The endocrine system HBS 3A

The Endocrine system This consists of glands (secretory cells) and hormones (chemicals) The endocrine glands secrete hormones into the blood or body fluids. Hormones are chemicals that travel through the bloodstream or body fluids and change the activity of target cells or organs Target organs contain cells with receptor sites for particular hormones. These hormones can then trigger a change in activity in these cells, and so control body activity. Paracrines secrete hormones which bind to receptors in nearby cells and affect their function eg intestinal mucosal hormones.

Endocrine glands The endocrine glands include hypothalamus pituitary gland thyroid gland thymus parathyroid glands adrenal glands pancreas ovaries testes The placenta and embryo can also act as endocrine glands

Hypothalamus and pituitary glands The hypothalamus is part of the brain (nervous tissue) which also produces hormones in special nerve cells called neurosecretory cells which are released from the posterior pituitary gland. It also secretes a number of hormones called releasing and inhibiting factors that control the release of hormones in the anterior pituitary gland. The pituitary gland (hypophysis) is found just below the hypothalamus. The anterior pituitary (front section – also known as the adrenohypophysis) produces and releases a number of hormones under the control of the hypothalamus. The posterior pituitary (back section – also known as the neurohypophysis) releases the hormones made by the hypothalamus.

Anterior pituitary hormones 1 Gonadotrophins (affect gonads) Follicle stimulating hormone (FSH) In females the target organs are the ovaries - FSH stimulates the development of the follicle, leading to increased oestrogen production In males the target organs are the testes - FSH stimulates the production of sperm in the seminiferous tubules and maturation of sperm in the epididymis The release of FSH is controlled by the hypothalamus by releasing factors (FSHRF - increases FSH) and inhibiting factors (FSHIF -decreases FSH) Luteinising hormone (LH) In females the target organs are the ovaries - LH brings about ovulation and formation of corpus luteum, leading to increased progesterone and oestrogen production In males the target organs are the testes - LH stimulates the secretion of testosterone from the interstitial cells in the testes The release of LH is controlled by the hypothalamus by releasing factors (LHRF - increases LH) and inhibiting factors (LHIF - decreases LH)

Anterior pituitary hormones 2 Growth hormone (GH) The target organs are body cells – especially bone, cartilage and muscle cells Stimulates the growth of body cells especially the skeleton. It increases the rate of amino acid uptake by the cells. It maintains the size of organs after maturity. The release of GH is controlled by the hypothalamus by releasing factors (GHRF - increases GH) and inhibiting factors (GHIF - decreases GH) Thyroid stimulating hormone (TSH) The target organ is the thyroid gland. Stimulates the production and release of the thyroid hormone thyroxine, from the thyroid gland The release of TSH is controlled by the hypothalamus by releasing factors (TSHRF - increases TSH) and inhibiting factors (TSHIF - decreases TSH)

Anterior pituitary hormones 3 Prolactin Target organs are the breasts Initiates (starts) and maintains milk secretion from the mammary glands in females The release of prolactin is controlled by the hypothalamus by releasing factors (prolactin RF - increases prolactin) and inhibiting factors (prolactin IF - decreases prolactin) Adrenocorticotrophic hormone (ACTH) The target organs are the adrenal cortex Stimulates the adrenal cortex to release cortisol (or cortisone) and aldosterone The release of ACTH is controlled by the hypothalamus by releasing factors (ACTHRF - increases ACTH) and inhibiting factors (ACTHIF - decreases ACTH)

Posterior pituitary hormones These hormones are produced in the hypothalamus and released from nerve endings in the posterior pituitary Oxytocin The target organs are breasts and uterus Causes contraction of smooth muscles of the uterus during labour, and milk ducts of the breasts to release milk in milk let down The release of oxytocin is controlled by the hypothalamus in response to stretch and suckling. Antidiuretic hormone The target organs are the nephrons in the kidney. It makes the nephrons more permeable to water, increasing water reabsorption and so removing water from urine making the urine more concentrated The release of ADH is controlled by the hypothalamus in response to the water concentration and osmotic pressure in the blood

Thyroid gland THYROXIN Target organs are the body cells It increases metabolic rate Its release is controlled by TSH from the anterior pituitary (which is controlled by the hypothalamus) CALCITONIN Target organs are bone, intestine and kidney. It decreases blood calcium levels by causing increased storage in bone, decreased uptake by intestines and increased excretion by kidneys. Its release is controlled by negative feedback based on blood levels of calcium and phosphate

Parathyroid gland Parathyroid hormone (PTH) Target organs are bone, intestine and kidney. It increases blood calcium levels by causing decreased storage in bone, increased uptake by intestines and decreased excretion by kidneys. Its release is controlled by negative feedback based on blood levels of calcium and phosphate

Thymus Thymosins These stimulate the development and differentiation of T lymphocytes. They play a role in regulating the immune system by stimulating other kinds of immune cells as well. The release of these hormones is controlled by a number of factors, including the presence of foreign organisms, secretions from white blood cells, levels of pituitary and other hormones (eg ACTH, cortisol, oestrogen) and nervous stimulation.

Pancreas Insulin (made in beta cells) The target organs are liver, muscle and fat cells It decreases blood glucose (sugar) by increasing glucose uptake and storage (as glucagon). It is released when the blood glucose level rises. Glucagon (made in alpha cells) It increases blood glucose (sugar) by increasing glucose release and glucagon breakdown. It is released when the blood glucose level falls.

Adrenal cortex Aldosterone Target organs are the nephrons in the kidney It acts on kidney nephrons to decrease sodium and increase potassium in the blood Its release is controlled by sodium and potassium concentrations of blood, and the release of ACTH from the hypothalamus Cortisol Target organs are body cells, especially fat cells It promotes normal metabolism and increases resistance to stress Its release is controlled by stress levels, and the release of ACTH from the hypothalamus

Adrenal medulla Adrenaline (Epinephrine) Target organs include heart, bronchioles, arterioles, liver, sweat glands, muscles Helps increase the ‘fight or flight response’ – increased heart rate and force of contraction, increased blood flow to muscles, bronchiodilation, increased sweating It is released due to nerve impulses from the sympathetic nervous system under control of the medulla oblongata in the brain NORADRENALINE (Norephinephrine) Similar to Adrenaline, especially in increasing heart rate and strength.

Testes Androgens (Testosterone) The target organs are body cells including testes, skeleton, muscles and skin Its effects are the development and maintenance of male secondary sexual characteristics Its release is controlled by FSH and LH released from anterior pituitary under the control of the hypothalamus.

Ovaries Oestrogen (Estrogen) The target organs are body cells including ovaries, uterus, breasts, skeleton, muscles, fat and skin Its effects are the development and maintenance of female secondary sexual characteristics and uterine development Its release is controlled by FSH and LH released from anterior pituitary under the control of the hypothalamus. Progesterone The target organs are uterus and breasts Its effects are the development and maintenance of the uterus and breast tissue to maintain pregnancy

Interactions of the hypothalamus and pituitary glands The hypothalamus is both nervous and endocrine tissue. It controls the release of posterior pituitary hormones by nervous stimulation (ADH and oxytocin) and anterior pituitary hormones by releasing factors (FSH, LH, TSH, ACTH, GH and prolactin). As well as monitoring hormone levels (oestrogen, progesterone, testosterone, cortisols, thyroxine), the hypothalamus monitors blood levels of glucose, ions and water concentration and body temperature. Changes in these lead to changes in hormone production (feedback). Many of the anterior pituitary hormones control other glands (FSH, LH, TSH and ACTH). This is why the pituitary is often called the ‘master gland’