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The Endocrine System
Hormones
The endocrine system is responsible for the delivery of chemical messengers or hormones. These messengers are secreted directly into the blood (thus endo-crine) especially by a gland, without passing through a duct in order to enter the blood.
Hormones are grouped together by their function, not by their structure. Some are made of protein, such as insulin, while others are steroids (adreno-corticoid hormones), glycoproteins (FSH, LH, TSH) and derivatives of single amino-acids (T4, T3). All hormones, however, are produced in a gland and then transported to an area or organ they control.
Hormones play an important role in homeostasis and are also responsible for the development and growth of the human body. In addition to this, the sex hormones control aspects of reproduction, i.e. the menstrual cycle in women.
The following describes which hormones each gland is responsible for producing, its function and control:
Hormone | Function | Control | ||
Hypothalamus | Releasing and inhibiting hormones | Control of anterior pituitary | Hormone and metabolite feedback | |
Posterior Pituitary | Oxytocin |
Ejection of milk Uterine contraction (birth) |
Hormone and nerve feedback | |
ADH |
Anti-diuretic Hormone (aka vasopressin) |
Reduces urine volume | Blood concentration (osmosis) | |
Anterior Pituitary | FSH | Follicle Stimulating Hormone |
Spermatogenesis (males) Follicular development (females) |
Plasma oestrogen and progesterones levels |
LH (females) ICSH (males) |
Luteinising Hormone Interstitial Cell Stimulating Hormone |
Ovulation and maintenance of corpus luteum (females) Testosterone secretion (males) |
Plasma oestrogen Plasma testosterone |
|
Prolactin | Stimulates milk production | Hypothalmic hormones | ||
TSH | Thyrotrophin | Synthesis and release of thyroid hormones | Plasma thyroxine levels | |
ACTH | Adrenal Cortex Hormone | Synthesis and release of adrenal cortex hormones | Plasma ACTH | |
HGH | Human Growth Hormone | Protein synthesis, growth (bones) | Hypothalmic hormones | |
MSH | Melanocyte Stimulating Hormone | Melanin synthesis and secretion | Sympathetic Nervous System | |
Parathyroid | Parathormone |
Increases blood Ca Reduces blood PO4 |
Plasma Ca and PO4 levels | |
Thyroid | Thyroxin |
Regulation of BMR (base metabolic rate) Growth and development |
TSH | |
Triiodothyronin |
Regulation of BMR Growth and development |
TSH | ||
Calcitonin | Decrease blood Ca | Plasma Ca levels | ||
Thymus | T lymphocytes | Immunity | Plasma proteins | |
Pineal | Melatonin | Melanin production | Sympathetic Nervous System | |
Seratonin | Control circadian rhythms | Sympathetic Nervous System | ||
Adrenal Cortex | Corticosteroids | Metabolism of proteins and carbohydrates in response to stress. Stress Adaptation | ACTH | |
Mineralocorticoids (Aldosterone) |
Na retention in kidney NA and K levels, reaises BP |
Plasma Na and K levels Low blood pressure |
||
Adrenal Medulla | Adrenaline | Increased heart rate. Dilation of skeletal arterioles. Increased blood glucose etc. | Sypmathetic Nervous System | |
Noradrenaline | Constriction of arterioles therefore BP raises | Nervous System | ||
Islets of Langerhans | Insulin (beta cells) | Blood glucose converted to glycogen. Increased cell uptake, i.e. reduces blood glucose | Plasma glucose and amino acid levels | |
Glucagon (alpha cells) | Breakdown of glycogen to glucose, i.e. increases blood glucose levels | Plasma glucose levels | ||
Ovarian Follicle | Oestrogen | Secondary sexual characteristics. Oestrous cycle | FSH and LH | |
Progesterone | Gestation. Inhibition of ovulation | LH | ||
Testis | Testosterone | Secondary characteristics | FSH and LH (ICSH) |