Endocrine system: Structure and function

The endocrine system is made up of endocrine glands that secrete chemicals called hormones into the bloodstream. When hormones reach their target cell, they bind to a receptor on the cell’s membrane or within that cell, and, in response, the target cell changes what it’s doing.

So, at the end of the day, the endocrine system helps regulate all sorts of things, like cellular metabolism, which is how cells work; growth and development of the body; reproductive function; and even blood sugar levels after a meal!

Now, endocrine glands are scattered throughout the body, kind of like a remote work environment. Underneath the brain, there’s the hypothalamus and the pituitary gland.

The hypothalamus and the pituitary are partners, working together to release hormones that regulate all sorts of things, such as body temperature; blood osmolarity, or how concentrated or dilute the blood is; how the body responds to stress; or even if there’s some sort of danger.Now, sometimes the pituitary will release a hormone that acts as a messenger that tells another organ what to do.

For example, the pituitary sends hormone messengers to the thyroid gland, telling it to secrete thyroid hormones; to the adrenal glands telling them to secrete cortisol; and it tells the gonads, which are the ovaries in females and testes in males, to secrete sex hormones, like estrogen, progesterone, and testosterone.

Other times, the pituitary releases hormones that go straight to its target tissue. So, antidiuretic hormone’s main role is to tell the kidneys to retain water, depending on the body’s needs.

Oxytocin dilates the cervix and stimulates uterine contractions during childbirth; it also makes the muscle cells in the breasts contract to eject the milk during breastfeeding.

So, aside from motherhood, its levels are generally pretty low, but they do increase a little bit during pleasant social interactions, hugs, and physical contact.

Another hormone, growth hormone, makes our bones and tissues grow, especially during childhood and adolescence. And finally, prolactin plays a role during breastfeeding.Ok now, let’s look at the other endocrine glands.

The pineal gland is also located on our brain, behind the hypothalamus and the pituitary. It secretes melatonin during the night and helps regulate the sleep-wake cycle.

Next, there’s the thyroid gland at the front of the neck, and it’s made up of a left and a right lobe and looks kind of like a bowtie.

The thyroid gland makes thyroid hormones, and there’s two of them: triiodothyronine, or T3, and thyroxine, or T4. Thyroid hormones speed up cellular metabolism, helping us think and talk faster, so you can think of them like a shot of caffeine!

Thyroid hormone production is regulated by the hypothalamus and the pituitary via negative feedback. Negative feedback means that when thyroid hormone levels are low, the pituitary secretes more Thyroid stimulating hormone or TSH, which increases thyroid production of T3 and T4.

When thyroid hormone levels increase to a certain level, this sends a negative feedback signal to the pituitary to make less TSH, keeping thyroid hormone levels in an optimal range.Now, in between the thyroid follicles, there are also parafollicular cells, or C- cells, which secrete a hormone called calcitonin.

And on the back of each thyroid lobe, there are also two parathyroid glands: one above and one below, so 4 in total. They make parathyroid hormone, or PTH.

Both calcitonin and parathyroid hormones are involved in calcium, phosphate, and bone metabolism. Next up, there are the adrenal glands, one above each kidney.

Each adrenal gland is made up of an outer layer, called the cortex, surrounding a core, called the medulla. The cortex of the adrenal gland can further be divided into three zones that produce steroid hormones, like aldosterone, cortisol, and sex hormone precursors.

Aldosterone prevents loss of water and sodium in the urine and increases the elimination of potassium. Cortisol secretion is stimulated by pituitary Adrenocorticotropic hormone, or ACTH, during acute stress, or “fight or flight” situations.

Cortisol increases blood pressure and blood sugar levels. Finally, the adrenal medulla is also involved in the “fight or flight” response, and it secretes adrenaline and noradrenaline: together known as catecholamines.

Catecholamines increase blood pressure, heart rate, and breathing rate, which in turn, increase oxygen delivery to tissues, like the muscles.

Okay, next up, let’s look at the gonads, which are the testes in biologically male individuals and the ovaries in biologically female individuals.

Follicle-stimulating hormone or FSH and Luteinizing hormone or LH travel from the pituitary to the gonads and regulate the production and maturation of gametes, which are sperm for the testes and oocytes for the ovaries as well as the production of sex hormones, like testosterone, estrogen, and progesterone.

As a general rule, sex hormones also send a negative feedback mechanism back to the pituitary. The exception is that in females, right before ovulation, estrogen levels get really high, and they make the pituitary even more sensitive to hypothalamic Gonadotropin-releasing hormone, or, GnRH.

So this acts as a positive feedback signal, leading to a massive surge of FSH and LH that leads to ovulation. You can think of negative feedback like the sensation of being less hungry after you’ve eaten, which makes you stop eating.

In contrast, positive feedback is like wanting to eat more chocolate after you’ve just had a piece. Finally, there’s the pancreas, which has an exocrine and an endocrine part.

The exocrine function of the pancreas involves secreting digestive enzymes directly into the duodenum, which helps break down food into nutrients the small intestine can absorb.

The endocrine function is carried out by beta and alpha cells that secrete insulin and glucagon respectively in response to blood sugar levels.

So, when there’s high blood sugar, like right after a meal, beta cells secrete insulin into the blood, which helps our cells take in glucose and use it for energy, lowering our blood sugar levels after meals.

With low blood sugar, on the other hand, the alpha cells take charge and secrete glucagon. Glucagon knows the liver has a glucose “stash” called glycogen, so it binds to a receptor on liver cells, which initiates a process that releases glucose into the bloodstream.

Finally, remember that just like our other organs and systems, the endocrine system is also subject to the effects of aging.

So, as we grow older, the endocrine glands produce slightly less hormones at a slightly lower rate. For example, less thyroid hormones means that our basal metabolism slows down and explains why weight is easier to gain and tends to stay on as you age.

Also, the gonads slow their activity with age, and this means different things depending on biological sex. So in females, the ovaries gradually decrease their sex hormone production until it grinds to a halt when menopause sets in; in males, on the other hand, sex hormone production doesn’t stop entirely, but it does decrease, which affects sexual drive and function.

All right, as a quick recap… The endocrine system is made up of various endocrine glands scattered throughout the body. One of the most important ones is the pituitary gland, which releases multiple hormones that affect many parts of the body.

Other endocrine glands include the pineal gland, which regulates the sleep wake cycle; the thyroid, which regulates cellular metabolism; the adrenal glands that control the fight or flight response and water and sodium retention; the gonads that produce sex hormones; and the pancreas, which regulates blood glucose level.

The release of hormones can be regulated either by negative feedback, where higher levels of a hormone in the body inhibits further release, or positive feedback, where higher levels of a hormone causes more of it to be released.