Endocrine System

Endocrine System Anatomy

Displayed here is an illustration depicting the human endocrine system, a complex network of glands that release hormones directly into the bloodstream for transportation to distant target organs. The system plays a crucial role in regulating metabolism, growth, development, tissue function, and mood.

At the top, we see the hypothalamus, located in the brain below the thalamus, above the brain stem. This region links the nervous system to the endocrine system via the pituitary gland. The pituitary gland, often termed the “master gland,” is depicted just beneath the hypothalamus. It secretes hormones that regulate several other hormone glands in the body.

To the right of the hypothalamus and pituitary gland is the pineal gland, also in the brain. It’s small and shaped like a pinecone, hence its name, and it produces melatonin, which regulates sleep patterns.

Moving down, we see the thyroid gland in the neck, which produces hormones that regulate the body’s metabolic rate, heart and digestive function, muscle control, brain development, and bone maintenance.

Further down, the image shows the thymus, located just behind the sternum and between the lungs. The thymus is where T lymphocytes or T cells mature. These cells are crucial for the adaptive immune response.

The adrenal glands are shown above the kidneys. These are responsible for producing hormones like cortisol and adrenaline, which help control blood sugar, burn protein and fat, react to stressors like a major illness or injury, and regulate blood pressure.

Central in the image is the pancreas, which lies in the abdomen and plays an essential role in digestion and blood sugar regulation. It produces insulin and glucagon, hormones that help manage blood glucose levels.

Finally, the image also includes the gonads, which are represented by the testicles in males and ovaries in females. These reproductive glands produce hormones that influence sexual development and reproductive functions. The testicles produce testosterone, while the ovaries produce estrogen and progesterone.

Each gland is depicted with a detailed illustration, emphasizing their structure and location relative to the human body. 

Hormone Physiology

This diagram provides an overview of the hormones produced by different glands and organs within the human endocrine system. The endocrine system is integral to regulating a vast array of bodily functions through the secretion of hormones, which are chemical messengers that travel through the bloodstream to tissues or organs.

Starting at the brain:

  • The hypothalamus, positioned at the base of the brain, is responsible for producing releasing and inhibitory hormones, such as TRH, CRH, GHRH, Somatostatin, Dopamine, and Vasopressin. These hormones regulate the secretion of pituitary hormones.

  • Just below the hypothalamus is the pituitary gland, often called the “master gland,” which produces hormones like GH (Growth Hormone), TSH (Thyroid Stimulating Hormone), ACTH (Adrenocorticotropic Hormone), FSH (Follicle-Stimulating Hormone), LH (Luteinizing Hormone), Prolactin, Oxytocin, and Vasopressin that have far-reaching effects on growth, metabolism, and reproduction.

  • The pineal gland, found within the brain, secretes melatonin, which is involved in regulating sleep-wake cycles.

  • The thyroid gland, located in the neck, produces thyroid hormones (T3 and T4), Calcitonin, and PTH (Parathyroid Hormone) which regulate metabolism, calcium balance, and bone metabolism.

Moving to the thorax:

  • The thymus, situated behind the sternum, secretes Thymopoietin, which is involved in the development of the immune system’s T-cells during early life.

In the abdomen:

  • The liver, while not traditionally considered an endocrine gland, does produce hormones such as IGF (Insulin-like Growth Factor) and THPO (Thrombopoietin), which are involved in growth and the production of blood platelets.

  • The adrenal glands, located atop the kidneys, produce androgens, glucocorticoids (like cortisol), adrenaline, and noradrenaline, which are crucial in stress response, metabolism, and the fight or flight response.

  • The pancreas has both exocrine and endocrine functions and produces Insulin, Glucagon, and Somatostatin, which are essential for regulating blood glucose levels.

In the reproductive system:

  • The ovaries in females produce estrogens and progesterone, which are vital for the menstrual cycle, pregnancy, and secondary sexual characteristics.

  • The testes in males produce androgens like testosterone, which are essential for sperm production and secondary sexual characteristics.

  • The placenta, which develops during pregnancy, produces hormones such as Estrogens and Progesterone, supporting the fetus and mother during pregnancy.


  • The kidneys produce Calcitriol, Renin, and Erythropoietin, involved in calcium metabolism, blood pressure regulation, and red blood cell production, respectively.

  • The stomach secretes hormones like Gastrin, Ghrelin, and Somatostatin, which are involved in digestion and appetite regulation.

  • The uterus produces Prolactin and Relaxin, especially important during pregnancy and childbirth.

Understanding the endocrine system is crucial for comprehending how the body maintains homeostasis and responds to internal and external stimuli. Each hormone has a specific function and target, indicating the complexity and precision of the hormonal regulatory mechanisms within the body.

Exocrine and Endocrine Gland Functions

The image depicts two types of glands in the human body: exocrine and endocrine glands, each with distinct functions and methods of secretion.

