In the human body, the role of the respiratory system is to supply vital oxygen for absorption into the blood and the subsequent dissemination of this oxygen is carried out by the cardiovascular system, which controls the circulation of blood. The flow of hormones in the bloodstream is regulated and controlled by the endocrine system - hormones are thus signal compounds which are used by the body to direct it self. Many vital factors in the body are controlled by these hormones; they play a vital part in making humans what they are. Some of the very important functions determined by the presence of hormones are the rate of growth and the rate of maturation in the body. Hormones also have a direct influence on the level of intelligence; they control and regulate physical agility, and are responsible for sexual drives. Growth of any kind in the human body is impossible without these signal compounds. The production and secretion of hormones from the endocrine system is a function shared with the nervous system in the human body - the brain is the originator of all signals to release hormones. This interdependence between the two systems in the human body is a recent scientific discovery; at one time scientists believed that the functioning and regulation of the endocrine system was independent to that of the nervous system. Activities in the body are regulated by the endocrine system in conjunction with the nervous system, the role of the two is however, fundamentally different. Messages in the brain or nervous system depend on the transmission of electrical impulses generated by chemical messages, whereas the only role of the endocrine glands is to send their chemical messengers - hormones - through the bloodstream, to reach target tissues in the body. The nervous system depends on a network of specialized cells called neurons. The transmission of impulses in the brain was thought to be solely electrical; this view has changed with new light shed by modern research into the brain. While the signals traveling in the neurons are always electrical albeit mediated by intermediary compounds called neurotransmitters, the neurons themselves are also known to secrete chemicals called neuron-hormones. These compounds have a similar function to the hormones in the body. It is also recognized nowadays that the hormones released by some endocrine glands can directly influence the signaling activity of the nervous system itself. The level of interdependence between these two systems is great and therefore, the basic distinction may not be very real as the functioning of the two systems is always dependant on the action of the other. Neurotransmitters are hormone like substances that jump between neurons to start the electrical impulse message - it is wrong to believe that electricity is the only way neurons communicate. The basic regulatory structure of hormone release in the human body can be seen in this way-the brain receives external and internal stimuli to some event, it sends a message to the endocrine system, the system responds by releasing a hormone which will travel to the target organ in the body and induce changes in response to the event originally detected by the brain. The endocrine system and the nervous system therefore have continuous feedback between them, they work together to signal the glands regarding the amount of hormones which are circulating in the bloodstream of the person. This system will send a signal to a specific gland when it finds that the level of any hormone in the blood has gone down - thus the system guides, directs or regulates all hormonal activity in the body. Glandular production is inhibited by another signal to a specific gland when the levels of a particular hormone become too high. The feed back system that regulates and operates the hormones is called negative feedback control - it is the biological equivalent of an automatic master control system. Lowering of hormonal levels results in signals that increase glandular secretion, correspondingly any rise in hormone levels will attract signals that depress glandular secretion - this feedback system is appropriately names, as it works on one principle, namely to reversing any excess or deficit of hormone levels and thus maintain a balance in hormonal levels within the body of a human being. The regulation of most hormone levels is thought to be regulated through the negative feedback control system. The regulatory apparatus is thus, similar to the regulation of water levels in a tank. For example, when water levels in a water tank are low, they sensor deep in the tank becomes dry and a filling mechanism will automatically operate to allow the water to enter the tank. Another sensor at the top of the tank detects water and shuts off the mechanism once the tank is full. The regulation of blood levels of a few hormones operates on the principle of positive feedback control. Here, regulation is via overlapping mechanisms, and the presence of one hormone in a target tissue will stimulate the production and secretion of another in that tissue. An excellent example for this form of hormonal control is seen in the hormone systems that regulate the menstrual cycle in females. An egg is produced in the normal human female body once every month and one of the ovaries will release this egg. The ovarian tissues release the hormone estrogen into the blood stream as the egg matures. As levels of estrogen rise in the blood, they trigger the pituitary gland in the brain, to release an additional female sex hormone called the LH- luteinizing hormone. The arrival of the LH in the ovary stimulates it to release the now fully mature egg into the passages of the nearby oviduct - the passage for eggs into the womb or uterus. A process known as ovulation is the final part of this process. Ovulation starts as a chain reaction which can be said to have begun with a single chemical message sent out by the endocrine system along with positive feedback triggering of hormones. The question often asked, is whether the hormones released by the glands know or recognize appropriate target cells or tissues. The answer is that hormones are not directed, and have no chemical path finders letting them reach the appropriate target tissues - they are blind messengers. Hormones are released and carried by the bloodstream, the blood takes the hormone to different parts of the body. The target tissues have recognition molecules which detect the presence of the hormone and are programmed to respond to the presence of the hormone - thus the hormone itself is a passive entity and does not act in anyway. The target cells and tissues react to the hormones in the blood