Endocrine System and Hormones Page 997

Nervous system: coordinates rapid responses to stimuli via action potentials (electrical signal) Endocrine system: coordinates long-term responses using chemical signals (hormones)

The endocrine system is made up of glands that release their products (chemicals called hormones) into the bloodstream. These hormones travel in the bloodstream to deliver messages throughout the body, affecting the activities of certain cells.

They do this by binding to specific chemical receptors on certain cells called target cells. If a cell has no receptors or does not have the specific receptors for a specific hormone, the hormone has no effect on it.

A nerve impulse may take only a fraction of a second to reach and affect its target cells. The effects of a hormone may take minutes, hours or days but are longer-lasting.

Glands A gland is an organ that produces and releases a substance or secretion.

There are two types of glands:

Exocrine glands where secretions pass through tube-like structures called ducts.

Endocrine glands which release their secretions (hormones) directly into the blood stream.

Exocrine glands Endocrine glands

Glands

Hormones: Steroid and non-steroid Hormones may be classified as belonging to one of two groups:

I. Steroid hormone which is produced from a lipid called cholesterol I. Steroid hormone which is produced from a lipid called cholesterol. It: a. crosses a cell membrane easily and passes directly into the cytoplasm.

b. once inside, it binds to steroid receptor protein (found only in the target cells) to form a hormone receptor complex.

c. The hormone receptor complex enters the nucleus where it binds to a DNA control sequence.

d. This binding starts the process whereby specific genes are copied into mRNA.

e. The mRNA moves into the cell and directs protein synthesis.

2. Non-steroid hormone which generally cannot pass through the cell membrane. a. A non-steroid hormone binds to receptors on the cell membrane.

b. The binding of the hormone activates an enzyme on the inner surface of the cell membrane.

3. This enzyme activates secondary messengers that carry the message of the hormone inside the cell.

4. These messengers can activate or inhibit a wide range of other cell activities.

Prostaglandins All cells except red blood cells produce small amounts of hormone-like substances called prostaglandins. They usually only affect cells close by. They are made of modified fatty acids.

Control of the endocrine system Like most systems of the body, the endocrine system is controlled by feedback mechanisms that function to maintain homeostasis.

Negative Feedback

Positive feedback

Pineal Located in the brain Releases melatonin which is responsible for body rhythyms, particularly sleep-wake cycles.

Pituitary Gland Bean sized structure Base of brain Secretes 9 hormones that regulate many body functions and control the actions of other glands. Human growth hormone (HGH) Too much, gigantism; too little, pituitary dwarfism.

Hypothalamus Part of the brain Above and attached to the pituitary gland Controls the secretions of the pituitary gland

Thyroid

Located at the base of the neck and wraps around the trachea. Has the major role in regulating metabolism Cells in the thyroid gland produce thyroxine which is made of the amino acid tyrosine and the mineral iodine. Thyroxine affects every cell, increasing the rate of protein, carbohydrate and fat metabolism, the rate of cellular respiration.

Adrenal Two pyramid share structures that sit on top of each kidney. Help the body prepare for and deal with stress.

Pancreas Has both exocrine and endocrine functions. Exocrine: Digestive enzyme released into the small intestine.

Pancreas The hormone producing part of the pancreas consists of clusters of cells that resemble islands. They are called the Islets of Langerhans after the German anatomist who discovered them.

Islets of Langerhans

Each islet includes beta cells which secrete the hormone insulin and alpha cells which secrete a hormone called glucagon Insulin and glucagon help to keep the level of glucose in the blood stable.

Insulin stimulates the cells in the liver, skeletal muscles, and fat to remove sugar from the blood and to store it as glycogen or lipids. Glucogen stimulates the liver and skeletal muscles to break down glycogen and release glucose back into the bloodstream. It also stimulates the release of fatty acids from stored fats.

Diabetes Mellitus Diabetes mellitus occurs when the pancreas fails to produce or properly use insulin. The amount of glucose in the blood may rise so high that the kidneys actually excrete glucose in the urine. Extremely high levels of glucose can cause damage to body systems.

Review of Endocrine System http://www.youtube.com/watch?v=HrMi4GikWwQ&feature=fvw

Reproductive Glands The gonads are the body’s reproductive glands. They serve 2 major functions: 1. the production of gametes 2. the secretion of sex hormones

Female gonads The female gonads are the ovaries. Ovaries produce eggs or ova (ovum singular) and estrogen and progesterone.

Estrogen Progesteran Development of eggs Formation of “female” physical characteristics. Progesteran - Prepares uterus for arrival of embryo

Male gonads The male gonads are testes (testis). Testes produce: Sperm Promote the development of “male”characteristics

Reproduction Reproduction is the formation of new individuals. In vertebrates, the reproductive system produces, stores and releases specialized sex cells.