The Urinary System
Chapter 15
The Anatomy of the Urinary System
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The urinary system is composed of
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Kidneys
Ureters
Urinary bladder
Urethra.
Functions of the Urinary System
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The urinary system functions to regulate
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blood ionic composition
blood volume
blood pressure
pH.
Red blood cell production
Vitamin D synthesis
Excretion of wastes.
The Physiology of the Kidneys
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The story of the urinary system begins with the blood, for it is the blood
which is regulated by the action of the kidneys.
Our blood contains waste products which we need to eliminate from
the body.
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The most important of these wastes is a nitrogen-containing compound
called urea. Whenever we convert the amino acids of proteins into the
energy molecule of glucose we have a problem. What do we do with the
nitrogen found in amino acids since glucose does not have nitrogen
present? In the conversion process, which occurs in the liver, amino acids
are converted into glucose with a resulting byproduct of urea. This
compound must then be lost from the body before it accumulates to the
level that would be toxic to the body’s tissue.
The Physiology of the Kidneys:
Glomerular Filtration
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Blood is sent to the kidney where it passes to the functional
units of the kidney called nephrons.
The afferent arteriole which carries blood to the nephron has a
larger diameter than the efferent arterole which carries blood
away from the nephron.
This difference in vessel diameter increases the local blood
pressure of the nephron.
It is this pressure which will help to push substances through
the membranous filter (glomerulus) of the nephron.
This high pressure pushes substances through the capillary
walls of the glomerulus much as a leaky garden hose looses
more water when the water pressure is increased.
This process is referred to as glomerular filtration.
The Physiology of the Kidneys:
Glomerular Filtration
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The filtration slits of the glomerulus are too small to
allow the formed elements (cells and cellular
components) and proteins of the blood to pass
through thus the filtrate is only composed of water,
glucose, electrolytes and, of course, urea.
This is the reason that presence of cells and protein
in the urine may be a cause of concern as it could
indicate a kidney disfunction.
The Physiology of Kidneys: Tubular
reabsorption
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The filtrate which had been strained through the glomerular
membrane will now pass into the proximal convoluted tubule,
so called because it is closest tubule to the glomerulus
(proximal) and that it is coiled (convoluted).
Glucose readily passes through the filtration slits of the
glomerulus and so is present within the filtrate which will
ultimately be the urine lost from the body.
Why then do we consider presence of glucose in urine to be an
indication of non-normal functioning of the body (diabetes)?
After all, shouldn’t glucose normally be found in the urine if it is
found in the filtrate of the kidney tubules?
The Physiology of Kidneys: Tubular
reabsorption
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Since we go through so much trouble to obtain glucose (we
obtain food, we cook food,we eat food, we digest food, we
absorb food) it would be a very inefficient process for our body
to simply then loose it to the outside.
So our kidney nephrons retrieve glucose from the filtrate as it
passes along the proximal convoluted tubule.
Along with active transport of glucose across the proximal
convoluted tubule lining other important substances such as
sodium ions and chloride ions are also transported across to be
returned to the blood.
This process is referred to as tubular reabsorption.
The Physiology of Kidneys:
Concentration of the Filtrate
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When the filtrate reaches the loop of Henle it has
now lost many of its solutes and has become more
dilute.
As the filtrate passes down the descending limb of
the loop of Henle it passes through increasingly
greater salt content in the surrounding tissue.
This tends to pull water out of the renal tubule due to
osmosis.
Thus the filtrate becomes more and more
concentrated due to the loss of water to the
surrounding tissues.
The Physiology of Kidneys:
Concentration of the Filtrate
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As the filtrate passes up the ascending limb of the
Loop of Henle the walls of the ascending limb is
impermeable to water thus water is unable to pass
back into the tubule even though on the return trip to
the cortex of the kidney it is passing through tissue
with less and less saltiness.
This process represents urine concentration.
The Physiology of Kidneys: Tubular
Secretion
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Fluid leaving the ascending limb of the loop
of Henle now passes through the distal
convoluted tubule where substances are
directly transferred from the surrounding
capillaries into the renal tubule. This process
is referred to as tubular secretion.
Further concentration of filtrate
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The end of the kidney nephron brings us to the
collecting ducts which collect the filtrate from
hundreds of nephrons.
Here we have a second chance to concentrate our
urine.
If antidiuretic hormone (ADH) is produced due to us
not drinking enough water, its presence will open
channels which allow water to go out into the
surrounding kidney tissue as the collecting duct
passes into the saltier medulla of the kidney.
Micturition
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From the collecting duct the filtrate which now can
be considered urine, passes into the minor calyx, the
major calyx, the renal pelvis, and the ureter, in order
to pass and be stored in the urinary bladder. It will
remain here until we feel we need to
er...ah...ahem...powder our noses. (I really mean the
process of urination or micturition.)
Fluids and Electrolytes
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Our body has a very delicate balance of its components.
Fluid from our extracellular compartment are able to enter our
intercellular compartment and vise versa.
This movement of water from outside the cell to inside the cell
is controlled by relative concentrations of electrolytes in the
various fluid compartments.
The relative concentration of the electrolytes in our various fluid
compartments is illustrated in a diagram in your textbook.
Their functions within the body are described in your text
narration.
Acidosis and Alkalosis
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For normal functioning of our cellular enzymes we need to have
normal pH (acidity) in our fluids.
We have a variety of buffer systems which tend to keep our
acids and bases in balance (homeostasis).
If our blood becomes too acidic the condition is called acidosis.
If our blood becomes too basic the condition is called alkalosis.
We can go into respiratory acidosis if we hypoventilate (lack of
breathing will allow for the accumulation of carbon dioxide in
our blood which reacts with water to form carbonic acid).
Respiratory alkalosis is caused by hyperventilation (breathing
too rapidly leads to a loss of too much carbon dioxide which
leads to our pH going to high).
Acidosis and Alkalosis
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Medications can lead to these pH problems as well.
A person who seeks to commit suicide by ingesting a bottle of
aspirin (salicylic acid) is creating acidosis and will die due to
enzyme malfunction unless appropriate medical attention is
delivered.
A nursing school instructor once explained it to me this way. “If
you don’t give the appropriate medication to a patient it is like
being on the loosing team of a baseball game. When you hit the
ball and begin to run around the baseball diamond, those bases
are going to kick your ass...ids.”