squeezing the pressure is too high and it tells the heart slow down and the vessels to stop squeezing and need to relax, open up the vessels and drop pressure Arterial baroreceptors, located in the carotid sinus and aortic arch, monitor arterial pressure the control “centre” is located in the medulla During exercise blood pressure increases but the baroreflex still operates to keep blood pressure essentially constant – the set-point is increased This involves changing the central controller within the medulla – resetting the baroreflex to operate at a higher set-point Likewise, in high blood pressure (hypertension) the baroreflex still operates, but at a higher setpoint Control of body temperature: Temperature receptors are located in the skin and in the hypothalamus Loss of heat to the environment is brought about by an increase in sympathetic sudomotor (sweating) activity and a decrease in cutaneous vasoconstrictor activity Gain of heat is brought about by an increase in cutaneous vasoconstriction and increase in activity of the skeletal muscles (shivering) Control of body temperature: Gain of heat is brought about by an increase in cutaneous vasoconstriction and increase in activity of the skeletal muscles (shivering) A pyrogenic fever results in a change in temperature set point, resulting in a perception of being cold Shivering occurs in an attempt to increase the core temperature