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And so we've kind of got an idea now about what hormones are, and what organs make them, but one thing that confused me for a really long time is the idea of how much of these hormones are in our blood, and how they actually get from point A to point B. And so I'll make a category for concentration. And my first thought when I started thinking about hormones, are that these must be just kind of infinitely present in our blood vessels. There are an unimaginable amount of blood cells, and I just kind of categorized hormones with that level of red blood cells, just a ton of them flowing through our body. And while there is a wealth of hormones flowing through our body at any given time, it's actually a fairly small amount. You're talking about picograms per milliliter, which is one millionth of one millionth of a gram. So really small amounts. Let's say that the pituitary is wanting to stimulate the adrenal glands. It's releasing that amount of hormones, but it's not a straight shot. There is no straight shot to the adrenal gland. So it's really just dumping this hormone into the blood. And then it's going everywhere, almost like your body's showering in this hormone. And so very similar to radio waves, which are going everywhere throughout the air, but are only picked up by those that are tuned into them, the endocrine hormones are flowing all throughout your body, but their effect is really determined by where the receptors are located. And so when you have these autocrine signals, they're autocrine signals because their receptors are right there at the cell that are making them. And paracrine signals, they're paracrine signals because their receptors are pretty close to the cells that are making them, and the same thing with endocrine system. That just means their receptors are a really long distance away from where they're being made. And so that concentration again is smalll, but it's everywhere in the body and it's being regulated kind of by two main ideas. You've got the idea of metabolism, that for every one of these hormones that actually reaches its destination, 99 or so percent are actually metabolized by either the liver or the kidney. And the liver breaks these down and ultimately makes bile, and the kidneys breaks these hormones down and ultimately excretes them into urine. But for every one of these hormones that are made, thousands of more are being metabolized and excreted in one way or another from the body. And so the metabolism is affecting concentration, but secretion is also affecting concentration. And secretion affects concentration really through this important idea known as a feedback loop, and so I went ahead and pre-drew a feedback loop that I'll walk us through. And so the idea behind a feedback loop is that the products that are being made and causing that simulation in different parts of the body also stimulate the cessation or the stopping of the production of the hormones themselves. So that's a little bit of a confusing idea and I'll walk us through an example. So say we need more thyroid hormones and our hypothalamus starts releasing thyroid releasing hormones, which is how it communictates to the anterior pituitary that we need these hormones in our blood. The anterior pituitary then releases thyroid stimulating hormone, which stimulates the thyroid's production of the thyroid hormones like we talked about, T3 and T4, triiodothyronine and thyroxine. So as these thyroid hormones travel through the blood, again they're going everywhere in the blood, not just from point A to point B. They're in all of the blood vessels, and they're just [unintelligible] the body's tissues and these hormones. They eventually reach receptors on the hypothalamus and the anterior pituitary, which when that thyroid hormone is received, signals the anterior pituitary to stop the production. I'll draw that in red so it's just really clear. It signals to stop the production of these hormones. And it does that in the anterior pituitary, and also in the hypothalamus. It says "we don't need these hormones anymore, "there are plenty of them." And that's a really beautiful effect, when you think about it, that the body is kind of naturally wired to not only elicit an effect, but also to kind of know when that effect is no longer needed. And so that's the idea of a negative feedback loop. And the negative feedback loop is the main way that secretion is controlled. And so you have these hormones, and you have these organs that are making them, and you have this idea of regulation. And ultimately what that leads into is the endocrine system's involvement with the nervous system and behavior. And if behavior is just the coordinated internal and external response of our body in response to its environment, then we can see that the hormones that are signalling these effects in different parts of the body, plays a pretty integral role in behavior. And then behavior ultimately forms the bridge between the physiology of the endocrine system and the psychology and the study of the mind and how we respond in attitude and personality. And all of these effects that we talked about in behavior, psychologically, the basis for those effects are the coordinated hormonal responses that are happening through the endocrine system. And so in this bridge between the physiology and psychology, that's where you start getting into ideas of cognitive behavioral therapy. And cognitive behavioral therapy is this idea that you can actually, you know, control parts of what your body's doing physiologically with your mind. It's under conscious control, these little hormones, and so you can imagine that when you're really afraid, I'll draw like being really afraid there, that you have epinephrine just coursing through your blood vessels, causing an increased heart rate and increased respiratory rate, and diverting energy away from your digestion and your immune system, and into this kind of fight or flight response. But as you become conditioned to that fearful stimulus, and as you are no longer afraid of it, that you can actually become more consciously calm in that and resulting in less adrenaline flowing throughout your body. And so this entire mass of hormones that are circulating through your body at any given moment, are in some way under your control. And that's actually a pretty neat concept.