Endocrine System

∞ generated and posted on 2018.09.02 ∞

Hormones and the endocrine glands that produce hormones.

Certain epithelial tissues can secrete chemicals into the blood, known as hormones, that then circulate through the blood to other tissues, where they interact with specific cells and change tissue and therefore bodily functioning in well-defined ways.

This page contains the following terms: Hormone, Paracrine signaling, Autocrine substance, Endocrine gland, Pineal gland, Melatonin, Thalamus, Hypothalamus, Pituitary gland, Antidiuretic hormone, Oxytocin, Thyroid gland, Thyroxine, Parathyroid gland

The above video introduces signaling between cells (cell-to-cell signaling) particularly from the perspective of the signal-receiving cell, with discussion of endocrine signaling, paracrine signaling, and also juxtacrine signaling, which is another name for contact-dependent signaling; the emphasis in this chapter is on endocrine signaling.


Chemical signaling molecules that function systemically, that is, following entrance into the blood.
We've all heard the word "hormone" but that doesn't mean that we necessarily understand what the word means. In fact, hormones are specific types of chemical substances within the body. They are made by the body and released by specific body tissues. All hormones following their release then enter the bloodstream, which allows them to move about much of the body.

They then, to a large extent, move out of the blood into the interstitial fluid that bathes the body's cells. From there they bind to receptors associated with those cells, whether as found on the surface of cells or instead inside of cells, depending on the hormone. Not all cells have receptors for all hormones and different cells may respond in different ways to hormone binding to receptors.

In any case, hormones are not nutrients but instead are explicitly signaling molecules. In addition, and unlike paracrine substances, hormones do not generally act on cells that are found immediately locally to where the hormone is produced and released, but instead function throughout the body.

Neurons also provide such long distance signaling, but the target of neuronal signaling is much more narrowly defined, spatially, than are the targets of hormonal signaling. Neurons also generally can convey their signal faster than can hormone-based signaling.

A list of some 70 human hormones, what they do, where they are produced, their abbreviation, their target tissues, etc., can be found at Wikipedia.

Links to terms of possible interest: Blood circulation, Hormone, Hormone secreting cell, Hormone target cell, Secretion

The above video focuses on insulin, it does a good job of introducing the idea of hormones.

Though the above video provides a nice introduction to what hormones are all about as well as much of what the rest of this chapter is all about.

Paracrine signaling

Localized cell-to-cell communication mediated by intentionally cell-produced molecules.
A paracrine substance, like a hormone, is a chemical signal produced by one cell and which acts upon a second cell. Also as with hormones, that second cell must possess one or more receptors for the signaling molecule, for that molecule to have an impact on the second cell's physiology.

Unlike hormones, paracrine substances are not released into the blood and thereby do not act at a substantial distance away from the releasing cell. Instead, the paracrine substance diffuses within the interstitial environment to interact with cells that are immediately local to the releasing cell.

A special example of a paracrine substance is a neurotransmitter. Neurotransmitters move from a presynaptic cell, across the synaptic cleft, and then bind to receptors that are found on the surface of the postsynaptic cell.

Paracrine substances are important intercellular signaling molecules, that is, acting between cells, for processes that are especially local in their relevance and functioning, such as blood clotting, wound repair, and also immune system functioning.

Links to terms of possible interest: Hormone, Hormone secreting cell, Hormone target cell, Paracrine substance (local mediators), Paracrine signaling

My point in posting the above video is to draw attention to the various signaling molecules supplied by the "General contractor stem cells" (as the video puts it). These locally signaling molecular signals are paracrine substances, that is, they are both released and act locally. This makes sense in this context because the need to repair damaged tissue is (ideally) only a highly localized process.

Autocrine substance

Hormone-like chemicals that are released from the same cell that they act upon.
Autocrine signaling is particularly relevant within the immune system, that is, with immune system cells auto-stimulating themselves.

Tumor cells can stimulate their own replication by producing and releasing growth factors that then pathologically signal the producing cell to divide. These cells thereby divide out of control where out of control cell replication is a hallmark of cancer cells.

Links to terms of possible interest: Autocrine, Endocrine, Hormone, Paracrine

Endocrine gland

Epithelial tissue that releases hormones directly into the blood.
By "directly", the meaning is that this release does not involve passage through ducts. By "hormone", this means that the chemical released is not released out of the body nor released in a manner that does not result in entrance into the blood.

Major endocrine glands include the adrenal gland, the ovaries, the pancreas, the parathyroid gland, the pituitary gland, the testes, the thymus, and the thyroid gland. The hormones released by these different organs and glands often are unique to those tissues, though in certain cases these hormones are also used elsewhere as neurotransmitters.

Links to terms of possible interest: Adrenal gland, Endocrine gland, Endocrine tissue, Exocytosis, Hormone, Hypothalamus, Kidney, Ovary as endocrine gland, Pancreas, Parathyroid gland, Pineal gland, Pituitary gland, Testes as endocrine gland, Thymus, Thyroid gland, Vesicle

The above video provides an effective comparison between the two basic gland types, endocrine gland and exocrine gland.

