The other hormones and the glands that secrete them include the following: •
Pineal gland—located in the brain and makes melatonin in response to darkness so that it helps to regulate the sleep-wake cycle.
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Thyroid gland—this is located in the neck and secretes two hormones (thyroxine and triiodothyronine) that regulate metabolism on a cellular level.
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Parathyroid glands—these are located behind the thyroid gland and make parathyroid hormone that increases the calcium level in the bloodstream.
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Adrenal glands—the adrenal glands are located above the kidneys. They make mineralocorticoids that regulate salt excretion and blood pressure, sex hormones, and cortisol, which has many effects on the immune system and on blood sugar values.
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Pancreas—this is located in the abdomen and regulates blood sugar by secreting insulin to lower blood sugar and glucagon to raise blood sugar.
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Ovaries—these are located in the female pelvis and are responsible for producing estrogen and progesterone under the influence of the pituitary gland.
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Testes—these are located in the scrotum of males. They secrete testosterone, which is the major male hormone.
REPRODUCTIVE SYSTEMS There is both sexual and asexual reproduction in the animal kingdom. There are advantages and disadvantages to both methods of reproduction. Asexual reproduction has the capacity to release large numbers of offspring that are genetically identical to the parent organism. It leads to highly adapted offspring during situations where the environment is stable. If the environment is unstable; however, the offspring might not be able to adapt to the changing environment. Sexual reproduction produces an organism that requires two parents but that creates a genetically unique organism. This creates diversity in the environment, which is potentially helpful in ensuring survival of the population. Usually, this involves the
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