Physiology Block 3 Week 16 17 Temperature Regulation Flashcards Preview

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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

•Blood Flow
- 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
-large volume
-low flow velocity
-close proximity to body surface
-can regulate surface area for heat conduction through the skin

Arteriovenous anastomoses


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


Cutaneous Veins

Very compliant

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
sympathetic stimulation


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
Convection--air currents


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
--inhibit shivering


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


Behavorial Thermoregulation

Internal temperatures lead to feelings of overheating or cold discomfort

Only effective mechanism for heat control with severely low environmental temperatures


Temperature Increasing Mechanisms

Cutaneous Vasoconstriction
– Via stimulation of posterior hypothalamic
sympathetic centers

Ex. Nursing homes are hot because those patients feel cold (loss of cutaneous vasoconstriction)

Piloerection "goose bumps"

Increased Heat Production
– Shivering
– 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


Insulator Systems

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)

Brain lesions
--surgery in area of hypothalamus
--brain tumors


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


What is now considered a high core temperature?

Above 102 degrees is not normal

103 in adults is high

Children can spike high fevers to over 106 degrees--requires immediate ice bath to cool body down