Flashcards in Physiology Block 3 Week 16 17 Temperature Regulation Deck (27)
Key Points about Cutaneous Circulation
•Control of Skin Blood Flow
- Storage of Blood
- Very low to approximately 1/3 of cardiac output
- Conduction of heat away from body
- Very little, if any autoregulatory control
Anatomy of Skin Circulation
Superficial to Deep
Dense system of capillary loops
Subpapillary Venous Plexus
-low flow velocity
-close proximity to body surface
-can regulate surface area for heat conduction through the skin
Neural Control of Skin Circulation
Tonic sympathetic outflow to arterioles
--tonic constrictor activity high at normal body and skin temperatures
Neural control decreases with rising core temperature and skin temperature
Sympathetic cholinergic vasodilator fibers
Large volume and blood reservoir
Dense adrenergic innervation
-rich in alpha (few beta)
Under thermoregulatory reflex control
How is the body like a radiator?
The internal body temperature remains stable despite wide changes in atmospheric temperature.
At moderately cold to very warm, the body is able to maintain a core body temperature
When it is hot, what happens to cutaneous vessels and blood flow?
Cutaneous vessels dilate allowing blood to flow to the skin, taking heat along with it to be released into the atmosphere.
Similar to when exercising.
Withdrawing sympathetic drive.
When it is cold, what happens to cutaneous vessels and blood flow?
Cutaneous vessels constrict so blood remains near the core to elevate and maintain the core temperature
Increasing sympathetic drive, vasoconstriction
How is heat produced?
Metabolic rate of the body
How is Heat Lost?
Determinants of heat loss
• Speed of conduction from deep organs to skin
• Speed of transfer to environment
Radiation--infrared heat rays: gain or loss depends on relative temperature
When you are older, what happens to the speed of your temperature adjustments?
The older you are (loss of blood vessels), the speed of temperature change is diminished
Ex. During a hot day, an old person can get heat exhaustion
--need to increase volume in order for more volume to carry heat away from core
How can you decrease temperature?
--inhibition of sympathetic vasoconstriction
--8x increase in heat loss
--evaporative water loss when core temp above set point
Decrease in heat production
How is sweat made?
Sweat gland innervated by ACh-secreting sympathetic nerve
Primary secretion is mainly protein free filtrate, but most of the electrolytes are reabsorbed in the duct, leaving a dilute, watery secretion
Removes heat via evaporative cooling
Control of Sweating
• Stimulation of preoptic area of anterior hypothalamus
• Sympathetic cholinergic fibers
• Circulating catecholamines
• Acclimatization of sweating mechanism
– 1 L/hr to 3 L/hr after 1-6 weeks in hot weather
– Increased aldosterone secretion, conserves NaCl
Internal temperatures lead to feelings of overheating or cold discomfort
Only effective mechanism for heat control with severely low environmental temperatures
Temperature Increasing Mechanisms
– Via stimulation of posterior hypothalamic
Ex. Nursing homes are hot because those patients feel cold (loss of cutaneous vasoconstriction)
Piloerection "goose bumps"
Increased Heat Production
– Chemical Thermogenesis: Brown Adipose Tissue
– Thyroxine Secretion (long term adaptation): cooling of anterior preoptic region increases TSH release
How does shivering occur?
Primary motor area for shivering
Activation of motor neurons
Muscle spindle oscillations
Fat, SubQ, Skin
--Fat conducts heat 1/3 as well, so maintains warmer internal temp
--skin temp can approach environmental temp
--traps air, decreases convective heat loss
Heat Stroke Induced by Exercise
Heat generated by exercise cannot be dissipated adequately
What can cause a fever?
--from toxic bacteria or inflamed cells/tissues
--interleukin-1 inc temp by acting on hypothalamus
--prostaglandins (constriction of smooth muscle)
--surgery in area of hypothalamus
What happens if you change the set-point of the internal temperature?
If suddenly raised the set-point of the hypothatlmic temperature controller to a high value, it would results in a gradual rise in internal body temperature, however, you would feel chills instead!
Your body is getting very hot, but your hypothalamus thinks you are cold.
--epinephrine secretion (sympathetic)
How is a fever a physiological adaptation?
Usually related to stimulation of the body's immune response
Fever can support the immune systems attempt to gain advantage over infectious agents, i.e. bacteria, viruses, fungi etc., creating an adverse environment for replication.
What is the mechanism of action of endogenous pyrogens?
Microorganism stimulates leukocytes to release endogenous pyrogens (IL-1, IL-6, TNF-alpha, lymphotoxins) which involve production of prostaglandins which stimulate hypothalamus to increase the set point, leading to fever
What is the mechanism of action of exogenous pyrogens?
Exogenous pyrogens are primarily represented by bacterial toxins or the microbes themselves
Live or Dead gram negative bacteria and live gram positive bacteria stimulate leukocytes and elicit fever when injected into the blood stream
Can only bacteria cause a fever?
Endogenous and exogenous pyrogens are not the only way to get a fever.
Double stranded RNA of specific viruses can cause fever
What is the mechanism of action of endogenous and exogenous pyrogens?
Both enter the brain and mediate release of prostaglandins (PGE2), causing the change in hypothalamic set point, resulting in fever
Why are NSAIDs taken for fevers?
NSAIDs block production of prostaglandins, which would cause an increase in the hypothalamic set point, resulting in fever