Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Physiological aspects of hunger and thirst. Appetite

Skipping meals triggers the sensation of hunger. Hunger is a visceral sensation projected in The Stomach region as a burning feeling, a "hollow ache," or pain, accompanied by hyperexcitability, salivation, and occasionally dizziness, headaches, and generalized weakness. Hunger induces specific psychological shifts and typically carries a negative emotional valence. The behavior of an individual experiencing this sensation focuses on seeking ways to satisfy this vital physiological need. As a rule, the longer hunger persists, the more intensely these symptoms manifest. Closely related to hunger is appetite, which represents the desire to eat. However, unlike hunger, appetite is generally characterized by Specificity—that is, a craving for a particular type of food. The sensation of hunger is provoked by the following factors:

✵ a drop in the concentration of glucose and other nutrients in the Blood and CEREBROSPINAL FLUID (their levels are monitored by chemoreceptors located in the Hypothalamus), a process especially accelerated by heavy physical labor demanding significant Energy Expenditure;

✵ the absence of food or chyme in the gastrointestinal tract, leading to the onset of hunger contractions (peristalsis).

These factors activate the hunger center. Specifically, a deficit in metabolic substrates for the Cells of this center directly stimulates it. The hunger center is located in the lateral hypothalamic area and is closely interconnected with the satiety center (ventromedial hypothalamus). The sensation of satiety is promoted by the following factors:

✵ stimulation of Oral Cavity receptors during the chewing and swallowing of food;

✵ gastric distension caused by food masses;

✵ irritation of gastrointestinal chemoreceptors and those monitoring blood glucose levels;

✵ an increase in bodily reserves of Proteins, fats, and CARBOHYDRATES, along with elevated body Temperature.

Pathologies involving altered appetite are quite common. An abnormally heightened desire for food is known as bulimia. Conversely, a complete lack of desire to eat is termed anorexia. Excessive food consumption that is disproportionate to metabolic rate and energy demands leads to obesity. Conversely, food deprivation results in emaciation.

The hunger and satiety centers combine to form the feeding center (or appetostat). Morphologically, it incorporates regions from various structures of the Central Nervous system (the hypothalamus, limbic system, and cortical structures). The feeding center acts as the regulator of food intake, portion size, specificity, and quality. It governs feeding behavior, the establishment of dietary routines, and food Selection.

An insufficient Water content in the body produces the sensation of thirst, which manifests as dryness in the Mouth and Pharynx, generalized weakness, and a strong urge to drink. The thirst center is also located in the hypothalamus. The onset of this sensation is linked to the activation of osmoreceptors localized in this area of the Brain, which respond to shifts in osmotic pressure. A water deficit elevates the concentration of solutes, altering the osmotic pressure between the intra- and extracellular spaces.

The sensations of hunger and thirst are general perceptions and innate drives aimed at satisfying fundamental biological needs. They are essential for maintaining Homeostasis within the body's internal environment. The hunger, thirst, and satiety centers influence the secretion of Hormones by the hypothalamohypophysial system, driving characteristic behavioral responses.

Concluding this section, a few words should be said about dietary routines and composition. Meal timing refers to the frequency and regularity of food intake, while diet refers to the Qualitative and quantitative composition of food consumed over the course of a day. For optimal gastrointestinal function, meals should be eaten at consistent times. A 3-to-4-meal schedule is generally considered most appropriate, with the largest portion of food ideally consumed at lunch. The daily diet must include proteins, fats, and carbohydrates in specific proportions (approximately 1:1:4). An essential requirement is the intake of Vitamins AND MINERALS necessary for normal physiological function.

It must be emphasized that food must be balanced in its qualitative and quantitative composition, mechanically, chemically, and thermally sparing, and of high quality. Furthermore, it should undergo appropriate culinary Processing. Boiled foods, dairy, baked goods, fruits, and berries are the most readily digestible. For normal development and vital activity, the diet must be nutritionally complete and diverse.