Hunger and Thirst Flashcards
What is homeostasis?
- the process of actively maintaining internal conditions, particularly with respect to food and water availability and body temperature
What do cells require for survival?
- a viable temperature and food and water
- temperature cannot be too hot or cold
- food and water availability must be above some threshold
What happens when the body is too cold?
- basal metabolic rate increases; calories are burned to generate heat
- the body shivers, a way of burning calories to generate heat
- peripheral blood vessels constrict, moving blood to the interior of the body so less heat is lost through the skin
What happens when the body is too hot?
- animals sweat or pant like a dog (breathe heavily); water evaporation has a cooling effect
- peripheral blood vessels expand; blood moves closer to the skin so body heat can dissipate into the surrounding air
What are endotherm animals?
- warm-blooded
- temperature around 37 degrees Celsius
What are ectotherm animals?
- cold-blooded
- not very good at maintaining their body temperature
- their ability to move and function is highly dependent on the ambient temperature
What is the thermostat metaphor?
- thermostat measures temperature
- when it falls below threshold, turns on heater or when it goes above it turns off the heater
- similar to how brain works with eating and drinking
- brain monitors water and calories
- falls below level, triggers hunger and thirst
- when eat or drink, know it is coming so can relax hunger and thirst
What happens when a need becomes satisfied?
- experience relief or pleasure
What is a need state?
- when our body temperature becomes uncomfortable, consciously experience a need state
- motivating
- drive us, push us to correct the specific problem
What can motivate us?
- anticipation of pleasure can motivate us (pull us) to perform an action, even in the absence of a corresponding need
How do we lose water?
- urinating
- sweating
- breathing
When do we consciously experience thirst?
- when there is not enough water inside cells
- when there is not enough blood (liquid) in our circulatory system
What are the steps in the regulation of thirst and fluid intake?
- body loses water
- detectors signal loss of water
- drinking occurs (can be inhibition even before goes to stomach)
- stomach fills with water, sends signal to brain
- safety mechanism inhibits further drinking
OR - water is absorbed, body fluids back to normal
What is tonicity?
- the relative concentration of dissolved molecules (solutes in solution) on either side of a membrane that is permeable only to the solution, not to the solutes dissolve in it
- tonicity describes the direction solvent will flow across a membrane that is only permeable to the solvent
- the concentration of dissolved solutes
What is diffusion?
- the process by which molecules move from areas of high concentration to areas of low concentration
What is osmosis?
- the movement of a solution (solvent) from areas of high concentration (low tonicity) to areas of low concentration (high tonicity)
What is an isotonic solution?
- similar concentrations of solute on either side of the membrane
- cell will neither gain nor lose water
What is a hypotonic solution?
- solute is less concentrated outside the cell than in, so water will enter the cell
What is an hypertonic solution?
- solute is more concentrated outside the cell than in, so water will leave the cell
How much intracellular fluid is there?
- 67%
What are the parts of extracellular fluid?
- interstitial fluid
- intravascular fluid (blood plasma)
- cerebrospinal fluid
How much interstitial fluid is there?
- 26%
How much intravascular fluid is there?
- 7%
How much cerebrospinal fluid is there?
- less than 1%
What do cells need?
- cells take in salts and other solutes as needed from extracellular fluid
- across time, intracellular solute concentrations are fairly stable, while extracellular solute concentrations vary according to what we eat and drink
What happens when we drink water?
- it lowers the tonicity of extracellular fluid, causing cells to expand in size as water moves into them from the extracellular fluid
- excess water is eliminated by urine production
What happens when we consume salt?
- it increases the tonicity of extracellular fluid, causing cells to shrink in size as water moves out of them
- this physical contraction of cells triggers osmometric thirst
What is osmometric thirst?
- not enough water inside cells
- hypertonic (salty) solutions cause cellular dehydration (cells lose water and shrink in size)
What are osmoreceptors?
- neurons whose membrane potential is sensitive to the size of the cell
- release of neurotransmitter from osmoreceptors relates to the volume of these cells
What is volumetric thirst?
- when there is not enough blood circulating in the body
- people feel an intense thirst after they lose a lot of blood
What is low blood pressure?
- causes cells in the kidneys to release an enzyme called renin
- initiates a cascade of chemical reactions in the blood
What is hypovolemia?
- not enough volume of blood
- reduced flow of blood to kidneys
What does the kidney do when hypovolemia?
