Overview of the Endocrine System
The endocrine system coordinates functioning between different organs through hormones, which are chemicals released into the bloodstream from specific types of cells within endocrine (ductless) glands. Once in circulation, hormones affect function of the target tissues, which may be another endocrine gland or an end organ. Some hormones exert an effect on cells of the organ from which they were released (paracrine effect), some even on the same cell type (autocrine effect).
Hormones bind selectively to receptors located inside or on the surface of target cells. Receptors inside cells interact with hormones that regulate gene function (eg, corticosteroids, vitamin D , thyroid hormone). Receptors on the cell surface bind with hormones that regulate enzyme activity or affect ion channels (eg, growth hormone, thyrotropin-releasing hormone).
The pituitary and its target organs
Peripheral endocrine organ functions are controlled to varying degrees by pituitary hormones. Some functions (eg, secretion of insulin by the pancreas, primarily controlled by the blood glucose level) are controlled to a minimal extent or are independent of pituitary control (eg, secretion of parathyroid hormone Overview of Parathyroid Function There are typically four parathyroid glands. As the name implies, they are located near the thyroid gland in the neck, although the number and particularly the location are quite variable. These. read more by the parathyroid glands, primarily in response to blood calcium levels), whereas many (eg, secretion of thyroid or gonadal hormones) are controlled to a great extent. Secretion of pituitary hormones is controlled by the hypothalamus.
The interaction between the hypothalamus and pituitary (called the hypothalamic-pituitary axis) is a feedback control system. The hypothalamus receives input from virtually all other areas of the central nervous system and uses it to provide input to the pituitary. In response, the pituitary releases various hormones that stimulate certain endocrine glands throughout the body. Changes in circulating levels of hormones produced by these endocrine glands are detected by the hypothalamus, which then increases or decreases its stimulation of the pituitary to maintain homeostasis.
The hypothalamus modulates the activities of the anterior and posterior lobes of the pituitary in different ways. Neurohormones synthesized in the hypothalamus reach the anterior pituitary (adenohypophysis) through a specialized portal vascular system and regulate synthesis and release of the 6 major peptide hormones of the anterior pituitary (see figure The pituitary and its target organs The pituitary and its target organs ). These anterior pituitary hormones regulate peripheral endocrine glands (the thyroid, adrenals, and gonads) as well as growth and lactation. No direct neural connection exists between the hypothalamus and the anterior pituitary.
In contrast, the posterior pituitary (neurohypophysis) comprises axons originating from neuronal cell bodies located in the hypothalamus. These axons serve as storage sites for 2 peptide hormones, vasopressin (antidiuretic hormone) and oxytocin , synthesized in the hypothalamus; these hormones act in the periphery to regulate water balance, milk ejection, and uterine contraction.
Virtually all hormones produced by the hypothalamus and the pituitary are released in a pulsatile fashion; periods of such release are interspersed with periods of inactivity. Some hormones (eg, adrenocorticotropic hormone [ACTH], growth hormone, prolactin) have definite circadian rhythms; others (eg, luteinizing hormone and follicle-stimulating hormone during the menstrual cycle) have month-long rhythms with superimposed circadian rhythms.