Question

Endocrine System

• Describe the general features of the endocrine system, including how hormones are transported between an endocrine gland and its target organs.
• Describe the action of insulin in lowering blood glucose concentration and the action of glucagon in increasing blood glucose concentration.
• Explain what is meant by the half-life of a hormone.
• Explain the functions of aldosterone, ADH, ACTH, growth hormone, and thyrotropin releasing hormone.
• Explain the endocrine functions of the hypothalamus.
• Describe how lipid-soluble hormones reach their target cell receptors and the type of cellular change they initiate.
• Define up-regulation and down-regulation.
• Compare autocrine and paracrine signaling that occurs through local hormones.
• Explain how the hypothalamus controls the release of hormones from the anterior pituitary and the general function of each.

Answer 1

1. The endocrine system includes a number of glands that secrete hormones helping to regulate metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood. The endocrine system consists of the pituitary gland, thyroid gland, adrenal glands, pancreas, ovaries (in females) and testicles (in males). Hormones are the body's chemical messengers secreted directly into blood. They transfer information from one set of cells to another in order to coordinate the functions of various parts of the body. The hypothalamus is the neural control center of all endocrine systems in vertebrates.

  Hormones are released into the bloodstream which leads to target sites. The target cell has a specific hormone receptors which are activated by a lipid - soluble hormones (permeable to the plasma membrane) or water - soluble hormones (binds to a cell - surface receptor). Hormones diffuse into the target cell via the plasma membrane and bind to the receptor protein. The stimulation of the receptor results in a change in cell activity that can send feedback to the original cell that produces hormones.

2. Insulin and glucagon are hormones that help control our body's blood glucose levels or sugar levels. Glucose from the food we eat, moves through the bloodstream to fuel our body.

Foods containing carbohydrates are transformed into glucose during digestion. Most of this glucose is sent into the bloodstream, which causes blood glucose levels to increase. This increase in blood glucose signals insulin secretion in our pancreas. Insulin tells cells to take glucose from the bloodstream throughout the body. As glucose moves into cells, blood glucose levels decrease. Glucose is used as energy by some cells. Other cells, such as liver and muscles, store excess glucose as a glycogen, which fuels body between meals.

As the glucose levels in blood decrease, it triggers pancreas to produce glucagon. This hormone signals cells in liver and muscles to convert the stored glycogen back into glucose to fuel the body cells. This cycle keeps the blood sugar level steady.

3. Half - life of a hormone is the time required to degrade half the hormone concentration. For example, the hormone cortisol derived from lipid has a half - life of about 60 to 90 minutes and the hormone epinephrine derived from amino acid has a half - life of about one minute. The biological half - life of luteinizing hormones is 20 minutes, which is shorter than that of follicle - stimulating hormones (three to four hours) and human chorionic gonadotropin (24 hours). Hormone blood levels show a balance between synthesis / secretion and degradation / excretion. The liver and kidneys are the main organs that degrade hormones in urine and feces with breakdown products.

4.   Aldosterone affects the heart and blood vessels directly. The aldosterone hormone is secreted by adrenal gland. The hormone acts primarily in the kidney's functional unit to help preserve sodium, secrete potassium, retain water and stabilize blood pressure.

ADH is produced in the brain by the hypothalamus and stored in the pituitary gland at the back. It determines the amount of water to be conserved by signaling kidneys. ADH regulates the amount of water in the blood and balances it. Higher water levels increase blood volume and pressure.

ACTH, secreted by anterior pituitary gland, enhances secretion of glucocorticoid steroid hormones from adrenal cortex cells, especially in the zona fasciculata of the adrenal glands. ACTH acts by binding ACTH receptors on the cell surface located on adrenocortical cells in adrenalcortex.

Growth hormone, produced by the pituitary gland, stimulates growth in children and adolescents. It also helps in regulation of  body composition, body fluids, muscle and bone growth, metabolism of sugar and fat and heart function.

Thyrotropin-releasing hormone (TRH), also called thyrotropin-releasing factor (TRF) or thyroliberin, is a releasing hormone, secreted by the hypothalamus, that stimulates the release of thyrotropin (thyroid-stimulating hormone or TSH) and prolactin from the anterior pituitary. TRH is also found throughout the brain and spinal cord, where it is thought to serve as a neuromodulator.