Body Temperature Temperature Regulation Dr Waleed R Ezzat
Body Temperature & Temperature Regulation Dr. Waleed R. Ezzat
Lecture Objectives: ● ● ● Explain how the body produces heat and exchanges energy with the environment. Explain how sweating, skin blood flow, and metabolic heat production help regulate body temperature. Explain how thermoregulatory set point varies cyclically and is elevated during fever. Explain the reflex control of physiologic hermoregulatory responses. Explain the process of acclimatization to hot and cold environments. Explain the adverse effects of excessive heat stress, heat injury, and excessive cold stress.
Normal body temperature: ● Core temperature → fixed at 36. 9 ± 0. 6º C (orally), range 36. 3 -37. 5º C ● Humans are able to regulate their internal body temperatures within a narrow range, despite wide variations in environmental temperature ● Skin temperature is directly related to the temperature of the surroundings
Note: 1. Rectal temp. is 0. 6º C higher than oral temp. 2. Axillary temperature recording is used infrequently as it needs long time to hold the upper arm firmly against the chest to close the axilla to record reliable temperature 3. Infrared ear (aural) thermometers are convenient and widely used in pediatric clinic to detect the temperature of a patient's eardrum. The eardrum is an extremely accurate point to measure core body temperature 4. In exercise, temp. can temporarily go as high as 40º C 5. Exposure to extreme cold can lower body temp. to 35º C
Principles of heat balance: ● ● ● Heat balance is based on the rate of heat production and heat loss Heat production is a by-product of metabolism (metabolic rate) especially in the liver, brain, and heart Extra rate of metabolism comes from muscle contraction (e. g. during exercise and shivering), effect of T 3 & T 4, and effect of E and NE or Sympathetic stimulation to brown fat (uncoupling of oxidative phosphorylation in neonate) → Nonshivering thermogenesis. Brown adipose tissue is brownish because it contains high concentrations of iron-containing respiratory enzymes. Brown adipose deposits are located symmetrically in the supraclavicular and the neck regions with some additional paravertebral, mediastinal, para-aortic, and suprarenal (but no interscapular) localizations and respond with increased activity to sympathetic stimulation and exposure to cold.
Principles of heat balance (cont. ) ● Heat loss is mainly via the skin ● The rate of heat loss is determined by two factors 1) how rapidly heat can be conducted from production sites to the skin 2) how rapidly heat can be transferred from the skin to the surroundings
The skin: ● ● The subcutaneous fat is the main heat insulator for the body. Skin circulation allows the heat transfer and preservation. This is achieved through the presence of: 1) arteriovenous anastomosis between subcutaneous arteries and the venous plexus that is highly controlled by the sympathetic system. When anastomosis are fully opened 30% of cardiac output can pass through 2) the sympathetic system that sets the degree of vasoconstriction of the arterioles and the arteriovenous anastomosis 3) The sympathetic reflex that is controlled by changes in body core temperature and environmental temperature
The skin (cont. ) ● ● ● Heat conductance during full vasodilation is 8 X the heat conductance during full vasoconstriction. When skin temp. is higher than the surroundings, heat is lost from skin to surroundings through radiation (60%), conduction & convection (3+15%), and evaporation (22%). Each gram of water needs 0. 58 Calorie to be evaporated; about 450 -600 ml of water is evaporated from the body daily from skin and lungs (insensible evaporation). This is a basal uncontrolled evaporation. Evaporation due to sweating is extra and controllable. When surroundings’ temp. is greater than that of skin, the body gets rid of heat by evaporation only (sweating). Sweating results from stimulation of anterior hypothalamus-preoptic area in the brain → sympathetic cholinergic outflow.
Control mechanism: ● ● ● In humans, the behavioral control of temperature is the most powerful control system. Temp. regulation depends entirely on nervous feedback mechanism, through temperature-regulating centers located in the hypothalamus. Heat-sensitive neurons (receptors) as well as about third as many cold-sensitive neurons are located in the preoptic and anterior hypothalamic nuclei of the hypothalamus. Both sets of neurons increase their firing rate when stimulated. Stimulation of heat-sensitive neurons → generalized sweating + cutaneous vasodilation. There are peripheral temp. receptors in the skin (cold is 10 X that of warmth) and in few specific deep tissues of the body (mainly cold receptors too). Their reflex prevents hypothermia.
Control mechanism (cont. ) ● Thermoreceptors are free nerve endings of small superficial myelinated Aδ (for cold) and dermal unmyelinated C fibers (for warm). ● The response of most peripheral thermosensors shows a powerful dynamic (phasic) component ● Warm receptors are active when the skin temperature is between 30°C and 43°C. The steady-state firing rate of warm receptors reaches a peak at temperature of approximately 42°C. ● Warm receptors transiently increase their firing rate when current skin temperatures increase, and decrease their firing rate when current skin temperatures decrease. ● Cold receptors are active when the skin temperature is between 15°C and 38°C. The steady-state firing rate of cold receptors reaches a peak at temperature between 23°C and 28°C. ● Cold receptors transiently increase their firing rate when current skin temperatures decrease, and transiently decrease their firing rate when current skin temperatures increase.
Control mechanism (cont. ) ● ● Reflex from both peripheral and central receptors end in the posterior hypothalamus where they are combined and integrated to control the heat-producing and heatconserving reaction of the body. Cold signals → stimulation of primary motor centre for shivering in dorsomedial portion of the posterior hypothalamus (normally inhibited) → increases skeletal muscle tone throughout the body → when tone increases beyond a certain critical value → shivering begins (feedback oscillation of the muscle spindle stretch reflex mechanism). The body has a set-point temperature. That is, all the temp. control mechanisms continually attempt to bring the body temp. back to this set-point level. The set-point threshold increases for sweating as the skin temp. decreases. Whereas, the shivering threshold is lowered when the skin becomes cold.
Fever: ● ● ● (def. ) It’s a state where the hypothalamic set-point increases and the body temperature rises above the usual range of normality. Fever is due to (1) abnormalities in the brain itself (local pressure), or (2) toxic substances (e. g. the pyrogens, lipopolysaccharide toxins released from bacterial cell membrane). The suggested mechanism of bacterial induced fever is: Phagocytized bacteria → release of the cytokine interleukin-1 (by blood leukocytes and tissue macrophages) → release of prostaglandins (mainly prostaglandin E 2) → acts on hypothalamus → Fever Other cytokines, particularly tumor necrosis factor, interleukin-6, and the interferons, are also pyrogenic in certain circumstances. The above mechanism explains why aspirin, which inhibits the formation of prostaglandins, can reduce the level of fever. Moderate fever is beneficial. Fever inhibits the growth of many microorganisms, increases antibody production, and stimulates leukocytes and macrophages activities.
Test Question Q. Antipyretics such as aspirin are not used to counteract the increase in core temperature that occurs during exercise. Which of the following best explains why it is inappropriate to use antipyretics for this purpose? A. The increase in core temperature during exercise stimulates metabolism, helping to support the body’s increased metabolic energy demands B. A moderate increase in core temperature during exercise is harmless, so there is no benefit in preventing it C. Antipyretics are ineffective during exercise because they act on a mechanism that operates during fever, but not to a significant degree during exercise D. Antipyretics increase skin blood flow so as to dissipate more heat, increasing circulatory strain during exercise E. The increased heat production during exercise greatly exceeds the ability of antipyretics to stimulate the responses for heat loss
- Slides: 18