The use of Chemical Signals to Maintain Homeostasis Hormones

Endocrine System: a system of glands that secrete hormones to regulate body function Hormones: chemical messengers or regulators they are released by cells in one part of the body and affect cells in other parts of the body to speed up or slow down processes. Endocrine Hormones – are produced in glands and secreted directly into the blood and distributed by the circulatory system.

Makes hormones and regulates the pituitary gland Pineal gland (makes melatonin which regulates sleep patterns) Not a gland

How do Hormones Signal Cells? Note: Hormones do not affect ALL cells Depends on whether or not that cell has a receptor for the particular hormone. There are 2 types of hormones: 1. Steroid Hormones 2. Protein Hormones

Steriod Hormones made from cholesterol (lipid); Complex ring of C, H, and O Soluble in fat but NOT water - ex: sex hormones - ex: cortisol (stimulates the conversion of amino acids to glucose by the liver)

Steroid Hormones HORMONE RECEPTOR

Protein Hormones Made of chains of amino acids Soluble in water Ex: insulin, growth hormone The protein hormones attach to receptor sites on the cell membrane The hormone-receptor complex leads to the production of the enzyme adenylyl cyclase which causes the cell to convert ATP into cyclic adenosine monophosphate (cyclic AMP) Cyclic AMP then activates enzymes in the cell.

Protein Hormones

Control Systems The body relies on the nervous system and the endocrine system for control of organs and tissues The nervous system allows the body to adjust quickly to environmental changes The endocrine system is designed to maintain control over longer durations

The hypothalamus regulates the pituitary gland through nerve stimulation However, endocrine glands – which are stimulated by the pituitary gland – secrete chemicals that feedback to affect nerve activity of the hypothalamus

Pituitary Gland (The Master Gland) “master gland” because it controls the other endocrine glands Located at the base of the brain; connected to the hypothalamus by a stalk The pituitary produces and stores hormones The hypothalamus stimulates their release when necessary

2 Lobes of the Pituitary Posterior Lobe Releases hormones that are actually made by the hypothalamus The hormones travel from the hypothalamus to the pituitary via special nerve cells They are stored in the pituitary and released into the blood when necessary Ex: ADH (antidiuretic hormone), oxytocin

2 Lobes of the Pituitary Anterior Lobe Produces its own hormones The release of these hormones is stimulated by other hormones produced in the hypothalamus These anterior pituitary hormones will travel through the blood to target cells

Epinephrine/ adrenaline

Hormones That Affect Metabolism

3 Glands Affect Metabolism Thyroid gland Parathyroid gland Anterior pituitary

THYROID GLAND

Thyroid Gland Located at the base of the neck in front of the trachea Makes 2 important hormones: thyroxine (T4) and triiodothyronine (T3) They regulate body metabolism, growth, and differentiation of tissues. (Thyroid also makes hormone calcitonin – which acts on bone cells to lower the amount of calcium in the blood)

Thyroxine and Metabolism People who secrete higher levels of thyroxine will breakdown sugars and other nutrients at a faster rate. For these individuals, ~60% of glucose oxidized is released as heat 40% is converted into ATP and is usually consumed during daily activity. Thus, these individuals don’t gain weight

Hypothyroidism In contrast, individuals who produce lower levels of thyroxine do not break down sugars as quickly. Excess blood sugar Is converted into glycogen, and the extra sugar into fat. The slower blood sugar is used, the faster fat is stored.

Negative Feedback control When metabolic rate decreases (b/c low thyroxine) the hypothalamus is activated. The hypothalamus releases thyroid-releasing hormone to the anterior pituitary which causes the release of thyroid-stimulating hormone by the pituitary. TSH goes to the thyroid and signals the release of thyroxine. Thyroxine inhibits the release of TRH.

Thyroid Disorders - Goiter Iodine, obtained from one’s diet, is an important component of T3 and T4 When not enough iodine is obtained from one’s diet, the thyroid enlarges to produce a goiter Lack of iodine→ decrease in T3/4 → TRH → TSH → thyroid stimulation – but with no hormones being produced, no negative feedback to shut off the thyroid stimulation so thyroid enlarges

Parathyroid Glands

Parathyroid Glands 4 small glands within the thyroid gland Usually nerves or other hormones regulate the endocrine glands but the parathyroid glands respond directly to chemical changes in their immediate surroundings. Produces parathyroid hormone (PTH) which regulates blood calcium levels

Low calcium → PTH release → kidneys and intestines retain absorb (retain) calcium → calcium is released by bones When calcium levels reach normal levels again, the parathyroid glands are inhibited. Abnormally high levels of PTH would cause prolonged breakdown of bones