Hormones: Endocrine Glands, Function, and Regulation
Endocrine Glands and Hormonal Regulation
In vertebrates, hormones regulate diverse activities such as growth, metabolic rate, nutrient utilization, and reproduction. They are crucial for regulating water balance and blood homeostasis, aiding the body in managing stress.
Hypothalamus and Pituitary Gland
The hypothalamus secretes hormones that regulate the anterior pituitary gland (adenohypophysis). These neurohormones travel through special capillaries and veins connecting the hypothalamus to the anterior lobe. Within the pituitary lobe, the veins divide into a second set of capillaries, allowing hormones to pass through the capillary walls into the adenohypophysis tissue, regulating the production and secretion of pituitary hormones.
The posterior pituitary (neurohypophysis) releases two peptide hormones: oxytocin and antidiuretic hormone. These are produced by specialized nerve cells in the hypothalamus and reach the posterior lobe via axons connecting the hypothalamus and neurohypophysis.
The adenohypophysis secretes growth hormone, prolactin, and several tropic hormones, which stimulate other endocrine glands.
Thyroid and Parathyroid Glands
The thyroid gland is located in the neck, in front of the trachea, below the larynx. It produces thyroid hormone. Two key hormones, thyroxine (T4) and triiodothyronine (T3), are synthesized from tyrosine and iodine. Thyroxine has four iodine atoms per molecule, while T3 has three.
Thyroid hormone secretion is mainly regulated by a negative feedback system between the adenohypophysis and the thyroid gland. Excess thyroid hormone in the blood also affects the hypothalamus, inhibiting the secretion of TSH-releasing hormone.
The parathyroid glands are embedded in the connective tissue surrounding the thyroid. They secrete parathyroid hormone, which regulates calcium concentration in the blood and tissue fluid.
Pancreas
Besides secreting digestive enzymes, the pancreas is an important endocrine gland. Its hormones, insulin and glucagon, are secreted by cells in small clusters or islands scattered throughout the pancreas, known as the islets of Langerhans. Beta cells secrete insulin, and alpha cells secrete glucagon.
Insulin reduces blood glucose concentration, while glucagon acts as an antagonist to insulin. Glucagon and insulin secretion are directly controlled by blood glucose concentration. After a meal, when glucose levels rise, beta cells increase insulin secretion. As cells extract glucose from the blood, reducing its concentration, insulin secretion decreases accordingly. Therefore, insulin and glucagon maintain blood glucose levels within normal ranges.
Adrenal Glands
The adrenal glands are small masses of tissue located near the upper ends of the kidneys. Each gland has a central part, the adrenal medulla, and an outer portion, the adrenal cortex. Both secrete hormones that regulate metabolism and support the body in handling stress.
The adrenal medulla secretes two hormones: epinephrine and norepinephrine. This secretion is controlled by sympathetic nerves. These hormones are chemically similar, belonging to the catecholamine group (derived from amino acids).
The adrenal cortex releases steroid hormones synthesized from cholesterol.