The above video introduces exocrine glands without dwelling greatly upon the hormones that they produce.

Pineal gland

Vertebrate animal producers of the hormone melatonin.
Melatonin is a derivative of the neurotransmitter, serotonin. It is involved in the control of patterns of sleep as well as circadian rhythms and seasonal rhythms.

The gland is small (less than 10 mm) and is found dorsally (behind) the thalamus and superior to the midbrain. Unlike the brain generally, the pineal gland is not protected by the blood-brain barrier but, by volume, instead is one of the most vascularized regions of the body.

Melatonin is released by the pineal gland in the absence of detection of light.

Links to terms of possible interest: Brain stem, Cerebellum, Cerebral cortex, Cerebrum, Colliculus, Geniculate body, Hindbrain, Hypothalamus, Medulla oblongata, Midbrain, Pineal body, Pineal gland, Pituitary gland, Pons, Spinal cord, Thalamus, Vermis,


Hormone that is released at different levels throughout the day thereby impacting circadian rhythms.
Melatonin secretion is suppressed by exposure to daylight. The hormone also is absorbed during digestion and as a consequence is available both in various foods as well as in drug form.

In addition to contributing to daily physiological rhythms (circadian rhythms), melatonin levels also can be used in various animals to give rise to seasonal rhythms, e.g., giving rise, for example, to mating seasons.

Links to terms of possible interest: Circadian rhythms, Melatonin, Pineal gland


Conduit of information between the cerebrum and the midbrain.
The thalamus does not possess endocrine functions but is situated within the brain superior to the hypothalamus.


Region of the brain that controls the release of hormones by the pituitary gland.
The hypothalamus exists as right and left structures that are found immediately above the brain stem and below the thalamus. They control such things as the temperature of our bodies, sleep, and hunger.

Towards control of homeostasis, the hypothalamus monitors body aspects including blood hormone levels as well as factors found external to the body, such as through association with the olfactory sense as well as to light. The hypothalamus additionally receives signals via neurons originating from the peripheral body as well as from other parts of the brain.

The hypothalamus, in terms of its endocrine function, releases hormones which also are described as neurohormones. Those neurohormones, which are released above the pituitary gland, have the effect of either stimulating or inhibiting the release of hormones by the anterior pituitary gland. The neurohormones are released from the axon terminals of neurons whose cell bodies are found in the hypothalamus.

Alternatively, neurohormones are released from neurons whose cell bodies are also found in the hypothalamus but from axon terminals that are found in the posterior pituitary gland. These latter neurohormones then both circulate to and have an impact outside of the pituitary gland.

Links to terms of possible interest: Anterior pituitary, Hypothalamus, Hypothalamo-pituitary portal vessels, Infundibulum, Median eminence, Posterior pituitary, Thalamus

The above video provides a rather detailed introduction to the hypothalamus, with a focus on its anatomy.

The above video provides a continuation of the detailed introduction to the hypothalamus, here emphasizing its introduction with the pituitary gland.

Pituitary gland

Hypothalamus-associated gland responsible for releasing numerous hormones including oxytocin and growth hormone.
The pituitary gland is traditionally distinguished into what is known as the posterior pituitary gland (the back side) versus the anterior pituitary gland (the front side), and different hormones are released from each. In addition, while the anterior pituitary gland is controlled hormonally by the hypothalamus, the posterior pituitary gland instead releases hormones from neurons that originate in the hypothalamus (i.e., what are known as neurohormones).

Both antidiuretic hormone (ADH) and oxytocin are released from the posterior pituitary gland. From the anterior pituitary gland comes, for example, follicle stimulating hormone (LH), growth hormone (GH), prolactin, and thyroid stimulating hormone (TSH).

Links to terms of possible interest: ADH, Anterior pituitary, Capillary bed, Corticotropin-releasing hormone Endocrine cell, Gonadotropin-releasing hormone, Growth hormone-inhibiting hormone, Growth hormone-releasing hormone, Hormone, Hypophyseal portal system, Hypothalamus, Hypothalamo-pituitary portal vessels, Infundibulum, Median eminence, Neurosecretory cells, Osmoreceptor stimulation, Oxytocin, Paraventricular nucleus, Portal vein, Posterior pituitary, Prolactin-inhibiting hormone, Prolactin-releasing hormone, Sensory stimulation, Supraoptic nucleus, Thalamus, Thyrotropin-releasing hormone

The above video is a catchy but way too rapid an overview of the pituitary gland.

The above video also is a really quick introduction mainly to the pituitary gland.

Discussed in this video is pituitary gland-thyroid gland interaction, providing illustration of "long-loop" feedback involving thyroxine.

Antidiuretic hormone

Regulator of the reabsorption of water during urine formation such that the urine becomes less dilute.
An important component of kidney function is to avoid excessive excretion of water from the body. This is accomplished not by preventing water from entering into urine as it is forming but instead via the removal or reabsorption of this water out of forming urine. The result is concentrated urine versus dilute urine (or more concentrated versus more dilute). It is especially when the water content of blood is already relatively low that water reabsorption is required.

