Chemical coordination and regulation: Endocrine glands and hormones

Introduction

The human body has secretory organs called glands. There are two kinds of glands in the body of multicellular organisms, namely exocrine and endocrine glands. The physical body consists of two interdependent integrating systems, called nervous and endocrine systems. Endocrine glands and hormones go hand in hand, where the human endocrine system modulates several body processes through the function of hormones. The endocrine system secretes hormones that further monitor the body’s functioning. In this way, the human endocrine system monitors and coordinates all body systems using hormones.  

The Endocrine System and Glands

Endocrine glands and the hormone production of localised scattered tissues/cells situated within various parts of our body make up an endocrine system. Although many parts of the body produce hormones, the major glands that compose the endocrine system include:

• hypothalamus
• pituitary
• thyroid
• parathyroids
• adrenals
• pineal body
• the ovaries
• the testes                       

1. Hypothalamus 

The hypothalamus is the part of the forebrain that regulates a wide range of bodily functions. It contains multiple groups of nerve cells known as nuclei that produce hormones. These hormones regulate how pituitary hormones are synthesised and secreted. 

These hormones released by the hypothalamus reach the anterior pituitary gland through the portal circulation system and regulate its function. The hypothalamus directly imposes control over the posterior pituitary. 

2. Pituitary Gland 

The pituitary gland lies within a bone cavity referred to as sella turcica. A stalk connects it with the hypothalamus. Though small in size, it is dubbed as the ‘master gland’. This gland is divided into two parts- adenohypophysis and neurohypophysis

The anterior lobe (that secretes seven hormones) and the posterior lobe (releases hormones into the circulation), together, form the pituitary gland. 

-Pituatory hormones

Pituitary hormones send signals to other endocrine glands to stimulate or inhibit the production of their hormone. The anterior lobe secretes hormones when it receives secretion or inhibition of hormones from the hypothalamus. These hormones from the hypothalamus tell the anterior lobe whether to release more of a particular hormone or to stop producing the hormone.  

• Anterior Lobe Hormones
• Growth Hormone (GH):  It maintains a healthy body composition and is essential for the growth of a child in his early years. GH supports healthy bone and muscle mass in adults and influences fat distribution.
• Prolactin: It offers stimulation to release breast milk. 
• Thyroid-stimulating Hormone (TSH):  It stimulates the thyroid gland to release hormones. 
• Adrenocorticotropic Hormone (ACTH): It vitalises the adrenal glands to release hormones.
• Luteinizing Hormone (LH): LH works with FSH to keep the ovaries and testicles functioning properly.
• Follicle-stimulating Hormone (FSH): It with LH to ensure normal ovarian and testicular function.
• Melanocyte-stimulating Hormone (MSH)- It works on melanocytes and regularises skin pigmentation. 

The posterior lobe contains the endings of neurons coming from the hypothalamus. The hypothalamus delivers the hormones straight into the posterior lobe using these nerves, and the pituitary gland releases them. 

• Posterior Lobe Hormones
• Oxytocin: It is implicated in various processes such as the uterine contraction during childbirth and stimulation of breast milk production. 
• Vasopressin [Anti-diuretic Hormone (ADH)]: This hormone causes the kidneys to increase water uptake in the bloodstream. 

3. Pineal Gland 

The pineal gland lies on the dorsal side of the forebrain. It produces melatonin, which helps maintain the circadian rhythm and regulates reproductive hormones. For instance, it helps to maintain the natural rhythms of the sleep-wake cycle, body temperature. Moreover, melatonin also affects metabolic pigmentation, menstrual cycle and defence ability. 

4. Thyroid Gland

The thyroid consists of two lobes that flank the trachea. A thin flap of connective tissue, called the isthmus, connects both lobes. The thyroid consists of follicles and interstitial tissue. Each thyroid follicle consists of follicular cells that surround a cavity. These follicular cells make two hormones, tetraiodothyronine or thyroxine (T4) and triiodothyronine (T3).

Iodine is necessary for the normal rate of hormonal formation in the thyroid gland. Lack of iodine in our diet can lead to hypothyroidism and an enlarged thyroid gland, commonly known as a goitre. 

Thyroid hormone plays an important role in regulating basal metabolic rate. These hormones also support the process of erythropoiesis and control the metabolism of carbohydrates, proteins and fats. Maintaining a balance between water and electrolytes is also affected by thyroid hormones. 

5. Parathyroid Gland 

Located behind the thyroid gland, it secretes a peptide hormone called parathyroid hormone (PTH). Parathyroid hormone (PTH) leads to the concentration of calcium ions in the blood. It also helps reabsorb calcium from the kidney tubules and digestive tract.

6. Thymus 

Situated on the heart’s and the aorta’s dorsal side, the thymus releases the peptide hormone thymosins that aid in T-lymphocyte differentiation for cellular immunity. It also stimulates the production of antibodies that provide humoral immunity.

7. Adrenal Gland 

It is located on the front of each kidney and consists of two types of tissue in the centre of the adrenal medulla and outside the adrenal cortex. The adrenal medulla secretes the hormones epinephrine and norepinephrine, commonly referred to as catecholamines and are also known as emergency hormones. These hormones increase attention, pupil dilation, sweating, heart rate, respiratory rate, and glycogenolysis.

8. Pancreas 

The pancreas is a complex gland that functions as both an exocrine gland and an endocrine gland. The pancreas has the dual function of releasing hormones into the blood and secreting enzymes through ducts. The endocrine pancreas consists of “islets,” which contain alpha cells and beta cells. Both hormones are involved in maintaining blood sugar levels. 

• Insulin- This hormone regulates blood sugar by allowing many cells in the body to absorb and utilise glucose. This, in turn, lowers blood sugar levels. In the absence of insulin, glucose is removed from the blood and excreted by the kidneys. This is a condition called diabetes.
• Glucagon- The second hormone in the pancreas, glucagon, stimulates the breakdown of glycogen into glucose in the liver.

9. Testis

The male scrotum (outside the abdomen) has a pair of testicles. The testes serve a dual function, not only as an endocrine gland but also as a major genital organ. The testicle is made up of seminiferous tubules and stromal or interstitial tissue. Leydig cells or stromal cells located in the intertubular space produce a group of hormones called androgens, which are mainly testosterone.

Androgens play a major stimulatory role during spermatogenesis (the formation of sperm). Androgens act on the central nervous system and affect male sexual behaviour (sexual desire). These hormones have anabolic (synthetic) effects on the metabolism of proteins and carbohydrates.

10. Ovary

The ovaries are the major female genitals that produce one egg during each menstrual cycle. In addition, the ovaries also produce two groups of steroid hormones called oestrogen and progesterone.

The ovary consists of ovarian follicles and interstitial tissue. Oestrogen is mainly synthesised and secreted by growing follicles. After ovulation, the ruptured follicle transforms into a structure called the corpus luteum, which mainly secretes progesterone.

Conclusion

Hormones play a major role in the human body’s communication system. They are responsible for the healthy functioning of the human body from head to toe. Hence, this endocrine glands and hormones study material may function as an overall guide for anyone who needs precise information about the functioning of endocrine glands and hormones in the human body.