Biomedical Engineering Reference
In-Depth Information
epithelial cells. However, the epithelial layer does not affect the flow within the blood ves-
sel but functions to partially regulate the blood filtration (see
Section 12.2
). The glomerular
capillary epithelial cells are surrounded by Bowman's capsule, which is connected to the
nephron tubule system. All fluid and compounds that are filtered from the glomerular
capillaries must pass through the endothelial cell barrier and the epithelial cell barrier
before entering Bowman's capsule. The entire glomerulus structure lies within the cortex
of the kidney. A term given to the glomerulus and Bowman's capsule is the renal
corpuscle.
Any fluid that enters Bowman's capsule is passed directly into the continuous renal
tubule. The renal tubule is a narrow-diameter hollow cylinder that is composed of a single
layer of epithelial cells, which are attached to a basement membrane. There are 10 different
types of epithelial cells that are found within the nephron. Each of these epithelial cell
types form a continuous segment of the nephron, each with a slightly different function
during urine formation. These 10 segments are typically grouped into 5 tubule systems,
each with their own common function. The first epithelial cell type is found within
Bowman's capsule (the first tubule system) and functions to shunt the plasma filtrate into
the proximal tubule. The proximal tubule (the second tubule system) is broken up into
two segments, each with its own specific epithelial cell composition. The first segment is
the proximal convoluted tubule, and the second segment is the proximal straight tubule.
The third tubule system is termed the Loop of Henle, which forms a very sharp 180-degree
turn within the nephron. The Loop of Henle is broken up into the descending thin limb,
the ascending thin limb, and the thick ascending limb. Very specialized cells, termed the
macula densa cells, are located within the thick ascending limb of the Loop of Henle
(more details regarding this will be discussed later). The fourth tubule system is termed
the distal convoluted tubule, which is only composed of one segment given the same
name. The last tubule system is termed the collecting duct system and is composed of the
connecting tubule, the cortical collecting tubule, and the medullary collecting tubule. As
the reader can imagine, each epithelial cell type has a different permeability toward com-
pounds within the glomerular filtrate, thus altering the particular segment's function from
the preceding segment and the following segment.
In subsequent discussions, it will become apparent that there are important differences
in nephron function based on the regional location within the kidney. For reference, the
renal corpuscle, the proximal convoluted tubule, the distal portion of the thick ascending
Loop of Henle, the distal convoluted tubule, the connecting tubule, and the cortical collect-
ing tubule are all located within the kidney's cortex. The proximal convoluted tubule, the
thin descending limb, the thin ascending limb, the proximal portion of the thick ascending
Loop of Henle, and the medullary collecting tubule are all located within the kidney's
medulla. It is also important to note that the entire tubule system is surrounded by the
peritubular capillaries (although these are not shown in
Figure 12.2
).
One of the most unique locations within the nephron unit is termed the juxtaglomerular
apparatus (
Figure 12.3
). At this location, the thick ascending Loop of Henle passes in
between the afferent arteriole and the efferent arteriole, which feeds the glomerular
capillaries and the peritubular capillaries of the same nephron. In the thick ascending
Loop of Henle, there are specialized epithelial cells termed the macula densa cells, which
are in direct contact with specialized cells present within the afferent arteriole. The afferent
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