On the left, the exocrine gland is shown with a duct leading to the skin surface. Exocrine glands secrete chemical substances via ducts to the outside of the body or onto another surface within the body. These secretions include substances like sweat, saliva, and digestive enzymes. The diagram shows the secretory cells of the gland producing chemicals that travel through the duct and are released on the skin surface.

On the right, the endocrine gland is depicted without any ducts, illustrating its ductless nature. Instead of secreting chemicals through ducts, endocrine glands release hormones directly into the surrounding blood capillaries. These hormones are then carried away by the bloodstream to target organs where they exert their effects. The image shows the secretory cells releasing hormones (indicated by small dots) directly into the bloodstream within the capillaries (indicated by the red circle).

This contrast highlights the primary difference between the two gland types: exocrine glands use ducts to deliver their products, while endocrine glands do not, instead using the vascular system for hormone distribution. Both gland types are essential for various physiological processes, from digestion to regulation of metabolism and growth.

Pituitary Hormone Physiology

The diagram presents the hormones produced by the anterior and posterior lobes of the pituitary gland and their target organs.

The anterior pituitary lobe, located at the bottom half of the circle, secretes several hormones. These include:

  1. FSH (Follicle-Stimulating Hormone) and LH (Luteinizing Hormone), which target the ovaries and testes. FSH is involved in the maturation of ovarian follicles and stimulates spermatogenesis in the testes. LH triggers ovulation and the production of testosterone in males.

  2. TSH (Thyroid Stimulating Hormone) acts on the thyroid gland to stimulate the production of thyroid hormones.

  3. ACTH (Adrenocorticotropic Hormone) targets the adrenal cortex and stimulates the release of cortisol.

  4. GH (Growth Hormone) has several target tissues, including bones and muscles, stimulating their growth and metabolism.

  5. Prolactin (PRL) primarily targets the mammary glands to stimulate milk production.

The posterior pituitary lobe, shown in the top half of the circle, stores and releases hormones produced by the hypothalamus:

  1. ADH (Antidiuretic Hormone), also known as vasopressin, targets the kidneys to regulate water balance by reducing urine volume.

  2. Oxytocin targets the uterus to stimulate contractions during childbirth and the mammary glands to promote milk ejection during breastfeeding.

The pituitary gland, often termed the “master gland,” is centrally located within the circle, indicative of its central role in the endocrine system as it regulates the function of other endocrine glands through the hormones it secretes. This complex feedback system is crucial for maintaining homeostasis within the body.

Thyroid and Parathyroid Anatomy

The diagram describes humoral stimulation, specifically how the parathyroid glands respond to low calcium levels in the blood, a prime example of a feedback mechanism in the endocrine system.

The thyroid gland is shown in the center of the diagram with a rich network of blood capillaries. The parathyroid glands, which are small, round structures, are located on the back side of the thyroid gland. These glands monitor the calcium concentration in the blood flowing through the capillaries.

In part (A) of the diagram, it is indicated that the capillary blood has a low concentration of calcium ions (Ca²⁺). Low calcium levels in the blood can lead to a condition known as hypocalcemia, which can affect muscle function and lead to symptoms such as muscle cramps or spasms.

Part (B) illustrates the body’s response to this condition. In response to low blood calcium levels, the parathyroid glands secrete parathyroid hormone (PTH). PTH is a key regulator of calcium levels in the body. Once secreted, PTH enters the bloodstream (represented by blue dashed lines) and acts to increase blood calcium levels. It does this through several mechanisms:

  1. Stimulating the conversion of vitamin D to its active form in the kidneys, which helps increase calcium absorption from the digestive tract.
  2. Increasing calcium reabsorption in the kidneys, reducing the amount lost in urine.
  3. Mobilizing calcium from bone, where most of the body’s calcium is stored.

The feedback loop is completed when the rise in blood calcium levels eventually inhibits further secretion of PTH by the parathyroid glands, thus preventing hypercalcemia, which is an excess of calcium in the blood.

This diagram serves as an educational tool to explain the role of parathyroid hormone in regulating calcium homeostasis, a critical aspect of human physiology.

Neural Stimulation of Adrenal Glands

The diagram provides an illustration of neural stimulation of the adrenal gland, highlighting the pathway by which the body’s sympathetic nervous system activates the adrenal medulla during the “fight or flight” response.

At the top of the image is the spinal cord, the central conduit for signals traveling between the brain and the rest of the body. Emerging from it are preganglionic sympathetic fibers, which are part of the sympathetic nervous system. These fibers extend from the spinal cord to the adrenal gland, which is depicted in the middle of the image. Specifically, these nerve fibers connect to the adrenal medulla, the inner part of the adrenal gland.

Part (A) of the diagram notes that these preganglionic sympathetic fibers stimulate adrenal medulla cells. When these cells are activated, they respond by secreting catecholamines, which are adrenaline (epinephrine) and noradrenaline (norepinephrine), as noted in part (B).

