Two Stages of Satiety
Food intake triggers a complex functional feeding system. This process is strictly divided into two sequential stages.
Primary (Sensory) Satiety This is implemented exclusively through neural mechanisms and is a rather fragile process. If a person is distracted during a meal by reading or watching television, or consumes alcohol, the normal course of this phase is disrupted. As a result, the amount of food eaten ceases to correspond to real physiological needs, which leads to a disorder in the subsequent processing of nutrients. Clinical application: some methods of body weight regulation are based on this phenomenon. If you eat frequently but in small portions, you can "fool" the feeding centers of the ventromedial hypothalamus. With this approach, the expenditure of substances from internal depots exceeds their intake from the outside, which leads to a systematic weight loss.
Secondary (Metabolic) Satiety When the sensory phase ends, the metabolic phase begins. It includes several steps:
- Processing of the food bolus by gastrointestinal tract enzymes.
- Absorption of digested elements into the bloodstream.
- Replenishment of current metabolic needs of tissues.
- Restoration of reserves in depots that were spent by the body during the primary satiety stage.
At this point, the cycle of the feeding system ends. It will resume in a few hours: the depletion of reserves will lead to the formation of "hungry blood," which will again cause a feeling of hunger and prompt the search for a new portion of food.
Classification of Digestion by Localization
Depending on where exactly the hydrolysis of food substances takes place, there are two main types of digestion.
1. Intracellular Digestion In this variant, chemical breakdown of substances is carried out after they are captured by the cell via phagocytosis or pinocytosis (for example, entry into an enterocyte). In humans, such a mechanism is characteristic not for mass food assimilation, but for the work of specialized cells: leukocytes and elements of the lymphoreticulohistiocytic system.
2. Extracellular Digestion This is the main method of nutrient processing in humans. It, in turn, is divided into two fundamentally different categories:
- Distant (Luminal/Cavitary): Enzymes are secreted as part of digestive secretions and work at a considerable distance from their site of formation—directly in the cavities of the gastrointestinal tract.
- Contact (Brush border, Membrane): This unique mechanism was discovered by Academician A.M. Ugolev. Hydrolysis is carried out by enzymes that are tightly fixed to cell membranes.
Stages of Hydrolysis in the Small Intestine
In the small intestine, the structural basis for membrane hydrolysis is the glycocalyx. The process of nutrient breakdown here is strictly structured and goes through three consecutive stages:
- Luminal stage. This represents the initial hydrolysis of complex molecules directly in the intestinal lumen. The main role here is played by enzymes secreted by the pancreas.
- Glycocalyx zone. Oligomers formed in the first stage penetrate into the structures of the glycocalyx. There they continue to be broken down by pancreatic enzymes that have adsorbed onto this surface.
- Membrane stage. This occurs in the immediate vicinity of the cell membrane, on the microvilli. At this stage, dimers are broken down into monomers. An important difference of this stage is that it is carried out by intestinal proper enzymes. Enterocytes themselves synthesize these proteins and embed them into the membranes of their microvilli.
Principles of Regulation
All the described activity of the digestive system is under strict control. The regulation of digestive processes is carried out by the coordinated work of neural and humoral mechanisms, which ensure the timely release of digestive juices and adequate motility at each stage of nutrient processing.