hormonal communication Flashcards
endocrine glands
The endocrine system is made up of endocrine glands. An endocrine gland is a group of cells which are specialised to secrete chemicals - these chemicals are known as hormones, and are secreted directly into the bloodstream. Examples of endocrine glands include the pancreas and adrenal glands.
Figure 1 shows the positions of the major endocrine glands in the body and the hormones they secrete. The pituitary gland at the base of the brain makes several hormones, which in turn control the release of other hormones. The close proximity of the pituitary gland to the hypothalamus ensures that the nervous and hormonal responses of the body are closely linked and coordinated.
pituitary gland
Pituitary gland - produces growth hormone, which controls growth of bones and muscles; anti-diuretic hormone, which increases reabsorption of water in kidneys; and gonadotrophins, which control development of ovaries and testes.
thyroid gland
Thyroid gland - produces thyroxine which controls rate of metabolism and rate that glucose is used up in respiration, and promotes growth.
adrenal gland
Adrenal gland - produces adrenaline which increases heart and breathing rate and raises blood sugar level.
testis
Testis - produces testosterone which controls sperm production and secondary sexual characteristics.
pineal gland
Pineal gland - produces melatonin which affects reproductive development and daily cycles.
thymus
Thymus - produces thymosin which promotes production and maturation of white blood cells.
pancreas
produces insulin which converts excess glucose into glycogen in the liver; and glucagon, which converts glycogen back to glucose in the liver.
ovary
produces ostrogen, which controls ovulation and secondary sexual characteristics; and progesterone, oestrogen, which controls ovulation and secondary sexual characteristics; and progesterone, which prepares the uterus lining for receiving an embryo.
hormones
Hormones are often referred to as chemical messengers because they carry information from one part of the body to another. They can be steroids, proteins, glycoproteins, polypeptides, amines, or tyrosine derivatives.
Although they are chemically different, they share many characteristics.
Hormones are secreted directly into the blood when a gland is stimulated. This can occur as a result of a change in concentration of a particular substance, such as blood glucose concentration. It can also occur as the result of another hormone or a nerve impulse.
Once secreted, the hormones are transported in the blood plasma all over the body. The hormones diffuse out of the blood and bind to specific receptors for that hormone, found on the membranes, or in the cytoplasm of cells in the target organs. These are known as target cells. Once bound to their receptors the hormones stimulate the target cells to produce a response. You will find out more about how blood glucose concentration is maintained in Topic 14.3, Regulation of blood glucose concentration.
steroid hormones
Steroid hormones are lipid-soluble. They pass through the lipid component of the cell membrane and bind to steroid hormone
lipid-soluble steroid hormone passes through membrane
receptors to form a hormone-receptor complex. The receptors may be present in the cytoplasm or the nucleus depending on the hormone. The hormone-receptor complex formed acts as a transcription factor which in turn facilitates or inhibits the transcription of a specific gene. Ostrogen is an example of a hormone which works in this way.
non-steroid hormones
Non-steroid hormones are hydrophilic so cannot pass directly through the cell membrane.
Instead they bind to specific receptors on the cell surface membrane of the target cell.
This triggers a cascade reaction mediated by chemicals called second messengers.
Adrenaline is an example of a hormone which works in this way.
hormonal system v nervous system
hormonal system-communication is by chemicals called hormones, transmission is by the blood system,transmission is usually relatively slow, hormones travel to all parts of the body, but only target organs respond, response is widespread, response is slow, response is often long-lasting, effect may be permanent and irreversible.
nervous system-communication is by nerve impulses, transmission is by neurons, transmission is very rapid, nerve impulses travel to specific parts of the body, response is localised, response is rapid, response is short-lived, effect is temporary and reversible.
adrenal gland
The adrenal glands are two small glands that measure approximately 3 cm in height and 5 cm in length. They are located on top of each kidney and are made up of two distinct parts surrounded by a capsule:
• The adrenal cortex - the outer region of the glands. This produces hormones that are vital to life, such as cortisol and aldosterone.
• The adrenal medulla - the inner region of the glands. This produces non-essential hormones, such as adrenaline which helps the body react to stress.
adrenal cortex
The production of hormones by the adrenal cortex is itself controlled by hormones released from the pituitary gland in the brain. There are three main types of hormones produced by the adrenal cortex:
-Glucocorticoids. These include cortisol which helps regulate metabolism by controlling how the body converts fats, proteins, and carbohydrates to energy. It also helps regulate blood pressure and cardiovascular function in response to stress. Another glucocorticoid hormone released is corticosterone. This works with cortisol to regulate immune response and suppress inflammatory reactions.
The release of these hormones is controlled by the hypothalamus.
•
Mineralocorticoids. The main one produced is aldosterone which helps control blood pressure by maintaining the balance between salt and water concentrations in the blood and body fluids. Its release is mediated by signals triggered by the kidney.
-Androgens. Small amounts of male and female sex hormones are released - their impact is relatively small compared with the larger amounts of hormones, such as ostrogen and testosterone, released by the ovaries or testes after puberty, but they are still important, especially in women after the menopause.
