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Endocrine Glands (Glands of Internal Secretion)

• The body uses chemical messengers called "Chemical Coordinators" to coordinate functions.
• These coordinators, also known as "Hormones," are substances released into the body's fluids (blood) from cells. They regulate the internal environment.
• "Humoral Substances" are a broader category, encompassing various chemicals that can sometimes be difficult to distinguish from hormones.

Hormones

• Hormones are produced in specific areas of the body, by specialized glands or cells.
• They enter the bloodstream and have effects in other parts of the body. The effect is not in the same area as the hormone's creation.
• Hormones do not directly release energy.
• They play a critical role, but in small quantities.

Hormone Action

• Hormones can be either stimulatory (activating) or inhibitory (suppressing) depending on factors like the amount of hormone and the tissue being affected.
• The presence of other hormones or vitamins also plays a role.
• The exact mechanism of hormone action on tissues isn't fully understood, but a connection exists between hormones, enzymes, and vitamins.
• Optimal hormone levels result in a healthy and active life.
• Insufficient or excessive hormone levels cause a variety of health issues.

Example Hormone (Thyroid Hormone)

• Three states of thyroid hormone (thyroxin) action are recognized:
  • Euthyroidism: Adequate hormone levels, functioning normally
  • Hypothyroidism: Insufficient hormone levels, causing various symptoms
  • Hyperthyroidism: Excessive hormone levels, causing various symptoms

Hormone Structure

• Some hormones are proteins (e.g., insulin).
• Others are steroids (e.g., hormones from the adrenal cortex and sex hormones).
• A third group consists of derivatives of phenols (e.g., adrenaline from the adrenal medulla).
• Isolation of hormones from natural sources often yields only small amounts.

Hormone Preparation

• Scientists create synthetic hormone analogs, often superior to natural hormones in their potency.

Types of Endocrine Glands (in Vertebrates)

• Two main classifications exist:
  • True Endocrine Glands: Secrete solely hormones; examples include the thyroid, parathyroid, adrenal, and pituitary glands.
  • Other Endocrine Glands: Secrete hormones and perform other functions; examples include gonads (testes and ovaries), pancreas, lining of the stomach and intestines, thymus gland, kidneys, placenta, hypothalamus, and pineal body.
• The specific locations, microscopic structure, and chemical composition of hormones within these glands vary.
• Blood regulates endocrine activity.

Pituitary Gland (Master Gland)

• This is the main regulator of other endocrine glands.
• Divided into three lobes: anterior, posterior, and intermediate.

Anterior Pituitary Hormones

• Thyroid Stimulating Hormone (TSH): Stimulates the thyroid to produce thyroxine, regulating metabolism.
• Growth Hormone (GH): Regulates growth in children and adolescents.
• Prolactin (PRL): Stimulates milk production.
• Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex.
• Follicle-Stimulating Hormone (FSH): Stimulates the growth of ovarian follicles and sperm production.
• Luteinizing Hormone (LH): Stimulates ovulation and testosterone production.

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH) or Vasopressin: Increases water reabsorption in the kidneys and regulates blood pressure.
• Oxytocin: Stimulates uterine contractions during childbirth and milk ejection during breastfeeding.

Thyroid Gland

• Produces thyroxine, vital for proper metabolism.
• Deficiency can lead to intellectual disability, stunted growth, reduced fertility.

Parathyroid Glands

• Located on the thyroid, regulating calcium and phosphate levels.
• Insufficient levels can cause muscle contractions and spasms.

Adrenal Glands

• Located above the kidneys, composed of a cortex and medulla.

Adrenal Cortex

• Aldosterone: Regulates sodium and water balance. Crucial for blood pressure.
• Cortisol: Regulates metabolism and stress response. Crucial for use of proteins, carbohydrates, and fats.
• Sex Hormones: (Androgens and Estrogens). Contribute to secondary sex characteristics.

Adrenal Medulla

• Produces adrenaline and noradrenaline which help manage the body's stress response.

Pancreas

• Exocrine gland, producing digestive enzymes.
• Endocrine functions:
  • Glucagon: Converts glycogen to glucose and increases sugar levels in the blood.
  • Insulin: Converts glucose to glycogen and lowers sugar levels; vital for proper metabolism.
  • Somatostatin: Regulates both glucagon and insulin levels.

Gonads (Ovaries and Testes)

• Ovaries:Produce female sex hormones (estrogen and progesterone), essential for reproductive function and growth of secondary sex characteristics.
• Testes:Produce the male sex hormone (testosterone) responsible for male development and secondary sex characteristics.

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