514
UNIT THREE
Recall that the hypothalamus receives information
from nearly all body parts, including visceral receptors, the
cerebral cortex, the reticular formation, and limbic system.
At times of stress, the hypothalamus responds to incom-
ing impulses by activating the “fight-or-flight” response.
SpeciF cally, sympathetic impulses from the hypothalamus
raise blood glucose concentration, the level of blood glycerol
and fatty acids, heart rate, blood pressure and breathing rate,
and dilate the air passages. The response also shunts blood
from the skin and digestive organs into the skeletal muscles
and increases secretion of epinephrine from the adrenal
medulla. The epinephrine, in turn, intensiF
es these sympa-
thetic responses and prolongs their effects
(f g. 13.37)
.
At the same time that sympathetic activity increases, the
hypothalamus’s release of corticotropin-releasing hormone
(CRH) stimulates the anterior pituitary gland to secrete
Types of Stress
Stressors may be physical or psychological. They may also
be a combination.
Physical stress
threatens tissues. Extreme heat or cold,
decreased oxygen concentration, infections, injuries, pro-
longed heavy exercise, and loud sounds inflict physical
stress. Unpleasant or painful sensations often accompany
physical stress.
Psychological stress
results from thoughts about real or
imagined dangers, personal losses, unpleasant social inter-
actions (or lack of social interactions), or any threatening
factors. ±eelings of anger, fear, grief, anxiety, depression,
and guilt cause psychological stress. Psychological stress
may also stem from pleasant stimuli, such as friendly social
contact, feelings of joy or happiness, or sexual arousal. The
factors that produce psychological stress vary greatly from
person to person. A situation that is stressful to one person
may not affect another, and what is stressful at one time may
not be at another time.
Responses to Stress
The hypothalamus controls response to stress, termed the
gen-
eral stress
(or
general adaptation
)
syndrome.
This response,
evoked to stress of any type, maintains homeostasis.
Sympathetic impulses
CRH released
ACTH released
Norepinephrine
released
Epinephrine and
norepinephrine
released
Cortisol released
Long-term adjustment or resistance stage
• Increase in blood concentration of amino acids.
• Increased release of fatty acids.
• Increased glucose formed from
noncarbohydrates—amino acids (from
proteins) and glycerol (from fats).
Short-term “fght or flight” or alarm stage
• Blood glucose increases.
• Blood glycerol and fatty acids increase.
• Heart rate increases.
• Blood pressure rises.
• Breathing rate increases.
• Air passages dilate.
• Pupils dilate.
• Blood flow redistributes.
Anterior pituitary
Hypothalamus
Adrenal cortex
Adrenal medulla
Neural signals
Hormonal signals
Signals from
sensory receptors
Stress
results from changes
in the external environment
FIGURE 13.37
During stress, the hypothalamus helps prepare the body for “F
ght or ±
ight” by triggering sympathetic impulses to various organs.
It also stimulates epinephrine release, intensifying the sympathetic responses. The hypothalamus also stimulates the adrenal cortex to release
cortisol, which promotes longer-term responses that resist the e²
ects of stress.
In±
ammation is the immune system’s generalized response to limit
the effects of injury or infection. However, inflammation is painful
and possibly destructive. The endocrine system keeps the immune
system in check by increasing secretion by the pituitary and adrenal
glands to temper in±
ammation. This is an example of how homeosta-
sis operates between organ systems as well as within them.
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