Endocrine System

 

The endocrine system is a regulatory system composed of ductless glands that emit chemical messengers known as hormones, which travel throughout the body via the bloodstream and affect distant organs. Ductless glands are known as Endocrine Glands, and they produce hormones directly into the bloodstream for distribution throughout the body. In contrast, exocrine glands have ducts and produce materials into them for delivery to bodily cavities. However, other organs and tissues, like the hypothalamus and thalamus, are not categorized as endocrine glands but do include hormone-secreting cells. As a result, the endocrine system includes both endocrine glands and hormone-secreting cells.

Hormones are chemical messengers (aka signal molecules). Hormones are endocrine signal molecules that are released into the bloodstream by endocrine gland cells and then delivered to distant target organs. Hormones are low-molecular-weight, chemically diverse non-nutrient compounds produced in trace amounts that serve as intercellular communication molecules. Despite the fact that hormones are distributed throughout the body via the blood, only certain target cells can respond to each hormone since they contain receptors for binding with the specific hormone.

Hormones are chemically varied and originate from a variety of precursors, including amino acids, cholesterol, and phospholipids. Chemically, they fall into two categories: those that are soluble in lipids and those that are soluble in water. The lipid-soluble hormones include steroid hormones and thyroid hormones:

1. Steroid hormones are derived from cholesterol
2. Thyroid hormones (T3 and T4) are synthesized by attaching iodine to the amino acid tyrosine.

The water-soluble hormones include amine hormones, peptide and protein hormones.

1. Amino hormones are synthesized by modification of certain amino acids. epinephrine, norepinephrine and dopamine are synthesized by modifying amino acid tyrosine. Histamine is synthesized from the amino acid histidine by mast cells and platelets. Serotonin and melatonin are derived from tryptophan.
2. Peptide hormones and Protein hormones are amino acid polymers. Peptide hormones include antidiuretic hormone and oxytocin; protein hormones include human growth hormone and insulin. Several of the protein hormones, such a s thyroid-stimulating hormone, have attached carbohydrate groups and thus are glycoprotein hormones.

Hypothalamus

It is placed at the base of the forebrain's diencephalon and is regarded as a neuroendocrine structure rather than an endocrine gland itself. It secretes neurohormones. These hormones begin in hypothalamic neurons, travel through axons, and are released from nerve terminals. These hormones enter the pituitary gland via a portal circulatory system and control the anterior pituitary's functions. The hypothalamus directly regulates the posterior pituitary gland.

These hormones regulate the synthesis and secretion of pituitary hormones. Depending on their actions, these hormones are called releasing hormones or inhibiting hormones. It produces six hormones that have an influence on the anterior pituitary. These hormones are:

1. TRH (Thyrotropin-releasing hormone) - Stimulates secretion of TSH and prolactin
2. GnRH (Gonadotropin-releasing hormone) - Stimulates secretion of FSH and LH
3. GHRH (Growth hormone-releasing hormone) - Stimulates release of growth hormone
4. Somatostatin - Inhibits secretion of growth hormone, TSH, GI hormones, insulin, glucagon, and oppose the effects of GHRH.
5. CRH (Corticotropin-releasing hormone) - Stimulates secretion of ACTH
6. PIH (Prolactin-inhibiting hormone) - Inhibits prolactin secretion

Pituitary Gland

The pituitary gland (or hypophysis) is situated in the Sella turcica of the sphenoid bone and is connected to the hypothalamus of the infundibulum. In humans, the pituitary gland is divided into two primary parts: the massive anterior pituitary and the small posterior pituitary.

Anterior Pituitary:

Anterior pituitary consists of two portions, pars distalis and pars intermedia. However, in humans, the pars intermedia is almost merged with pars distalis. Anterior pituitary secretes seven hormones and the release of these hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus. Para distalis secretes growth hormone (GH), prolactin (PRL), thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), luteinizing hormone (LSH), and follicle stimulating hormone (FSH). Pars intermedia stimulates only one hormone called melanocyte stimulating hormone (MSH).

Posterior Pituitary:

Although the posterior pituitary does not synthesize hormones, it does store and release two hormones: oxytocin and vasopressin. The hypothalamus synthesizes these hormones, which are then delivered axonally to the posterior pituitary.

Pineal Gland

The pineal gland is a tiny endocrine gland located on the top of the third ventricle of the forebrain. The gland is made up of neuroglia and secretory cells known as pinealocytes. The pineal gland secretes melatonin, the hormone of darkness. Melatonin appears to help set the body's biological clock. Melatonin secretion increases up to tenfold during the dark of night and subsequently decreases to low levels during the light of day. Melatonin is supposed to promote sleepiness since it is produced more in darkness than in light. Melatonin is also an effective antioxidant that may protect against harmful oxygen free radicals. In animals, the pineal gland is positioned within the brain and so cannot receive direct light stimulation. Light from the eyes activates the gland through the optic nerve.

