The human endocrine system is an intricate network of ductless glands that secrete hormones directly into the bloodstream. These hormones act as chemical messengers, traveling to distant target organs to initiate specific metabolic or physiological responses. For NEET, a deep understanding of the hypothalamus-pituitary axis, adrenal functions, and reproductive endocrinology is vital.
1. The Pituitary Gland (Hypophysis)
Known as the 'Master Gland', the pituitary is located in a bony cavity called Sella turcica. It is divided anatomically into an adenohypophysis and a neurohypophysis. The adenohypophysis consists of the Pars distalis and Pars intermedia.
- Pars distalis: Secretes Growth Hormone (GH), Prolactin (PRL), Thyroid Stimulating Hormone (TSH), Adrenocorticotrophic Hormone (ACTH), Luteinizing Hormone (LH), and Follicle Stimulating Hormone (FSH).
- Pars intermedia: In humans, it is almost merged with the pars distalis. It secretes Melanocyte Stimulating Hormone (MSH). MSH acts on the melanocytes (melanin-containing cells) in the skin and regulates the distribution of pigment, thereby affecting skin color.
- Neurohypophysis (Pars nervosa): It does not synthesize hormones but stores and releases Oxytocin and Vasopressin (ADH), which are actually produced in the hypothalamus.
2. The Adrenal Glands
Located at the anterior part of each kidney, these glands are composed of two types of tissues: the outer Adrenal Cortex and the inner Adrenal Medulla.
- Adrenal Medulla: It secretes two hormones called Catecholamines—Adrenaline (epinephrine) and Noradrenaline (norepinephrine). These are called "3F hormones" (Fight, Flight, or Fright) because they are secreted during emergency situations. They increase alertness, pupillary dilation, heart rate, and the rate of respiration. They also stimulate the breakdown of glycogen, resulting in an increased concentration of glucose in the blood.
- Adrenal Cortex: Divided into three layers (zona reticularis, zona fasciculata, and zona glomerulosa). It secretes corticoids like cortisol (glucocorticoid) and aldosterone (mineralocorticoid).
3. Reproductive Hormones: Ovary and Testis
The ovaries are the primary female sex organs. Each ovary is composed of ovarian follicles and stromal tissues. Two major groups of steroid hormones are produced: Estrogens and Progestogens.
- Corpus Luteum and Progesterone: After ovulation, the ruptured follicle is converted into a structure called the Corpus luteum. It secretes large amounts of Progesterone. Progesterone is essential for the maintenance of the endometrium (uterine lining), which is necessary for the implantation of the fertilized ovum and other events of pregnancy. It also acts on mammary glands and stimulates the formation of alveoli (sacs which store milk) and milk secretion.
- Relaxin: Towards the end of the gestation period, the hormone Relaxin is produced by the ovary. Its primary role is to relax the pubic symphysis and the pelvic joints, making the birth canal wider for the delivery of the baby.
4. Hormonal Interaction and Feedback Mechanisms
Hormone secretion is tightly regulated by negative feedback loops. For example, high levels of progesterone inhibit the secretion of GnRH from the hypothalamus and LH from the pituitary, preventing further ovulation during pregnancy. Similarly, the catecholamine response is a rapid-onset system that resets once the perceived threat or stressor is removed. Any imbalance in these systems leads to clinical conditions like Addison's disease (adrenal cortex deficiency), Cushing's syndrome (excess cortisol), or reproductive disorders.
5. Comparison Table for Quick Revision
| Hormone | Source | Primary Function |
|---|
| MSH | Pars intermedia | Skin pigmentation |
| Catecholamines | Adrenal Medulla | Emergency response (Fight/Flight) |
| Progesterone | Corpus luteum | Pregnancy maintenance |
| Relaxin | Ovary/Placenta | Pelvic ligament relaxation |
| ADH | Hypothalamus | Water reabsorption in kidneys |
6. Key Points for MCQs
- Location: Don't confuse the adrenal cortex with the adrenal medulla. Catecholamines are strictly medullary.
- MSH Source: While many glands are master glands, MSH comes specifically from the pars intermedia.
- Progesterone vs. Estrogen: Estrogen is secreted by growing follicles; Progesterone is by the Corpus Luteum.
- Emergency Hormones: Adrenaline and Noradrenaline are collectively called catecholamines.