Control of Urine VolumeOur urine is produced not only in order to eliminate many of the cellular waste products, but also to control both the amount and the composition of the extracellular fluid in the body. Controlling the amount of water and chemicals in the body is essential to life, and our body does so by producing various amounts of urine so that we can either excrete the "extra" water and chemicals (mainly sodium) or conserve the water and chemicals when they are in short supply. Therefore, the volume of urine that we excrete everyday is a reflection of how much extracellular fluid and sodium our bodies have to spare. The kidney tubule regulation of the salt and water in our bodies is the most important factor in determining urine volume. Too much water and salt in our bodies is dangerous and too little water and salt in our bodies is dangerous. Therefore, the level of water and salts excreted in urine - the urine volume - is adjusted to the needs of the body. As a general rule, however, and under optimum conditions, the body produces urine at a rate of about 1 ml/min.
Control of Urine Volume through HormonesOne of the mechanisms that the body uses to control such things is through the action of hormones, chemical messengers that travel through the blood system and act as regulators of many of the body's internal activities. Hormones are secreted by specialized glands in the body. By definition, a gland is any cell or organ that secretes some substance. But not all glands secrete hormones. Let's find out which ones do.
The body contains two types of glands, classified according to the way they secrete. One type is the exocrine gland (exo = outward), whose secretions move outward, usually by way of ducts, to some body surface, whether it be to the skin itself or, for example, the lining of the digestive or respiratory tract. Among the exocrines are the sweat and sebaceous glands, whose respective secretions of water and oil are evident on the skin; the mucus glands, whose mucus moistens the digestive and respiratory tract; the salivary glands, whose saliva softens food when it enters the mouth; and the mammary glands, which provide milk for the nursing infant.
The second type is the endocrine gland (endo = into), whose secretions move into the bloodstream. The endocrines are the great chemical regulators of bodily function. The substances which they secrete serve as chemical messengers and are called hormones, from the Greek hormon, meaning "arouse to activity." Compared with such organs as the heart or the lungs, the endocrines seem ridiculously small and unimpressive. They are bits of tissue tucked away in obscure corners of the body (Figure 4); all of them together weigh no more than about five ounces. But in the widespread control they exercise over the body, they act like giants. The key to their extraordinary power lies in the hormones they secrete - among the most powerful and remarkable substances in the body.
The two endocrine glands that are concerned with the regulation of fluid balance within the body are the adrenal glands and the pituitary gland. Let's first examine the pituitary gland, one of the tiniest endocrines of all. The pituitary hangs from the bottom of the brain by a little stalk just above the nasal passage (Figure 4b). It was originally and mistakenly believed that it discharged mucus into the nose, and therefore its name is derived from the Latin word for "nasal secretion." The pituitary is insignificant in appearance (about the size of an acorn) but it packs an extremely powerful punch. The pituitary is actually two glands in one - posterior and anterior (we will not discuss the anterior pituitary in this chapter). Of the two, the posterior pituitary manufactures no hormones of its own, but stores two that are initially secreted in a part of the brain known as the hypothalamus. These hormones are oxytocin (a hormone responsible for stimulating labor at the end of pregnancy), and anti-diuretic hormone (ADH), which helps the body retain its fluids. Of the two hormones secreted by the posterior pituitary, our focus for this chapter will be on ADH.
Control of Urine Volume through ADH (anti-diuretic hormone)
A diuretic is a substance that acts to increase urine production. Sometimes when a person is feeling "bloated" (which often occurs when a woman is about to begin menstruation), it usually means that she is retaining water. If the condition is extremely uncomfortable to the woman, a doctor may prescribe a diuretic to eliminate extra water and reduce the bloating. An anti- diuretic, on the other hand, is a chemical that inhibits urine formation. ADH produces its anti-diuretic effect by acting on the kidneys and causing them to reduce the amount of water they excrete. In this way, ADH is important in regulating the water concentration of body fluids, which ultimately helps to maintain an appropriate sodium concentration in the body. In fact, the sodium concentration in the extracellular fluid is the factor that either stimulates the pituitary to release ADH or to inhibit the release of ADH. This process operates as one of our "famous" negative feedback loops (Figure 5). Let's take a look at how this loop operates.
The release of ADH is regulated by the hypothalamus. Apparently certain sensors in this part of the brain are sensitive to changes in the salt concentration of body fluids, particularly the blood. If, for example, a person is sweating and losing large amounts of water, the sodium in the blood becomes more and more concentrated. The hypothalamus can sense this high concentration of sodium and it signals the posterior lobe of the pituitary to release ADH. The ADH is transmitted by the blood to the kidneys causing the kidney tubules to reabsorb more water into the blood stream to dilute the sodium. As a result of this effect, less urine is produced. This action conserves water in the body. The mechanism of action seems to be that the presence of ADH makes distal and collecting tubules in the nephron more permeable to water. That is, water is moved more easily through the tubule membrane back into the blood and less urine is produced. For this reason, ADH is described as the "water-retaining hormone." You might also think of it as the "urine-decreasing hormone."
