3.2 Primary motives THIRST

3.2 Primary motives THIRST

Thirst - physiology

• Our bodies are mostly water-about two-thirds.

• When our water volume falls by about 2% we feel thirsty.

• Dehydratation does not occur until the person loses 3% of water.

Thirst

• consciously experienced motivational state

• perform behaviour to replenish water deficit

• arise from low homeostatic level of water

Double-depletion model

• thirst can be triggered by two different kinds of body fluid deficits →

• intracellular dehydration

• extracellular dehydration

Extracellular fluids

• 20% of body weight

• volumetric thirst (fluid los, low blood or plasma volume)

• arise from bleeding or vomiting 

Intracellular fluid

• 40% of body weight

• Osmomentric thirst (high solute concentration in blood)

• arise from sweating, urination, breathing, sneezing.

Osmometric thirst

• The primary cause of thirst activation

• Thirst comes mostly from dehydrated cells.

Thirst Activation

• When body water levels are low

• Main control center: Hypothalamus

  • Monitors intracellular shrinkage (cells lose water).

  • Detects when body fluids are concentrated.

Two Main Responses Triggered by the Hypothalamus

• Hormonal response 

• Behavioural response 

Hypothalamus

• monitors intracelular shrinkage

Alcohol lead to → 

inhibit hypothalamus to produce hormones → leading to dehydration 

thirst satiety 

• occurs only when water reach the cells. 

Hormonal response

• Hormone involved: Vasopressin (ADH – antidiuretic hormone)

• Target organ: Kidneys

• Effect:

  • Kidneys conserve water by reducing urine output.

  • Helps restore internal water balance.

Behavioural response

• Trigger: Activation of thirst sensation.

• Effect: Leads to drinking behavior, which replenishes body fluids.

Thirst Satiety (When thirst is reduced after drinking)

• Prevent overdrinking by turning off the thirst drive.

1. Weak Thirst Inhibitory Mechanism

2. Strong Thirst Inhibitory Mechanism

Weak Thirst Inhibitory Mechanisms

• act quickly but temporarily

• Mouth: Wetting the mouth signals the brain that fluid intake has started.

• Stomach: Stretch receptors detect fluid entering, giving early satiety signals.

Strong Thirst Inhibitory Mechanism

• Acts more slowly but is long-lasting

• Cellular negative feedback mechanism:

• Once water reaches the bloodstream and cell hydration is restored, osmoreceptors signal that the body’s water needs are met.

• Fully stops thirst.

ENVIRONMENTAL INFLUENCES of drinking

• Taste

• Flavoured water → drinking behaviour changes based on the incentive value of the fluid.

Drinking occurs for three reasons

• replenishment of water

• sweet taste

• attraction or addiction to the substance in the water

amount of water you need depends on several factors

• body size & composition

• activity level

• temperature & humidity

• diet (high salt, caffein or alcohol increase fluid needs).