Respiratory acidosis: Acidosis refers
to a process that lowers blood ph below 7.35 and respiratory refers to the fact
that it's a failure of the respiratory system to carry out its normal ph
balancing job. Now normally during an inhalation the diaphragm and chest wall
muscles contract to pull open the chest and that sucks in air like a vacuum
cleaner then during exhalation the muscles relax allowing the elastin in the
lungs to recoil pulling the lungs back to their normal size and pushing that
air out ultimately. The lungs need to pull oxygen into the body and get rid of
carbon dioxide; carbon dioxide binds to water in the blood and forms carbonic
acid which then dissociates into hydrogen ions and bicarbonate ions so in order
to prevent ph fluctuations the carbon dioxide concentration or the partial
pressure of carbon dioxide called pco2 needs to be kept within a fairly narrow
range.
For this reason, the lungs maintain
the ventilation rate they need to get rid of carbon dioxide at the same rate
that it's created by the tissues, if pco2 starts to rise and ph starts to fall;
chemoreceptors that are found in the walls of the carotid arteries and in the
wall of the aortic arch start to fire more and that notifies the respiratory centers
in the brain stem that they need to increase the respiratory rate and the depth
of breathing as the respiratory rate and the depth of each breath increases the
minute ventilation increases and that's the volume of air that moves in and out
of the lungs in a minute. The increased ventilation helps move more carbon
dioxide out of the body, reducing the pco2 in the body and raising the ph of
respiratory acidosis. The normal mechanism of ventilation is disturbed and
minute ventilation becomes inadequate to balance the ph this can be due to a
number of problems.
The carbon dioxide accumulates in
the blood so the partial pressure of carbon dioxide rises usually above 45mm of
mercury, this causes a decrease in blood ph often reducing it below 7.35 to
compensate for this decrease the body has designed several mechanisms, if the
respiratory centers are working then they try to increase the rate and depth of
ventilation if that doesn't work then some of the excess carbon dioxides
diffuses across cell membranes, especially into red blood cells where it reacts
with water molecules and forms carbonic acid which eventually gets converted
into hydrogen ions and bicarbonate ions. The key here is that this bicarbonate can
quickly escape into the circulation trying to counteract the increased partial
pressure of carbon dioxide and keeping the ph from getting too low at the same
time though free hydrogen ions are generated which could very well make the
intracellular environment.
Acidic fortunately they can be bound
and neutralized by various basic molecules within the cells mainly exposed nh2
or amine groups in proteins like hemoglobin the concentration of these proteins
though is too low compared to the amount of excess carbon dioxide molecules floating
through the blood what this means is that if all of the carbon dioxide
molecules tried to hide inside the cells they'd give rise to a whole lot of
hydrogen ions that have no spare protein to bind to and therefore this would
mess with the intracellular ph so essentially only a small amount of carbon
dioxide molecules find their way into the cells as a result the amount of
bicarbonate that's generated is too little about 1ml equivalent per liter for
each 10mm of mercury increase in partial pressure of carbon dioxide, in order
to have a substantial effect on ph for example if partial pressure of carbon
dioxide has an acute rise of 20mm of mercury let's say it moved from 40 to 60
then this mechanism could only result in a rise of plasma bicarbonate by 2ml equivalents
per liter, from its reference value of 24 up to 26 which doesn't have a big
impact on ph.
Therefore the ph remains very low
during this acute phase of the disorder fortunately if minute ventilation
hasn't decreased to life-threatening levels then within about three to five
days the kidney starts sensing that ph is too low and steps up to help correct
the imbalance more specifically the cells of the proximal convoluted tubule
start generating and reabsorbing more bicarbonate into the bloodstream, in fact, the kidneys are pretty effective in doing this since they managed to increase
the concentration of bicarbonate about 4ml equivalents per liter for each 10mm
of mercury increase in partial pressure of carbon dioxide, so if the partial
pressure of carbon dioxide went up from 20 to 40mm of mercury which is a 20mm
of mercury increase plasma bicarbonate would increase by 8ml equivalents per liter,
going from its reference value of 24 to 32 ml equivalents per liter this can lead
to a substantial increase in the ph bringing it closer to its normal range again.
All right as a quick recap, respiratory
acidosis happens when the lungs fail to eliminate excess carbon dioxide which
builds up in the blood causing blood ph to fall below 7.35 it's divided into an
acute and a chronic phase according to the absence or presence of renal
compensation respectively which raises bicarbonate concentration in the blood.
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