If, on the other hand, a person drinks an excess of water, the sodium in the body fluids, including the blood becomes more dilute and the release of ADH is inhibited. The lack of ADH causes the distal and collecting tubules to become practically impermeable to water, and little or no water is reabsorbed from them back into the blood. Consequently, the kidneys excrete more watery urine until the water concentration of the body fluids returns to normal.
You can probably relate this hormone activity to your own experience. If you have ever been urinating and notice that the urine is extremely yellow, this means that ADH is probably circulating in your body, causing less water to be excreted and more water to be reabsorbed into the bloodstream to stay in your body. Therefore, the urine becomes concentrated with urea and all of the other chemicals, causing it to turn yellow. On the other hand, if you notice your urine to be very watery, you can probably win a bet that says very little ADH is circulating in your body. With less ADH, your kidneys will reabsorb less water back into the bloodstream and more of it will be excreted in the urine.
Control of Urine Volume through Adrenal Glands
The phenomenon of thirst is equally important for regulating body water and sodium concentration as is the ADH system. Thirst is the primary regulator of intake of water and the ADH system is the primary regulator of output of water. Thirst is not stimulated through the release of a specific hormone but instead, it is stimulated by the hypothalamus in response to too much salt and is expressed through a conscious desire for water. The phenomenon of thirst is not only important, it is very interesting! A thirsty person receives relief from thirst immediately after drinking water, even before the water has been absorbed from the stomach into the blood system and the rest of the body. You might wonder what the value of this temporary relief from thirst could be, but there is good reason for its occurrence. After a person has drunk water, as long as one half to one hour may be required for all the water to be absorbed and distributed throughout the body. Were the thirst sensation not temporarily relieved after dringing water, the person would continue to drink more and more. When all this water should finally become absorbed, the body fluids would be far more diluted than normal - too much so. Whoever put the human body together really knew what they were doing!
So far we have discussed the role of the pituitary gland (through the release of ADH) and thirst in the regulation of body fluids. These are the two primary mechanisms that are responsible for making sure that there is an appropriate water and sodium level in the blood. The second set of glands that participate in the regulation of body fluids is the adrenal glands. The adrenals sit on top of each kidney like a cap and, although they usually vary somewhat in size and shape, they generally look like pyramids. Like the pituitary, each adrenal gland is really two glands in one (Figure 4c). The central portion is termed the adrenal medulla, and the outer part is the adrenal cortex. The two sections represent distinct glands that secrete different hormones.
The adrenal medulla secretes both epinephrine (more popularly referred to as adrenalin from the days when it was thought to be the only hormone made by the adrenals) and norepinephrine. Both play a part in helping the body respond to emergency situations. Our main interest in this chapter leads us to focus more on the adrenal cortex, which is responsible for the production of at least 30 hormones. (Don't worry! We will only be looking at one!) All of the hormones secreted by the adrenal cortex are steroids, after a Greek word meaning "solid," or "firm." (You may have heard of the steroids that musclebuilders use to become larger. Those are synthetic steroids whose use can be extremely dangerous.) Because they are manufactured by the adrenal cortex, they are called "adrenocorticosteroids."
There are three groups of adrenocorticosteroids, each secreted by cells in a different layer of the cortex:
Figure 7. Aldosterone secreted by the adrenal cortex causes the kidney to excrete potassium (K 1 and the kidney to reabsorb sodium back into the blood stream.Of the thirty adrenocorticosteroids that exist, we will only be looking at the most important mineralocortocoid - aldosterone. This hormone acts mainly through the kidneys to maintain the homeostasis of sodium and potassium ions. More specifically, aldosterone causes the kidneys to conserve sodium ions (Na+) and to excrete potassium ions (K+). At the same time, it promotes water conservation and reduces urine output.
The mechanism that controls the secretion of aldosterone is primarily responsive to the potassium ion (K+) concentration in body fluids. This mechanism operates as a negative feedback loop (Figure 7) to detect high levels of K+ and to release aldosterone so that the K+ level can be reduced. Or, if the K+ level is too low, less aldosterone will be released so that K+ is allowed to accumulate. This is how it works when the body contains too much K+:
SummaryUrine volume control is influenced by a combination of many factors, and the precise regulation of urine volume is important and essential for a variety of different reasons. Ultimately, the body's main requirement is to maintain a balance of fluids and a balance of many chemicals (especially sodium) that flow throughout the system. From previous chapters, you are well aware of the fact that fluid volumes are affected by space flight. Overall fluid volume is decreased in microgravity. It is important to find out how microgravity affects the regulatory systems that control the fluid volumes and electrolyte concentrations in our bodies.
Let's move on to our investigation into how space flight affects the kidneys and the fluid regulating systems of the body.