Thermoregulation Flashcards
What is normal body temperature?
Traditionally 37 C or 98.6 F
36.8 C or 98.2 F is now average with a -/+ 1 C difference
When does body temperature peak?
Between 4-6 pm
What is thermoregulation?
The balance between heat production and heat loss to maintain a steady body temperature
What are the four types of heat dissipation?
Radiation
Conduction
Convection
Evaporation
How do we produce heat?
Through metabolism!
What generally does the BMR do?
(Basal metabolic rate) is the metabolic heat needed to compensate for heat loss/gain through skin or respiration
What hormones can increase metabolic rate?
Epinephrine and thyroid hormones
What is DIT?
Dietary induced thermogenesis, result of increased metabolism
How is BMR measured?
Amount of heat given off per unit of time; at rest it depends on proportionality of body surface area
What determines the rate at which the body loses heat?
The surface to volume ratio (Surface/Volume)
- greater surface area ratio implies and increase in heat conduction from body core to the air
What is the effect of evaporation?
Cools the body
What is heat radiation?
Objects exchanging heat with the air; warmer objects loose heat to colder objects
What is conduction?
Direct transfer of heat when objects of different temperatures come into contact
What is convection?
The contact of fluid or air around a surface, facilitating conduction and evaporation
Mathematically explain conduction
(efficiency of heat movement)
J = k(Tc-Tsk)/L
J=conduction; Tc=temp core; Tsk=temp skin
What is the benefit of having low and high conductivity tissues and what are examples of each?
Low conductivity: insulates (ex: muscle and fat)
High conductivity: facilitates convection from core to skin to air (blood and sweat)
How does air convection improve heat conduction?
Convection thins the surface layer of poorly conducting air, improving the conduction of skin to air.
What are the three skin layers?
Epidermis, dermis, and hypodermis
What is circulatory convection?
Constriction/dilation of vessel layers in dermis determine the degree of convection to epidermis
- regulates dry release of conductive, convective, and radiation heat
What vessels regulate blood convection in retention or loss of heat?
A-V (arterio-venous) anastomoses among subpapillary and cutaneous blood vessels
What is the role of A-V anastomoses?
To shunt blood from arterioles to veins
- Dilation: blood shunted away from superficial capillary bed (retaining heat)
- Constriction: blood enters superficial capillaries and slows (permits conductive, convective, and radiation of heat)
What controls the look and color of the skin?
Sympathetic activity
How does circulatory convection respond to a cool environment?
- vasoconstriction and dilation of A-V anastomoses (reduce convection)
- Cooling outer shell (less heat gradient, less heat loss)
- low S/V ratio in inner core and low heat conduction from interior (conserving heat)
How does circulatory convection respond to a warm environment?
- vasodilation and constriction of A-V anastomoses (increase convection)
- warming outer shell (greater thermal gradient b/w skin and air: more heat loss)
How do venae comitantes aid in heat control?
- countercurrent temp. regulation to minimize heat loss
- when cold, heat is transfered from arteries to veins, warming the returning venous blood
How do superficial veins aid in heat control?
- dilate to release excess heat
- constrict with excess cold and shunt blood to venue comitantes via perforating branches
What are the effects of cold on vascular convection?
induces vasoconstriction of arterioles and veins, dilates AV shunts, reducing heat loss thru skin
- stimulates alpha-2 receptors producing vasoconstriction
- stimulates cold receptors to do sympathetic constriction from hypothalamus and brain stem
What is CIVD?
(Cold induced vasodilation)
- 15-30 min cold inhibits vasoconstriction to improve perfusion; persistent cycles of vasodilation and constriction are called the Lewis Hunting Response
What is Raynaud’s Disease?
Increased cold sensitivity of the alpha-2 receptors, causing vasoconstriction, ischemia and pain
What are the effects of heat on vascular convection?
- reduced thermal gradient which reduces heat loss
- a need to bring core heat to the surface (forced gradient) enhancing heat dissipation
How does heat stimulate sensory pathways for vasodilation of arterioles?
- Sympathetic withdrawal (alpha-2 receptor inhibition)
- sympathetic cholinergic vasodilation reflex (ACh, NO, VIP, histamine, prostanoids, substance P)
Explain the process of evaporation
- Eccrine glands secrete water
- Water is highly conductive, and its heat becomes latent heat of evaporation
- evaporation depends on pressure gradient, not temperature gradient
How does humidity effect evaporation?
the pressure gradient b/w skin and humid air is low, limiting evaporation
sweat drips without heat loss or evaporation
What are the types of sweat glands?
Eccrine (merocrine) and Apocrine sweat glands
- both are tubular, but apocrine is larger in mass and luminally
Describe the secretory portion of sweat glands
cuboidal or low columnar epithelia, coiled into a sphere at lower border of dermis
_______ constrict to facilitate release of sweat. These cells are between _______ and _________.
Myoepithelial cells…..epithelium…basement membrane
Eccrine sweat glands
(merocrine)
- all over body
- do merocrine exocytosis of vesicles
- hypotonic sweat, pH neutral (contains water, Na, K, Cl, urea, ammonia, lactic and uric acid)
- duct reabsorbs water and solutes via aldosterone
What innervations do eccrine glands have? in response to what?
