Thirst Flashcards
How does extreme thirst affect us? (9)
• Whilst one can last an extended period of time without food, dehydration rapidly kills
– The speed of death is heavily dependent upon climatic conditions, but most people die when their water deficit is between 15-25% of bodyweight (12 litre loss)
– Strong thirst ensues at around a 2% deficit
– A fit adult can still function at 3-4% deficit
– At 5-8% severe fatigue and apathy ensue
– At 10%+ gross physical and mental deterioration are evident. Delirium may ensue, followed by coma and death
• Dehydration is problem in children and a serious (sometimes fatal) problem in the elderly
– Both may have insensitive interoception (i.e., don’t feel thirst)
– In elderly, blunted receptor sensitivity
What are the 2 kinds of thirst?
• We can define two kinds of thirst and hence of motivations to drink
– The first is homeostatic drinking (the kind we have been thinking of so far). Here the body attempts to maintain a set point, with behaviour kicking in to regulate water consumption when deviations from the set point occur
– The second is non-homeostatic drinking, such as anticipating future water needs
What is homeostatic drinking?
• There are two main types
– Eating
• If a person is water deprived they eat less. Similarly if they are food deprived they drink less
• A ratio of 1 part water to 1 part food (stomach) and 3 parts of water to 1 part food (intestine) are needed for digestive purposes
• This represents a type of set point that the body uses to maintain adequate levels of hydration
• Blood, plasma and cells
– Fluid in the body can be categorized into two compartments
• Fluid inside cells
• Fluid outside cells (blood & plasma)
– Loss of fluid from within cells can cause thirst (e.g., increased salt in blood plasma, draws water out of the cell and produces thirst)
– Loss of fluid from outside cells can also cause thirst (e.g., blood loss)
• Both represent further and separate set points
What is non homeostatic drinking? (4 types)
• Time
– Some animals prefer to drink mainly at night even if they eat in the day
• Anticipatory drinking
– Most of us drink whilst we eat
– But the need for water caused by food can not produce an immediate need for water, hence the name anticipatory drinking
– Animals show this too in topping up whilst water is available (drink while you can)
• Schedule induced polydipsia* (SIP)
– If a rat is given a food pellet at the rate of 1/min for 3 hours, the animal will consume up to 50% of its bodyweight in water if it is given free access to water during this time
– SIP can also be observed in humans playing on slot machines and experimentally too
• Polydipsia in psychiatric patients
– 6-17% of chronic psychiatric patients are polydipsic
– Of these, 70% have schizophrenia
• Some of this is delusion related (e.g., inner cleansing)
– Such patients may drink 20 litres/day
– It is hard to control and can be fatal
What mechanisms underpin homeostatic and non-homeostatic drinking?
– Peripheral mechanisms (outside the CNS)
– Central mechanisms (CNS)
– Combined mechanisms
• Even though thirst only involves one type of stimulus - water - it turns out to be rather complicated
What is the dry mouth theory?
• The dry mouth theory was first proposed by Walter Cannon. It is an expressly peripheral model
• When we are thirsty our mouth becomes dry, and, ergo, a dry mouth causes thirst
What is the evidence for the dry mouth theory? (4)
• Saliva levels in the mouth strongly correlate with levels of water deficit
• Anaesthetizing the mouth of thirsty dogs (and humans) alleviates thirst
• If water is placed directly into a thirsty animal’s stomach, the animal will still drink a roughly equivalent amount of water. The net amount drunk will exceed that drunk by an animal without
water placed in the stomach
– It is like the mouth has to feel the water and become less dry for thirst to be assuaged
• Thirsty camels will drink 30% of their bodyweight in water in 10 minutes
• That is 200 litres of water at 20L/minute!
• This is long before that water could have physiologically relieved their water deficit
• In sum, a dry mouth (and lack of) appears a good index of when to start and stop drinking
What is the evidence against the dry mouth theory? (4)
• In 1925 a man attempted suicide by cutting his own throat. He failed, but did manage to make a gaping hole in his oesophagus
– When he drank, water just spilled out the hole
– He became very very thirsty and so tried pouring water directly into his oesophagus (i.e., into his stomach by-passing his mouth) which relieved his thirst
• Animals and humans who lack salivary glands (and have a chronic dry mouth) drink normally
• It cannot account for non-homeostatic drinking
• Subjective sensation of thirst and the insula cortex
• This sensation is supported in part by the insula, but damage to the insula does not disrupt normal drinking
• Dry mouth is a signal (i.e., correlate) of, not a cause of, thirst
How can a central mechanism be responsible for thirst?
• Such an area would have to meet at least two criteria for it to be a contender
– It should collect information about water levels in the body – the information criterion
– It should be able to start and stop drinking – the control criterion
How is the hypothalamus responsible for thirst? (6)
• Information criterion - (detect water deficit)
– Specific cells in the hypothalamus are sensitive to variations in salt levels in blood plasma
– The hypothalamus reacts to other peripheral signals too (e.g., vasodilation)
– The hypothalamus receives inputs from other sensory receptors located outside of the blood brain barrier that monitor bodily systems
• Control criterion - (correct water deficit)
– Saline injected into the hypothalamus triggers drinking
– Electrical stimulation of the hypothalamus does likewise
– Lesions of the hypothalamus disrupt drinking
How does the hypothalamus control drinking?
– The hypothalamus controls release of Antidiuretic Hormone (ADH)
– ADH release, increases water retention by kidneys (conserving water) and raises BP
– ADH is released when:
– Loss of fluid from within cells (e.g., from increasing salt levels)
– Loss of fluid from blood or plasma (e.g., from falling blood pressure)
How do the kidneys affect thirst?
• Arterial blood pressure falls as water deprivation ensues and salt levels increase
– The kidney detects these changes and releases a hormone called renin
– Renin in the blood causes release of a further hormone, angiotensin
– Angiotensin then affects the CNS (drinking)
– In addition, angiotensin
• Constricts blood vessels (raising blood pressure)
• Increases release of ADH
What is the link between blood pressure and thirst?
• Note the close link between blood pressure and thirst
– African-Americans have heightened rates of high blood pressure
• It has been suggested they may be ‘super-sensitive’ to angiotensin
• This may have arisen from a selection pressure, in which only those individuals who could survive the dehydration on a slave ship (i.e. those most sensitive to angiotensin) lived to reproduce
– Dietary salt and blood pressure
• High levels of dietary sodium (i.e., salt) seems to increase blood pressure and hence risk for stroke as people age
• This is why there is pressure in many countries to reduce the amount of salt we eat, especially hidden salt in processed foods
How do we explain non homeostatic drinking? (3)
• SIP (schedule induced polydipsia)
– SIP appears maladaptive, but is not so in the environment
– If an animal is very hungry, it might continue to search for food totally disregarding ALL other needs
– Consequently, animals show a phenomenon called displacement, in which one activity can temporarily replace another
– This allows a hungry animal to engage in other useful activities (such as drinking) which might not occur if the animal could not switch from one goal to another
• Anticipatory drinking
– This often occurs when we eat
– Chemical changes that take place when we see/smell food and in the early stages of eating may trigger thirst
• Histamine is released upon the sight of food and triggers drinking
• Insulin release also triggers drinking and is released during food intake
• Learning (e.g., desert nomads)
