The Thymus Gland (The Immunity Training Camp)

The thymus gland is an important lymphoid and endocrine organ of the body. It plays a major role in the development of immunity, especially during childhood.

Location

It is a soft, lobular structure located between the lungs, safely tucked behind the sternum (breastbone) on the ventral (front) side of the aorta.

Hormone

It secretes peptide hormones collectively called Thymosins.

Function:

  • The thymus is essentially the training ground for your immune system.
  • It promotes the differentiation and maturation of T-lymphocytes, which are responsible for Cell-Mediated Immunity (destroying infected cells).
  • It also helps in the production of antibodies (Humoral Immunity).

The Ageing Effect

Important Fact: The thymus is large in infants but gradually shrinks and degenerates as we age. This results in a decreased production of thymosins, which is why the immune responses of old persons become weak.

The Adrenal Gland

nThe adrenal glands are paired endocrine glands associated with the kidneys.

Location

We have a pair of adrenal glands. They sit like little caps right at the anterior (top) part of each kidney.

Structure

If you slice it open, you'll see it has two completely distinct tissues:

  1. Adrenal Medulla: The central, inner core.
  2. Adrenal Cortex: The outer wrapping tissue.

Hormones of the Adrenal Medulla (The Emergency Center)

Hormones

The medulla secretes two rapid-acting hormones: Adrenaline (Epinephrine) and Noradrenaline (Norepinephrine). Together, these are called Catecholamines.

Function (The 'Fight or Flight' Response)

  • These are emergency hormones. They are rapidly pumped into the blood during stress or dangerous situations.
  • Physical Effects: They instantly increase alertness, cause pupillary dilation (wide eyes), piloerection (goosebumps/raising of hairs), sweating, faster heartbeat, stronger heart contractions, and a rapid rate of respiration.
  • Metabolic Effects: To fuel this emergency response, they stimulate the breakdown of glycogen (dumping glucose into the blood for instant energy), as well as lipids and proteins.

Hormones of the Adrenal Cortex (The Life Sustainers)

The cortex is divided into three distinct layers (from the outside in):

  1. Zona Glomerulosa (Outer layer)
  2. Zona Fasciculata (Middle layer)
  3. Zona Reticularis (Inner layer)

Types of Corticoids (Steroid Hormones)

  • Glucocorticoids (e.g., Cortisol): As the name suggests, they are involved in glucose (carbohydrate) metabolism.
    • Metabolism: They stimulate gluconeogenesis (making new glucose from non-carbs), lipolysis (fat breakdown), and proteolysis (protein breakdown).
    • Cellular effect: They actively inhibit cells from taking up and using amino acids (saving them for gluconeogenesis).
    • Cortisol: The main glucocorticoid. It maintains the cardiovascular system, produces powerful anti-inflammatory reactions, suppresses the immune response (which is why cortisol creams are used for rashes), and stimulates RBC production.
  • Mineralocorticoids (e.g., Aldosterone): Regulate water and mineral (electrolyte) balance.
    • Target: Acts mainly at the renal tubules of the kidneys.
    • Action: Stimulates the body to reabsorb Na+Na^+ (Sodium) and water, while telling it to pee out (excrete) K+K^+ (Potassium) and phosphate ions. This helps maintain blood volume and pressure.
  • Androgenic Steroids (Sex Corticoids): Secreted in very small amounts. They play a role in the growth of axial hair, pubic hair, and facial hair during puberty.

Disorders of the Adrenal Cortex

  • Addison's Disease: A serious condition caused by the underproduction (deficiency) of hormones by the adrenal cortex. Without these hormones, carbohydrate metabolism is altered, leading to acute, severe weakness and extreme fatigue.

Thymus and Adrenal Glands

1. The Thymus Timer:

  • Thymus trains T-cells.
  • It shrinks with Time (Ageing = Weak Immunity).

2. Adrenal Cortex Layers (Outer to Inner): G-F-R

  • Glomerulosa (makes Mineralocorticoids/Aldosterone)
  • Fasciculata (makes Glucocorticoids/Cortisol)
  • Reticularis (makes Sex Steroids/Androgens) (Trick: "Go Find Rex, Make Good Sex" matches the layers to the hormones).

3. Medulla vs. Cortex Functions:

  • Medulla: Short-term stress (Fight or flight →\rightarrow Adrenaline).
  • Cortex: Long-term stress & survival (Sugar/Salt balance →\rightarrow Cortisol/Aldosterone).

💡 Questions and Answers

Q1. Why is the immune system weak in old people?

An1: In old people, the thymus gland degenerates. Because of this, the production of thymosins decreases, and the maturation of T-lymphocytes becomes weaker. As a result, immune responses become weak.

Key points:

  • Thymus shrinks in old age
  • Less thymosin
  • Weaker T-cell immunity

Q2. Name the 'fight or flight' hormones.

A2: The fight or flight hormones are Adrenaline and Noradrenaline. They are secreted by the adrenal medulla during stress or emergency.

Key points:

  • Adrenaline + Noradrenaline
  • Secreted by adrenal medulla
  • Help in emergency response

Q3. Which hormone stimulates gluconeogenesis?

A3: Glucocorticoids, especially Cortisol, stimulate gluconeogenesis, which means formation of glucose from non-carbohydrate sources.

Key points:

  • Cortisol = major glucocorticoid
  • Stimulates gluconeogenesis

Q4. What is the main function of Aldosterone?

A4: The main function of Aldosterone is to maintain water and electrolyte balance. It increases reabsorption of sodium and water and increases excretion of potassium from the kidneys.

Key points:

  • Aldosterone acts on kidney tubules
  • Increases Na⁺ and water reabsorption
  • Increases K⁺ excretion

Q5. Which disease is caused by the deficiency of adrenal cortex hormones?

A5: Deficiency of hormones of the adrenal cortex causes Addison's disease.

Key points:

  • Addison's disease = adrenal cortex hyposecretion
  • Causes weakness and fatigue