Physiology of Cold: Lessons from Everest PDF

LESSONS FROM EVEREST The physiology of cold Objectives ¨ Explain the mechanisms of body temperature regulation. ¨ Describe the physiological responses to exercise in the cold. ¨ Describe how cold stress influences fuel utilization ¨ Describe the health risks associated with hypother...

LESSONS FROM EVEREST The physiology of cold Objectives ¨ Explain the mechanisms of body temperature regulation. ¨ Describe the physiological responses to exercise in the cold. ¨ Describe how cold stress influences fuel utilization ¨ Describe the health risks associated with hypothermia Words in red are hyperlinks for information on that topic or objective; just click on one and it will take you where you want to go! The arrow icon will take you back to the previous slide The home icon will take you back to this slide Explain the mechanisms of body temperature regulation. Remember, humans are homeotherms-that is we need to maintain our body temperature in a certain range- approximately 37°C or 98.6°F regardless of the environment Our thermostat is located in the hypothalamus (preoptic-anterior hypothalamus—POAH, to be exact!) Peripheral thermoreceptors in the SKIN “report” to the POAH for a physiological response and to the cerebral cortex for a behavioral response (we perceive the temperature and act accordingly) Central thermoreceptors in the hypothalamus, other regions of the brain, and spinal cord, monitor BLOOD TEMPERATURE and respond to changes as small as 0.01° C or 0.018°F! The effector organs are the skin arterioles, endocrine glands, skeletal muscle, and eccrine sweat glands ENVIRONMENTAL CHALLENGE: To Maintain a Steady-State Core Temperature, the Body Must Balance Heat Gain With Heat Loss Heat Click here to learn more Loss  Heat Loss: Removal of Heat From the Skin Radiation: heat is given off in the form of infrared rays Sweat Evaporation Convection: moving heat from one place to another by the motion of gas or liquid across a heated surface Thermogram of someone wearing socks?! Conduction: the transfer of heat from one solid material to another though direct molecular contact Evaporation Primary avenue for heat dissipation during exercise Evaporation accounts for 80% of heat lost during exercise vs. 10-20% at rest As body core temperature increases, sweat production increases Sweat must evaporate to provide cooling Overview of the Physiological Response to Cold  SKIN Hairs become erect: muscle erectors Temperature contract and make receptors: Free-nerve the hairs erect and stand up vertically endings. trapping air in the gaps between them. This acts as insulation to reduce heat loss! Subcutaneous fat: is an excellent insulator & its capacity to transfer heat (thermal conductivity) from deep tissues to the body surface is relatively low  SKIN ARTERIOLES-1st line of defense Peripheral vasoconstriction is our 1st line of defense against hypothermia Keeps blood from SNS activates the going to the skin vascular smooth surface and being cooled muscle cells (VSMC) Receptors are Adrenergic Cholinergic- what does this mean? ECCRINE SWEAT GLANDS These glands are activated by the SNS in response to an INCREASE in core or skin temperature Not needed in response to the cold unless you’re over-dressed! Even with a windchill of -35 I am starting to sweat! WINDCHILL: the cooling power of the environment AIR AIR MOVEMENT WINDCHILL TEMPERATURE (WIND) ie -10°C ie. 15 km/hr Cools like: -17°C Windchill is an index based on the cooling effect of wind. It is NOT the temperature of the wind. 2nd Line of Defense: Non-Shivering Thermogenesis Which of the following hormones would increase body temperature by increasing metabolic rate? Select all that apply A. Thyroxine B. Epinephrine C. Norepinephrine D. Growth Hormone  Thyroxine Yes! Thyroxine can increase cellular metabolism by 60% - 100%! Remember that we are not incredibly efficient. When we do work ~60% of the energy used is given off in the form of heat But that is not the only hormone that can help you stay warm.  