- cholinergic responding to heat
- adrenergic sympathetic responding to emotion
What regulates the sympathetic control of eccrine sweat glands?
Anterior (preoptic) hypothalamus and medullary, also pontine nuclei in brain stem
How do sympathetic neural pathways stimulate most eccrine glands?
- muscarinic cholinergic receptors
How does adrenal medullary epinephrine stimulate eccrine glands and where?
- adrenergically in hands and soles (early stress response)
Sweating is controlled primarily and secondarily by what?
Primarily by brain temperature
Secondarily by skin temperature
Apocrine Sweat Glands
- located in axilla, external genitalia, anus
- excretion into upper hair follicle, not skin surface
- sympathetic innervation; adrenergic when responding to emotional and sensory stimuli (not heat)
Explain apocrine secretion
- apical part and contents secrete into lumen
- milky, proteinaceous, odorless
- the smell comes from bacteria breaking down the the sweat from armpits
- only activated by puberty, influenced by sex steroids
How is body temperature controlled homeostatically and by what?
(by hypothalamus)
- anterior (preoptic) and posterior nuclei regulatee autonomic control of sweating, blood flow to skin, and metabolic activity related to heat generation
- posterior hypothalamus to brain stem to preganglionic sympathetic neurons in spinal chord
What are integration sites?
anterior and preoptic regions of hypothalamus that integrate thermal/non-thermal input
What are effectors?
- posterior regions of hypothalamus that generate thermoregulatory responses
- each is stimulated at a different threshold of core temp.
Explain temperature autoregulation
- maintained internal homeostasis despite changes in ambient temperature
Short term homeostatic control is maintained by what types of mechanisms?
hypothalamic mechanisms
- specific threshold temperatures indicate when each specific effector should initiate a response
How is long term thermoregulation maintained?
By endocrines (thyroid hormone, estrogen, etc)
Explain sweating as a heat defense
- starts at a higher threshold than vasodilation
- second line of defense to increased blood flow
- higher threshold b/c water is precious and saved for greater stresses
Explain shivering as a cold defense
Only at a lower threshold than vasoconstriction
What is the interthreshold zone?
Temp range where only vascular changes regulate core temperature
- vascular responses are less noticeable here
- autonomic sweating or shivering responses can be activated here
- can be thermally or non-thermally regulated
What is Cold Uresis?
Cold inhibits ADH release and vasoconstricts peripheral vessels to increase core volume
- triggers pressure natriuresis
How do we induce increased heat production?
- shivering
- neuroendocrine thermogenesis: NE, EPI, and thyroid hormone enhance metabolic generation of heat
What is hypothermia?
Cold depressing neuronal metabolic activity, inhibiting hypothalamic heat production responses
What are the responses to heat?
- sweating, vasodilation, decrease in heat production
What is heat syncope?
Systemic vasodilation causing lightheadedness, pooling of venous blood, lowered CO and decreased perfusion
What is heat stroke?
Therrmoregulatory responses are depressed causing dizziness, nausea, vomiting, and delirium
Explain the negative feedback of temperature control
- typical type when responding to core temp change
- when ambient conditions change core temp, the latter elicits thermoregulatory responses in hypothalamus
Explain feedforward temperature control
- ambient conditions alter hypothalamic integrator activity, producing responses prior to changes in core temp
- resetting of thresholds for generating heat loss/retention
How does skin temperature alter thermoregulatory equilibrium?
Cutaneous temp receptors provide feedforward signals that coordinate heat loss/conservation responses
- brief exposure changes temp eq point, but not core temp
How do non-therman stimuli alter thermoregulatory thresholds?
- Motor activity increases core temp, which increases sweating
- Glucose receptors (some) can detect hypoglycemia, minimizing motor activity and causing shivering
How does hypoglycemia affect temp threshold?
Shifts for shivering at a lower temp level
How does dehydration affect temp threshold?
(detected by baroreceptors, osmoreceptors) raising temp threshold for sweating to preserve water
What is the effect of motion sickness at low temp?
increases vasodilation
What is fever?
Pyrogens elevating temp eq. point
- exogenous pyrogen release endogenous pyrogens, cytokines, from leukocytes and macrophages
- trigger PGE release from hypothalamic neurons
What initiates fever?
- cytokines and PGE increase eq. point
- shivering increases core temp to match eq.
What is the steady state of fever?
(plateau)
- no sense of hot or cold
- higher body temp enhances metabolic rate and immune response
What is the defervescence of fever?
(crisis)
- decreased eq. point after reduction in pyrogens
- sweating and vasodilation return core temp to normal
What effect do neuroendocrine changes in the brain have?
- altering of the threshold zone
- changes in ambient temp elicit a non-vasculatory thermo-response
What are hot flashes?
- sweating, peripheral vasodilation, heat perception
- triggered with slight increases in body temp reaching upper side of narrowed threshold zone
- zone is narrowed due to high NE release in cerebral cortex or decreased estrogen levels (reversed by clonidine and HRT, respectively)
