8.3 : Primary Motives: Hunger and Thirst

Hunger and thirst are fundamental physiological drives crucial for maintaining homeostasis and ensuring the survival of both humans and animals. These drives are regulated through complex interactions between the brain, hormones, and sensory receptors.

Hunger arises when the brain detects changes in the body's nutrient levels, including glucose, lipids, amino acids, and hormones such as ghrelin and leptin. The hypothalamus plays a central role in hunger regulation. The lateral hypothalamus acts as the hunger center, initiating the drive to eat when the body requires energy. Conversely, the ventromedial hypothalamus signals satiety, indicating when the body has consumed enough nutrients. Hormonal signals like ghrelin, which stimulates hunger, and leptin, which promotes satiety, provide feedback to the hypothalamus, ensuring energy balance. Environmental stimuli, such as the sight, smell, and taste of food, can also influence hunger by triggering the desire to eat even when the body may not need nutrients.

Thirst motivation is explained by the double-depletion hypothesis, which proposes that both cellular and extracellular dehydration drive the need to drink water. Cellular dehydration occurs when water is lost from cells, leading to an increased concentration of solutes within them. This imbalance is detected by osmoreceptors in the hypothalamus, which then stimulate mechanisms to restore water balance. Extracellular dehydration, on the other hand, involves the loss of water from the extracellular fluid, resulting in reduced blood volume and lowered blood pressure. This triggers the release of hormones like the antidiuretic hormone (ADH) from the pituitary glands and renin from the kidneys, both of which act to conserve water and promote the sensation of thirst. Together, these mechanisms help maintain proper hydration and fluid balance.