Homeostasis:
Your Body’s Constant
Drive to Stay in
Balance
Homeostasis
• A tendency to maintain a balanced or
constant internal state
• The regulation of any aspect of body
chemistry, such as blood glucose, around a
particular level
• Any change in levels, up or down, results
in being motivated to bring the level back
to normal.
Homeostatic Regulation
An Example
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
Homeostatic Regulation
ENERGY BALANCE
BASAL METABOLIC RATE
SET POINT
Basal Metabolic Rate
• The rate at which the body uses energy
for vital functions while at rest
• Factors that influence BMR
– Age
– Sex
– Size
– Genetics
– Food intake
There is a steep decline in the rate at which your body uses energy
for vital functions, such as heartbeat, breathing, and body heat.
Your BMR continues to decrease by about 2 to 3 percent during each
decade of adulthood.
At all points in the lifespan, women’s metabolic rate is 3 to 5 percent
lower than men’s
Basal Metabolic Rate (BMR)
• BMR = The
body’s resting
rate of energy
expenditure
• BMI = Measure
of your weight
• Want to calculate
your own Body
Mass Index?
Click here to try
Energy Balance
•Positive energy balance occurs when caloric intake exceeds
calories expended for energy. The excess glucose is converted
to body fat.
•Negative energy balance occurs when caloric intake falls short
of the calories expended for energy. Body fat stores shrink as the
reserve energy in fat cells is used
Set Point
• The point at which an individual’s
long-term “weight thermostat” is
supposedly set.
• When the body falls below this
weight, increased hunger and a lower
basal metabolic rate (BMR) may act
to restore the lost weight.
Set Point
Physiology of Hunger
Hypothalamus – The Control Center
Drives as States of the Brain
Cerebral cortex
Hypothalamus
• The hub of
many central
drive systems
lies in the
hypothalamus
Portion of
limbic system
Pituitary
gland
Brainstem
Self-Stimulation in Rats
An electrode has been placed in the lateral hypothalamus of this rat’s
brain. By accident, the animal presses the lever at the far end of the
cage, which stimulates the electrode and the animals reward system.
He presses the lever at a constant rate, neglecting to eat and drink.
Click here
to view
Hunger Drive
• Two areas of the
hypothalamus,
the lateral and
ventromedial
areas, play a
central role in
the hunger drive
Hypothalamus
Hypothalamus
Lateral Area
• Increases Hunger
• Chemical lesions
to specific cell
bodies reduce
hunger drive as
well as general
arousal
Hypothalamus
Hypothalamus
Ventromedial Area
• Decreases Hunger
• Lesions alter digestive
and metabolic
processes
• Food is converted into
fat rather than energy
molecules, causing
animal to eat much
more than normal
and gain weight
Hypothalamus
Hypothalamus
Hypothalamus & Hunger:
Remembering the Difference
• Lateral Hypothalamus = Start Eating
• Ventromedial Hypothalamus = Stop Eating
– L comes before V just like Eat comes before
Full
– L is for Lunch. V is Vomit (I’m so full I could
vomit)
Hunger Drive
A variety of stimuli act on the brain to
increase or decrease hunger:
– satiety signals from the stomach (CCK)
– signals indicating the amount of food
molecules in the blood (insulin)
– leptin, a hormone indicating the amount
of fat in the body
– internals vs. externals
Physiological changes that
predict eating (Short Term)
• About 30 minutes before you eat, you
experience…
• A slight increase in blood levels of insulin and
• A slight decrease in blood levels of glucose.
• Even if you do not eat, glucose will return to its
baseline level.
• Prior to eating, body temperature increases and
metabolism decreases.
• As the meal is consumed, this internal
physiological pattern reverses.
Glucose
• Form of sugar which circulates through the body
• Glucose is converted by the liver and stored as
glycogen
• One feels hunger when the levels become low.
Insulin
• Hormone which allows the body to
use glucose for energy or fat
production
• Secreted by the pancreas
• Controls blood levels of glucose
and promotes the uptake of
glucose by the muscles and other
body tissues
• As insulin levels increase, glucose
levels decrease.
•
Satiation signals: Sensing when to
stop
Satiation is the feeling of fullness and diminished
desire to eat that accompanies eating a meal
Satiation signals include:
1. taste buds that activate chemical signals in the
stomach and intestine to prepare for digesting food
2. stretch receptors in the stomach that communicate
sensory information to the brainstem
3. hormone cholecystokinin (CCK) that is secreted by
the intestines; it promotes satiation and reduces or
stops eating by slowing the rate at which stomach
empties
4. sensory-specific satiety - the reduced desire to
continue consuming a particular food
Long-Term Signals That Regulate
Body Weight
• Leptin is secreted by the body’s adipose (fat)
tissue, and insulin is secreted by the pancreas,
both in direct proportion to the amount of body
fat.
