Introduction

The endocrine system consists secretory organs called glands that release hormones into the blood vessels Figure 1. Major endocrine glands include pituitary, pineal gland, thyroid gland, adrenal gland, pancreas and reproductive glands(Guyton and Hall, 2001).

Hypothalamus

• Growth-hormone-releasing hormone - Stimulate growth-hormone (GH) release from anterior pituitary.

• Gonadotropin-releasing hormone – stimulates pituitary to release hormones that stimulate gonads viz. follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release from anterior pituitary.

Pituitary gland

Also known as hypophysis, pituitary manifests as pea sized protrusion from bottom of hypothalamus at the bottom of the brain. Pituitary gland is divided into two lobe viz. anterior and posterior lobes. The anterior lobe is controlled by hormones secreted by hypothalamus and secretes following hormones,

• Follicle-stimulating hormone (FSH): an essential hormone for sexual development. Stimulates primordial germ cells in both males and females. Causes Sertoli cells in males to secrete androgen binding proteins that binds to testosterone while in females FSH induces follicular growth in ovary.

• Prolactin: also referred to as luteotropic, stimulates mammary glands to produce milk.

Thyroid gland is situated in front of the neck, below the Adam’s apple. Morphologically it consists of two lobes connected by an isthmus. The activity of thyroid is regulated by Thyroid stimulating hormone secreted by pituitary. Thyroid is known to produce three hormones viz. thyroxine/T4 and triiodothyronine/T3 collectively referred to as thyroid hormones, and calcitonin.

Thyroid hormones regulate multiple biological processes including

Calcitonin is secreted in response high calcium levels in blood. Calcitonin decreases the activity of osteoclasts, cells involved in braking down bone, thereby reducing the amount of calcium secreted to the blood.

Parathyroid glands

• Aldosterone – regulates salt balance and blood volume. It acts on distal convoluted tubule and collecting ducts of kidney and increases resorption of sodium ions and excretion of potassium and hydrogen ions. Aldosterone production is regulated by angiotensin II and extracellular potassium levels. The increased uptake of sodium ions caused by aldosterone affect the extracellular volume and influences blood pressure. Since it regulates mineral uptake aldosterone is considered as a mineralocorticoid.

• Cortisol – is a Glucocorticoid that increase circulating level of glucose by mobilization of amino acids from protein and synthesis of glucose from these amino acids in the liver.

In addition to its primary function kidney also secrets two endocrine hormones erythropoietin and renin. Erythropoietin stimulates red blood cell production in bone marrow during hypoxia while renin is an enzyme that cleaves angiotensinogen into angiotensin I. angiotensinogen I is precursor to angiotensinogen II which induce production of aldosterone.

Pancreas

Together the opposing actions of insulin and glucagon are responsible for maintaining homeostatic regulation of blood glucose level Figure 2.

3. Evaluate the effects of a hormone imbalance

Conclusion

Unlike single celled organism multicellular organisms need to coordinate action between its different cells. In simple organisms this is easily achieved through simple cell to cell communication meditated molecules that acts as messengers. However, in complex organisms this communication calls for targeted and efficient delivery of such chemical messengers, sometime over large distances. Endocrine system in higher organisms allows for long distance regulation of biological processes by co-opting circulatory system to deliver messenger molecule, hormones, to target organs.

References

GUYTON, A. C. & HALL, J. E. 2001. Textbook of Medical Physiology, Saunders.