Chapter 74

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What is the temperaature of deep tissues of body (the core)?

It is relatively constant (within +- 0,6 degrees Celsius)

When can core temperature change?

Due to febrile illness

What is making the skin temperature different from core temperature?

It rises and falls with temperature of surroundings

What is the average normal core temperature considered to be?

Between 98 Fahrenheit and 98,6 Fahrenhei WHEN MEASURED ORALLY AND ABOUT 1 FAHREnheit when measured rectally

What happens with body temperature during exercise?

It increases to as much as 101-104 Fahrenheit when excessive heat is produced in the body by strenous exercise

What happens when reate of heat production in body is greater than rate at which heat is being lost?

Heat builds up in body and body temperature rises

Which are the factors of heat production?

1) basal rate of metabolism of all cells of body 2) extra rate of metabolism caused by muscle activity including muscle contractions 3) extra metabolism caused by thyroxine and growth hormone 4) extra metabolism caused by epinephrine, norepinephrine 5) extra metabolism caused by increased chemical activity in cells 6) extra metabolism needed for digestion, absorption

Where is most of heat produced in body?

Deep organs, (liver, brain and heart and skeletal muscles during exercise)

What happens with the generated heat?

It is transferred from deeper organs to skin, where it is lost to air and other surroundings

Rate at which heat is lost is determined by which factors?

1) how rapidly heat can be conducted from where it is produced in the body core to skin 2) how rapidly heat can be transferred from skin to surroundings

What is acting as a heat insulator for the body?

The skin, subcutaneous tissues and fat of subcutaneous tissues

What about fat and its conduction of heat?

It conducts heat only one third as readily as other tissues

What is the insulation beneath the skin?

It is an effective means of maintaining normal internal core temperature, even though it allows the temperature of skin to approach temperature of surroundings

How are blood vessels distributed?

They are distribuetd profusely ( a lot) beneath skin, especially important is a continuous venous plexus supplied by inflow of blood from skin capillaries

What happens in hands, feet and ears?

Blood is also supplied to the plexus (continuous venous plexus) from the small arteries through highly muscular arteriovenous anastomoses

What does a high rate of skin flow cause?

It causes heat to be conducted from core of body to skin with great efficiency

What does environmental air temperature have on conductance of heat from core to the skin surface and then conductance into the air?

There is an approximate eightfold increase in heat conductance between fully vasoconstricted state and fully vasodilated state

Which are the methods by which heat is lost from the skin to surroundings?

Radiation, conduction and evaporation

What does radiation cause?

Heat lsos in form of infreared rays

How much is total heat loss by radiation?

About 60% of total heat loss is by radiation

What about heat rays (radiation)?

Human body radiates heat rays in all directions, and heat rays are being radiated from walls of rooms and other objects toward the body

What about conduction?

Only small quantities of heat 3% are lost from body by direct conduction from surface of body to solid subjects such as chair, but to air is about 15%

What happens once temperature of air adjacent to skin equals temperature of skin?

No further loss of hat occurs in this way because now an equal maount of heat is conducted from air to body

What does convective heat loss result from?

It results from air movement

How does convection occur?

Heat from skin is conducted to air and then carried away by convection air currents (15% of heat loss occurs by this process)

What role does wind have?

It has a cooling effect (body exposed to wind leads to air adjacent to skin, and heat loss by convection increases)

What is the cooling effect of wind at low velocities?

It is about proportional to square root of wind velocity, for instance, a wind of 4 miles per hour is about twice as effective for cooling as a wind of 1 mile per hour

What is heat condutivity in water?

It is very great in comparison with that of air

What is evaporation?

It is when water evaporates from body surface, 0,58 Calorie of heat is lost for each gram of water that evaporates
Een when a person is not sweating, water still evaporates insensibly from skin and lungs

When can radiation and conduction occur?

As long as skin temperature is greater than temperature of surroundings

How can the body rid itself of heat?

By evaporation

What happens if we prevent evaporation when the surrounding temperatuer is higher than skin temperature?

It will cause internal body temperatuer to rise (occurs in humans who are born with congenital absence of wweat glands) can tolerate cold temperatures as well as normal people can

What does clothing reduce?

it reduces conductive and convective heat loss

What does clothing entrap?

It entraps air next to the skin in the weave of the cloth, thereby increasing thickness of so-called private zone of air adjacent to skin and also decreasing flow of convection air currents (heat loss is reduced)

What causes sweating?

Stimulation of anterior hypothalamus-preoptic area in brain

What are the sweat glands innervated by?

Cholinergic nerve fibers (fibers that secerte acetylcholine but that run in sympathetic nerves along with adrenergic fibers)

What can sweat glands also be stimulated by?

By epinephrine or norepinphrine circulating in blood (important during exercise)

What is the structure of sweat glands?

They have
1) coiled portion that secretes wweat
2) duct portion that passes outward through dermis and epidermis of skin

What does the secretory portion of sweat gland secrete?