Adrenaline and noradrenaline are powerful hormones that have wide-ranging effects on the body. Their release into the bloodstream, indicated by the dashed red line leading to a capillary, prepares the body to respond to a stressor. The effects include increasing heart rate, elevating blood pressure, expanding the air passages of the lungs, enlarging the pupil in the eye, redistributing blood to the muscles, and temporarily shutting down gastrointestinal and immune system functions.

This neural stimulation mechanism is rapid and prepares the body to handle acute stress, providing the energy and focus needed for either fighting the stressor or fleeing to safety. The diagram succinctly encapsulates this critical biological response, emphasizing the direct neural control over hormone secretion in the adrenal medulla.

Hierarchy of Hormonal Stimulation

The diagram illustrates the hierarchical structure of hormonal stimulation in the body’s endocrine system, showing how the hypothalamus, pituitary gland, and other endocrine glands interact to regulate various bodily functions.

At the top of the diagram is the hypothalamus, a region of the brain that serves as a crucial control center for the endocrine system. It secretes hormones (part A) that regulate the pituitary gland, which is shown just beneath it.

These hormones travel a short distance to the anterior pituitary gland (part B) and stimulate it to produce and release its own hormones. The dashed blue and red lines represent the pathway of hormone signals.

The anterior pituitary gland secretes several hormones that act on various endocrine glands, depicted as part C in the diagram. Three of these target glands are shown:

  1. The thyroid gland, located in the neck, which regulates metabolism, energy production, and neurodevelopment.
  2. The adrenal cortex, which is part of the adrenal gland situated above the kidneys. It produces corticosteroids that help regulate metabolism, the immune response, and salt and water balance.
  3. The gonads (testes shown in the image), which are responsible for producing sex hormones such as testosterone, playing a key role in reproductive functions and secondary sex characteristics.

The dashed lines with arrows indicate the direction of hormonal signals from the anterior pituitary gland to the target glands, prompting them to release their respective hormones into the bloodstream.

This system exemplifies a feedback loop where the hypothalamus regulates the pituitary gland, and the pituitary gland, in turn, regulates other endocrine glands. Hormones secreted by the endocrine glands can feedback to the hypothalamus and pituitary to modulate their activity, maintaining homeostasis in the body. The diagram serves as a didactic representation of the complex interactions within the endocrine system.

Anatomical Terms and Definitions

Adrenal GlandsGlands above the kidneys responsible for producing hormones like cortisol and adrenaline.
Adrenal MedullaThe inner part of the adrenal gland that secretes catecholamines in response to stress.
Adrenocorticotropic Hormone (ACTH)A hormone that stimulates the adrenal cortex to release cortisol.
AlveoliTiny air sacs in the lungs where gas exchange occurs.
Antidiuretic Hormone (ADH)A hormone that regulates water balance by reducing urine volume.
CalcitoninA hormone produced by the thyroid gland that lowers blood calcium levels.
CatecholaminesHormones, such as adrenaline and noradrenaline, that prepare the body for fight or flight response.
CortisolA glucocorticoid hormone produced by the adrenal cortex that regulates metabolism and stress response.
Endocrine GlandsGlands that secrete hormones directly into the bloodstream.
Exocrine GlandsGlands that secrete substances through ducts to an external or internal surface.
Follicle-Stimulating Hormone (FSH)A hormone that stimulates the maturation of ovarian follicles and spermatogenesis.
GlucagonA hormone produced by the pancreas that raises blood glucose levels.
Growth Hormone (GH)A hormone that stimulates growth, cell reproduction, and cell regeneration.
HormonesChemical messengers secreted by glands that regulate various bodily functions.
HypothalamusA region of the brain that links the nervous system to the endocrine system.
InsulinA hormone produced by the pancreas that lowers blood glucose levels.
Luteinizing Hormone (LH)A hormone that triggers ovulation and testosterone production.
MelatoninA hormone produced by the pineal gland that regulates sleep patterns.
OxytocinA hormone that stimulates contractions during childbirth and milk ejection during breastfeeding.
PancreasAn organ with both digestive and hormonal functions, producing insulin and glucagon.
Parathyroid GlandsGlands that regulate calcium levels in the blood.
Parathyroid Hormone (PTH)A hormone that increases blood calcium levels.
Pineal GlandA gland in the brain that produces melatonin.
Pituitary GlandThe "master gland" that regulates other hormone glands in the body.
ProlactinA hormone that stimulates milk production.
TestosteroneA hormone produced by the testes that influences male secondary sexual characteristics.
ThymusAn organ where T lymphocytes mature, crucial for the adaptive immune response.
Thyroid GlandA gland in the neck that produces hormones regulating metabolism and growth.
Thyroid Stimulating Hormone (TSH)A hormone that stimulates the thyroid gland to produce thyroid hormones.
TracheaThe windpipe, facilitating the passage of air to the lungs.

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