It is via the release of antidiuretic hormone (ADH) that water reabsorption, particularly from what are known as the collecting ducts of the kidney, is stimulated. It is mainly within the hypothalamus that low blood water content is detected and ADH serves as a neurohormone that is released from the posterior pituitary gland (i.e., there from neurons that originate within the hypothalamus).

Note that a diuretic – contrasting antidiuretic – is a substance that has the opposite effect of antidiuretic hormone, resulting in the production of urine that is more dilute, which could potentially dehydrate the body.

ADH is also known as vasopressin

Links to terms of possible interest: ADH, Antidiuretic hormone, Blood osmolarity, Collecting duct, Distal tubule, Hypothalamus, Osmolarity, Osmoreceptor, Permeability, Pituitary gland, Water reabsorption

The above video provides a quick introduction to antidiuretic hormone, a.k.a., ADH.


Hormone that affects the functioning of the uterus as well as mammary glands, resulting in contraction of associated smooth muscles.
Oxytocin release is stimulated particularly via the action of babies, either pushing their heads against the cervix, resulting in uterus muscle contraction that has the effect of expelling the baby from the mother's body, or instead by baby suckling on the mother's nipple, resulting in release (letdown) of milk to the baby. Oxytocin itself is released from the posterior pituitary gland as a neurohormone.

Links to terms of possible interest: Breastfeeding, Oxytocin

The above video talks about a lot more than oxytocin, but at least can be thought of how the action of oxytocin fits in with that of a bunch of other hormones, ultimately that can contribute to reproductive success.

Thyroid gland

Source of an iodine-containing hormone that is necessary for maintaining normal metabolic rate, growth, and development, as well as source of the hormone, calcitonin.
The hypothalamus stimulates the anterior pituitary gland to produce thyroid stimulating hormone (TSH) which in turn stimulates the thyroid gland to produce thyroid hormones. These hormones are thyroxine (or tetraiodothyronine, T4) and triiodothyronine (a.k.a., T3, where the numbers in the subscripts refer to the number iodine atoms associated with each hormone). T4 is a not-active hormone that is converted to the active, T3.

In addition to thyroxine, and triiodothyronine, the thyroid gland also secretes calcitonin which has the effect of reducing blood calcium levels, particularly towards the production of new bone. This process is known as ossification, where bone serves not just as body support structures but also as calcium storage tissues.

Links to terms of possible interest: Carotid artery, Esophagus, Larynx, Parathyroid glands, Pharynx, Thyroid gland, Trachea

The above video provides a nice, quick overview of what the thyroid gland is all about.

Thyroid gland associated diseases are discussed in the above video.

The above video provides a nice, quick overview of the physiological roles of the thyroid gland and hormones it produces.

Though the above video is a second part to a previous video, one introducing hormones, here focus is on the thyroid gland and the impact of thyroid hormones.


Iodine-containing hormone that is necessary for maintaining a normal metabolic rate.
Thyroxine (T4) is water insoluble so must be transported to target cells in association with a carrier protein. This water insolubility, however, allows passage of thyroxine across the plasma membrane of target cells, that is, just as steroid hormones do, which also are highly lipid soluble and therefore are able to enter cells as well by passing directly across the plasma membrane. T4 is then converted intracellularly to the active, T3 form. The target of T3 action is within the nucleus, directly impacting gene expression.

Links to terms of possible interest: Adrenal cortex, Androgens, Endocrine organ, Estrogens, Glucocorticoids, Hormone target cells, Hypothalamus, Inhibin, Negative feedback, Ovaries, Pituitary gland. Pituitary hormone. Progestins, Testes, Thyroid gland, Thyroid hormones

Parathyroid gland

Source of a hormone that has the effect of increasing blood calcium levels when those levels otherwise are low.
The maintenance of a relatively constant concentration of calcium in blood is an important aspect of homeostasis. When blood calcium levels are insufficient, then osteoclasts can increases blood calcium levels by dissolving bone. Alternatively, the sequestration of calcium to form bone or instead the loss of calcium via excretion within urine can serve to reduce blood calcium levels.

In response to such reductions, the parathyroid gland, actually endocrine glands found in association with the thyroid gland, will release parathyroid hormone (PTH). PTH has the effect of stimulating osteoclast activity, which reabsorb bone, thereby releasing stored calcium. PTH also reduces calcium loss that is otherwise due to calcium excretion from the kidneys and increases calcium absorption from the small intestine.

Contrast the impact of PTH on blood calcium levels with the impact instead of the hormone calcitonin.

Links to terms of possible interest: Bone, Ca2+, Calcitonin, Calcium, Calcium homeostasis, Kidney, Osteoblast, Osteoclast, Osteocytes, Parathormone, Parathyroid hormone (PTH), Parathyroid gland, Thyroid gland

Remarkably fast (42-second) introduction to the parathyroid gland. Left out though is the calcitonin side of the blood-calcium homeostasis equation.