Endocrine Glands And Hormones: Function, Organs, Diseases

What Are Endocrine Glands?

The endocrine glands in the human body are special organs that produce and secrete hormones directly into the bloodstream, thereby controlling such a wide array of physiological processes as those related to growth, metabolism, and homeostasis. Those include major glands such as the pituitary, thyroid, adrenals, and pancreas, which participate in the maintenance of internal balance.

Unlike exocrine glands, which release secretions through ducts to specific locations like sweat glands or salivary glands, the endocrine glands are ductless and require the circulatory system for transmission. This direct release into the circulatory system results in a diffuse action, systemic in nature, required for the coordination of activities of the body over long periods and responses to internal and external stimuli. The endocrine system plays a vital role in maintaining homeostasis, affecting growth and development, and regulating reproduction, therefore, keeping a person healthy and fit.

Major Endocrine Glands

The major endocrine glands are:

Pituitary Gland

A pea-sized endocrine gland located at the base of the brain directly behind the socket of the eye and just below the hypothalamus; it is enclosed within a bony prominence of the sphenoid bone, the sella turcica.

Hormones produced:

• Growth Hormone – GH: Stimulation of growth and cell reproduction.

• Prolactin: Milk production within the mammary glands.

• Adrenocorticotropic Hormone – ACTH: Stimulation of the adrenal cortex to release cortisol.

• Thyroid-Stimulating Hormone – TSH: Stimulation of the thyroid gland to produce thyroid hormones

• Follicle-stimulating hormone – FSH: It regulates the reproductive process. This includes activities such as maturation of ovarian follicles and spermatogenesis.

• Luteinizing Hormone - LH: Ovulation stimulation and maintenance of sex hormone secretion.

• AT or ADH: Water amount regulation in the body; water reabsorption in kidneys

• Oxytocin: Uterine contractions at birth; Milk letdown, lactation.

Functions and effects on the body

This is sometimes thought of as the "master gland" and does act in a manner somewhat similar to the pituitary gland. Such functions, through the release of hormones, actually do different facets of the body to control other endocrine glands. It has a very vital role in growth, metabolism, stress response, and reproductive processes.

Hypothalamus

Connection with the pituitary gland

The hypothalamus lies just above the pituitary gland and communicates with it through the hypothalamic-pituitary portal circulation, a system of blood vessels that enables the direct exchange of hormones between the two.

Hormones produced:

• Thyrotropin-Releasing Hormone (TRH): Stimulates TSH release from the pituitary.

• Corticotropin-Releasing Hormone (CRH): Stimulates ACTH release from the pituitary.

• Gonadotropin-releasing hormone (GnRH): Controls the release of FSH and LH from the pituitary.

• Growth Hormone-Releasing Hormone: stimulates growth and development.

• Somatostatin: Inhibits GH and TSH release.

• Dopamine: Inhibits prolactin release.

Role in homeostasis and hormone regulation

Through its control over the activities of the pituitary, the hypothalamus indirectly regulates many of the body's functions, from overall growth and development to metabolic rate, responses to stress and reproductive activities.

Thyroid Gland

It is located anterior to the trachea in the neck. The thyroid has two lobes joined at an isthmus

Hormones produced:

Thyroxine and Triiodothyronine, T4 and T3: This mediates the metabolic processes, producing energy, and growth.

Calcitonin: This reduces blood calcium through the inhibition of bone resorption and with an increased calcium deposition within the bones.

Functions and effects on metabolism, growth, and development

As mentioned, thyroid hormones regulate the metabolic rate, which affects growth. In addition, they are also necessary for maturation of nervous tissue.

Parathyroid Glands

The parathyroid glands are four small glands on the posterior surface of the thyroid gland.

Hormone produced:

Parathyroid Hormone (PTH): As a consequence of increased bone resorption, improved renal calcium reabsorption, and stimulation of intestinal calcium absorption, PTH brings up the level of calcium in the blood.

Functions and regulation of calcium levels

PTH controls the level of calcium required to keep the proper working of the muscles, transmission of nerve impulses, and correct bone health.

Adrenal Glands

Two adrenal glands sitting at the top of each kidney are divided into two distinct parts—the cortex and the medulla.

Hormones produced:

Cortisol: Controls the metabolism, and immune response, and enables the body's response to stress.

Aldosterone: Controls the balance of sodium-potassium. The product of this event affects blood pressure.

Adrenaline or Epinephrine and Noradrenaline or Norepinephrine: These are sometimes referred to as "fight-or-flight" hormones. During dangerous situations, they increase heart rate, blood pressure, and energy supply.

Functions and response to stress

The adrenal glands are mainly concerned with the management of stress reactions and controlling a wide array of metabolic and physiological activities.

Pancreas

Dual role as an endocrine and exocrine gland

The pancreas acts both as an endocrine gland concerning the production of hormones and as an exocrine gland concerning the secretion of digestive enzymes.

Hormones produced:

• Insulin: It reduces blood sugar by increasing the uptake of glucose by cells.

• Glucagon: It increases blood sugar by the release of glucose stored in the liver.

• Somatostatin: This is against the release of both insulin and glucagon.

Functions and regulation of blood glucose levels

The pancreas controls the concentration of glucose in the blood through actions of insulin and glucagon. Hence this maintains homeostasis in energy

Pineal Gland

It is a very small, pea-shaped gland, which rests in the epithalamus of the brain close to the middle of the brain.

Hormone produced:

Runs an individual's sleep-wake cycle and is also responsible for seasonal biological rhythms.

Functions and regulation of sleep-wake cycles

Melatonin appears to play a role in the production of the 'circadian rhythm' and sleep-wake cycles and, therefore, acts as the 'body clock.'

Gonads (Testes and Ovaries)

Testes are in the scrotum whereas ovaries are in the pelvic cavity.

Hormones produced:

• Testosterone: It controls male reproductive functions as well as secondary sexual characteristics.

• Estrogen and Progesterone: They regulate female reproductive activity, the menstrual cycle, and the manifestation of secondary sexual characteristics.

Functions and roles in reproduction and secondary sexual characteristics

Gonads play a crucial role in reproduction and in expressing secondary sexual traits and, therefore impact the physical characteristics and behaviours defining sexual maturation.

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