The Endocrine System

The endocrine system, alongside the nervous system, plays a key role in regulating body functions, sending and receiving sensory information, and coordinating body responses. Together, they are referred to as the neuro-endocrine system.

Key Functions of the Endocrine System

• The endocrine system regulates body activities by releasing hormones into the bloodstream, which carry signals throughout the body.

• Hormonal responses can be instantaneous or may occur after several hours or days.

• Hormones have diverse and wide-ranging effects on the body.

Hormone Functions

Hormones are chemical messengers that regulate various physiological activities, ensuring proper body functioning. Their roles include:

1. Regulating the chemical composition and volume of extracellular fluid (internal environment).

2. Controlling metabolism and energy balance.

3. Regulating muscle contractions and glandular secretions, especially smooth and cardiac muscle.

4. Maintaining homeostasis, even during disruptions like infection, trauma, dehydration, starvation, hemorrhage, and temperature fluctuations.

5. Supporting immune system activities.

6. Promoting growth and development, ensuring smooth and sequential processes.

7. Facilitating reproduction, including gamete production, fertilization, embryonic nourishment, delivery, and infant care.

Exocrine and Endocrine Glands

The body contains two types of glands:

1. Exocrine Glands

• These glands secrete products into ducts, which carry substances to body cavities, the lumen of organs, or the body's surface.

• Examples: Sudoriferous (sweat) glands, sebaceous (oil) glands, mucous glands, and digestive glands.

2. Endocrine Glands

• Endocrine glands secrete hormones into the extracellular space, where they diffuse into the capillaries and are transported via the bloodstream.

• Major Endocrine Glands:

  • Pituitary gland

  • Thyroid gland

  • Parathyroid glands

  • Adrenal glands

  • Pineal gland

  • Pancreatic islets

• Other hormone-producing organs/tissues:

  • Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, small intestine, skin, heart, and placenta.

Hormones

• Hormones are chemical substances that act as messengers, influencing specific actions within target cells.

• Even in very low concentrations, hormones have powerful effects.

• The human body produces approximately 50 different hormones.

• The cells affected by specific hormones are called target cells.

• Hormones bind to receptors on target cells to initiate their actions.

The Pituitary Gland and Hypothalamus

• The pituitary gland (hypophysis) is attached to the hypothalamus at the brain's base.

• The hypothalamus serves as the primary link between the nervous system and the endocrine system.

• Together, the hypothalamus and pituitary gland regulate most aspects of growth, development, metabolism, and homeostasis.

Pituitary Gland Overview

• It is about the size of a pea, weighs 500 mg, and consists of two main parts:

  1. Anterior Pituitary (Adenohypophysis): An upgrowth of glandular tissue.

  2. Posterior Pituitary (Neurohypophysis): A downgrowth of nervous tissue connected to the hypothalamus via nerve fibers.

Blood Supply to the Pituitary

• Anterior Pituitary: Supplied indirectly via blood that passes through the hypothalamus.

• Posterior Pituitary: Supplied directly by arterial branches of the internal carotid artery.

Role of the Hypothalamus

• The hypothalamus regulates hormone release from both parts of the pituitary gland:

  • Anterior Pituitary: By producing releasing hormones (stimulate) and inhibiting hormones (suppress).

  • Posterior Pituitary: Stores and releases hormones produced in the hypothalamus.

Hormones of the Pituitary Gland

Posterior Pituitary Hormones

1. Antidiuretic Hormone (ADH): Retains body water, maintaining water balance.

2. Oxytocin (OT): Controls uterine contractions during labor and stimulates milk ejection during breastfeeding.

Anterior Pituitary Hormones

The anterior pituitary contains five principal cell types, which secrete seven major hormones:

1. Growth Hormone (GH)

   • Stimulates body growth, particularly bones and skeletal muscles.

   • Regulates metabolism in organs like the liver, pancreas, and intestines.

   • Secretion is influenced by GHRH (stimulates) and GHRIH (inhibits), and peaks during sleep and adolescence.

2. Thyroid Stimulating Hormone (TSH)

   • Stimulates thyroid gland activity and secretion of T3 (triiodothyronine) and T4 (thyroxine).

   • Regulated by Thyrotropin Releasing Hormone (TRH).

3. Adrenocorticotropic Hormone (ACTH)

   • Promotes synthesis and release of steroids from the adrenal cortex.

   • Controlled by Corticotropin Releasing Hormone (CRH).

4. Prolactin (PRL)

   • Prepares breasts for lactation and initiates milk production post-pregnancy.

   • Regulated by Prolactin Releasing Hormone (PRH) and Prolactin Inhibiting Hormone (PIH).

5. Gonadotropins

   • Follicle-Stimulating Hormone (FSH):

     • In Females: Stimulates ovarian follicle maturation and estrogen secretion.

     • In Males: Initiates spermatogenesis.

   • Luteinizing Hormone (LH):

     • In Females: Triggers ovulation, corpus luteum maintenance, and progesterone secretion.

     • In Males: Stimulates testosterone production.

6. Melanocyte-Stimulating Hormone (MSH)

   • Influences skin pigmentation.

Growth Hormone (GH)

• Stimulates cell growth and division, particularly in bones and skeletal muscles.

• Its release peaks during childhood and adolescence and is regulated by:

  • GHRH: Growth hormone-releasing hormone.

  • GHRIH (Somatostatin): Growth hormone-inhibiting hormone.

• Secretion is influenced by sleep, exercise, and low blood glucose levels.