Physiology of digestion Lecture 1
Plan:
1. Physiological bases of hunger and satiation.
2. Digestion in oral cavity.
a. Salivation.
b. Structure and properties of saliva.
c. Regulation of salivation.
3. Digestion in stomach.
a. Secretory activity of stomach.
b. Structure and properties of gastric juice.
Human organism during vital activity spends various materials and great deal of energy. Substances recovering plastic and energy demands of an organism should come from an outer .environment. Obviously insufficient supply with nutrients affects homeostasis and is incompatible with life. At the same time human organism is not capable to assimilate proteins, fats, carbohydrates and number of other substances from food without pretreatment. This major function in an organism is carried out by digestive system.
Digestion is the physical and chemical processing of food.
1. Physiological bases of hunger and satiation.
Hunger as physiological state (as opposite to starvation as pathological process) is an expression of needs of the organism in nutrients which it has been deprived for some time that resulted in decrease of the amount of these substances in depot and circulating blood.
Subjective manifestation of hunger are unpleasant sensations: "burning sensations", sinking sensation in the pit of the stomach, nausea, sometimes - giddiness, a headache, sense of general weakness.
External objective manifestation of hunger is the behavioral reaction of search for food directed to elimination of the reason caused hunger.
Subjective and objective manifestations of hunger are caused by excitation of neurons of various departments and levels of CNS. Academician I.P.Pavlov named the whole of these neurons the food center. Its functions are formation of food behavior directed to search and intake of food, and also regulation and functional integration of organs of digestive system.
Food center is a compound hypothalamolimbic-reticulocortical complex. The leading part which activates all food center are lateral nuclei of hypothalamus. At destruction of this nucleus there appears denial of food (aphagia), their stimulation leads to strengthened consumption of food (hyperphagia). The specified part of food center is the center of hunger.
Destruction of ventromedial nuclei of hypothalamus leads to hyperphagia and their stimulation - to aphagia. This part of food center is the center of satiation.
Hypothalamic nuclei of the food center are excited or inhibited depending on structure of blood and also entering of various signals from various peripheral receptors.
Theories of originating of sense of hunger:
Glucostatic theory according to which the sensation of hunger is connected with the decrease of glucose volume in blood.
Aminoacidostatic theory according to which excitability of neurons of food center is determined by the amino acids volume in blood.
Lipostatic theory considers that the irritant of hypothalamic nuclei is insufficiency of metabolites formed at mobilization of fat.
Thermostatic theory assumes suppression of food center due to rise in temperature of blood washing it which happens during eating.
Hydrostatic theory binds sense of hunger with water resources of the organism: decrease of water deposit in organism reduces consumption of food.
Metabolic theory unites all mentioned.
Eating produces state of satiation opposite to hunger. It precedes coming of nutrients digestion products into blood. Such satiation is called sensory (initial) satiation. It consists in inhibition of food center and has the complex reflex nature. The sensory satiation is replaced with metabolic (secondary, or true) satiation whose basic mechanism is coming of nutrients digestion products into blood.
However, for nutrients to come into blood to be utilized by the organism, food should pass through the complex mechanical and chemical process in gastrointestinal tract.