All glands in our body are divided into three groups: glands of external, internal and mixed secretion.
External secretion glands
Glands of external secretion or exocrine glands are called such glands that secrete their secretions along the ducts in the body cavity or on its surface.
Example: numerous glands of the digestive tract (salivary, gastric, intestinal, etc.) through the ducts remove the digestive juices formed in them into the cavity.
The endocrine system is formed by a set of interconnected glands of internal and mixed secretion.
Endocrine glands. Hormones
The endocrine glands include: pituitary gland, thyroid gland, parathyroid glands, thymus (thymus gland), adrenal glands, pineal gland.
The endocrine glands (endocrine glands) do not have excretory ducts and secrete special physiologically active substances – hormones – directly into the internal environment of the body (blood and lymph). Hormones move through the fluids of the internal environment and act on an organ or organ system.
Hormones are vital compounds synthesized in the cells of the endocrine glands and actively affecting all types of metabolic processes in living organisms.
Signs of hormones
Hormones include substances that:
- are formed in living cells;
- are not a source of energy;
- are allocated in very small quantities;
- not excreted through the ducts, but immediately enter the blood (internal environment);
- act on organs through special substances that are located in the nuclei of cells of target organs or on their outer membranes.
The effect of hormones on the work of the body is very diverse. They affect all life processes. Under their control, the internal organs function, metabolism is carried out, homeostasis is maintained, the growth and development of the organism occurs.
Hormones can act in one direction (both the thyroid hormone thyroxin and the adrenal hormone adrenaline increase blood sugar) and in the opposite direction (for example, insulin has the opposite effect on blood sugar – it lowers blood sugar).
Hormones are produced in microscopic amounts, which, however, are sufficient to keep all the work of the human body under control, carrying out humoral regulation.
Properties of hormones
High biological activity – hormones act at negligible concentrations in body fluids:
- Distant action – hormones, as a rule, regulate the metabolism and functions of cells at a considerable distance.
- Strict specificity of action – hormones serve as chemical messengers, transferring the relevant information (signal) from the central nervous system to strictly defined and highly specific target cells of the corresponding organs or tissues.
- A relatively short half-life (usually less than an hour) – as a result of this, the effective action of hormones aimed at maintaining a certain state of the body is possible only with their continuous synthesis and secretion for the entire required time.
Mixed secretion glands
There are also glands in the body, the cells of which can secrete hormones, as well as secretions that flow through the ducts to the internal organs or to the external environment. Such glands are called mixed secretion glands.
The glands of mixed secretion include: part of the pancreas, genital (testes in men and ovaries in women) and some other glands.
Example: the pancreas, in addition to the hormone insulin, which regulates blood sugar levels, produces digestive juice that is secreted into the duodenum.
In the gonads, not only sex hormones are formed, but also sex cells (eggs, sperm).