Hormones in Animals: Types, Function and Classification

Animals are more complex than plants. They have several specialized organs that perform special functions for their control and coordination. In animals, this coordination is due to nervous coordination (nervous system) and chemical coordination (endocrine system). The endocrine and nervous system both works in a coordinated manner in animals to control various body activities. Let’s discuss the hormones in animals in detail with few important questions. 

Key terms: Testosterone, pituitary gland, oxytocin, endocrine gland, adrenal gland, estrogen

Read More: Difference between cerebellum and cerebrum

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What are Hormones?

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Hormones are chemicals secreted directly into the blood by various endocrine glands and transported to the respective organs, where they act by regulating various metabolic processes. The organ affected by hormones is called the target organ. Target cells have particular receptors to recognize the respective hormone.

There are 2 types of glands present in our body:

Exocrine gland: These glands have ducts to pass their secretions, for example, liver, sweat, etc.

Endocrine gland: These glands do not have ducts and transport their secretion through the blood to the site of action, for example, pituitary glands, adrenal glands, etc.

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Hormones and Their Functions

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Types of Hormonal Imbalance

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Hyperthyroidism: When the thyroid gland produces too much thyroxine hormone, it results in hyperthyroidism. Symptoms include sweating, fast heart rate, etc.

Adrenal Insufficiency: Addison’s disease occurs as a result of insufficient adrenalin secretion. Symptoms such as dehydration, fatigue, and skin changes indicate Addison’s disease.

Acromegaly (Gigantism): The hypersecretion of growth hormone from the pituitary gland in kids results in an abnormally huge body.

Hypothyroidism: When the thyroid gland does not produce much thyroxine, it results in hypothyroidism. Symptoms include constipation, goiter (enlargement of the thyroid gland), fatigue, dry skin, etc.

Cushing’s disease: Hypersecretion of ACTH from the pituitary gland can result in an overactive adrenal gland. This is a type of Cushing's syndrome, which occurs in humans with high corticosteroid levels.

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Classification of Hormones in Animals

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Based on chemical structure, hormones can be classified into the following:

Steroid Hormones

Steroid hormones are derived from cholesterol, a lipid-derived hormone. They are generated from precursors on demand and released from the parent cells through simple diffusion. These hormones bind themselves to proteins when carried in the blood and there is usually a targeted response involving new protein production. Steroid hormones are the primary group of lipid hormones. They are usually ketones or alcohol chemically, for example, testosterone and estrogen, which are secreted by the male and female reproductive systems respectively. These hormones are insoluble in water. They last longer than peptide hormones because they are carried by transport proteins in the blood. Example: the hormone cortisol. Its half-life ranges from 60 to 90 minutes.

Protein/Peptide Hormones

These hormones consist of polypeptide chains of linked amino acids. Peptide hormones are stored and synthesized in secretory vesicles. They are released from the parent cell through exocytosis and are found in the cell membrane. This group also includes smaller proteins such as growth hormone which are synthesized by the pituitary and larger glycoproteins such as FSH (follicle-stimulating hormone) which are also synthesized by the pituitary. After being stored in the vesicles, a stimulus triggers a response causing it to be released, for example, a high blood sugar level causes insulin to be secreted. These hormones are soluble in water and insoluble in lipid. The receptors are found on the cell surface of the target because they cannot permeate through the plasma membrane of the cells.

Amino-acid Derived Hormones

Amino-acid derived hormones are comparatively smaller molecules that are derived from tryptophan and tyrosine amino acid. Epinephrine and Norepinephrine are secreted by the adrenal gland, they are commonly known as catecholamines. Melatonin is secreted by the pineal gland located in the brain, which is responsible for controlling the sleep-wake cycle.

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List of Female Hormones

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Hormones that are responsible for female secondary sexual characteristics are called female hormones. Some of them are also naturally produced in men but a very small amount and are not effective. They play a role in women’s fertility because they are more effective in women’s bodies. Listed below are female hormones:

Estrogen: It plays an essential role in pregnancy, menopause, puberty, and, menstruation.

Progesterone: These hormones aid in pregnancy, prepare the lining of the uterus for the fertilized egg, and reduce the synthesis of estrogen after the ovulation phase.

Testosterone: Although it is considered to be a male hormone, it is similar to the amount of estrogen produced in men because a small amount of testosterone is also made in women.

Human Chorionic Gonadotropin (HCG): secreted by the placenta.

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Hormonal Regulations of Body Processes in Animals

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Blood Glucose

Glucose is the primary energy source for most animal cells, and it is distributed throughout the body through the bloodstream. The ideal or target, blood glucose concentration is approximately 90 mg/100 mL of blood, which is equivalent to approximately 1 teaspoon of glucose per 6 quarts of blood. After a meal, carbohydrates are broken down during digestion and absorbed into the bloodstream. The amount present after a meal is typically more than the body needs at that time, and so the excess glucose must be removed and stored for later use. The opposite phenomenon occurs after a period of fasting. Insulin and Glucagon are two hormones that are primarily responsible for maintaining proper blood sugar levels.

Hunger Management

The immediate form of energy for most animal cells is glucose, and excess glucose is stored as glycogen which is easily broken down into glucose is stored as glycogen which is easily broken down into glucose when needed. Long-term stores of energy are stored as fat in cells called adipocytes. Too little fat means there may not be enough energy reserves at times when food is less available and this makes the animal hungry, however, too much fat is generally unhealthy and likely to satisfy the animal. The hormone leptin helps in maintaining the proper amount of fat stores in the body.

Growth and Metamorphosis

Invertebrate species that undergo metamorphosis, such as amphibians, are responsible for the recognization of the development of internal organ systems, the development of new structures, and other processes that occur during metamorphosis. In insects, metamorphosis is controlled by a set of hormones that determine whether the animal progresses to the next larval stage or turns into an adult as it grows older. The corpus allatum, an endocrine gland in the brain, secretes a hormone called juvenile hormone during all larval stages, which maintains the animal’s larval state. As the larva grows, another endocrine gland in the brain releases a prothoracicotropic hormone, which signals the prothoracic gland to release the hormone ecdysone. Ecdysone promotes either molting (shedding the exoskeleton) or metamorphosis depending on juvenile hormone levels. Ecdysone in combination with higher juvenile hormones results in molting in the next larval state, ecdysone in combination with low juvenile hormone results in metamorphosis in an adult.

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Things to Remember

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• Hormones are chemicals secreted directly into the blood by various endocrine glands and transported to the respective organs, where they act by regulating various metabolic processes. 
• When the thyroid gland produces too much thyroxine hormone, it results in hyperthyroidism. Symptoms include sweating, fast heart rate, etc.
• Addison’s disease occurs as a result of insufficient adrenalin secretion. Symptoms such as dehydration, fatigue, and skin changes indicate Addison’s disease.
• The hypersecretion of growth hormone from the pituitary gland in kids results in an abnormally huge body. When the thyroid gland does not produce much thyroxine, it results in hyperthyroidism. Symptoms include constipation, goiter (enlargement of the thyroid gland), fatigue, dry skin, etc.
• Hypersecretion of ACTH from the pituitary gland can result in an overactive adrenal gland. This is a type of Cushing's syndrome, which occurs in humans with high corticosteroid levels.
• Based on chemical structure, hormones in animals can be classified into, steroid hormones, peptide hormones and amino-acid derived hormones.
• Hormones that are responsible for female secondary sexual characteristics are called female hormones. Female hormones are, Estrogen, Progesterone, Testosterone, and Human Chorionic Gonadotropin (HCG).

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