The endocrine system consists of cells, glands and tissues that secrete
hormones into the bloodstream that affect physiological and behavioral function and
activities [1]. This is in contrast to the exocrine system that secretes substances into
ducts. The hypothalamus connects the nervous and endocrine systems to each other
through the pituitary gland, or hypophysis. It helps control body temperature, hunger,
parenting and attachment behavior, thirst, fatigue, sleep, circadian rhythms and other
activities of the autonomic nervous system. Darkness causes the pineal gland to secrete
N-acetyl-5-methoxytryptamine, which is better known as melatonin. Melatonin is part
of the system that regulates the sleep-wake cycle. It causes drowsiness and lowers the
body temperature. The anterior lobe of the pituitary gland secretes growth hormone
(GH), beta-endorphin, luteinizing hormone (LH), follicle stimulating hormone (FSH),
melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH),
thyroid stimulating hormone (TSH) and prolactin (PRL). The posterior lobe stores
vasopressin, also known as antidiuretic hormone (ADH) and oxytocin (OXT). The
thyroid gland helps control how fast a body uses energy and makes proteins as well as
influencing the sensitivity of the body to other hormones. It does this by producing T3
and T4, which are made from tyrosine and iodine. The stomach, duodenum, liver,
pancreas and kidneys all secrete hormones. The kidneys secrete renin, erythropoietin,
calcitrol and thrombopoietin. They regulate pH, electrolytes and blood pressure. The
adrenal glands produce hormones in response to stress by synthesizing corticosteroids
such as cortisol and catecholamines, such as adrenaline (epinephrine) and noradrenaline
(norepinephrine). They also produce androgens in their innermost cortical layer. The
adrenal glands affect kidney function by secreting aldosterone. The testes, ovarian
follicle and corpus luteum are in the endocrine system, as are the placenta and uterus
when a woman is pregnant. The testes secrete androgens (mostly testosterone),
estradiol and inhibin. They stimulate or control the development and maintenance of
male characteristics by binding to androgen receptors. This includes the activity of the
male sex organs and development of male secondary sex characteristics. Androgens are
also the original anabolic steroids and the precursor of all estrogens. The endocrine
system also regulates the concentration of Ca2+. The parathyroid gland secretes the
parathyroid hormone (PTH), which stimulates Ca2+ release from bones, stimulates
osteoclasts and Ca2+ reabsorption in the kidneys. Calcium regulation also occurs in the
skin, which secretes the prehormone calcidiol (25-hydroxyvitamin D3), the inactive
form of vitamin D. The major endocrine systems are the TRH-TSH-Y3/T4, the GnRH
LH/FSH-sex hormones, the CRH-ACTH-cortisol, the renin-angiotensin-aldosterone
system and the leptin vs insulin system. The TRH-TSH-Y3/T4 system is also called [1].
Keywords: Adrenal, Endocrine system, Hypothalamus, kidneys, Parathyroid,
Pituitary, Reproductive system, Thyroid.
