Hormones are powerful chemicals that work by altering the activity of their target cell. Although a hormone does not initiate a cell’s biochemical reactions, it adjusts to the rate at which these biochemical reactions occur. The ones that are secreted into the blood stream by the endocrine cells travel through the blood stream and act on cells and tissues in various parts of the body.
How they work
The hormones come into contact with every possible cell in the body. However, they produce an effect only on specific cells, which are called the target cells. These target cells have the receptors that the hormone recognizes and binds to. The entire process helps to trigger a response inside the cell. Here, each hormone can only affect specific target cells that possess the right kind of receptor for that hormones. For example, the thyroid-stimulating hormone only binds with receptors on cells of the thyroid gland.
Hormones can be divided into two parts
- Water-soluble:Water-soluble hormones are unable to pass through the cell membrane, which has fatty layers. They bind to receptors on the surface of the cell to act on target cells. Most of the hormones in the human body are water-soluble.
- Fat-soluble: The fat-soluble ones are able to pass through the cell membrane. They produce their effect by binding with receptors in the cell. Sex hormones and thyroid hormones are fat-soluble ones.
Factors that are responsible for stimulating the production and release of hormones keep varying. Some endocrine glands are stimulated by the presence of nutrients of minerals in the blood, while many endocrine glands respond to hormones produced by other endocrine glands.
Hormonal stimulation leads to a rhythmic release, with hormone levels rising and falling in a particular pattern. There are few cases wherein the release of hormones is triggered by signals from the nervous system. And there are few times when the secretion bursts rather than occurring in a rhythmic way.
Hormones are considered to be powerful and tend to affect target organs at low concentrations. However, the duration of their action is limited – it varies from seconds to several hours. Therefore, the blood levels need to be kept within limits, tailored to the needs of the specific hormone and the body. Many of them are regulated by negative feedback mechanisms. These work like a thermostat-controlled heating system. The thermostat is set at the desired temperature and its sensor monitors the air. If the temperature drops, a control unit in the thermostat triggers the boiler to go on. And when the desired temperature is reached, the control unit triggers the boiler to go off. If the level of a hormone in the blood drops lower than optimum, it triggers the endocrine gland to turn on and release them. Once the blood level rises, the endocrine gland is triggered to turn off.
The blood levels of some hormones may be different on different days of the month as they tend to vary due to several reasons. Levels of female sex hormones follow a monthly cycle, regulated by the rhythmic release of gonadotrophin-releasing hormone (GnRH) from the hypothalamus.
GnRH regulates release of hormones from the pituitary gland:
- Follicle-stimulating hormone, which is responsible for the development of egg follicles
- Luteinizing hormone, which triggers the release of eggs.
Growth hormone (GH), cortisol from the supra-renal gland and melatonin from the pineal gland follow diurnal cycles. GH and melatonin are highest at night, while cortisol increases in the morning. The rhythm of diurnal hormone are linked with either sleep-wake or light-dark cycles.