Fig 1.
It is estimated that 60% of the total heat loss from a naked man sitting in a room kept at 33 C is by radiation.
Radiation 60%
Evaporation 20%
Conduction/ Convection 20%
Evaporation
Conversion of liquid to a vapour. Under typical conditions, about 22% of heat loss occurs through evaporation of about 700 ml of water per day – 300ml in exhalation and 400 ml from the skin surface. When water loss is through the skin and mucous
membranes of the mouth and respiratory system it is termed insensible water loss. The rate of evaporation is inversely related to relative humidity, the ratio of the amount of moisture in the air to the maximum amount it can hold at any given temperature. The higher the rate of relative humidity - the lower the rate of evaporation. Evaporation provides the main defence against over heating in exercise. Under extreme conditions, a maximum of about 3 litres of sweat can be produced each hour, removing 1700 calories of heat if all evaporates.
Hypothalamic Thermostat
Control centre that functions as the body’s thermostat. This is a group of neurons in the anterior part of the hypothalamus known as the preoptic area. It receives impulses from thermoreceptors in the skin, mucous membranes and from the hypothalamus. Neurons of preoptic area generate nerve impulses of a higher frequency when blood temperature increases, and at a lower frequency when blood temperature decreases. Nerve impulses propagate two other parts of the hypothalamus, the heat centre and the cold centre. When stimulated by the preoptic area sets into operation a series of responses that lower and raise body temperature, respectively.
Thermoregulation
The hypothalamus detects, through nerves temperature of the blood that passes through it. If core temperature increases or decreases, the mechanisms that help conserve or get rid of heat, increase or decrease heat production via several negative feedback (fig 2.) loops to raise or lower the body temperature back to normal. The heat centre acts like a thermostat. It switches on heat loss mechanisms when the temperature of the blood is higher than normal. Conversely it switches on heat conservation mechanisms when the temperature of the blood is lower than normal. The heat centre inhibits the cold centre; and becomes active when receptors in the skin signal that the environment is getting cooler.
Fig 2.
Negative feedback mechanism for conserving heat and increase heat production.
Stimuli disrupts
Homeostasis by
Decreases
Body temperature
Return to homeostasis
When response brings
Body temp back to
normal
Receptors
Thermoreceptors in skin
And hypothalamus
Input Nerve impulses
Control centre
Preoptic area, heat
And cold centre
Output Nerve impulses and TSH
Effectors
Vasoconstriction Adrenal medulla Skeletal muscles Thyroid gland
Decreases heat release hormones contract in release hormone
Loss through increases cellular repetitive cycle increases metabolic
Skin metabolism - shivering rate
Increase in body temp
If core temperature rises above normal, negative feedback will be the opposite of fig 2.
Regulating Heat Loss
Skin plays a centre role in controlling body temperature (fig 3.). Thermoreceptors in the skin send messages to the hypothalamus, triggering a response so the body feels hot or cold and reacts to the change in temperature. Heat from the body circulates in the blood and is lost through the skins surface. The heat is lost where the surface area to volume ratio is high, for example in the arms and legs. The regulation of body temperature is via our blood supply to the skin. This happens by, vasoconstriction or vasodilation.
Fig 3.
Diagram showing some of the components of skin.
arteriole
hair
cold receptor
epidermis hair follicle
tough receptor
dermis sebaceous gland
subcutaneous adipose tissue
layer
sudorfic gland
pressure receptor
erector- pilli muscle
Vasoconstriction
Sympathetic stimulation of the arterioles by nerve impulses causes vasoconstriction. Arterioles leading to the skin constrict and reduce the blood flow to the skin, reducing heat loss via radiation and conduction.
Fig 4.
Vasoconstriction in the skin reducing epidermal heat loss to a minimum.
Epidermis
Capillary
Arteriole (constricted)
Venule
Vasodilation
Heat is taken into the dermis by blood in numerous arterioles and capillaries. Arterioles expand if the body temperature rises and more blood flows close to surface of the skin via capillaries. Heat is lost through the epidermis by radiation. The skin becomes pink and the body is cooled.
Fig 5.
Vasodilation in the skin leading to heat loss through the epidermis.
Epidermis
Capillary
Fat
Arteriole (dilated)
Venule
Hair
Most mammals are covered with hair, helping thermoregulation. By keeping a layer of air next to the skin. Air is a poor conductor of heat because it is kept in contact with the skin and is not readily replaced by cold air from the surroundings. The temperature gradient between the skin and trapped air is small. When we are cold the skin pulls the hairs upright to try and trap air for insulation, resulting in goose pimples. The human body does not have an abundance of hair. This is why we rely on clothes to trap air above our skin – keeping us warm in cold weather.
