Zusammenfassung der Ressource
Temperature Regulation
Anmerkungen:
- Humans are homeothermic. Maintain body temperature at a narrow range due to the need of a stable environment for enzymes to function. Basically, the temperature is stable for most parts especially for the rectal temperature. Oral temperatures are susceptible to fluctuations fue to people drinking different temperature stuff or such.
- Variations in Core Temperatures
Anmerkungen:
- Core temperatures tend to be stable, but it also follows a few fluctuations.
- Circadian fluctuations
Anmerkungen:
- Day and night cycles, a normal human with normal sleeping patterns will have the coldest core temperature at around 36.2deg.
- Phase of Menstrual Cycles
Anmerkungen:
- Women generally have lower temperature range in the follicular phase than the luteal phase.
- Age
Anmerkungen:
- Children tend to have higher body core temperatures.
- Activity
Anmerkungen:
- Exercise tends to generate higher body heat.
- Emotion/Excitement
Anmerkungen:
- Excited people have higher metabolic activity. Hence, generate more heat.
- Constitutional Hyperthermia
Anmerkungen:
- Adults that have original basal temperature above the normal range.
- Core and Shell
Anmerkungen:
- Body is divided into warm inner core and a cool outer shell.
Skin temperatures varies more than the core temperature. The shell has a variable size depending on the temperature of the environment.
Heat produced within the body is generated in the deep organs and skeletal muscles during exercise.
It is transferred to the skin t be lost to the surroundings.
- Cool Subject
Anmerkungen:
- Skin blood floor is low.
Convection is low.
Core-to-skin transfer dominated by conduction. Conduction is decreased as the shell is large.
- Warm Subject
Anmerkungen:
- Skin blood flow is high.
Convection is increased.
Conduction increased as the shell is thin.
Fat has little effect on convection.
- Heat Balance
Anmerkungen:
- Body temperature is determined by the balance between heat production and heat loss.
- Heat Production
Anmerkungen:
- Measures using oxygen consumption.
For a basal man 70kg.
Each litre of oxygen consumed releases 4.82kcal.
- Processes that Contribute to Basal Metabolic Rate (BMR)
Anmerkungen:
- Minimum energy required for the body to survive.
It is measured under standard condition.
Such as, fasting for 12 hours as there are thermic effect on food.
Well rested body
Awake
and air temperature of 25degrees.
- Male vs Female
Anmerkungen:
- Male produce more BMR as they have more muscle hence are more metabolically active.
Females on the other hand have more fat and hence do not need to have a higher BMR.
- Muscle Contraction
Anmerkungen:
- Accounts for 90% of the heat production in exercise.
- Thermic Effect of Food
Anmerkungen:
- Up to 10 to 30 percent increase in heat production.
Obligatory energy expenditure that assimilation of food.
- Hormones
Anmerkungen:
- Catecholamines: Short term
Thyroid Hormones: Long term
- Brown Fat
Anmerkungen:
- One of the 2 kinds of adipose tissues.
Brown fat is important for chemical thermogenesis.
Locates at the sternum, nape of neck, and around kidneys.
High density of mitochondria hence the fat looks brown.
Activated by, cold stress and trigger increase in adrenaline and thyroid stimulating hormone (TSH).
TSH generates thyroxine (T4) and adrenaline increases peripheral conversion of T4 to T3
- Thermogenesis
Anmerkungen:
- T4 activated by adrenaline to T3.
T3 up-regulates uncoupling proteins (UCP) or thermogenin.
Leads to oxidative phosphorylation. Instead of making ATP it makes heat.
- Heat Loss
Anmerkungen:
- Note that conduction, convection and radiation can be a route of heat gain.
70% Radiation and Conduction
27% Vaporisation of Sweat
2% Respiration
1% Urine and Faeces
- Conduction
Anmerkungen:
- Also known as direct heat loss
Heat exchange between objects at different temperatures which are in direct contact with each other. Thermal energy is transferred by collision of molecules.
Generally small heat loss
- Convection
Anmerkungen:
- Transport of heat by movement of heated substance such as air or liquid.
Greatly increases heat loss by conduction.
Heat loss by convection depends on few factors such as the temperature between skin and the ambient fluid.
Area of skin in contact
Velocity of the fluid
Type of fluid (water/air)
- Insulator Zone
Anmerkungen:
- Like a blanket or a feather/fur on animals.
Bind the air close to the skin. Reduce heat loss to the environment.