Thyroid Gland

The thyroid is the body's largest endocrine gland. It is found on both sides of the trachea. It is made up of left and right lateral lobes, one on each side of the trachea, which are joined by a fibrous connective tissue called the isthmus. It generates hormones such as thyroid hormone and calcitonin. The thyroid gland is made up of multiple spherical hollow sacs known as thyroid follicles and parafollicular cells. Follicular cells produce two hormones: thyroxine (tetraiodothyronine, or T4) and triiodothyronine (T3). T3 and T4 are referred to as thyroid hormones. T has three atoms of iodine, while T4 includes four atoms. The parafollicular cells are located between follicles and emit a hormone known as Calcitonin or Thyrocalcitonin.

Thyroid hormones regulate the basal metabolic rate (BMR). It boosts BMR in most tissues (excluding the brain, spleen, and testes) by encouraging the utilization of cellular oxygen. When BMR increases, so does cellular metabolism of carbs, fats, and proteins. As cells make and use more ATP, they generate more heat, causing the body temperature to rise. This phenomenon is known as the Calorigenic effect. Thyroid hormones, like human growth hormone and insulin, promoted body growth, notably in the brain and skeletal systems.

Calcitonin is a peptide hormone secreted by thyroid parafollicular cells, which are different from thyroid follicles. Thyroid cells create calcitonin in response to elevated blood calcium levels. It reduces plasma calcium concentration by reducing calcium mobilization from bones, hence increasing osteoblastic activity.

Parathyroid Gland

The parathyroid glands are four tiny glands located on the rear of the thyroid gland, one pair in each of the two lobes. They secrete parathyroid hormone (PTH), which raises calcium levels in the blood. Its secretion is controlled by the blood calcium level. Bone tissues operate as a calcium storage reservoir, and PTH increases calcium removal from the bone, increasing levels in the blood. It enhances calcium reabsorption by the renal tubules of the kidney, resulting in decreased urine loss, but also stimulating phosphate loss. It also stimulates the kidney to produce calcitriol, which enhances calcium absorption from digested food in the stomach. PTH is thus a hypercalcemic hormone, which raises blood calcium levels.

Thymus Gland

The thymus is placed behind the sternum, between the lungs. The thymus produces four hormones: Thymosin, Thymic Humoral Factor, Thymic Factor, and Thymopoietin. These hormones stimulate the maturation of T cells (a type of WBC that eliminates foreign substances) and may slow the aging process. Thymosins are thymus hormones that drive the formation and differentiation of T-lymphocytes, or T-cells. They help to regulate the immune system by stimulating different types of immune cells. It also regulates childhood growth.

Pancreas

The pancreas functions as both an exocrine and an endocrine gland. The pancreas' exocrine cells are organized in clusters called Acini. The acini create digesting enzymes, which enter the GI system via the pancreatic duct. Islets of Langerhans (also known as pancreatic islets) are clusters or islands of endocrine cells found throughout the pancreas between the acini. The islets account for around 1% to 2% of the overall pancreatic mass. Each pancreas has four types of hormone-secreting cells:

1. Alpha or A-cells constitute about 17% of pancreatic islet cells and secrete Glucagon.
2. Beta or B-cells constitute about 70% of pancreatic islet cells and secrete insulin.
3. Delta or D-cells constitute about 7% of pancreatic islet cells and secrete somatostatin
4. F-cells constitute the remainder of pancreatic islet cells and secrete pancreatic polypeptide.

Adrenal Glands

Our bodies have one pair of adrenal glands, one for each kidney, and they are placed superior to them. They are separated into two regions: a large, peripherally positioned Adrenal Cortex (80–90%) and a tiny, centrally located Adrenal Medulla.

The adrenal cortex is divided into three sections, each of which secretes a separate hormone. It produces three types of steroid hormones, including aldosterone and glucocorticoids (including cortisol), which are generally referred to as corticoids.

The adrenal medulla is a modified version of the sympathetic nervous system. It is made up of modified postganglionic sympathetic neurons. The adrenal medulla produces two primary hormones: epinephrine (or adrenaline) and norepinephrine (or noradrenaline), which are both released by chromaffin cells from the amino acid tyrosine. Epinephrine and norepinephrine are referred to as Catecholamines.

Female & Male Gonadas 

The basic reproductive organs are known as the Gonads, which include the Testes in men and the Ovaries in women. Gonads are the organs that create gametes, namely sperm in men and oocytes in females. In addition to this, the gonads release hormones.

The basic reproductive organs are known as the Gonads, which include the Testes in men and the Ovaries in women. Gonads are the organs that create gametes, namely sperm in men and oocytes in females. In addition to this, the gonads release hormones.

The ovaries also conduct endocrine functions. It secretes several types of female hormones, including estrogens, progesterone, inhibitin, and relaxin. Graffian follicles are responsible for producing the majority of estrogen. Its secretion is stimulated by pituitary hormones LH and FSH. Progesterone is produced by the Corpus Luteum. The adrenal cortex and placenta secrete a modest quantity of progesterone. The ovaries also generate Inhibin, a protein hormone that prevents the release of follicle-stimulating hormone (FSH). During pregnancy, the ovaries and placenta release a peptide hormone known as relaxin.

I'll explain all the hormones, mentioned here in detail in my upcoming blog, so stay tuned.