- release renin into blood
- renin converts angiotensinogen into angiotensin I
- angiotensin II signals not enough blood
- retention of sodium
- retention of water
- increase in blood pressure
- salt appetite
- drinking
Feelings of thirst relate to neural activity in what brain regions?
- few different regions
- particularly a hypothalamic area known as anteroventral tip of the third ventricle (the AV3V region)
Where are the neurons that are activated by feelings of thirst?
- feelings of thirst activate neurons in the AV3V region as well as anterior cingulate cortex
What does drinking do?
- quenches feelings of thirst
- some thirst related neural activity immediately dissipates upon drinking (before water reaches the relevant cells)
- AV3V neurons generally remain active until the water reaches them (long after people have stopped drinking)
What do cold sensors and sensory fibers do?
- cold sensors in the mouth and sensory fibers in the stomach are part of the rapid satiety feedback mechanism
- make mouth cold then feel less thirsty
What is the main satiety mechanism?
- may be a learned association between the act of drinking and the dissipation of thirst
What does food mostly consist of?
- sugars (carbs)
- lipids (triglycerides)
- amino acids (proteins)
What does the pancreas do?
- monitors blood glucose levels
What does the pancreas do when blood glucose is high?
- the pancreas releases insulin
What does the pancreas do when blood glucose is low?
- the pancreas releases glucagon
What does insulin do in terms of blood glucose?
- causes blood glucose to be stored as glycogen (in liver and muscle cells)
What does glucagon do in terms of blood glucose?
- causes glycogen to be broken down into glucose
What is glycogen?
- represents our short-term storage of glucose
- build up glycogen levels when we eat (when insulin is released)
- deplete glycogen levels between meals
- can store up to 2000 calories
How do cells in the brain absorb glucose?
- cells in the brain can always take in glucose (using a glucose transporter)
- glucose transporter works in the absence of insulin, so can always internalize sugar
How do cells outside the brain absorb glucose?
- use a glucose transporter that requires insulin to be functional
- use a glucose transporter that requires insulin to be functional
What do cells outside the body do in the absence of insulin?
- cells in the body cannot take in glucose
- they can only take in ketones (made from break down of fatty acids into glucose?) for energy
What does insulin do in terms of blood lipids?
- causes fatty acids to be stored as triglycerides in adipose tissue (fat cells)
What does glucagon do in terms of blood lipids?
- causes triglycerides to be broken down into fatty acids
What are triglycerides?
- represent our long-term storage of energy
What does the liver do?
- converts glycerol into sugar and fatty acids into ketones
What happens in the presence of insulin?
- all cells can use glucose for energy
- glucose is stored for later use
- when digestive system contains food (absorptive phase)
What happens in the presence of glucagon?
- glycogen is broken down into glucose for cells in the brain
- cells in the body switch to using ketones (from fatty acids) for energy
- when digestive system is empty (fasting phase)
What do cells in the liver monitor?
- glucose levels
- this information is brought to the brain by the 10th cranial nerve (the vagus)
What are some controllers of hunger?
- blood glucose levels
- stomach releases different signaling molecules when empty and when full; some reach brain and influence hunger
How is an empty stomach communicated?
- by the stomach’s release of a peptide called ghrelin
- levels of circulating ghrelin increase with hunger and fall with satiation
- exogenous administration of ghrelin increases hunger and food intake
What can reduce hunger?
- swelling of the stomach can slightly reduce, but it mostly just causes a bloated feeling
What are the peptides that are released by the stomach and intestines when food is consumed?
- the hormones CCK and GLP-1
What do CCK and GLP-1 do?
- regulate the release of digestive enzymes and insulin
- released by the intestines in proportion to the number of calories ingested
- their entry into the brain elicits feelings of satiety
What does CCK do?
- repeated administration of CCK to healthy people does not reliably cause sustained weight loss
- sometimes decreases meal size, but people typically respond by eating small meals more frequently
What does GLP-1 do?
- GLP-1 agonists have recently proven to be highly effective in reducing hunger and weight in most people
- these drugs were initially developed to boost insulin signaling in diabetics
What does the body monitor?
- blood glucose
- food in the stomach
- fat levels
Why does the body monitor fat levels?
- body wants to ensure there is enough fat to make it between meals
What happens when animals are force fed?
- become heavier than normal
- reduce food intake once it regains control over how much it eats
- body weight goes up but food intake goes down
- after stop being force fed, body weight will go back down and food intake back up until it is regulated
What is leptin?
