There are only two major sets of paired organs in the body, the kidneys and the lungs. It seems that in the original plans for the human body, the architect (whoever that may be in each of our minds) recognized the crucial nature of the kidney's and lung's assigned tasks and included an "extra" organ in each case to assure that the respiratory and excretory gateways of the body could operate with some kind of a back-up system. In a spacecraft, many of the main computer systems that are extremely crucial to the safe operation of the spacecraft have back-up systems that are included just in case the main system breaks down. These are what are known as redundant systems (redundant = serving as a duplicate for preventing failure of an entire system). Many systems that operate with redundancy utilize the back-up system only if the primary system breaks down. However, the body utilizes a different approach. Under normal conditions, the body will utilize both organs, but if one of the organs is damaged in some way, the other organ can take over the entire function and operate as if both organs were present. Many human beings are alive and thriving today, despite damage or actual removal of one lung or kidney, because the remaining organ fills the breach and continues to do the work of both. In this chapter, we will be focusing on the structure and fluid-regulating function of the kidneys.
| Figure 2.(a) The position of the kidneys in the body. |
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We have already seen in the last chapter how the kidneys are a major
participant in the regulation of red blood cell (RBC) production.
Remember, they act as sensors to detect low levels of
oxygen in the blood, then they release the hormone erythropoietin, which
is the effector that travels to the bone marrow to
stimulate RBC production. Remember further that this is a negative
feedback loop. In this chapter we will examine how the kidneys
play an essential role in the regulation of an appropriate fluid balance
within the body. Again, through their release of hormones, the kidneys
participate in this other negative feedback loop, which operates to
regulate the amount of water in the body. With their multiple roles within
the body, the kidneys are a very good example of how everything in the
body is somehow connected or integrated. That is, on the
one hand, the kidneys must pay close attention to the RBC volume in the
body to ensure that oxygen levels are correct and, on the other hand, they
must pay close attention to the amount of water in the body. The kidneys
even perform another function that we will mention later.
Let's first discuss the kidney's structure. At first sight, a kidney looks
like a four-or-five-inch-long bean (Figure 2a)!
| Figure 2. (b) The inside of the kidney has two main parts, the cortex and the medulla. The medulla is composed of medullary pyramids (so named because of their shape). |
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| Figure 2. (c) Within the cortex and the medulla of each kidney are about a million nephrons responsible for the production of urine through the processes of filtration, reabsorption, and secretion. |
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The outer skin of the kidney is called the renal capsule (renal refers to the kidney). If you were to slice through a kidney from side to side and open it like the pages of a book, you would see two main parts, a central part and an outer, peripheral part (Figure 2b). The peripheral part consists of the cortex and the medulla, with the medulla consisting of medullary pyramids; the inner, central part consists of a group of tubes (the calyx and the pelvis) that lead out of each pyramid and into the ureter (the tube that drains urine into the bladder).
Within the cortex and the medullary pyramids are found millions of tiny structures called nephrons (Figure 2c). The nephrons are responsible for filtering out of the bloodstream an estimated 43 gallons of water a day- about twice the body's entire weight in fluid - through an intricate network of tubules (little tubes
The shape of a nephron is unique, unmistakable, and admirably suited to
its function of producing urine (Figure 2c).It looks a little like a big
"mouth" hooked to a really long and winding neck. The "mouth" is filled
with what looks like a round lawbreaker that is so big that the "mouth"
cannot close! In reality, the nephron is composed of two main parts: the
renal corpuscle (the mouth and the jawbreaker) and the
renal tubule (the long and winding neck). The "mouth" of
the renal corpuscle is actually called Bowman's capsule
and the " jawbreaker" is actually called the glomerulus
from the Latin word for "small ball" (the plural form is glomeruli). The
glomerulus is a network of blood capillaries that is surrounded, first, by
a double membrane (the glomerular capsular membrane) and
then is surrounded by Bowman's capsule. The renal tubule (the long and
winding neck) consists of the proximal tubule (the thick,
winding part of the neck that extends just up to where it begins to make a
U-turn); the loop of Henle (the thinner part of the neck
that actually makes the U-turn); and the distal tubule
(the last, thicker part of the neck that travels away from the U-turn,
winds all around, and eventually leads into collecting
tubules). Now that you are familiar with the structure of the
kidney and, in particular, the nephron, let's see how the whole system
works to produce urine so that waste products can be removed from the
body.