Adipose Tissue
Human bodies have a subcutaneous fat layer of skin below the dermis (see fig 3.) Known as adipose tissue, helping to insulate the body by conducting heat more slowly to the surroundings due to lower thermal conductivity. The adipose tissue in the skin that insulates against most heat loss is made up of white fat. Some adipose tissue is made up of brown fat. Brown fat cells contain many mitochondria and have a high metabolic rate. Babies and thin people have a relatively large amount of brown fat, whereas obese people tend to have little but extensive reserves of white fat.
Sweat
Evaporation of sweat from the skins surface is a means of increasing heat loss. When it is necessary to conserve heat, sweating stops. Autonomic nerves control the glands. These impulses come from the thermoregulation in the brain. If the body is sweating profusely heat cramps may occur. This is a result form the water and salt removal from the body. Salt loss causes painful contractions of the muscles- cramps, and in severe cases heat exhaustion can occur.
To combat the body being to cold the body tries to conserve heat. This can include –
- The body secretes the hormone adrenaline, which raises metabolic rate and increases heat production.
- Shivering as muscular activity generates heat.
- Adding layers of clothes to reduce heat loss, by convection trapping layers of hairs.
- Eating more as eating stimulates heat production by respiration.
- Food can also convert to subcutaneous fat for insulation.
- Turning up the heating. To receive heat via convection.
- Curl up in the foetal position to reduce the surface area.
If the body’s temperature falls below 24 C the heat mechanisms fail to work hypothermia can set in. Hypothermia is a lowering of core body temperature. Hypothermia affects cells in the brain and heart, leading to coma or death. Metabolism slows down and the muscles cannot work properly. Hypothermia is common in the elderly because they have reduced metabolic protection and perception against cold environments.
To combat the body being too hot, the body tries to increase heat loss –
- A hot person becomes lethargic and tends to lie down with their limbs spread out, this decreases heat production and increases heat loss.
- During a fever the body’s temperature resets 2 - 3 C higher, this will denature proteins in the infecting bacteria more rapidly than they affect human proteins.
- Heat loss is also enhanced by wearing loose clothing, fanning and cool drinks.
When heat stroke occurs, the temperature and relative humidity are high, making it difficult for the body to lose heat by radiation, conduction or evaporation. Blood flow to the skin is decreased, perspiration is reduced and core temperature rises sharply. If the temperature becomes too high brain cells affected and hypothalamic thermostat fails to work.
Experiment 1
Aim
To determine the relationship between heat loss and body size.
Body Size
The body of a baby is small, therefore will have a high surface area to volume ratio. Consequently the baby will lose heat quickly as there is a greater amount of surface area, which will lose heat in relation to body size. A baby is not very active so cannot generate heat to keep warm. The brown fat in babies has many mitochondria in their cells that oxidise in away to produce more heat than ATP. The heat acts as a furnace, generating heat to protect against the cold. This heat maybe one reason why babies don’t shiver when they are cold. A baby needs to feed well and often to get energy to keep warm. The body will have to work a lot harder to retain core temperature. Babies need to be wrapped in a few layers in a room at a constant temperature. The layers will reduce heat loss using convection by trapping layers of air next to the skin.
Surface Area And Body Volume
The amount of heat generated in tissue respiration depends mainly on the volume of the body. Most heat is lost through our skin. The surface area of the skin determines the amount of heat loss. As we grow our volume increases in three dimensions but our surface area increases by two dimensions. Therefore, relative to volume our surface area increases at a slower rate.
Plan
In a controlled experiment using different sized round bottom flasks of differing volumes, to represent different sizes of the human body. Hot water will be used and the temperature drop will be recorded over a certain period of time.
Prediction
The largest flask will retain heat better than any of the other flasks, and the smallest flask will cool quicker – a baby is smaller than an adult, and loses heat more quickly. Due to the surface area of the baby being higher. An adult holds more volume, but less surface area in comparison to a baby, resulting in the baby losing heat quicker.
Apparatus
Round bottom flask 50ml
Round bottom flask 500ml
Round bottom flask 1000ml
Round bottom flask 2000ml
Mercury thermometer
Kettle
Stopwatch
Water
Pen and paper
Risk Assessment
The solution used is very hot water. Care needs to be taken for it not to spill. The kettle and round bottom flasks need to be away from the edge of the table. If there is contact of hot water on the skin, rinse with cold water immediately. Also care should be taken when using a mercury thermometer, as mercury is poisonous.
