Endocrine System and Hormone Function

Endocrine System Fundamentals

What is Endocrinology?

Endocrinology is the science studying the function and pathology of the ductless or endocrine glands, whose secretions are called hormones.

Components of the Endocrine System

The endocrine system is a control system responsible for various bodily functions. It maintains the body's internal environment, participates in growth, sexual differentiation, and reproduction, and intervenes in reactions. Closely related chemically with the nervous system, the endocrine system functions in an integrated way. Endocrine interrelations between glands are most often governed by the hypothalamic-pituitary axis.

The main endocrine glands include the pituitary gland, thyroid gland, endocrine pancreas, parathyroid glands, placenta, testes, ovaries, adrenal glands, and pineal gland.

What are Hormones?

Hormones are chemical substances produced by specialized cells in endocrine glands. They are expressed in small doses and transported through the blood. They regulate physiological functions by acting on specific cells called target or white cells. Hormones are inactivated in the target cells, liver, or kidneys and eliminated, or their metabolites are eliminated, in urine or feces.

Hormone Classification

By Origin

• Glandular Hormones: Made by endocrine glands; act anywhere in the body.
• Tissue Hormones: Produced in various organs; act exclusively locally (e.g., gastrin, secretin, histamine, serotonin).

By Chemical Nature

• Protein Hormones: Thyroid, pancreatic.
• Steroid Hormones: Corticosuprarenal, sexual.
• Amines: Those from the adrenal medulla, neurohypophyseal, and pineal glands (nerve origin).

How Hormones Act

Hormones only act on an effector or target organ, meaning it has specific hormone receptors for each hormone. They work in two main ways:

1. Steroid Hormone Mechanism: Once the hormone binds to its receptor, it enters the interior of the target cell to its nucleus, leading to the selective activation of certain genes that will give rise to the formation of new specific proteins. This mechanism is used by steroid hormones.
2. Protein Hormone Mechanism (Second Messenger): When the hormone binds to its membrane receptor, it stimulates adenylate cyclase, which causes ATP to become cyclic AMP (cAMP). cAMP is the second messenger that enters the nucleus and gives the order. This mechanism is used by protein hormones.

Physiological Roles of Hormones

Hormones have various physiological actions:

• Morphoregulatory: Regulates the harmonious development of the body.
• Biochemical Process Regulation: Regulates both anabolic and catabolic reactions.
• Neuroregulatory: Regulates the state of excitement of the nervous system.
• Psychological Equilibrium: Influences the psyche, which needs perfect hormonal balance.
• Regulation of Vital Phenomena: Regulate various vital phenomena.

Regulating Hormone Levels

Hormonal activity is regulated through different mechanisms:

Nervous System Control

The nervous system controls the endocrine system globally through different mechanisms:

• Direct nerve impulses (e.g., on the adrenal medulla).
• Chemical substances produced through the nervous system (e.g., by the hypothalamus on the pituitary).

Feedback Mechanisms

This is the main regulatory mechanism and is very accurate. When a hormone is released into the blood, it sends direct or indirect signals to the gland that produced it, inhibiting its secretion. This ensures that there is always the necessary and sufficient quantity of hormone in the blood.

Hormone Secretion Patterns

Hormones are secreted with a particular rate of secretion for each, which is called the heart rate (or pulsatile rate). Most hormones, except those from the neurohypophyseal and adrenal medulla, are synthesized following what is known as the hypothalamic-pituitary axis.

The Hypothalamic-Pituitary Axis

Hypothalamic Hormones

These are called factors. Some are well-studied and known, while others exist but have not yet been discovered or characterized. They reach the pituitary via the hypothalamic-pituitary stalk, acting on the secretion of the adenohypophysis. They are named ending in RF (Releasing Factor/Hormone) if they enhance secretion and RH (Inhibiting Hormone) if they inhibit secretion. The neurohypophysis also secretes hormones.