Endocrine System Overview

Study Notes

The endocrine system is a collection of glands that secrete hormones directly into the bloodstream.
Hormones are chemical messengers produced by endocrine glands, affecting various tissues and organs.
Hormones are classified into two main groups: proteins and steroid hormones.
Protein hormones are mainly composed of amino acids, while steroid hormones are derived from lipids.
Hormones are extremely potent and effective in very small amounts.
Hormone secretion is stimulated when there's a need for their effects, with each hormone having a specific trigger.
Negative feedback mechanisms regulate the secretion of most hormones.

Target cells possess receptors for specific hormones.
Receptors are protein molecules located either on the cell membrane or inside the cell.
Protein hormones interact with membrane receptors, activating a chain reaction that ultimately leads to a cellular response.
Steroid hormones can easily pass through cell membranes and bind to receptors within the cytoplasm or nucleus.
Upon binding, steroid-receptor complexes either remain in the cytoplasm or migrate to the nucleus, ultimately activating gene expression and influencing cellular functions.

The pituitary gland is located beneath the hypothalamus and is often referred to as the "master gland".
It produces and releases several hormones that influence various organs and glands throughout the body.
The pituitary gland is divided into two lobes: the anterior pituitary lobe and the posterior pituitary lobe.

Growth Hormone (GH): Promotes overall body growth by increasing both skeletal and soft tissue mass.
- Excess GH before puberty leads to gigantism.
- Deficiency of GH before puberty results in dwarfism.
Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland's growth and the production of thyroxine (T4) and triiodothyronine (T3).
Adrenocorticotropic Hormone (ACTH): Regulates the secretion of hormones from the adrenal cortex.
Prolactin: Responsible for lactation in females.
Follicle-Stimulating Hormone (FSH): Involved in reproductive functions in both sexes.
- In females, FSH stimulates follicle growth within the ovaries.
- In males, FSH initiates sperm production in the testes.
Luteinizing Hormone (LH): Critical for ovulation in females.
- LH triggers the release of a mature ovum from the ovary.
- Ruptured follicular cells form the corpus luteum, which produces estrogen and progesterone.
- In males, LH stimulates testosterone secretion from the testes.

Antidiuretic Hormone (ADH): Increases water reabsorption in the kidneys, reducing urine production.
Oxytocin: Primarily responsible for uterine contractions during labor and milk release from the mammary glands (breast).

The thyroid gland is a butterfly-shaped structure situated above the trachea.
It consists of numerous follicles composed of follicular cells surrounding a cavity filled with colloid, the source of thyroid hormones.
The thyroid gland produces two key hormones: thyroxine (tetraiodothyronine or T4) and triiodothyronine (T3).

Thyroid hormones increase overall body metabolism, leading to increased heat production.
They play a crucial role in the development of the nervous system during fetal and neonatal stages.

Usually four in number, the parathyroid glands are embedded on the dorsal surface of the thyroid gland.
They secrete parathyroid hormone (PTH), which regulates serum calcium levels.

Specialized hormone-producing cells within the pancreas, termed islets of Langerhans, are responsible for insulin and glucagon secretion.
Alpha cells within the islets produce glucagon, while beta cells produce insulin.
Insulin facilitates glucose transport from the bloodstream into cells.
Insulin deficiency leads to diabetes mellitus.
The primary effect of glucagon is to elevate blood glucose levels.

Two adrenal glands are located on top of each kidney.
Each adrenal gland is composed of two parts: an inner adrenal medulla and an outer adrenal cortex.

It secretes epinephrine (adrenaline) and norepinephrine (noradrenaline), hormones that prepare the body for "fight or flight" responses.

The adrenal cortex produces three main types of steroid hormones: aldosterone, cortisol, and sex hormones.
Sex hormones produced by the adrenal cortex are present in small amounts and their specific significance is not fully understood.

Aldosterone promotes sodium reabsorption and potassium excretion by the kidneys.

Cortisol enhances gluconeogenesis (glucose formation from amino acids and glycerol), providing glucose for brain cells and promoting the use of fatty acids as an energy source by other body tissues.
This process, known as glucose sparing, conserves glucose for the brain.
Cortisol also exhibits an anti-inflammatory effect, minimizing tissue damage.