Chapter 5: Thirst, Hunger, Elimination Flashcards
Extracellular Thirst
Accounts for 1/3 of total water in body
Water outside cells (in blood, CSF, body cavities)
Induced by perspiration, blood loss, diarrhea, heavy menstrual bleeding
Blood volume and blood pressure decrease
Requires replacement of electrolytes and water
Extracellular Thirst Mechanisms
Drop in blood pressure lead to…
Leads to activation of baroreceptors (blood pressure receptors) in kidneys
Activation of baroreceptors triggers release of renin from kidneys
Renin is an enzyme that produces angiotensin in blood
Angiotensin causes vasoconstriction
Angiotensin causes hypothalamus and pituitary to increase production of vasopressin (anti-diuretic hormone –prevents urination)
Angiotensin causes adrenal gland to release aldosterone from adrenal cortex
Aldosterone causes kidneys to increase sodium reabsorption
Vasopressin causes kidneys to increase water reabsorption
How does angiotensin stimulate production of vasopressin
Subfornical Organ
• Located outside BBB
• Contains osmoreceptors and also responds to angiotensin
• Neurons in subfornical organ project to hypothalamus
Cellular Thirst
Accounts for 2/3 of total water in body
Water inside cells
Induced by excess salt consumption or severe thirst (dehydration)
Excess salt consumption or severe thirst leads to increased extracellular sodium
Osmotic force draws water out of cells (cell shrinks)
Extracellular thirst will occur before cellular
Concentrated saline (salt) applied around third ventricle …
induces drinking
Distilled water applied around third ventricle …
causes drinking cessation
Feeding (Absorptive) Phase
Insulin release from pancreas
Glucose from blood moves into cells, glucose–>glycogen
Energy stored
Parasympathetic
Fasting (Utilization) Phase
Glucagon release from pancreas
Glycogen stores from cells metabolized to glucose and released into blood
Liberates stored energy
Sympathetic
Ghrelin
- Secreted by epithelial cells lining empty stomach, as well as intestines and pancreas
- Ghrelin levels in blood rise during fasting
- Signals hunger & stimulates feeding
- Inhibits vagus nerve (signals parasympathetic system)
- Crosses BBB via transport system (get into hypothalamus)
Cholecystokinin (CCK)
- Released when food reaches the intestines (duodenum)
- Stimulates digestion of fat and protein
- Injecting CCK reduces feeding and food seeking behaviour
- Receptors in many areas, influence on hypothalamus directly and through vagus nerve
- Receptors in many areas of brain stimulate vagus nerve
Orexin
- Produced in the Lateral Hypothalamus
- Causes craving of food
- Increases meal size by suppressing inhibitory feedback
Neuropeptide Y (NPY)
- NPY neurons found in arcuate nucleus of hypothalamus
- NPY active during hunger
- NPY less active in well-fed state
- Stimulates hunger
- Injecting NPY into hypothalamus of rats leads to ravenous and frantic eating behaviours
- Obesity can be associated with excessive NPY
- Chronic stress and a high fat, high sugar diet are associated with excess NPY in studies of mice and monkeys
Leptin
- Secreted by adipose tissue
- Decreases food intake
- Increases metabolism
- Involved in long-term regulation of body weight and fat stores
- Does not necessarily have immediate effects on hunger and satiety
Lesion the ventromedial hypothalamus (VMH)
Hyperphagia (overeating)
Therefore, Natural role of VMH is to inhibit/reduce feeding
Lesion the lateral hypothalamus (LH)
Aphagia (failure to eat/drink)
Therefore, Natural role of LH is to stimulate/promote eating behaviour
Leptin vs Ghrelin
- For NPY & Orexin
Leptin
Inhibits NPY secreting neurons in the arcuate nucleus of the hypothalamus
Inhibits Orexin producing cells
Decreases food intake
Ghrelin
Activates NPY secreting neurons in the arcuate nucleus of the hypothalamus
Stimulates Orexin producing cells
Increases food intake
Glucose
Decreased glucose in cells stimulates hunger
Hunger and feeding correlate with low blood sugar
Glucose in hepatic portal vein reduces appetite
Liver information delivered to the hypothalamus via the vagus nerve
Blood Glucose
- Levels detected by hepatic portal vein
- Info sent to hypothalamus via vagus nerve
- Involved in hunger regulation
- NOT used as a source of energy
Intracellular Glucose
- Used as a source of energy
- Cells capable of signalling in response to low intracellular glucose
- Involved in hunger regulation (when we are hungry)
Insulin
- decrease blood glucose by increasing intracellular glucose
* decrease in intracellular glucose increases hunger
Diabetes
- increase in blood glucose
- decrease in intracellular glucose
- increase hunger
Social influences on Hunger
Herman et al. (2003)
- All female, eat 1-3 hrs before study
- Rate how hunger they are right now
- In high category, told people before them rated high hunger
- In low category, all fake data provided said they were not hungry
- Participants believed their hunger levels should match those that are around them
- Both high and low ate the same amount of cookies
Social influences on Hunger De Castro (1994)
- Individuals are not alone when eating (unlike last study)
- Participants had diet journals
- When with other people (partners/family), eat more food and faster
- When eating with friends, eat slower but still more than when alone
- Females eat more when with a male than other female friends
- No difference in males
Sweetness
Sweet foods provide rapid energy with little metabolic cost
Tongue can directly sense sweetness
Newborns (smile with sugar water, innate preference) Anencephalic infants (born w/o forebrain)--prefer sugar water over distilled water
Saltiness
Tongue can directly sense saltiness
Herbivores will travel great distances for salt; Carnivores usually obtain sufficient dietary salt; Omnivores vary depending on the environment
Salt deprivation will cause animals to drink very high salinity solutions
Avoidance
Learning to avoid sickening foods is rapid and single-trial, and does not follow conventional laws of learning
Dietary Neophobia
Animals tend to avoid foods that they have never previously eaten
Area Postrema
Controls vomiting reflex in response to toxins in food
Strong release of vasopressin from posterior pituitary can induce vomiting
