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Excretory System
V. Excretory System = Urinary System
A. Overall functions
1. Water homeostasis
2. Cleanses body of waste such as urea (urea is waste from protein metabolism)
3. pH homeostasis of blood
B. Anatomy
1. Kidney (renal, "nephr-" both roots mean kidney) - Your two kidneys are located
against your lower back region.
2. Ureters - These are the tubes that take the urine made by the kidneys to the
urinary bladder for storage.
3. Urinary bladder - Located within your pelvic cavity, the urinary bladder stores
the urine until it is time to urinate. It can hold 600-800 mls of urine.
4. Urethra - The urethra is the tube carrying urine from the bladder to the outside
opening of the body. It travels through the penis in the male, but is short and
empties in the vulva region, near the vaginal opening in the female.
C. Urine formation - Occurs in the kidneys at the microscopic level.
a. Nephron - The nephron is the functional unit of the kidney. These millions of
microscopic structures make urine by filtering blood.
1. Glomerulus - The first part of the nephron starts with a clump of
blood capillaries. This should make sense, since urine is filtered blood.
At this level, anything that is small enough to fit through the wall of the
glomerulus capillaries is filtered. The process is based on size, not on
need. For example, RBCs are too large and are not filtered. Glucose,
water and amino acids are small and are filtered.
2. Glomerular or Bowman's Capsule - The material filtered out of
the glomerulus passes into a cup-like structure, called the glomerular
capsule. It "catches" what is being filtered.
3. Renal Tubules - Next, the filtered material passes through a series
of tubules, or little tubes, in which substances NEEDED by the body,
that should not be urinated out, can be reabsorbed out of the tubules
and back into the bloodstream. For example, you do not want to
urinate your favorite energy source, glucose, so it is selectively
reabsorbed back into the bloodstream. Water, nutrients and salt are
also reabsorbed. This process turns the filtered material into waste.
[Substances can also be secreted directly from the blood to the tubules
as well, to allow for getting rid of larger molecules that cannot be
filtered.]
4. Collecting ducts - After the selective reabsorption, many nephrons
dump their contents into collecting ducts. Here, some more water can
be reabsorbed, and then urine is in its final form and ready to exit the
kidney.
b. Controls
1. Blood pressure - As blood pressure increases, more blood moves
through the glomerulus, and filtration & urine production increases.
2. Aldosterone - This is a hormone made by the adrenal gland. It
causes sodium (salt) to be reabsorbed back into the bloodstream from
the renal tubules. Since water loves to follow the salt, it indirectly
results in water being reabsorbed as well. As more water goes into
your bloodstream, it as though someone turned up the faucet on a hose,
and the blood pressure rises.
3. Antidiuretic hormone (ADH) - This hormone is made by a part of
the brain called the hypothalamus. It results in water reabsorption
from the renal tubules. This causes you to conserve water in your
bloodstream, and urinate dark and concentrated urine. Drinking
alcohol directly blocks the release of ADH, and thus you cannot
conserve your water, and you urinate a lot of dilute urine. It can result
in dehydration.
D. Urinary Pathology
1. Cystitis - "Cyst" means bladder. This is a bladder infection. It is usually the
result of an ascending infection - bacteria migrating from the outside of the body,
up the urethra, to the bladder. It is more common in women, due to the shorter
urethra. Increasing urination, through increases fluid consumption, can help
"rinse" bacteria out of the bladder and urethra. It can be diagnosed through a
urinalysis which is an analysis of the urine in which bacteria and white blood cells
would be numerous.
2. Renal failure - You have many nephrons, so have some leeway in losing
nephrons through time. However, you reach a critical level if more than 2/3 of your
nephrons are destroyed. Then, there are not enough nephrons to rid you body of
waste. This is renal failure.
Can you live without ONE kidney if the other one is functioning properly? This
would cause a loss of 1/2 of your nephrons. The answer is yes.
a. Uremia - This is the build up of toxic waste products, such as urea,
in the bloodstream as a result of the kidneys not working.
b. Treatment
1. Dialysis - This process filters your blood for you, since
your failing kidney cannot do it.
2. Transplant - Kidney transplants are an option for people
with renal failure. The procedure is one of the more
successful transplants performed.