Method 1
The circumference of the round bottom flask was measured and then filled with 50ml of hot water, the experiment commenced when the hot water temperature was at 90 C. A stopwatch was used and the temperature recorded every minute for a period of 20minutes. During the experiment the thermometer was placed in the same position and the flask was stirred at regular intervals to maintain a constant temperature through out. The experiment was repeated with the following sized flasks –
500ml
1000ml
2000ml
Recording of results
Different sized round bottom flasks were used to represent humans with different body sizes.
Fig 6. Table of recordings showing how temperature was maintained over a 20minute period using different sized round bottom flasks
Circumference Of Flasks
50ml = 16cm 500ml = 32.5cm 1000ml = 41cm 2000ml = 52.5 cm
Fig 7. Table showing surface area to volume ratio of the different sized flasks
( see appendix 2 page 20 for calculations.)
Summary
The experiment has fallen in line with the prediction. The small flask loses heat quicker due to a larger surface area to volume ratio. Using the graph (Fig 5.) and the table (fig 6) with surface area to volume ratio, it should be seen that the larger the volume of the flask (human) the smaller surface area through which heat is lost through the environment. This is linked to the homeostasis in humans.
Experiment 2
Aim
To see if there is any correlation between heat loss, body size and different types of insulating materials.
Apparatus
50cm round bottom flask
2000cm round bottom flask
1 layer bubble wrap
2 layers of nylon
Kettle
Water
Thermometer
Pen and paper
Method 2
Method 1 was repeated but this time different types of insulating materials were used.
The independent variable is the insulating materials used. The dependent variable is the measurement (temperature change) that is affected by the independent variable.
Fig 8.
Table showing how temperature was maintained over a 20minute period using different sized round bottom flasks with one layer of bubble wrap insulation
Temperature started at 90 degrees Celsius
Fig10.
Table showing how temperature was maintained over a 20minute period using different sized round bottom flasks with 2 layers of nylon insulation.
Temperature was started at 90 degrees Celsius
Fig 12.
This table represents the different ranges of drops in temperature from 90C using the different sized flasks – non- insulated and insulated (see appendix 1 page 19 for calculations)
Results
In both experiments the heat was lost through convection, this was due to the hot water being in contact with the cooler air. The cooler air rises and then heats up drawing in more cool air to the hot water, the cool air keeps circulating until the water is cool. From the table above it is clear that the large flask conserves heat better than the smaller flask with or without insulation. It also shows that using insulation conserves the heat. When the bubble wrap was used, the larger flasks heat was conserved by 32%, the smaller flasks heat was conserved by 15 %. Bubble wrap is a good insulator as it has pockets of air, which heat doesn’t flow through easily. Using the layers of nylon the heat was conserved by 19% in the large flask whereas the small flasks heat was conserved by 18%. Although both insulated, the flasks with bubble wrap appears to be the better insulator. These results can show that a bigger person wrapped up conserves heat better than a smaller person. It could also show that the smaller the creature the less important the type of insulation used.
Evaluation
Both experiments have been successful, however a few improvements could have been made.
- Repeating the tests again, to gain stronger evidence of results.
- Using temperature probes linked to a computer to record temperature, this would have achieved more accurate results.
- Making sure the temperature was recorded accurately every minute. It could have been possible that the experimenter was taking longer to read the temperature at certain times during the experiment.
- Stirring the water constantly to keep the temperature constant.
- The temperature of the room could have varied, thus cooling the water quicker.
By using these results and the knowledge of homeostasis it can be seen that body size and insulation play an important role in maintaining a stable body temperature.
Bibliography
Principles Of Anatomy And Physiology – Tortora Grabowski
- Advanced Human Biology – Simpkins and Williams
- Human Form and Function – Pamela Minett and David Wayne
- Class Notes
Internet Resources
Appendix 2
Calculations to show how percentages were obtained (see page 17,fig 10.)
2000cm flask with no insulation - at the end of the 20 minute experiment the temperature drop was 90 C- 79 C = 11 C
Temp drop with bubble wrap = 90 C – 82.5 C = 7.5 C
7.5 * 100 = 68.18% 100-68 = 32%
11
The bubble wrap conserved the heat by 32%
Temp drop with 2 layers of nylon was 90 C – 81 C = 9 C
9 *100 = 81.81% 100 – 81 = 19%
11
The nylon conserved the heat by 19%
50cm flask with no insulation - at the end of the 20 minute experiment the temp drop was 90 C – 59 C = 31 C
The temp drop with bubble wrap was 90 C – 63.5 C = 26.5 C
26.5 *100 = 85% 100 – 85 = 15%
31
The bubble wrap conserved the heat by 15%
The temp drop with 2 layers of nylon was 90 C – 63 C = 26 C
26 * 100 = 83% 100 – 83 = 17%
31
The nylon conserved the heat by 17%
