Chapter 9: Homeostasis

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Homeostasis

Relatively stable internal environment optimized for cellular activity

Factors of homeostasis

Acidity, salt, water, oxygenation, temperature, energy availability

Example of homeostasis

Thermoregulation

Mammals are endotherms, meaning we make hear inside our bodies using metabolism and muscular activity; there is a dedicated, precisely regulated system for creating warmth

Set point

Desired homeostatic value (98.6°F or 37°C)

Negative feedback

Deviation leads to compensatory action of the system

Restoring the set point turns off the response

Redundancy

Each system has multiple mechanisms for monitoring stores, conserving remaining supplies, obtaining new resources, and shedding excesses

Example: pre-optic and lateral areas of the hypothalamus controlling temp

Behavioral Compensation

Behaviors that change the exposure of the body surface, external insulation, and/or surroundings

Allostasis

Dynamic process whereby the system shifts responses depending on prior experience and the environment

DSM-identified eating disorders

Anorexia Nervosa, Bulimia Nervosa, Binge-Eating Disorder

Anorexia Nervosa

Voluntary self-starvation despite more response to the presence of food than controls

Complex combination of genetic, endocrine, personality, cognitive, and environmental factors

Similar symptoms to Bulimia Nervosa, often difficult to distinguish

Bulimia Nervosa

Bingeing and purging

Similar symptoms to Anorexia Nervosa, often difficult to distinguish

Binge-Eating Disorder

Tendency to gorge oneself way past the point of fullness

Causes not well understood but thought to involve over activation of dopamine reward mechanisms

Obesity

Determined based on Body Mass Index (BMI)

Body Mass Index (BMI) calculation

kg/mxm OR 703(lb/inxin)

Starvation BMI category

Under BMI 15

Underweight BMI category

BMI 15 to 18.5

Normal BMI category

BMI 18.5 to 25

Overweight BMI category

BMI 25 to 30

Obese BMI category

BMI 30 or higher

Weight loss treatment

Is often extremely difficult; minimal success rates

Should ideally involve both diet and exercise

Requires permanent, substantial lifestyle changes; minimal evidence of long term success

Utility of drugs/supplements or surgery; strictly complementary to lifestyle changes

How can drugs promote weight loss?

Decreasing appetite, increasing metabolism, inhibition of fat tissue, blocking absorption of calories, reducing reward/cravings for specific foods, decreasing meal size (make you feel fuller faster)

To perform these functions, drugs may act in the CNS or the digestive tract

Health at Every Size (HAES)

Weight loss doesn’t have to be a goal; instead, the idea is to support people in adopting healthy habits for the sake of health and well-being (rather than weight control)

Recognizes that health and fitness can exist at all sizes

Hypothalamus

Complex effects on ingestive behavior

Regulation of energy balance; regulation of metabolism; motor behaviors

Depends critically on interconnections with other brain regions

Two regions of hypothalamus implicated

Ventromedial hypothalamus (VMH) and Lateral hypothalamus (LH)

Two types of peptide hormones

Satiety peptides and Hunger peptides

Satiety peptides

Decreases appetite

Cholecystokinin (CCK), α-MSH, Somatostatin, Leptin, PYY_3-36, GLP-1

Hunger peptides

Increase appetite

Neuropeptide Y, Galanin, Orexin-A, Ghrelin

Basal metabolism

The majority of food energy is spent on things like heat, cellular activity, and maintenance of membrane potentials

Energy metabolism is under strict homeostatic control and can be adjusted

Short term storage

Insulin converts glucose to glycogen to store as reserve fuel

Glucagon converts glycogen back into glucose when stores are low

Long term storage

Insulin triggers lipid (far) storage in adipose tissues, liver, and muscles

Glucagon produces free fatty acids and ketones

Three phases of energy metabolism

Cephalic phase, Absorptive phase, Fasting phase

Cephalic phase

Preparatory phase, which is initiated by the sight, smell, or expectation of food

Insulin levels high; glucagon levels low

Absorptive phase

Nutrients from a meal meeting the body’s immediate energy requirements, with the excess being stored

Insulin levels high; glucagon levels low

Insulin levels high; glucagon levels low

Promotes utilization of blood glucose as a source of energy

Promotes conversion of excess glucose to glycogen and fat

Promotes conversion of amino acids to proteins

Promotes storage of glycogen in liver and muscle, fat in adipose tissue, and protein in muscle

Inhibits conversion of glycogen, fat, and protein into directly utilizable fuels (glucose, free fatty acids, and ketones)

Fasting phase

Energy being withdrawn from stores to meet the body’s immediate needs

Glucagon levels high; insulin levels low

Glucagon levels high; insulin levels low

Promotes conversion of fats to free fatty acids and the utilization of free fatty acids as a source of energy

Promotes conversion of glycogen to glucose, free fatty acids to ketones, and protein to glucose

Inhibits utilization of glucose by the body but not by the brain

Inhibits conversion of glucose to glycogen and fat, and amino acids to protein

Inhibits storage of fat in adipose tissue

Insulin

Required by glucose transporters in the body to allow use of glucose in the body

Diabetes Mellitus

Lack of insulin production or reduced sensitivity to insulin

Theories of hunger

Set point theory and Positive-incentive theory

Set point theory assumption

Hunger is a response to an energy need; we eat to maintain an energy set point

Set point theory problems

Not everyone is the same size

It doesn’t fit evolutionary pressures; energy storage necessary for survival

Reductions in blood glucose or body fat do not reliably induce eating

It doesn’t account for the influence of external factors on eating and hunger (taste, learning, social cues)

Positive incentive theory

We are drawn to eat by the anticipated pleasure of eating

We have evolved to “crave” food

Eating has a positive-incentive value; multiple factors interact to determine this

Accounts for the impact of external factors on eating behavior

Water Balance

A precise balance of fluids and dissolved salts surround all of the cells of the body to allow proper functioning; approximately like diluted seawater

Important to maintain a decent ratio of water in the intracellular compartment vs the extracellular compartments; happens via diffusion and osmosis

Diffusion

Passive spread of the salt components (ions) to evenly space them in the water

Osmosis

Movement of water molecules to equalize the concentration of two solutions on either side of a semi-permeable membrane

Two types of water imbalance (thirst)

Osmotic thirst and Hypovolemic thirst

Osmotic thirst

Volume of extracellular fluid is slightly decreased due to obligatory losses

Concentration of salt in the extracellular fluid increases

Results in water being drawn out of the cells

Detected by osmosensory neurons

Hypovolemic thirst

Caused by a more significant decrease in the overall volume of extracellular fluid

Due to vomiting, hemorrhage, or sustained diarrhea; detected by baroreceptors

Doesn’t change concentration, just volume