Concept and Chemical Nature
In physiology, secretion is not merely a passive release of substances, but a targeted process of synthesizing specific chemical compounds within cells followed by their extrusion. The product released by the cell as a result of this activity is called a secretory product or secretion.
Their chemical nature is extremely diverse. Depending on functional specialization, a secretion may consist of:
- protein structures and complex glycoproteins;
- various lipid classes;
- biological catalysts — enzymes;
- aqueous solutions of acids or bases;
- salt solutions.
Water is also a critical component ensuring proper consistency and physiologically adequate dissolution of the released chemical substances.
Classification of Released Substances
During their life cycle, cells release various substances. It is important to strictly distinguish them based on their origin and functional role in cellular metabolism. Three main groups are distinguished:
- Secretions. These are products of cellular anabolism (synthetic pathways). Their synthesis and subsequent release represent the primary, specific function of a secretory cell.
- Excretions. Unlike secretions, these are the end products of cellular catabolism (breakdown pathways). Essentially, these are biological "waste products" that the body no longer requires and must eliminate.
- Recretions. This is a special group of substances that the cell first takes up from the environment and then releases outward in an entirely unchanged form (without chemical transformation).
The Secretory Cycle
The formation and release of substances do not occur randomly. They follow a strict algorithm known as the secretory cycle. This process occurs sequentially and includes four mandatory stages:
- Uptake of precursors. At this initial stage, building blocks pass from the bloodstream directly into the secretory cell.
- Synthesis. Intracellular synthetic pathways are triggered, generating the secretion from the acquired precursors.
- Transport and maturation. The newly formed substance moves through the cytoplasm. During this time, final processing occurs, and the product is stored within the cell.
- Extrusion / Release. The final stage where the mature secretion is expelled from the cell (most commonly via exocytosis).
Regulation of Secretion
For a secretory cell to initiate or cease releasing its product, a regulatory signal is required. Regulation of secretion in the body occurs through two main pathways:
- Neural regulation. Mediated by the autonomic nervous system.
- Humoral regulation. Mediated by biologically active substances—specific hormones that provide chemical signals to start or inhibit product extrusion.