Epinephrine Correct! SNS stimulation of the adrenal medulla results in the secretion of Epinephrine (80%) and Norepinephrine (20%) into the blood. Thyroxine increases the synthesis of the beta receptors so all 3 of these hormones work synergistically to increase cellular metabolism Stimulation of metabolism by the SNS is known as non- shivering thermogenesis and is our 2nd line of defense against hypothermia Norepinephrine Right! NE is the main neurotransmitter released from the sympathetic nerve fibers. When SNS activation is high, NE can “spillover” into the blood, elevating plasma concentrations of NE. Of course NE is also released from the adrenal medulla along with epinephrine. Together they are known as the catecholamines and have similar actions. Stimulation of metabolism by the SNS is known as non- shivering thermogenesis and is our 2nd line of defense against hypothermia Growth Hormone GH doesn’t seem to play a large part in increasing cellular metabolism; MAYBE in children but not so much if you are an adult. Remember GH decreases considerably as we age. However, GH does increase substrate availability which is important if cellular metabolism has increased ¨ Fat metabolism: GH enhances the utilization of fat by stimulating triglyceride breakdown and oxidation in adipocytes. ¨ Carbohydrate metabolism: GH stimulates gluconeogenesis in the liver Protein metabolism: GH facilitates amino acid uptake, increases protein synthesis, and decreases protein oxidation… so GH effects on protein metabolism are not particularly helpful during cold stress  SKELETAL MUSCLE- 3rd line of defense Hypothalamus is Shivering is our 3rd vasoconstriction by the SNS is on but if I were coordinating my shivering response line of defense bigger I would have a smaller BSA to body but I need some insulating fat against hypothermia mass ratio & lose less heat to the environment I’m not shivering because I have more muscle mass on my limbs than him…but skin would be nice… Both inactive peripheral muscles and subcutaneous fat are excellent insulators Describe the physiological responses to exercise in the cold. We have already reviewed thermoregulation…Are there any unique responses or concerns for exercising in the cold? NO! YES!  If clothing insulation and exercise metabolism are sufficient to maintain core temperature, there shouldn’t be any problems NO! High intensity exercise Click the jacket to get creates greater tips on what to wear for metabolic heat so if cold weather exercise you are cold, just run faster!!!  Impaired muscle Muscle &/or function Metabolic Fatigue YES! Hypothermia! Prolonged eve Exercise ntua l ly Get cold Slow down There is Less heat limited generated ability of the body to Although this is an extreme case, adapt to exercise in the water does pose a cold thermal challenge— Heat loss by convection is FASTER in cold water than in cold air. Moving water speeds the heat loss through convective heat loss. Heat is lost 4x faster in water compared to air! Click the picture if you want to find out more about these crazy people! HYPOTHERMIA The lethal lower limit of body temperature is between 23 - 25°C or 73.4 - 77°F Breathing cold air does not freeze the respiratory passages or lungs when ventilation is low As cellular metabolism slows with the drop in Of course it helps to breathe core temperature, the through your nose rather than your mouth to maximize the warming hypothalamus begins effect losing the ability to regulate body Exposure to extreme cold temperature (this occurs decreases respiratory rate and at ~34.5 °C) By~29.5 °C: the volume hypothalamus has lost this ability completely Exposed skin can freeze quickly (FROSTBITE) This can lead to gangrene and loss of tissue Muscle cooling: slowed NCV àaltered Motor unit recruitmentà loss of Cardiac cooling: spontaneous force and power depolarization of the SA node decreasesà decreasing HR. SA node firing can slow to a complete stop (cardiac arrest) Describe how cold stress influences fuel utilization Epinephrine, norepinephrine and cortisol (the loss of body heat is a threat to homeostasis and so cortisol is released in response to cold stress) all stimulate lipolysis. BUT… Vasoconstriction of the arterioles to the skin reduces the availability of free fatty acids from subcutaneous fat Muscle glycogen seems to be used at a higher rate in the cold for thermogenesis