Kidney Structure, Filtration, Reabsorption, and Secretion
The human body contains multiple organs, which in turn those organs belong to systems. One
system that is very important to the health of the human body is the renal or urinary system. The
major organs included in the renal system are the kidneys. The kidneys are bean-shaped organs,
located in the back of the abdomen on both the left and right side of the body, that help filter and
secrete waste in the form of urine. The overall job of the kidneys is to filter the blood passing
through them in order to eliminate the waste found in the blood stream. This document provides
an introductory biology-level review of kidney structure and the three processes the kidney
experiences in order to regulate blood composition which is filtration, reabsorption, and
secretion.
Overview of the Kidney Structure
The kidney is comparable to the size of an
individual’s fist. As mentioned previously, the
kidneys are bean-shaped organs with a convex
and concave surface. On the concave side, the
renal artery enters the kidney and the renal
vein and ureter (tube in which the urine exits
the kidneys) exit the kidneys. There is a thin
fibrous layer or membrane outside the kidneys,
on its surface, known as the renal capsule. The
inside of the kidney can be divided into two
structures, the cortex and the medulla. The
cortex is the outer portion of the kidney located
between the capsule and medulla (see Figure
1). The medulla is the innermost part of the
kidney (see Figure 1) where there are pyramid
shaped lobes (number of lobes varies from 8 to
18 per kidney). Tiny tubular structures that
stretch from both the cortex and medulla are
called nephrons. The nephron is where the
filtration occurs. Nephrons consist of six
different components. The six parts include:
ascending and descending loops of Henle,
capillaries, a proximal tubule, glomerulus, and
a distal tubule (see Figure 1).
Figure 1: Diagram of a kidney with labels.
© 2001 How Stuff Works
Filtration of Blood through the Kidney
The first process in eliminating waste from the body and blood is filtration. The kidneys receive
about 20 percent of blood that needs to be “processed” or filtered. In general, humans contain
seven to eight liters of blood which gets filtered between 20 to 25 times per day. Blood travels
from other parts of the body, mostly from the heart, to the renal artery. The renal artery
(oxygenated blood) carries the unfiltered blood, containing toxins and wastes, into the kidneys.
Once the blood enters the kidneys, it begins the process of filtration. The blood is put under
pressure from the glomerular capillaries and the Bowman’s capsule (both are part of the
glomerulus) where it is filtered (see figure 1 above). The filtered blood is then passes back out
through the series of capillaries that are part of the nephron. After passing through the capillaries
of the nephron, the “clean” blood sent back to the heart via the renal vein. After which the
“clean” blood re-circulates throughout the body. The excess particles and molecules that are
filtered out generally consist of water, sodium ions, potassium ions, chloride ions, sugars, and
small proteins. The filtration process is analogous to making espresso (water under high pressure
through a sieve containing ground coffee).
Reabsorption of Particles and Molecules in the Kidney
The second process of the kidneys is reabsorption. The excess particles and molecules (water,
sodium ions, potassium ions, chloride ions, sugars, and small proteins), are grabbed by the
transporters in the nephrons as the particles and molecules pass by them. Transporters are
specialized proteins located in
various cells in the nephron that
collect the particles and
molecules. Each of the
transporters is designated to grab
certain particles or molecules
(generally one or two types of
particles and molecules per
transporter). Certain transporters
are located at different positions
throughout the nephron. Some of
the transporters require energy,
also known as active transport,
in order for the particles or
molecules to be reabsorbed.
Some of the transporters do not
require energy, which is known
as passive transport, for the
particles or molecules to be
reabsorbed. A form of passive
transport, known as osmosis, is
how water gets reabsorbed in the
nephron. The amount of water
that is reabsorbed depends on
how much sodium ions are
reabsorbed. In fact, most substances that are reabsorbed back into the blood stream are based on
the sodium ions that were reabsorbed. There are two factors that affect the reabsorption process.
The first factor is the concentration of the particles and molecules. The higher the concentration
the more particles and molecules can be absorbed. The second factor is the rate at which the
particles and molecules flow. The rate of flow affects the amount of time available for the
transporters to reabsorb the particles and molecules. Figure 2 illustrates the reabsorption process.
Secretion of Waste from the Kidney
The third process of the kidneys is secretion. The remaining and unused particles and molecules
not reabsorbed into the blood stream are considered as waste. The secretion function of the
nephron moves the remaining and unused particles and molecules from the blood in the
capillaries. Those remaining and unused particles and molecules are transferred to the urine
which is located in the distal tubules (see figure 2 above). Some particles and molecules that are
transferred to the urine include: ammonia, hydrogen ions, excess potassium ions, and any
possible drugs that are in a person’s system. The urine then moves into the ureters that are
attached to the kidneys which then transfer to the bladder.
Issues with the Kidneys and Conclusion
The kidneys are part of the urinary or renal system. The kidneys are bean-shaped organs, located
in the back of the abdomen on the left and right side of the body, that help filter and excrete
waste in the form of urine. There are three processes the kidney endures in order to regulate the
blood composition. The three processes are filtration, reabsorption, and secretion. However,
certain diseases can cause problems in one if not all of the three processes. A few of these
diseases include: diabetes, high blood pressure, acute and chronic renal failure, etc. There are
hundreds of diseases that cause the kidneys to not function properly. These diseases cause
problems that could ultimately lead to the need for a kidney transplant (Only if both the kidneys
are damaged; a person can live with one working kidney) or dialysis (the use of a machine to
filter the blood for a non-working or functioning kidney).
Overall, the kidneys are one of the most important organs in the human body. They help with
regulating the blood in the body, which contributes to a person’s blood pressure. The kidneys
assist in eliminating unnecessary and harmful substances from the body. They also aid in
maintaining homeostasis (balance and equilibrium) within the human body and all other systems.