The first thing that probably comes to mind when someone mentions 'steroids' is a muscle-bound bodybuilder who may look more like a work horse than a human being. Steroids aren't all about bulking your muscles up though- they're actually a class of naturally occurring hormones that are synthesized and secreted into your bloodstream by your endocrine system (your hormone secreting system). Steroids are vital to the proper functioning of your body and play a key role in everything from anti-inflammatory effects to sexual maturation and the regulation of events during pregnancy.
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There are a variety of endocrine glands in your body that synthesize a wide range of hormones but only four of those glands, the testis, ovaries, placenta (during pregnancy only), and adrenal gland, are steroid producing glands. Now, you're probably familiar with the steroids of the testis and ovaries/placenta; the testis produce testosterone and other androgens ('andro-gen' meaning 'masculinity generating' hormones), while the ovaries produce estrogens ('estrous generating', or menstrual cycle hormones) and both the ovaries and placenta produce progestins (meaning 'pro-gestation' or pregnancy hormones). However, you may not be familiar with the role of the adrenal gland so, before we discuss the steroids it produces, let's take a moment to explore the gland itself.
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The adrenal glands sit atop each of your kidneys, like a little hat, and is composed of an outer 'cortex' layer and an inner 'medulla' (middle) layer of cells. The inner layer produces hormones like adrenaline, noradrenaline, and dopamine in response to your sympathetic nervous system (an automated system that would, for example, be 'sympathetic' to your situation if you were being chased by a bear and so would stimulate your body's 'fight or flight' response). The outer 'cortex' portion of your adrenal glands is the steroid producing layer, which produces glucocorticoids (responsible for using and storing glucose sugars in metabolism) and mineralocorticoids (responsible for retaining minerals in the blood), of which the most notable is aldosterone.
Mineralocorticoids are so named because they help your body retain, or reabsorb, minerals such as sodium and chloride (NaCl), otherwise known as salt. The most notable mineralocorticoid is aldosterone, which works directly on the blood filtration system of the kidneys, called nephrons, to reclaim salts.
Your kidneys function like very efficient recycling plants whose job it is to purify your blood of metabolic wastes (like urea and creatinine) as well as excess water and electrolytes. If urea or creatinine were to build up in your blood it would actually make your blood toxic. This is why people whose kidneys fail must go for dialysis treatment, which is an artificial form of blood purification. In proper functioning kidneys, blood from your renal arteries is forced through structures, called glomeruli, that have millions of little pores, like a sieve, which small molecules (such as electrolytes, salts, glucose and metabolic wastes) easily pass through. These molecules are collected into something called a filtrate which, if not reabsorbed, will become urine and pass on to your bladder for excretion.
Now, the important thing to note is that not all of these little molecules are waste products and some must be reabsorbed back into your blood. So, once the small molecules are collected into a filtrate they pass into a series of tubules whose job it is to sort and 'recycle' the needed molecules (like glucose, salts, and water) back into the blood while leaving the waste products. The 'workers' that do this sorting are called transport carriers that either actively (requiring metabolic energy) or passively (through osmosis) return these important molecules back to the blood. One such active transport carrier, is triggered by aldosterone to reabsorb salt and, in the process, excrete excess potassium.
Aldosterone comes into play when your kidneys sense that your blood fluid volume is either too low or if your potassium levels are too high. Now, you may be wondering why too much potassium is bad because we're always being told that potassium rich foods, like sweet potatos, bananas, and dark leafy greens, are good for us. Well, while potassium rich foods may help reduce your risk of high blood pressure, stroke, and heart disease, excessive amounts of potassium can actually end up harming your heart and your nervous system so it's important to maintain a proper balance, and here is where aldosterone comes in.
Either too much blood potassium or too low of a blood fluid volume will trigger your adrenal glands to start producing aldosterone. These two events may seem unrelated but sodium and potassium are electrolytes that your body can trade, one for the other, so that it can retain one. If your blood potassium levels are too high, your adrenal glands will start synthesizing aldosterone which will, in turn, engage the transport carriers in the tubules of the nephron to trade the excess potassium in the blood for sodium. This process has a secondary effect of increasing your body's water retention as well.
You know when you eat too many potato chips and you start to get really thirsty? Well, that's because your blood's salt level has increased due to the salty potato chips and your body is asking you for more water so that it can absorb the water into your blood and balance, or equalize, the salt content. A similar process occurs in the tubules of the nephrons where, as salt is drawn out of the filtrate by the transport carriers stimulated by aldosterone, water is drawn out too to equalize the salt content. Therefore, aldosterone has the secondary effect of increasing your blood's fluid volume by retaining sodium ions.
Aldosterone is a naturally occurring steroid that is synthesized by a gland of your endocrine system called the adrenal gland. The adrenal gland is located on top of each of your kidneys. When your kidneys sense that your blood fluid volume is too low or if your blood potassium levels are too high, they start a chemical chain reaction that results in the your adrenal glands synthesizing aldosterone. This steroid, in turn, signals the transport carriers in the tubules of your kidney's nephrons to start exchanging excess blood potassium for salt which, in turn, draws water out of the filtrate and back into the blood, increasing your blood's fluid volume.
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