Animal Hormones

What are they?
Hormones are special chemical messengers in the body that are created in the endocrine glands. They are directly secreted into the blood, which carries them directly to organs and tissues of the body. There, they control major body functions from simple things like hunger to reproduction and mood. 

Different Endocrine Glands & What Hormones They Secrete:
Hormones are secreted directly from their ductless endocrine glands. The most notable of them are the:
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  • Hypothalamus: The hypothalamus is responsible for body temperature, hunger, moods and the release of hormones from other glands; and also controls thirst, sleep and the drive for sex. Heavily influences the anterior pituitary gland, and controls the hormones that it releases.
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  • Parathyroid: Controls the amount of calcium in the body by either releasing PTH or Calcitonin to increase or decrease the amount of Calcium in the blood.
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  • Thymus: Plays a role in the function of the adaptive immune system and the maturity of the thymus, and produces T-cells.
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  • Pancreas: Produces the insulin and glucagon that helps control blood sugar levels.
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  • Thyroid: Produces hormones like thyroxine that are associated with calorie burning and heart rate.
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  • Adrenal: Produce the hormones that control sex drive and cortisol, the stress hormone also known as adrenaline.
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  • Pituitary: Separated into the anterior and posterior, it is considered the "master control gland," the pituitary gland controls other glands and makes the hormones that trigger growth.
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  • Pineal: Also called the thalamus, this gland produces serotonin derivatives of melatonin, which affects sleep.
Sex Glands:
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  • Ovaries: Only in women, the ovaries secrete estrogen, testosterone and progesterone, the female sex hormones.
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  • Testes: Only in men, the testes produce the male sex hormone, testosterone, and produce sperm.
How Do Hormones Work?
While all cells are exposed to hormones circulating in the bloodstream, not all cells react. Only a hormone's "target" cells, which have receptors for that hormone, will respond to its signal. When the hormone binds to its receptor, it causes a biological response within the cell. Signaling ends when the circulating hormones are broken down and excreted by the body.  The number of receptors that respond to a hormone determines the cell's sensitivity to that hormone and the resulting cellular response. Additionally, the number of receptors that respond to a hormone can change over time, resulting in increased or decreased cell sensitivity. In up-regulation, the number of receptors increases in response to rising hormone levels, making the cell more sensitive to the hormone, allowing for more cellular activity. When the number of receptors decreases in response to rising hormone levels, called down-regulation, cellular activity is reduced. Usually after the receptor receives the message, a signal transduction pathway occurs, leading to a second messenger to receive the signal and either amplify it and or carry it on to its intended destination.
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