- a circulating hormone
- secreted by adipocytes (fat cells)
- leptin levels correlate with the amount of fat in the body
- to some extent, leptin levels regulate the sensitivity of hypothalamic neurons to short-term satiety signals (CCK and GLP-1)
How do leptin levels increase?
- as fat cells grow and proliferate, leptin levels increase
What happens if leptin levels fall below some threshold?
- feel intense hunger
What does exogenous leptin administration do?
- slightly decrease meal size in healthy people, but this effect is short-lived
- lifesaver for people who are unable to produce leptin due to a genetic mutation
What is congenital leptin deficiency?
- brain thinks there is no fat in body so think they’re in feeding emergency and eat a lot
- rare in humans
What is an ob mouse?
- strain of mice whose obesity and low metabolic rate are caused by mutation that prevents production of leptin
When are emergency hunger circuits activated?
- when a specific critical need to eat or not eat overrides energy homeostasis circuitry
What are the types of emergency hunger circuits?
- glucoprivation (hypoglycemia)
- lipoprivation
What is glucoprivation (hypoglycemia)?
- dangerously low blood glucose levels can cause intense hunger
- not enough immediately available sugar in the blood
- can result from excessive insulin signaling and from drugs that inhibit glucose metabolism
What is lipoprivation?
- dangerously low levels of fat
- not enough fat on the body or free fatty acids in the blood
- can be caused by drugs that inhibit fatty acid metabolism
What does the brain do when it senses that energy stores are dangerously low?
- insulin release is suppressed, and glucagon release is triggered
- short-term satiety signals are ignored
- energy expenditure slows (basal metabolic rate), halting growth and reproductive systems
- a potent and sustained feeling of hunger takes hold
What is diabetes?
- a condition where people are either insensitive to insulin signaling or they do release enough insulin
- results in high blood glucose levels and an inability to store glucose as fat
What happens if diabetes is not treated?
- leads to intense thirst and progressive weight loss
- as fat cells become depleted, leptin levels fall, and a lipoprivation- related feeding emergency takes hold, resulting in intense hunger, even if there is tons of glucose in the blood
- often led to death before insulin treatments were discovered
What is the hypothalamus?
- key regulator of hunger
What happens in the arcuate nucleus of the hypothalamus in terms of hunger?
- two cell populations have opposing influences on hunger
- AGRP/NPY neurons
- POMC neurons
What does stimulation of one cell population (AGRP and NPY) do?
- Stimulation of one cell population – the neurons that co-release the peptides AGRP and NPY – causes dramatic overeating
- Leptin and other satiety signals inhibit these neurons
What do AGRP/NPY neurons do?
- promote hunger
- inhibited by leptin and activated by ghrelin
What do POMC neurons do?
- inhibit hunger
- activated by leptin and inhibited by ghrelin
What do the two cell population do?
- Feelings of hunger partially relate the balance of activity between these two cell populations
- project to the paraventricular nucleus (PVN) of the hypothalamus
- Some neurons in this area stop firing when the body has dangerously low levels of fat (leptin)
What does PVN activity do?
- Artificially increasing PVN neuron activity does not substantially/reliably change hunger
- the inhibition of some cells in this area can generate intense hunger
- seem to trigger a lipoprivation response
What is Prader-Willi syndrome?
- a rare chromosomal abnormality in which up to 7 genes are deleted from chromosome 15
- one of these genes is critical for the development/survival of a population of PVN neurons
What do people suffer with Prader-Willi syndrome?
- born with very low muscle mass and have little interest in eating
- develop a heightened, permanent and painful sensation of hunger
- most die of obesity- related causes
- sensations of satiety to tell them to stop eating or to throw up
- can accidentally consume enough food in a single binge to fatally rupture their stomach
What is the modern obesity epidemic?
- world is changing much faster than our genes are, and some people’s genes are not well-suited to our current food environment
- hedonic aspect to hunger
- Food can be delicious and reinforcing even when people are not hungry
Why is there variability in body fat?
- About 50% of the variability in people’s body fat is due to genetic differences
- Natural variations in metabolic efficiency are one of the most important factors
What can treat obesity?
- pharmacological treatments to control weight
- surgeries have developed that limit the amount of food that can be eaten during a meal
What is bariatric surgery?
- modifies the stomach, small intestine, or both
- most effective form is called the Roux-en-Y gastric bypass (RYGB)
What is RYGB surgery?
- the second part of small intestine (the jejunum) is cut and attached to the top of the stomach
- the stomach is also stapled to make it much smaller
- results in reductions in hunger over time
