(Option H – Higher Level Human Physiology / Paper 3)
Hormones are chemical messengers Secreted directly into the blood from endocrine glands (adrenal glands, testes, ovaries) Transported by blood to target organs Effect the functioning in the organ, by influencing the metabolic activity of the cells
Steroids Testosterone, estrogen Peptides (proteins) Insulin, glucagon, ADH (vasopressin) Tyrosine Derivatives Thyroxine
Steroid Hormones Act directly inside the cell by entering the cell and acting on the DNA, by activating or inhibiting genes Peptide (Protein) Hormones Act indirectly, by attaching to surface receptors on the target cell Initiate the formation of a secondary messenger An example is cyclic AMP (cAMP) Will initiate the formation of an enzyme, through the process of transcription / translation, for example
Thyroxin The hypothalamus produces Thyroid Releasing Hormone. (TRH) Hormone stimulates the anterior pituitary to produce and release Thyroid Stimulating Hormone, or TSH. Thyroxin is released when the TSH gets to the thyroid. The thyroxin increases the metabolic rate and the basal temperature increases, along with the other factors previously mentioned in maintaining Homeostasis.
ADH or Vasopressin ADH is produced in the hypothalamus and secreted from the posterior pituitary gland. The hypothalamus contains osmoreceptors, which detect changes in the blood solute potential. If the water concentration is lower than usual in the blood, the osmoreceptors detect the high osmotic pressure (low water concentration). It is carried by the blood to the kidneys. The kidneys respond by decreasing urine output, and more water is retained. The ADH also decreases the rate of perspiration. ADH can also increase blood pressure caused by the constriction of arterioles. If the water concentration in the blood is higher than normal, the osmoreceptors will detect the low osmotic pressure, and the ADH secretion is reduced or stopped. The kidneys put out a large amount of urine.