Endocrine system

• Influence activity of cells by releasing chemical messengers (hormones). • Manages homeostasis (constant internal environment).

• Secrete into a duct that carries secretion to bodies surface or to a body cavity. • E.g. Sweat, Mucous, Salivary and Alimentary glands.

• Secrete hormones into extracellular fluid of surrounding cells. • Secretion passes into capillaries and transported by blood.

• A secretion from an endocrine gland. • Can be proteins, amines or steroids. • Transported in blood to cell/groups of cells called target cells or target organs. • Specific – target cells must have correct receptors. Saturation can also occur (when all receptors are used). • Once hormone has produced desired effect it must be ‘turned off’ by breaking down hormone molecules. • Can be done in target cells, but mostly in liver and kidneys. Degraded hormones are then excreted in bile or urine.

• Activate certain genes so a particular enzyme/protein is produced. • Hormone triggers cascading effect – Enzyme Amplification. • Change structure of enzymes. • Change rate of production of enzyme/protein by changing rate of transcription/translation.

• Attach/bind to receptors on surface of cell membrane of target cell. • Combination of hormone and receptor (hormone – receptor complex) causes a secondary messenger to diffuse through cell and activate particular enzymes in cytoplasm. • Faster acting hormones (seconds/minutes).

• Enter target cells and combine with receptor protein on nucleus inside cytoplasm of cell. • Lipid soluble, easily diffuse through cell membrane. • Hormone receptor complex forms inside cytoplasm/on nucleus. • Activates genes controlling formation of particular proteins. • Regulates gene expression. Slower acting hormones (hours/days).

Thyroxine: Most body cells - Increases metabolic rate, oxygen consumption and heat production. Calcitonin: Bones, Kidney - Decrease calcium and phosphate levels in blood.

Parathyroid hormone: Bones, Kidneys - Increases level of calcium in blood.

Thymosins: T-lymphocytes - Stimulates maturation of T lymphocytes.

Aldosterone: Kidneys - Increases reabsorption of Na+ and excretion of K+. Cortisol: Most body cells - Promotes metabolism; helps body deal with stress; promotes repair of damaged tissues.

Adrenaline and Noradrenaline: Most body tissues - Prepares body for fight or flight responses; reinforce effects of sympathetic nervous system.

Insulin: Most body cells - Lowers blood glucose level; Increase glucose in cell. Glucagon: Liver and fat storage tissues - Increase blood glucose level; stimulates breakdown of glycogen and fat.

Androgens - Testosterone: Testes, Bones, Brain - Stimulates sperm production; male sexual characteristics.

Oestrogen: Many tissues - Stimulate development of female sexual characteristics; regulates menstrual cycle. Progesterone: Uterus and mammary glands - Prepares mammary glands for milk secretion.

• Controls release of hormones from two lobes. • Effect of hormone is opposite to original stimulus. • Releasing or Inhibiting factors from hypothalamus regulate function of pituitary gland.

• Referred to as master gland. • Hormones released from pituitary control many other endocrine glands. • Produces and secretes hormones. • Lies under hypothalamus and is joined to it by a stalk (infundibulum).

Ovaries: Growth and maturation of follicle. Testes: Sperm production.

Ovaries: Ovulation and maintenance of CL. Testes: Secretion of testosterone.

All cells: Growth and protein synthesis.

Thyroid gland: Secretion of thyroxine/calcitonin from thyroid.

Adrenal cortex: Secretion of corticosteroid from Adrenal cortex.

Mammary glands: Milk production

Kidneys: reabsorption of water.