A hormone is an endogenous (produced inside the body) biochemical substance that acts as a messenger and exerts its effects on particular target cells. There are several hormones, and they vary based on their source, target tissues, chemical composition, and functions. However, they’re identified and characterised by the fact that they’re produced in one place (cell or gland), and they travel through the bloodstream until they reach their target cell. Then, specific receptors bind the hormones on or in their target cells. Once the hormone has attached to the receptor, a response happens within the cell.
Most hormones are released by the glands of the endocrine system. These hormones influence almost every cell, tissue, organ, and function in the body. The hormones released are pivotal in regulating mood, metabolism, tissue function, sexual and reproductive processes, and growth and development.
HAVE A LOOK AND SEE OTHER SOURCES OF HORMONES:
Endocrine glands
An endocrine gland is an organ that produces a hormone. It does so within a specialised type of cell. For example, the anterior pituitary gland has cells that specialise in the manufacture of growth hormone, adrenocorticotropic hormone (ACTH), and thyroid-stimulating hormone (TSH).
Other organs
Some organs not traditionally included within the endocrine system by physiologists and anatomists have cells and tissues specialised in manufacturing hormones. Below are some examples:
- Specialised cells in the testes and ovaries convert cholesterol molecules into testosterone and oestrogen molecules, respectively.
- The intestines and stomach produce and release hormones that regulate the chemical and physical aspects of digestion.
- Your pancreas may be conveniently divided into two main parts: one part is concerned with releasing enzymes to digest food. In contrast, the other part contains specialised cells that produce and release glucagon and insulin.
- Even your heart produces hormones that quickly affect fluid balance (blood volume).
Neurons
Or nerve cells produce neurotransmitters that are also hormones. Whether it’s a hormone or a neurotransmitter depends on where it’s made and the distance the messenger travels to the target cell. For example, epinephrine produced by a neuron in the central nervous system is usually called a neurotransmitter. At the same time, those from the adrenal glands are aptly designated as hormones.
HOW DO HORMONES COMMUNICATE WITH THEIR TARGET CELLS OR ORGANS?
Hormones communicate their messages through a process called cell signalling. Don’t worry. It’s just a fancy way of saying that the effects of hormones are dependent on where they’re released. Classically, hormones are released from a specialised cell and into the bloodstream until they bind to their receptors on the target cell or organ. Cell signalling is not limited to the endocrine system. Below are some other common types of cell signalling:
Endocrine
Classical type of cell signalling. The hormone is released into the bloodstream and acts on distant target cells or organs.
Paracrine
The hormone isn’t required to be released into the bloodstream and exerts its effect on neighbouring cells.
Autocrine
The hormone exerts its effect on the cell that released it. Examples include the signalling between cytokines and immune cells. Another would be how cancer cells signal or message each other.
Intracrine
The hormone exerts its effect on the inside of the cell that released it. Angiotensin and angiotensin II of the renin-angiotensin system are hormones that use intracrine cell signalling to exert their effects.
The endocrine system and its hormones are in charge of mostly gradually occurring processes. The nervous system controls quicker processes like body movement and breathing. They work closely together. The nervous system tells the endocrine system when it should release or stop the release of hormones. The hormones control every aspect of metabolism in your body. Too much or too little of a particular hormone can lead to a medical condition.
WHAT ARE HORMONES MADE UP OF?
Biochemically speaking, hormones may be classified into three types: those made up of peptides and proteins, those composed of amines, and those formed from lipids. Peptide hormones are composed of short chains of amino acids. Amine hormones are derivatives of the amino acids tryptophan and tyrosine. Lastly, lipid hormones are predominantly derived from fatty acids. See the table below to know what some critical hormones are made up of.