Zone could be increased by fluffing of feathers or erection of hairs (erector pili)
- Water vs Air
Anmerkungen:
- Water have higher specific heat capacity. It also have higher conductivity.
At moderate temperatures, the difference of heat loss of water is much greater to air in the same temperature.
Difference is negligible in the cold.
- Radiation
Anmerkungen:
- Transfer of heat by infrared EM radiation.
All objects not at absolute zero emits infrared rays.
Naked person lose about 60% of the total heat loss by radiation.
- Evaporation
Anmerkungen:
- Vaporization of water occur in the mouth and respiratory tract and skin.
1g of water removes 0.58kcal of heat. Latent heat of evaporation.
ONLY mode of heat loss when the surrounding environment is greater than the internal.
- Insensible
Anmerkungen:
- Unaware
Such as breathing in and out (moisture)
Water that is transfused thru the skin
- Sweating
- Urine and Faeces
- Equation: M= E + R + C + K + W +S
Anmerkungen:
- M: Metabolic rate/heat production
E: Evaporation
R: Radiation
C: Conduction
K: Convection (konvection?)
W: Heat loss thru work
S: Rate of heat storage
If it is heat gain, it will present itself as a negative value.
- Regulation of Body Temperature
Anmerkungen:
- Core temp can be maintained for several hours in dry air over a range of 15 to 50 deg
Thermoneutral zone (body does best) is 27 - 31 deg.
- Set Points
Anmerkungen:
- A base line.
The body will try to follow the baseline.
Regulatory centre will be in the hypothalamic regulating centres
Anterior Hypothalamic Area + preoptic area controls the area that regulate heat.
Posterior Hypothalamic Area regulate actions such as shivering.
- Afferents
- Temperature sensitive cells in hypothalamus
Anmerkungen:
- 75% heat sensitive, 25% cold sensitive. Nerves fire more according to the conditions.
Main function is to stop hyperthermia.
Deals with Core Heat
- Deep body temperature receptors
Anmerkungen:
- Spinal cord, abdominal viscera and great veins.
Mainly detect cold
Prevents hypothermia
- Cutaneous Temperature Receptors
Anmerkungen:
- Skin
10 Times as many cold receptors than warmth.
Deals with preventing hypothermia
- Efferents
- Cold Activated
- Increase heat production
Anmerkungen:
- Shivering: Controlled by dorsomedial of posterior hypothalamus, send impulses to anterior motor neurones to increase muscle tones.
In bursts. 4 -5 fold in heat production
Hunger: Thermic effect of food.
Increased activity
Sympathetic activation and secretion of catecholamines: Thermogenesis of brown fat
Thyroxine secretion
- Decrease Heat loss
Anmerkungen:
- Cutaneous Vasoconstriction
Curling up
Piloerection
- Heat Activated
- Increased heat loss
Anmerkungen:
- Cutaneous Vasodilation: Inhibits sympathetic centres which cause vasoconstriction.
Sweating: Rate increases sharply after 37 deg
Increased Respiration: Like dog panting.
- Sweat Glands
Anmerkungen:
- Flow rate determines how much sodium concentration in sweat.
High flow: NaCl not absorbed by body. Hence higher conc.
Low flow: Vice versa
- Decreased heat production
Anmerkungen:
- Anorexia: Stop/decreased intake of food
Apathy and Inertia: Lack of motivation to move
Inhibition of shivering and chemical thermogenesis
- Examples
- Heat Stroke Victims
Anmerkungen:
- Exercise in heat is particularly dangerous as blood goes to exercising muscles and skin.
Hence decreased venous return and stroke volume and increased heart rate.
Decreased cooling and increased in body temperature. and hence could lead to heat stroke.
- Heat Acclimatization
Anmerkungen:
- Train in hot environment.
Body increased in tolerance to heat.
Features: Lowered heart rate, rectal temperature/ core temperature.
Shows higher sweating rate. Increased in conc of NaCl.
Lowered threshold for cutaneous vasodilation and increased in plasma volume.
Lowered urinary sodium losses.
- Fever
Anmerkungen:
- Body temperature above basal range.
Indication of diseases
Shivering and vasoconstriction to increase the temperature, then sweating and vasodilation to decrease it.
- Pyrogens
Anmerkungen:
- Substances that cause the heat changes.
Caused by white blood cells creating cytokines at the preoptic area of the hypothalamus, prostaglandins to raise set points.
Aspirin inhibits prostaglandins and hence lower fever.