Hormones are chemical substances produced in the body for the control and regulation of activities of certain cells or organs.
Hormones are special chemical messengers in the body that are created in the endocrine glands. They control most major bodily functions, from simple basic needs like hunger to complex systems like reproduction, and even the emotions and mood.
Types of Hormones
Hormones can be grouped into three main types:
- Amines, these are simple molecules.
- Epinephrine (adrenal gland)
- Melatonin (pineal gland)
- Triiodothyronine (peripheral tissue of thyroid gland)
- Thyroxine (thyroid gland)
2. Proteins and Peptides – They are made from chains of amino acids.
- Amylin (or Islet Amyloid Polypeptide), Anti-Müllerian hormone (or Müllerian inhibiting factor or hormone), Adiponectin,Adrenocorticotropic hormone (or corticotropin), Angiotensinogen and angiotensin, Antidiuretic hormone (or vasopressin, arginine vasopressin), etc.
3. Steroids – They are derived from cholesterol.
- Endocrines glands
Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct. The major glands of the endocrine system include the pineal gland, pituitary gland, pancreas, ovaries, testes, thyroid gland, parathyroid gland, hypothalamus and adrenal glands.
These glands are also called ductless glands, since they lack excretory ducts. Instead, the secretors cells release their products, hormones, into the extra cellular space. From the extra cellular space, the hormones may enter the blood stream, by which they reach their target organs.
Endocrine Gland, Hormones and Their Functions
The hypothalamus is a part of the brain. It serves many different functions in the nervous system, and is also responsible for the direct control of the endocrine system through the pituitary gland.
The pituitary gland, also known as the hypophysis, is a small pea-sized lump of tissue connected to the inferior portion of the hypothalamus of the brain. It is also called “master gland” because its secretion controls other glands. The pituitary gland is actually made of 2 completely separate structures: the posterior and anterior pituitary glands.
is a small extension of the hypothalamus. The hypothalamus produced 2 hormones that are stored and released by the posterior pituitary:
- Oxytocin triggers uterine contractions during childbirth and the release of milk during breastfeeding.
- Antidiuretic hormone (ADH) prevents water loss in the body by increasing the re-uptake of water in the kidneys and reducing blood flow to sweat glands.
Anterior Pituitary: The anterior pituitary gland is the true glandular part of the pituitary gland. The function of the anterior pituitary gland is controlled by the releasing and inhibiting hormones of the hypothalamus. The anterior pituitary produces 6 important hormones:
- Thyroid stimulating hormone (TSH), as its name suggests, is a tropic hormone responsible for the stimulation of the thyroid gland.
- Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex, the outer part of the adrenal gland, to produce its hormones.
- Follicle stimulating hormone (FSH) stimulates the follicle cells of the gonads to produce gametes—ova in females and sperm in males.
- Luteinizing hormone (LH) stimulates the gonads to produce the sex hormones—estrogens in females and testosterone in males.
- Growth hormone (GH) also known as pituitrin affects many target cells throughout the body by stimulating their growth, repair, and reproduction.
- Prolactin (PRL) has many effects on the body, chief of which is that it stimulates the mammary glands of the breast to produce milk.
- Oxytocin stimulates both the release of milk from nipples and contraction of uterus during birth.
The pineal gland is found posterior to the thalamus of the brain. The pineal gland produces the hormone melatonin that helps to regulate the human sleep-wake cycle known as the circadian rhythm.
The thyroid gland is located at the base of the neck. The thyroid gland produces 3 major hormones: Calcitonin, Triiodothyronine (T3) and Thyroxine (T4).
The parathyroid glands are 4 small masses of glandular tissue found on the posterior side of the thyroid gland. The parathyroid glands produce the hormone parathyroid hormone (PTH), which is involved in the control of calcium content of the bone. PTH is released from the parathyroid glands when calcium ion levels in the blood drop below a set point. PTH stimulates the osteoclasts to break down the calcium containing bone matrix to release free calcium ions into the bloodstream. PTH also triggers the kidneys to return calcium ions filtered out of the blood back to the bloodstream so that it is conserved.
The adrenal glands are a pair of roughly triangular glands found immediately superior to the kidneys. The adrenal glands are each made of 2 distinct layers, each with their own unique functions: the outer adrenal cortex and inner adrenal medulla.
Adrenal cortex: The adrenal cortex produces many cortical hormones in 2 classes: glucocorticoids and mineralocorticoids
- Glucocorticoids have many diverse functions, including the breakdown of proteins and lipids to produce glucose. Glucocorticoids also function to reduce inflammation and immune response.
- Mineralocorticoids, as their name suggests, are a group of hormones that help to regulate the concentration of mineral ions in the body. The most important of these hormones is aldosterone which increases the re absorption of sodium ions by the kidney tubules.
Adrenal medulla: The adrenal medulla produces the hormones Adrenaline (epinephrine) and noradrenaline (norepinephrine) which are the emergency hormones. Both of these hormones help to increase the flow of blood to the brain and muscles to improve the “fight-or-flight” response to stress. These hormones also work to increase heart rate, breathing rate, and blood pressure while decreasing the flow of blood to organs that are not involved in responding to emergencies.
Pancreas is a large gland located in the abdominal cavity just inferior and posterior to the stomach. Within these pancreas are 2 types of cells—alpha and beta cells. The alpha cells produce the hormone glucagon, which is responsible for raising blood glucose levels. Glucagon triggers muscle and liver cells to break down the polysaccharide glycogen to release glucose into the bloodstream. The beta cells produce the hormone insulin, which is responsible for lowering blood glucose levels after a meal. Insulin triggers the absorption of glucose from the blood into cells, where it is added to glycogen molecules for storage.
The gonads—ovaries in females and testes in males—are responsible for producing the sex hormones of the body. These sex hormones determine the secondary sex characteristics of adult females and adult males.
- Testes: The testes are organs found in the scrotum of males that produce the testosterone in males after the start of puberty. During puberty, testosterone controls the growth and development of the sex organs and body hair of males, including pubic, chest, and facial hair.
- Ovaries: The ovaries are located in the pelvic body cavity in females. The ovaries produce the female sex hormones progesterone and estrogens. Progesterone is most active in females during ovulation and pregnancy where it maintains appropriate conditions in the human body to support a developing fetus. Estrogens are a group of related hormones that function as the primary female sex hormones. The release of estrogen during puberty triggers the development of female secondary sex characteristics such as uterine development, breast development, and the growth of pubic hair.
Thymus: produces hormones called thymosins that help to train and develop T-lymphocytes during fetal development and childhood. The T-lymphocytes produced in the thymus go on to protect the body from pathogens throughout a person’s entire life.
Plant Hormones and Their Functions:
Auxin: It promotes cell elongation e.g. tropic responses of plants. It stimulates cell division, e.g. root and fruit development. It also causes apical dominance and inhibits abscission.
Gibberellins: It promotes cell elongation, e.g. stem growth. It induces dormant seeds to germinate.
Cytokinins: It promotes cell division, e.g. stem, root and axillary bud growth. It inhibits ageing in leaves.