Important Hormones
| Hormone | Cell Signalling Type | Structure | Source | Function |
| Adrenocorticotropic hormone (ACTH) | Endocrine | Peptides | Pituitary gland | Stimulate the release of glucocorticoids and mineralocorticoids (see below) by the adrenal gland. |
| Antidiuretic hormone (ADH) | Endocrine | Peptides | Pituitary gland | It tells the kidneys to reabsorb water to prevent dehydration. |
| Calcitonin | Endocrine | Peptides | Thyroid gland | It tells the intestines, kidneys, and bones to decrease calcium levels in the blood. |
| Epinephrine and Norepinephrine | Endocrine | Amine | Adrenal gland | Stimulates the muscles and the heart during the fight-or-flight response. Increases glucose levels in the blood for a quick source of energy. |
| Glucagon | Endocrine | Peptides | Pancreas | It makes fat tissues, muscles, and liver release glucose into the circulation. |
| Glucocorticoids | Endocrine | Lipids | Adrenal gland | Activate the formation of glucose from proteins and fats. |
| Gonadocorticoids | Endocrine | Lipids | Adrenal glands | Activate the libido. |
| Growth hormone (GH) | Endocrine | Peptides | Pituitary gland | Stimulates the soft tissues and bones to increase cell numbers and size. |
| Insulin | Endocrine | Peptides | Pancreas | It makes fat tissues, muscles, and liver store glucose to reduce blood glucose levels. |
| Melatonin | Endocrine – pineal gland. Autocrine or paracrine – when the source is outside the pineal gland | Peptides | The primary source is the pineal gland | Stimulates a range of tissues to mediate regulation of biorhythms like sleep, wakefulness, metabolism, blood pressure, temperature, other hormone levels, urine production, etc. For example, melatonin tells your body that it’s time to go to bed and when it’s time to go to work. Oops, we mean wake up. |
| Mineralocorticoids | Endocrine | Lipids | Adrenal glands | Activates the kidney cells to reabsorb sodium and get rid of potassium to maintain the balance of electrolytes (ions). |
| Oestrogen | Paracrine | Lipids | Ovaries | Activate the maturation and release of ova from the ovaries. Stimulates skin, muscles, and bone to develop female secondary sex characteristics*. |
| Oxytocin | Endocrine | Peptides | Pituitary gland | Stimulate breasts to release milk and the uterus to contract during childbirth. |
| Parathyroid hormone | Endocrine | Peptides | Parathyroid glands | It tells the intestines, kidneys, and bones to release calcium to increase levels in the blood. |
| Progesterone | Endocrine | Lipids | Ovaries | Readies the uterus for implantation of the embryo and sustains the pregnancy. |
| Prolactin | Endocrine | Peptides | Pituitary gland | Activates the breasts to produce milk. |
| Testosterone | Paracrine | Lipids | Testes | Activates sperm production in testes. Stimulates skin, muscles, and bones to develop male secondary sex characteristics**. |
| Thyroid-stimulating hormone (TSH) | Endocrine | Glycoprotein | Pituitary gland | Activates the thyroid gland to produce and release thyroxine and calcitonin. |
| Thyroxine | Endocrine | Amine | Thyroid gland | Plays a role in the control of development and growth. Targets all tissues to increase metabolism. |
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Female secondary sex characteristics
Include rounding of hips, breast development, a menstrual cycle, increased body fat composition, thicker hair on the head in some cases, less body hair, ability to breastfeed, reduced ability to form muscle mass at a fast rate, and decreased upper body strength.
Male secondary sex characteristics
Include increased facial and body hair, ability to increase muscle mass faster, lack of rounded hips, and stronger upper body strength.
HORMONES MATTER ALWAYS
HORMONES ARE YOUR BODY’S BIOCHEMICAL MESSENGERS THAT ARE ESSENTIAL FOR EVERYDAY LIFE.
A hormone is an endogenous (produced inside the body) biochemical substance that acts as a messenger and exerts its effects on particular target cells. There are several hormones, and they vary based on their source, target tissues, chemical composition, and functions. However, they’re identified and characterised by the fact that they’re produced in one place (cell or gland), and they travel through the bloodstream until they reach their target cell. Then, specific receptors bind the hormones on or in their target cells. Once the hormone has attached to the receptor, a response happens within the cell.
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References
Barrett, K., Barman, S., Yuan, J., & Brooks, H. (2019). Ganong’s Review of Medical Physiology, Twenty-sixth Edition (26th ed.). McGraw Hill / Medical.
https://www.ncbi.nlm.nih.gov/books/NBK9924/ https://www.hopkinsmedicine.org/health/conditions-and-diseases/hormones-and-the-endocrine-systemHall, J. E., & Hall, M. E. (2020). Guyton and Hall Textbook of Medical Physiology E-Book (Guyton Physiology) (14th ed.). Elsevier.
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