and induce whatever changes are necessary. The actual mechanism for the detecting hormones is via a receptor molecule on the target tissue or cell. Most receptors are essentially large protein molecules, there is a factor known as receptor specificity and the receptor molecule is on a continual alert to react to a specific type of chemical substance in the blood. Hormone receptors react only to the specific hormone they interact with and ignore all other hormones in the blood. The passage of a specific hormone in the blood near a target tissue will be recognized immediately by the receptor molecule on the target tissue. The receptor can be said to attract the specific hormone and captures it - this process means the message has been passed and the tissue reacts. Most receptors are very efficient and sensitive to the availability of their specific hormone in the blood, this is important as most hormones are produced very minute quantities. For the full expression of the hormonal activity, the single most important factor may well be this receptor sensitivity. Hormones cannot affect target cells when or if the receptor molecule site breaks down, is blocked or is damaged. Many disorders are connected to the absence of receptors, causes can be genetic factors arising at birth. If the receptor is damaged, conditions and disorders which are similar to the ones caused by a lack of adequate hormone production in the body will begin to affect the person. The majority of hormones are too large to enter the target cell, a good examples is the blood sugar regulating hormone called insulin. The mechanism of hormone action is by direct attachment to the external receptor on the cell, the arrival of the hormone results in a transmission of a chemical message towards the interior of the cell. Small hormones are minute enough to penetrate the surface of a cell and hormones, such as the male sex hormone testosterone can fit easily into the cellular structure. The result is that these hormones are handled by internal receptors and the hormone may then travel into the nucleus to stimulate appropriate actions in the cell. The main function of a hormone is to send messages which activate specific genes in the nucleus -the genes then produce a protein to enact the relevant action. Once this action has been enacted, the target cells give a relevant respond and changes are wrought in the body. The type of cell will determine the exact nature of the response generated by the hormone. For example, hormonal action on muscle cells may lead to an increase or to a decrease in the rate of contraction. Hormones can act on epithelial cells, cells that form the surfaces of skin and which are also found in the walls lining many of the internal organs, the hormones can bring a change in the rate at which liquids diffuse through these cells to induce some specific action. Hormones can stimulate the gland cells to secrete more chemicals or reduce their secretions. The action of hormones thus allows the human body to carefully regulate all metabolic and biochemical processes, it enables the system to adjust levels of salt and water in the tissues on a constant basis. The regulation of sugar levels in the blood and the levels of salt in the form of sodium in the sweat is also determined by hormonal actions. Stimulus via hormonal reception is the way the body enacts specific actions in its organs and these constant changes aids the body in keeping the vital chemical balances in its tissues intact. The degradation of the hormone is usually the next step following hormone receptor contact and subsequent activation of target tissue. It is not essential for the hormone to remain in the blood and the hormone is normally excreted out and expelled from the blood by filtering through the kidneys or it may be degraded by the action of specific enzymes present in the blood, in the liver, in the kidney, the lungs, or any other target tissues that have already been stimulated. The need to quickly eliminate the hormone exists as the biological effect or stimulus of the hormone continues indefinitely and is in effect as long as the hormone is intact. The rate at which hormones are degraded and the rate at which glands release the hormone give the endocrinologists an ability to determine the actual hormone levels in the blood of a person-this ability to measure the hormone levels is a very useful diagnostic tool in the treatment of hormonal imbalances. Endocrinologists normally determine and measure the length of time it takes half a dose of some specific hormone to exit out from the circulatory system. The term half-life is given to this time interval, the measurement of a half-life serves as a means to predict the rate at which any hormone is eliminated or degraded in the body. Treatments for hormonal imbalances are usually based on the half-life measure of particular hormones - half-life is not measurable by counting the total elimination time of a substance from the body as such an event is heavily influenced by the initial amount or the starting concentration of the compound. Scientifically such a measurement is called the metabolic clearance rate - the amount of time it takes to completely disappear from the body, the measure is used in other treatment scenarios. Here, the measure is of the rate of hormone molecule eliminated by both the liver and the kidneys following stimulus of target tissue. This measurement also accounts for all the hormones which the target cells often ingest and then destroy after stimulus or activation of the gene is achieved. The measurement of the metabolic clearance rate of any substance in the body is often used in medicine -here doctors often determine the frequency of drug administration and can calibrate dosage requirements. At the basic level, seven overall stages can be said to be involved in hormonal communication within the body. A start is made by the nervous system or the endocrine system which sends a signal that stimulates the hormonal production in a specific gland. The second stage is the secretion and release of the hormone from the gland and the third stage is the delivery of this hormone to the target cells in the specified tissue via the blood. Recognition of the hormone by the receptors on the target cell can be said to form the fourth stage, stage five is the biological response of the target cells to the stimulus. Stage six occurs once the hormone has performed its specified function and the final stage is the destruction of the degradation of the chemical hormone and it can also include the signalling the elimination of hormone cells in the appropriate excretory or filtering organ - liver or kidney.