Endocrine System Overview: Glands, Hormones, and Functions
Endocrine System Overview
Hypothalamus
Located in the brain below the thalamus, the hypothalamus acts as the main integration center between the nervous and endocrine systems. It controls the autonomic nervous system and regulates vital functions such as body temperature, thirst, hunger, and emotional responses like fear and anger.
Pituitary Gland
A pea-sized structure (1.5 cm in diameter) situated in the sella turcica of the sphenoid bone, the pituitary gland connects to the hypothalamus via the infundibulum. It comprises two main parts:
Adenohypophysis (Anterior Pituitary)
Its action is stimulated by releasing and inhibiting hormones from the hypothalamus, establishing a crucial link between the nervous and endocrine systems. These hypothalamic hormones reach the adenohypophysis through a portal system (a network of capillaries connecting two regions without passing through the heart) via the superior hypophyseal arteries.
Growth Hormone
Produced by somatotrophic cells, growth hormone acts indirectly by promoting the synthesis and secretion of insulin-like growth factors (IGFs) in the liver, muscle, bone, and cartilage. IGFs are released into the blood or act locally (paracrine action). In children and adolescents, growth hormone increases the growth rate of bones and muscles. In adults, it helps maintain bone and muscle mass, promotes wound healing, and tissue repair. This hormone is primarily released during sleep.
- Hypersecretion (especially during childhood): Pituitary gigantism
- Hyposecretion in childhood: Pituitary dwarfism
- Hypersecretion in adulthood: Acromegaly
Neurohypophysis (Posterior Pituitary)
The neurohypophysis stores and releases two hormones synthesized in the hypothalamus: oxytocin and antidiuretic hormone (ADH). These hormones are produced in hypothalamic neurons and transported along axons to the neurohypophysis for release via exocytosis.
Oxytocin
Oxytocin acts on two target tissues during childbirth: the uterus and breasts. It promotes uterine contractions and milk ejection in response to nipple stimulation.
Antidiuretic Hormone (ADH)
ADH decreases urine production by causing the kidneys to reabsorb more water into the blood. It also reduces water loss by inhibiting sweating and causes arteriolar contraction, increasing blood pressure. Hyposecretion of ADH leads to diabetes insipidus. Factors like stress, pain, trauma, anxiety, and morphine can stimulate ADH release, while ethanol and anesthetics inhibit it, increasing urine output. The resulting dehydration can cause thirst and headaches (hangover).
Pineal Gland
Located in the brain, on the roof of the third ventricle, the pineal gland weighs between 0.1 and 0.2 grams. It secretes melatonin, a hormone released in darkness that induces sleepiness and contributes to the regulation of the biological clock (circadian rhythm).
Thyroid Gland
The thyroid gland, weighing approximately 30 grams, consists of two lobes connected by an isthmus and has abundant blood flow. Its main functions include:
- Regulating oxygen consumption and basal metabolic rate (increases during fasting by using oxygen for ATP production and heat generation)
- Stimulating protein synthesis and increasing glucose use in ATP production
- Increasing lipolysis and enhancing the action of catecholamines
- Influencing growth and development by accelerating growth and sexual maturation
Thyroid conditions include:
- Goiter: Enlargement of the thyroid gland
- Hyperthyroidism: Overactive thyroid
- Hypothyroidism: Underactive thyroid
Parathyroid Glands
These small, rounded masses are attached to the back of the thyroid’s lateral lobes. They produce parathyroid hormone (PTH), which stimulates osteoclast activity (bone resorption), releasing calcium and phosphate ions into the blood. PTH also promotes kidney reabsorption of calcium and magnesium, returning them to the blood. It has an antagonistic effect with calcitonin.
Adrenal Glands
Located atop the kidneys, the adrenal glands are flattened structures, 3-5 cm high, 1 cm thick, and 2-3 cm wide, weighing 3.5-5 grams. They have two main parts: the adrenal cortex and the adrenal medulla.
Adrenal Cortex
The adrenal cortex constitutes 80-90% of the gland’s weight and produces steroid hormones. It has three zones:
- Zona glomerulosa: Secretes mineralocorticoids (regulate sodium and potassium homeostasis)
- Zona fasciculata: Secretes glucocorticoids (regulate glucose homeostasis)
- Zona reticularis: Synthesizes androgens (steroids with masculinizing effects)
Adrenal Medulla
Located centrally and derived from ectoderm, the adrenal medulla produces catecholamines (adrenaline and noradrenaline). These hormones are released under stress, increasing heart rate, blood pressure, dilating airways, and raising blood glucose concentration.
Pancreas
The pancreas is a mixed gland with both exocrine and endocrine functions. It’s a flattened organ, 12.5-15 cm long, located behind and below the stomach. It has a head, neck, body, and tail. Its exocrine portion produces pancreatic juice, while its endocrine portion consists of the islets of Langerhans.
Islets of Langerhans
The islets contain four cell types:
- Alpha cells (20%): Secrete glucagon
- Beta cells (70%): Produce insulin
- Delta cells (5%): Secrete somatostatin (inhibits growth hormone)
- F cells: Secrete pancreatic polypeptide
Gonads
Ovaries
Ovaries produce female sex hormones: estrogen and progesterone. Follicle-stimulating hormone (FSH) from the pituitary stimulates follicle growth and development, while luteinizing hormone (LH) stimulates ovulation.
Testes
Testes produce testosterone, the primary androgen. This hormone regulates sperm production and stimulates the development and maintenance of secondary sexual characteristics. They are stimulated by FSH from the pituitary.