Naish Medical Sciences Ch1
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Physiological control mechanisms and homeostasis 1
Types of control system 1Negative feedback mechanisms 1Positive feedback mechanisms 3Feedforward control mechanisms 4Homeostatic imbalances cause disease 4
Acidbase balance 4Hydrogen ion concentrations 4Buffers 5
Biological buffer systems 6HendersonHasselbalch equation 7Control of pH 7Acidbase disturbances 8
Fluid balance 10Body fluids and fluid compartments 10Daily fluid balance 12Failures of fluid balance 13
Failures of homeostasis 13Diabetes and obesity 13High blood pressure 14
PHYSIOLOGICAL CONTROL MECHANISMS AND HOMEOSTASIS
The French scientist Claude Bernard (181378) first described the mileau interieur, and observed that this internal environ-ment remains remarkably constant despite changing condi-tions in external conditions. Shortly afterwards, the American physiologist Walter Cannon (18711945) first used the word homeostasis to describe this constancy.
Homeostasis is the physiological process by which the internal systems of the body are maintained at equilibrium despite variations in the external conditions. It comes from the word homeo-, which means the sameness, and stasis, that is, standing still. However, equilibrium is not an unchang-ing state, so this is not strictly true it is a dynamic state of equilibrium causing a dynamic constancy of the internal envi-ronment. It arises from the variation in response to changes in the external environment.
Homeostasis is responsible for maintaining physiological systems and there are many examples which can be used to describe the way in which systems work together. Two of the most well known and understood are the control of acidity of the body fluids, better know as acidbase balance, and the control of the fluid volumes of the body, fluid balance.
TYPES OF CONTROL SYSTEM
All control systems have a common basic structure. In control system theory the factor that is being controlled is called the variable. In order to control the value of a variable all control systems need at least three components:
n A sensor which monitors the current value of the variable
n A control centre which stores the desired value of the variable and can compare this to the current value as provided by the sensor
n An effector which changes the value of the variable in a way that is determined by the control centre.
NEGATIVE FEEDBACK MECHANISMS
The most common type of control system is negative feed-back (Fig. 1.1). A negative feedback system causes the vari-able to change in the opposite direction to the initial change, returning it to its set point. An everyday example of a nega-tive feedback system is a central heating system. The sensor is the temperature sensor on the wall that monitors the room temperature. The control centre is the thermostat that com-pares the measured temperature to the temperature set on the thermostat; if it is too low, the heating is turned on. In most domestic systems this control system can only function to increase the temperature when it gets too cold but in more sophisticated systems, if the temperature becomes too high, the thermostat could then turn on the air conditioning. In these circumstances the temperature is constantly main-tained at the set temperature.
Control of body temperatureA good example of a physiological homeostatic negative feedback system is the control of body temperature. Human beings live in a wide range of environments where the tem-perature may range from 50 to +50 C. But body tempera-ture is normally controlled between 36 and 38 C. Biochemical processes in the body will not function if the temperature becomes too low or too high. At very high temperatures enzymes lose their activity and at low temperatures there is insufficient energy to maintain metabolic processes. Hyper-thermia occurs when the core temperature rises above 40 C and hypothermia occurs below 35 C.
1 IntroductionPatricia Revest
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Thermoneutral zonesThe metabolic rate is at a minimum when heat gain or heat loss mechanisms are at a minimum. In naked humans who are resting, this thermoneutral zone occurs around 27 C. At ambient temperatures above and below this, energy must be expended to control the body temperature. Body tem-perature also rises during activity, as the series of chemical reactions that cause muscular contraction do not convert all the energy to mechanical energy. The efficiency is low and as much as 75% is lost as heat. Other mammals have a thermoneutral zone that is much colder. Arctic foxes can live comfortably at temperatures below 50 C, protected by their thick coats.
Heat loss mechanismsAn increase in body temperature is detected by sensors in the skin, which measure peripheral temperatures, and in the hypothalamus, which measure core temperature (Fig. 1.2). One of the effectors that enables the body to lose heat is the rings of smooth muscles in the walls of blood vessels in the skin, which can change the rate of blood flow in the region just under the skin surface. By relaxing the smooth muscle, the diameter of the blood vessels increases and the resistance to flow falls. By increasing the amount of blood flow, heat can be lost by radiation, reducing both the local temperature and the core temperature by returning cooler blood to the central blood volume. This can be clearly seen on pale-skinned people on a hot day when they become very red. If they have been exercising this redness can be widespread, but if the increase in heat is more localised, such as on bare arms in the sun, then the redness may be very local and show up as distinct lines.
Another effector that is particularly effective in hot, dry climates is the sweat glands in the skin. An increase in the rate of sweat produced causes more water to be evaporated from the surface of the body. As the latent heat of evapor-ation of water is high, large amounts of heat can be lost by this mechanism. However, in hot, wet climates, this mecha-nism is ineffective as evaporation requires a gradient of humidity between the skin and the air. In tropical climates the humidity may be 100% so evaporation does not occur. The other problem with this method of heat loss is that it requires large amounts of body water. If this is not replaced then dehydration can occur.
Heat gain mechanismsIf there is a fall in body temperature the responses are more varied. The same effectors the vascular smooth muscle and
the sweat glands that can reduce body temperature can work in reverse to increase the temperature. Constriction of vascular smooth muscle in the skin can reduce the blood flow in the upper layers of the skin to almost nothing. This prevents heat loss and gives the pale, slightly blue tinge to the skin surface. The pallor is due to the very small amounts of blood in the skin and the blue tinge is due to the fact that the rate of blood flow is so slow that most of the haemoglobin is deoxygenated and so appears bluish.
Two other mechanisms are also brought into play. In adult humans, a fall in core temperature can induce an increase in muscular activity. Voluntary activity includes stamping the feet, tapping the hands and generally increased fidgeting. Involuntary movements also occur as shivering; the rapid contraction and relaxation of skeletal muscle that is control-led by the autonomic nervous system. This can range from an increase in muscle tone to vigorous shaking.
Fig. 1.1 Negative feedback loop. An increase in the variable produces an effector response to decrease it and vice versa.
Control centre(set point)
Fig. 1.2 Control of body temperature by negative feedback. (A) Responses to an increase in body temperature; (B) responses to a decrease in body temperature.
Skin (peripheral)Hypothalamus (central)
Smooth muscle inwalls of blood vesselssupplying skin
Skin (peripheral)Hypothalamus (central)
Smooth muscle in walls ofblood vessels supplying skin
Brown and adipose tissue (brown fat)
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In the cold, humans also show piloerection, when the body hairs stand up. In mammals with thick fur this can sig-nificantly decrease heat loss by trapping more air close to the skin, where it is warmed and forms an insulating layer. However, most humans are not hairy enough for piloerection to affect their heat balance and the behavioural practice of wearing clothes produces the same effect much more effi-ciently. Humans who fall into cold water survive longer if clothed than if naked, as a similar blanket effect is produced by water trapped inside the clothes. Because of this, thrash-ing about, which disrupts this warm layer, causes the heat to be lost faster.
Long-term temperature controlLong-term mechanisms to control body temperature include changes in metabolic rate, changes in feeding, and behav-ioural mechanisms such as seeking shade in the heat or shelter in the cold. The wearing, and discarding, of clothes has reduced the need for much temperature regulation. However, babies become overheated quite easily if clothed too much, as they cannot discard clothes at will.
FeverThe set point for temperature control is not always fixed. In infection, toxins released from