Your body’s biology is centered around maintaining a homeostatic environment. We’re constantly doing things that would threaten this homeostasis and as such, your body has intricate mechanisms in place to keep its internal environment steady. This internal environment allows for a certain degree of variability around a setpoint. These are things like your body temperature, blood pressure, and blood glucose levels. When things get out of this range, we see disease. And so, homeostasis is a control process – it’s a highly dynamic state where changes are always taking place to stave off anything that might threaten your normal biological functioning.
Homeostatic control generally involves some input signal, an integrating center, and an output signal. Your endocrine system’s major role is communication – it’s the integrating center of homeostatic control, determining what the response should be to the given input. Your endocrine system is comprised of gland cells, like the pituitary and adrenal glands. These communicate with the rest of the body by secreting hormones into the blood.
Again, the primary goal of your endocrine system is maintaining homeostasis, or the optimal range of your physiological environment. Homeostasis involves adapting to whatever the external environment throws at us. An example of this could be the availability of food. When food is scarce, for example, there is a set of endocrine mechanisms that kick in to burn up your “fuel” storages and use energy differently. Leptin and ghrelin are two hormones involved in this. Stress and the stress response is another example. Injuries, the perception of danger, psychological and emotional stress, and even temperature changes can cause an endocrine system response.
Because it’s an integrating and communicating system, the endocrine system is involved in controlling the processes of many tissues, coordinating a sequence of events at the whole body level. This includes things like ion and fluid balance, energy metabolism, digestion, growth and development, reproduction
Defining a Hormone
“Hormones” – that word is thrown around quite a bit and, depending on who you’re talking to, will likely have different (and often skewed) definitions. When I was in the clinical setting, patients generally thought of hormones as something bad. I imagine this comes from hormones in food or their infamy in sports. It’s a word surround by palpable misconception.
Hormones came into discovery sometime around 1915 and garnered their name from the Greek meaning “to excite” (although we now know they sometimes inhibit things). There are three exceedingly important parts to a hormone’s definition. They are chemical messengers which:
1. are secreted by specific cells. Specificity is key concept of endocrinology. Not every cell is going to make a hormone – they need to have specific enzymes and genes activated in order to make hormones.
2. are released into the bloodstream in very small amounts (10-9 to 10-12, nanomolar to picomolar, respectively). Given this small amount, they’re quite powerful. Even though the existence of hormones has been known for over 100 years, only recently have we began to understand how these chemicals could circulate around the body and have such an impact in such small amounts.
3. communicate information to their target cells to regulate, not initiate, functions in their target cells. In other words, they can either increase or decrease a given function. There are exceptions to this rule (like during development), but throughout adulthood, hormones don’t typically start a process, they just regulate it.
“Target” cells, in this context, refers to the cells that can respond to hormones – that is, they have receptors specific to the hormone. When they bind, a variety of things can happen. Hormones can act on a cellular level by regulating cell division, differentiation, activation, apoptosis, motility, secretion and nutrient uptake. At the molecular level, hormones can regulate gene transcription, protein synthesis and degradation, enzyme activity and protein-protein interactions.
How the Endocrine System Communicates
There are four types of endocrine communication:
Here, the cell that produces the hormone also has a receptor for the hormone. This is a way for the cell to regulate itself, a term dubbed auto-regulation.
Here, when a hormone is released from the cell, it travels just outside the cell, acting on a target cell within its vicinity. It doesn’t go out into the bloodstream – its stays within the extracellular fluid within a tissue. This happens in the adrenal cortex and pancreas.
This is how most hormones work. They get released from the cell (typically a gland cell), go out into the bloodstream, and travel through the circulation until they encounters a cell that has a receptor for it (the target cell).
This is a subtype of endocrine signaling. This is an interaction between the brain and the endocrine system (pituitary gland specifically) where hypothalamic hormones are released from neurons. This triggers the pituitary gland to secrete hormones, which then go out into the blood and act on their targets.
Your pituitary gland is the “master gland” of the endocrine system. Its divided into the anterior and posterior pituitary. In the anterior pituitary, six hormones are secreted: follicle-stimulating hormone, luteinizing hormone, adrenocorticotropin, thyroid stimulating hormone, growth hormone, and prolactin. In the posterior pituitary, two hormones are secreted: antidiuretic hormone (also called vasopressin) and oxytocin.
The Different Types of Hormones
There are three main classes of hormones: steroids, proteins, and amine derivatives. We can distinguish the different types of hormones not only from where they are produced, by their chemical class as well. This also helps us understand how they work. A protein hormone will act different than a steroid hormone, for example.
Steroid hormones are derivatives of cholesterol. They’re synthesized in the adrenal cortex, gonads, placenta, and kidney (where vitamin D, a cholesterol derivative, plays an important role in calcium homeostasis). The main distinguishing feature of steroid hormones is they’re lipophilic – the cross membranes easily. As such, they have intracellular receptors (with a few exceptions). And, because they don’t like water, they are bound to a protein carrier when traveling in the bloodstream – this keeps them solubilized. They aren’t really stored in large amounts – more often, they’re released as soon as they’re made. Part of this is because they can easily pass through membranes. This property also allows them to be taken orally – a stark contrast to peptide hormones.
The biggest class are the peptide and protein hormones. Peptide and protein hormones are less than 20 amino acids and greater than 20 amino acids in length, respectively. There’s nothing too significant about this cutoff, there simply had to be one. Often, these are synthesized as prohormones or pre-prohormones and need to be cleaved to release the biologically active form. These, unlike the steroid hormones, are hydrophilic. So, they generally circulate unbound. Moreover, because of this (and because they’re comprised of polar amino acids), they do not cross the plasma membrane – their receptors, then, must reside on the cell surface. And, as you have likely figured out, these cannot be given orally. Just like any protein source, they get digested. So, these need to be injected.
The third class of hormones are amino acid derivates (predominately tyrosine derivatives). These include our thyroid hormones (which act like steroid hormones mechanistically). Epinephrine and norepinephrine, our catecholamines, also fall into this category. Although they’re neurotransmitters, we consider them hormones because they are released from the adrenal medulla.
This is a brief introduction into the endocrine system and the role hormones play in your body. In the next post in this series, I’ll go over the lifecycle of hormones – how they’re made, secreted, and transported. Later on, I’ll cover their actions, regulation, and parameters that impact their circulating concentrations.