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Hormones in Animals

Although we rarely think about them, the glands of the endocrine system and the hormones they release influence almost every cell, organ and function of our bodies. The endocrine system is instrumental in regulating mood, growth and development, tissue function and metabolism as well as sexual function and reproductive processes.
In general, the endocrine system is in charge of body processes that happen slowly, such as cell growth. Faster processes like breathing and body movement are monitored by the nervous system. But even though the nervous system and endocrine system are separate systems, they often work together to help the body function properly.
The foundations of the endocrine system are the hormones and glands. As the body's chemical messenger, hormones transfer information and instructions from one set of cells to another. Although many different hormones circulate throughout the bloodstream, each one affects only the cells that are genetically programmed to receive and respond to its message so we can say that hormones are target specific. Hormone levels can be influenced by factors such as stress, infection and changes in the balance of fluid and minerals in blood.
A gland is a group of cells that produces and secretes 'chemical messengers' called hormones. A gland selects and removes materials from the blood, processes the materials and secretes the finished chemical product to be used somewhere in the body. Some types of glands release their secretions in specific areas. Glands which are categorized as exocrine glands, such as the sweat and salivary glands, release secretions in the skin or inside of the mouth through ducts. Endocrine glands, on the other hand, release more than 20 major hormones directly into the bloodstream where they can be transported to cells in other parts of the body.
The major glands that make up the human endocrine system are the
  • Hypothalamus,
  • Pituitary,
  • Thyroid,
  • Parathyroids,
  • The Islets of Langerhans cells in the pancreas
  • Adrenals,
  • Pineal body
  • Ovaries and
  • Testes.
Although the endocrine glands are the body's main hormone producers, some non-endocrine organs such as the brain, heart, lungs, kidneys, liver, thymus, skin and placenta also produce and release hormones.
The hypothalamus, a collection of specialized cells that is located in the lower central part of the brain, is the primary link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary.
Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The production and secretion of pituitary hormones can be influenced by factors such as emotions and seasonal changes. To accomplish this, the hypothalamus relays information sensed by the brain (such as environmental temperature, light exposure patterns, and feelings) to the pituitary.
The tiny pituitary is divided into two parts: the anterior lobe  Adenohypophysis and the posterior lobe Neurohypophysis. 
The hormones secreted by the anterior lobe are:
  1. Somatotrophic hormone / Growth hormone
  2. Adrenocorticotropic hormone
  3. Thyroid-stimulating hormone
  4. Follicle-stimulating hormone
  5. Luteinizing hormone (females),
  6. Interstitial Cell Stimulating Hormone (males)
  7. Prolactin (females)
These induce the respective target glands/ cells ( namely body cells , adrenal glands , thyroid glands,gonads, ovaries, testes and ovaries to secrete their specific hormones respectively
The hormones secreted by the posterior lobe are
  1. Oxytocin which helps in uterine contractions during child birth and lactation
  2. Vasopressin / antidiuretic hormone (ADH) which promotes reabsorption of water by the nephrons of the kidneys
Along with the above stated hormones the pituitary also secretes endorphins, chemicals that act on the nervous system to reduce sensitivity to pain. In addition, the pituitary secretes hormones that signal the ovaries and testes. The pituitary gland also controls ovulation and the menstrual cycle in women.
The thyroid, located in the front part of the lower neck, is shaped like a butterfly and produces hormones called thyroxine that control the rate at which cells burn fuels from food to produce energy (Basal Metabolic Rate-B.M.R). As the level of thyroxin increases in the bloodstream, at the same time the speed at which chemical reactions occur in the body also increases. Thyroxine also plays a key role in bone growth and the development of the brain and nervous system in children. A hormone secreted by the pituitary gland controls the production and release of thyroxine.
Attached to the thyroid are four tiny glands, called parathyroids which regulate the level of calcium in the blood with the help of another hormone produced in the thyroid.
The pancreas consists of a special type of cells called Islets of Langerhans which produces (in addition to others) two important hormones, insulin- secreted by beta cells and glucagon- secreted by the alpha cells. They work together to maintain a steady level of glucose, or sugar. Insulin converts excess glucose into glycogen whereas when there is excess of glucose in the blood glucagon convert glycogen back to glucose, in the blood and to keep the body supplied with fuel to produce and maintain stores of energy.
The body has two triangular adrenal glands, one on top of each kidney. The adrenal glands have two parts, each of which produces a set of hormones and has a different function. The outer part, the adrenal cortex produces hormones that influence or regulate salt and water balance in the body, the body's response to stress, metabolism, the immune system, and sexual development and function.
The gonads are the main source of sex hormones. In males, they are located in the scrotum. Male gonads, or testes, secrete hormones called androgens, the most important of which is testosterone. These hormones regulate body changes associated with sexual development, including enlargement of the penis, the growth spurt that occurs during puberty, and the appearance of other male secondary sex characteristics such as deepening of the voice, growth of facial and pubic hair, and the increase in muscle growth and strength. Working with hormones from the pituitary gland, testosterone also supports the production of sperms by the testes.
The female gonads, the ovaries, are located in the pelvis. They produce eggs and secrete the female hormones estrogen and progesterone. Estrogen is involved in the development of female sexual features such as breast growth, the accumulation of body fat around the hips and thighs, and the growth spurt that occurs during puberty. Both estrogen and progesterone are also involved in pregnancy and the regulation of the menstrual cycle.


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