• Coordinated fluctuations in leptin and insulin
levels, acting on receptor sites in the
hypothalamus, influence chemical signals in the
brain like neuropeptide Y (start eating) and
corticotropin-releasing hormone (stop eating).
Long-Term Signals That Regulate
Body Weight
• During periods of negative energy balance and
weight loss, decreased leptin and insulin levels
promote the secretion of neuropeptide Y (NPY)
by the hypothalamus. NPY triggers eating
behavior, reduces body metabolism, and
promotes fat storage.
• When leptin and insulin levels increase,
corticotropin-releasing hormone (CRH) activity
increases in the hypothalamus and surrounding
areas. CRH reduces food intake and increases
body metabolism.
The Appetite Hormones
• Insulin: Hormone secreted by pancreas; controls blood
glucose.
• Leptin: Protein secreted by “bloated” fat cells; when
abundant, sends a message to “stop eating” to brain that
increases metabolism and decreases hunger.
• Orexin: Hunger - triggering hormone secreted by
hypothalamus. As glucose levels drop, orexin levels
increase and person feels hungry
• Ghrelin: Hormone secreted by empty stomach; sends "I'm
hungry" signals to the brain.
• Neuropeptide Y – Start eating hormone released by HT
• Corticotropin-Releasing Hormone (CRH): hormone in
hypothamalus that sends "I'm not hungry" signals to the
brain.
Regulating Appetite & Body Weight
• Appetite is stimulated
(+) by increased levels
of ghrelin, orexin, and
neuropeptide Y.
• Appetite is suppressed
(-) by increased levels of
leptin, insulin, CCK, and
CRH
Psychological factors that trigger
eating
• Through classical conditioning, the time of
day at which you normally eat acts as a
conditioned stimulus and elicits reflexive
internal physiological changes (the CR),
which increases your hunger
• Positive reinforcement plays a role in eating;
voluntary eating behaviors are followed by
the addition of a reinforcing stimulus—food
• Due to prior learning experiences, certain
tastes, especially sweet, salty, and fatty
tastes, hold greater positive incentive value
Variety of Factors Influence
Hunger
Environment and
Hunger
External Incentives
• Include the sight, sound, and smell of
food
Cultural Influences on Eating
• Cultural views on obesity can vary
• Eating disorders are more prevalent in the
West than in East although the more
developed a country becomes the more
people are seen to develop them.
• Culture influences the foods we like and
dislike.
Genetics vs. Environment
In countries with poorly developed economies and food supplies, the
prevalence of underfed people (BMI < 17) is higher than the
prevalence of obesity (BMI > 30).
But in countries with established or well-developed economies, such
as most Western countries, food is plentiful and easily attainable,
creating high-risk conditions for obesity.
The Obesity Epidemic
Excess Weight and Obesity
• Obesity—condition characterized by
excessive body fat and a BMI equal to or
greater than 30.0
• Overweight—condition characterized by
BMI between 25.0 and 29.9
Factors Contributing to Being
Overweight
• Highly palatable food—we eat because it tastes so
good
• SuperSize It—food portions are larger than
necessary for health
• Cafeteria Diet Effect—more food and more
variety leads us to eat more
• Positive Incentive Value – eat a certain food
because you know it will be pleasurable
• Snacking—does not cause us to eat less at dinner
• BMR—changes through the lifespan
• Sedentary lifestyles
Factors in Obesity
• Genetic susceptibility—some people are
more likely to be predisposed to obesity
• Leptin resistance—condition where higherthan-normal levels of leptin do not produce
desired physiological response (Body
ignores the high leptin levels)
• Weight cycling—repeated dieting, weight
loss and weight gain tends to result in
higher weight and reduced BMR.
We’re # 1!
Epidemic of Overweight
and Obese Americans
• The problem of
being overweight
escalates during
young and middle
adulthood.
• Males outpace
females in being
overweight in
each age group.
• However,
although the data
are not shown,
more women than
men are obese in
each age group.
Research on Weight
Regulation and Dieting
• No consistent personality trait differences
found between obese and non-obese people
(e.g., willpower, anxiety)
• Dieters and obese are more likely to eat in
response to stress than non-dieters
• Snacking between meals does help to decrease
calories – actually increases them.
• Family environment of little importance in
determining body weight; genetics plays a
large role
• Number of fat-storage cells is a major
determinant of body weight
Research on Weight
Regulation and Dieting
• Fat cells are determined by genetics and food intake
• They increase with weight gain, but merely shrink
with weight loss; may stimulate hunger
• Weight loss causes a decline in basal metabolism
Fat cells
Normal
diet
High-fat
diet
Return to
normal diet