Endocrine System Overview
Hormones:
- Secreted by specialized cells (usually epithelial cells)
- Transported via the blood
- Bind with receptors on other tissues
- Initiate physiological responses
- Responsible for long-term regulation of: metabolism, growth, development, reproduction, and the internal environment (i.e., temperature)
Key endocrine structures and examples of their products:
- Synthesizes anti-diuretic hormone (ADH, aka, vasopressin), corticotropin-releasing hormone (CRH), and gonadotropin-releasing hormone (GnRH), among others
Pituitary gland
- Posterior lobe of the pituitary gland does not synthesize hormones, it release antidiuretic hormone, which, as we've indicated, is synthesized by the hypothalamus
- Anterior lobe synthesizes adrenocorticotropic hormone (ACTH, aka, adrenocorticotropic hormone), growth hormone (GH), thyroid-stimulating hormone (TSH), and the gonadotropins (follicle-stimulating hormone and luteinizing hormone)
- Produces the thyroid hormones, T3 and T4, as well as calcitonin.
- Clinical correlation: enlarged thyroid gland is called goiter.
Parathyroid glands
- Secrete parathyroid hormone (PTH)
Heart
- Specialized cells within it secrete atrial natriuretic hormone (aka, atriopeptin); this hormone is secreted in response to atrial stretching to increase sodium and water excretion in the kidneys
Kidneys
- Secrete erythropoietin and renin; erythropoietin stimulates red blood cell production, and renin aids in blood pressure regulation
- Cortex: cortisol
- Medulla: catecholamines (norepinephrine and epinephrine)
- Specialized cells throughout the gastrointestinal tract secrete hormones that aid in digestion and metabolism; gastrin and secretion are two examples
- Pancreatic islets secrete insulin and glucagon, which are hormones that regulate blood glucose levels
Gonads
- Produce large quantities of the reproductive hormones; the testes are a major source of the androgens, particularly testosterone, in males, and the ovaries are a major source of progestins and estrogens
Hormone Classes:
Peptides and proteins
- Vary in size
- Are derived from amino acids
- Are synthesized throughout the body
- Examples: insulin (which comes from the pancreatic islets), growth hormone (from the anterior pituitary gland), and parathyroid hormone
Steroid hormones
- Are derived from cholesterol, which is present in food and produced endogenously
- Steroid hormones are primarily produced in the adrenal cortex and gonads
- Examples: cortisol and the reproductive hormones
Amines
- Are derived from tyrosine in the adrenal medulla and thyroid gland.
- Include the: catecholamines (epinephrine, aka, adrenaline, and, norepinephrine, aka, noradrenaline, and, T3 and T4).
Regulatory mechanisms that control the secretion of hormones and receptor response.
Hormonal feedback mechanisms are physiological processes that are influenced by their own products:
- In positive feedback, the hormonal pathway is reinforced by its own products to ensure additional secretion. This type of feedback is relatively rare.
- For example, past a certain threshold, circulating estrogen triggers events that ultimately increase estrogen release from the ovaries
- In negative feedback loops, physiological products inhibit the hormonal pathway to halt further secretion
- For example, testosterone limits its own secretion via its effects on the hypothalamus and pituitary gland
Receptor Regulation:
- Hormones can also influence their effects at target tissues by altering the number or affinity of receptors to regulate receptor response:
- In up-regulation, the presence of a hormone increases receptor response
- In down-regulation, receptor response decreases