Endocrine System: Hormones, Organs, and Functions

Endocrine System Overview

The endocrine system is a network of glands that produce and secrete hormones to regulate various bodily functions, including growth, metabolism, reproduction, and homeostasis. Hormones act as chemical messengers, traveling through the bloodstream to target organs.

Key Endocrine Organs and Their Hormones

• Pituitary Gland: Often called the "master gland," it releases hormones that control other endocrine glands. Key hormones include FSH (Follicle-Stimulating Hormone), LH (Luteinizing Hormone), and GH (Growth Hormone).

• Ovaries: Female reproductive organs that produce estrogen and progesterone, regulating the menstrual cycle and secondary sex characteristics.

• Testes: Male reproductive organs that produce testosterone, responsible for male secondary sex characteristics and sperm production.

• Pineal Gland: Produces melatonin, which regulates sleep-wake cycles.

• Endothelium: Produces nitric oxide, a vasodilator that helps regulate blood pressure.

• Muscle/Bone: Targeted by GH for growth and development.

Hormone-Organ Matching

Each hormone is primarily produced by a specific organ or tissue, and is responsible for distinct physiological effects.

Hormone	Primary Organ/Tissue
FSH (Follicle-Stimulating Hormone)	Pituitary gland
LH (Luteinizing Hormone)	Pituitary gland
Progesterone	Ovaries
Testosterone	Testes
GH (Human Growth Hormone)	Pituitary gland (targets muscle/bone)
Nitric Oxide	Endothelium
Melatonin	Pineal gland

Hormone Functions and Effects

FSH and LH

FSH and LH are released by the pituitary gland and play key roles in reproductive function:

• FSH: Stimulates follicle development in ovaries and sperm production in testes.

• LH: Triggers ovulation in females and stimulates testosterone production in males.

Estrogen and Progesterone

• Estrogen: Produced by the ovaries, it regulates the menstrual cycle, supports bone health, and develops secondary sex characteristics.

• Progesterone: Also produced by the ovaries, it prepares the uterus for implantation and maintains pregnancy.

Testosterone

• Testosterone: Produced by the testes, it is responsible for male secondary sex characteristics such as increased muscle mass, bone density, and deepening of the voice.

Growth Hormone (GH)

• GH: Released by the pituitary gland, it stimulates growth in muscles and bones, increases protein synthesis, and shifts metabolism toward building tissues.

Nitric Oxide

• Nitric Oxide: Produced by the endothelium, it acts as a vasodilator, relaxing blood vessels and lowering blood pressure.

Melatonin

• Melatonin: Produced by the pineal gland, it regulates the sleep-wake cycle by promoting sleep. Its production is suppressed by light, especially blue light.

Hormone Effects Table

Hormone	Main Effect	Target Tissue/Organ
FSH	Stimulates follicle/sperm development	Ovaries/Testes
LH	Triggers ovulation/testosterone production	Ovaries/Testes
Estrogen	Regulates menstrual cycle, bone health	Various tissues
Progesterone	Prepares uterus for pregnancy	Uterus
Testosterone	Develops male characteristics	Muscle, bone, reproductive organs
GH	Stimulates growth	Muscle, bone
Nitric Oxide	Vasodilation	Blood vessels
Melatonin	Regulates sleep	Brain

Hormonal Feedback Loops

Many hormones are regulated by feedback mechanisms. For example, FSH and LH work together in a feedback loop with estrogen and progesterone to regulate the menstrual cycle.

• Negative Feedback: Increased levels of estrogen and progesterone inhibit further release of FSH and LH.

• Positive Feedback: A surge in LH triggers ovulation.

True or False: Common Misconceptions

• FSH stimulates testosterone production in males: False. LH stimulates testosterone production in the testes.

• Progesterone levels decrease during pregnancy: False. Progesterone levels increase to maintain the uterine lining.

• Testosterone is only for males: False. Females also produce small amounts in the ovaries and adrenal glands.

• Melatonin production is suppressed by light: True. Especially blue light.

Short Answer Explanations

FSH and LH Feedback Loop

• FSH prepares the follicle; LH triggers ovulation. Both hormones work together in a feedback loop with estrogen and progesterone.

Testosterone in Females

• Females produce small amounts of testosterone in the ovaries and adrenal glands, influencing bone density, muscle development, and libido.

Growth Hormone (GH) Effects

• GH promotes growth, shifts metabolism toward building tissues, and channels nutrients into muscle and bone.

Melatonin and Sleep

• Melatonin regulates the sleep-wake cycle. Its production is suppressed by light and can be disrupted by irregular schedules, stress, aging, or jet lag.

Key Terms and Definitions

• Hormone: Chemical messenger produced by glands, affecting distant organs.

• Endocrine gland: Organ that secretes hormones directly into the bloodstream.

• Feedback loop: Regulatory mechanism in which a hormone's effect influences its own production.

• Secondary sex characteristics: Physical traits that distinguish the sexes but are not directly involved in reproduction (e.g., muscle mass, voice depth).

Relevant Equations

• Hormone Regulation (Generalized):

$\text{Hormone Output} = \text{Stimulus} - \text{Inhibition}$

• Negative Feedback Example:

$\text{FSH/LH Production} \xrightarrow{\text{Estrogen/Progesterone}} \text{Inhibition}$

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard Anatomy & Physiology knowledge.