Chapter 13 - Endocrine Glands
The endocrine system is composed of glands that synthesize and secrete products, called hormones, directly into the blood rather than through a duct. Hormones are transported throughout the body where they influence only those cells that have receptors for that hormone.
Hormones can be:
- Proteins/peptides (
e.g., insulin, growth hormone)
- Lipid derivatives - derived from cholesterol or fatty acids (e.g., steroids, eicosanoids)
- Amino acid derivatives - derived from tyrosine or tryptophan (e.g., epinephrine, melatonin, thyroid hormones)
Hormone effects are described as:
- Endocrine - released into blood and act at long distances
- Paracrine - diffuse a short distance through tissue fluids to nearby cells
- Autocrine - act on the same cell that produced the hormone
Endocrine glands are highly vascular and often contain fenestrated capillaries to facilitate the diffusion of hormones into blood.
The pituitary is often called the "master gland" of the body because it produces hormones that regulate other endocrine glands, as well as, have direct effects on target tissues.
The thyroid gland produces hormones, triiodothyronine (T3) and thyroxine (T4), that primarily influence the basal metabolic rate and protein synthesis.
Parathyroid glands secrete parathyroid hormone (PTH) in response to low blood levels of calcium. PTH secretion causes the release of calcium from bones by stimulating osteoclasts, inhibition of osteoblasts, and increased reabsorption of calcium in the kidney.
This specimen is unusual in that thyroid, parathyroid gland and thymus failed to migrate to their proper locations during development.
Adrenal glands produce a variety of hormones that help regulate metabolism, blood pressure, response to stress, and other essential functions.
Pancreatic islets (or islets of Langerhans) are 'islands' of endocrine cells located within the pancreas. They secrete hormones (insulin and glucagon) important in the regulation of glucose in blood.