It secretes primary secretion (constituents in the fluid are then modified as the fluid flows through the duct)

Primary secretion has what composition?

It is similar to that of plasma, except that it does not contain plasma proteins (concentration of sodium is 142 and chloride is 104)

What happens when the primary secretion flows through duct portion of gland?

It is modified by reabsorption of most of sodium and chloride ions (degree of reabsorption depends on rate of sweating)

What happens when sweat glands are stimulated only slightly?

Primary secretion passes through duct slowly, and all soduum and chloride ions are reabsorbed, falling to low levels

What happens when sweat glands are strongly stimulated by sympathetic nervous system?

Large amounts of primary secretion are formed, and duct reabsorbs only more than half sodium chloride

What happens when a person is unacclimatized to heat?

Significant lsos of sodium chloride occurs in the sweat, much less electrolyte loss occurs

What is associated with acclimitazation?

Better conservation of body salt, because of increased secretion of aldosterone by adrenocortical glands, which results from a slight decrease in sodium chloride concentration in extracellular fluid and plasma

What happens with an unacclimatized person who sweats profusely?

It often loses 30 grams of salt each day for the first few days
After 4 to 6 weeks of acclimatization, the loss is usally 3 to 5 g/day

What effects do atmospheric temperature have on body ''core'' temperature of a nude person?

It is increasing the body core temperature, and the temperature of body is regulated entirely by nervous feedback mechanisms, and almost all tehse mechanisms operate through temperatureregulating centers located in hypothalamus

What does anterior hypothlamic-preoptic area contain?

Large numbers of heat-sensitive neurons and cold-sensitive neurons (which are controlling the body temperature)

What is the skin endowed with?

Both cold and warmth receptors (skin has more cold receptors than warmth receptors)

Deep body temperature receptors are found where mainly?

In spinal cord, abdominal viscera and around great veins in upper abdomen and thorax

What about the deep body temperature receptors, what are they exposed to?

They are expsoed to body core temperataure rather than body surface temperature

What do deep body temperature receptors detect?

They detect mainly cold rather than warmth

What is preventing hypothermia (low body temperature)?

Skin and deep body receptors

What does body use to reduce body heat when body temperature becomes too great?

1) Vasodilation of skin blood vessels 2) Sweating 3) Decrease in heat production (mechanisms that cause excess heat production such as shivering are inhibited strongly)

What happens when the body is too cold, with the temperature control system?

It institutes opposite procedures which are
1) Skin vasoconstriction throughout body 2) Piloerection (hairs standing on end), 3) increase in thermogenesis (heat production)

What about the primary motor center for shivering in hypothalamus?

It is normally inhibited by signals from heat center in anterior hypothalamic-preoptic area but is excited by cold signals from skin and spinal cord

What activates primary motor center for shivering?

When body temperature falls below a critical temperaturer level, transmitting signals causing shivering through bilaterla tracts down the brain stem, into lateral columns of spinal cord and finally to anterior motor neurons (causing shivering)

What happens during maximum shivering?

Body heat production can rise to four to five times normal

What is chemical thermogenesis (nonshivering thermogenesis)?

When an increase in either sympathetic stimulation or circulating norepinephrine and epinephrine can increase rate of cellular metabolism

Degree of chemical thermogenesis occuring in animals is what?

It is directly proportional to amount of brown fat in animal's tissues

What is brown fat richly supplied with?

With sympathetic nerves that release norepinephrine, which stimulates mitochondrial uncoupling protein (called thermogenin) and increases thermogenesis

What does effect the intensity of chemical thermogenesis+


What happens with cooling of anterior hypothalamic-preoptic area?

It increases production of thyrotropin-relasing hormone by hypothalamus, and this hormone is carried by way of hypothalamic portal veins to anterior pituitary gland, where it stimulates secretion of thyroid-stimulating hormone

What happens if we epose animasl to extreme cold?

It can cause their thyroid glands to increase in size 20 to 40% percent

Metabolic rates increase when?

IN the arctic, indigenous people

What hapepns at a critical body core tmeperature of 37,1 Celsius?

Drastic changes occur in rates of heat loss and heat production (at temperatures baove this level rate of heat loss is greater than that of heat production) at temperatures below 37,1 Celsius makes rate of heat production greater than that of heat loss so body temperaature rises and approaches 37,1 C level, which is called set point of temperature control mechanism

What happens when internal body temperature becomes too high?

Signals from temeprature-controlling areas in brain give person a psychic sensation of being overheated, conversely, when body becomes too cold, signals from skin and from deep bodyreceptors elicit feeling of cold discomfort

What are pyrogens?

Lipopolysaccharide toxins which can cause set point of hypothalamic thermostat to rise

What happens when a brain surgeon operates in a the region of hypothalamus?

Severe fever almost always occurs, and another condition that frequently causes prolonged high temperature is compression of hypothalamus by a brain tumor