Download 18 Renal Acid-Base Balance

Renal Physiology
PART
THREE
Renal Acid-Base Balance
1
Acid
•
•
•
•
An acid is when hydrogen ions accumulate in a solution.
It becomes more acidic
[H+] increases = more acidity
CO2 is an example of an acid.
HCl
2
H+
H+
Cl-
H+
H+
ClH+
7
ClpH
ClCl-
As concentration of hydrogen
ions increases, pH drops
Base
• A base is chemical that will remove hydrogen ions from
the solution
• Bicarbonate is an example of a base.
NaOH
Na+ OH- H +
ClH+
ClNa+ OHH+
ClCl- Na+ OHH+
ClNa+ OH-
H+
2
7
pH
Acids and basis
neutralize
eachother
A change of 1 pH unit corresponds to
a 10-fold change in hydrogen ion
concentration
2
Na+
ClNa+
H+
Na+
7
OH-
Na+
Cl-
ClH2O
Cl-
pH
Acids are being created constantly
through metabolism
• Anaerobic respiration of glucose produces
lactic acid
• Fat metabolism yields organic acids and
ketone bodies
• Carbon dioxide is also an acid.
Acids must be buffered, transported away
from cells, and eliminated from the body
Phosphate: important renal tubular buffer
HPO4- + H+
H2PO 4
Ammonia: important renal tubular buffer
NH3 + H+
NH4+
Proteins: important intracellular and plasma buffers
H+ + Hb
HHb
Bicarbonate: most important Extracellular buffer
H2O + CO2
H2CO3
H+ + HCO3 -
Buffering is good, but it is a temporary
solution. Excess acids and bases must be
eliminated from the body
gas
H2O + CO2
aqueous
H2CO3
H+ + HCO3 Kidneys can remove excess
non-gas acids and bases
Lungs eliminate
carbon dioxide
Excessive Acids and Bases can cause
pH changes---denature proteins
• Normal pH of body fluids is 7.40
• Alkalosis (alkalemia) – arterial blood pH rises above 7.45
• Acidosis (acidemia) – arterial pH drops below 7.35
• Acidosis:
– too much acid
– Too little base
• Alkalosis
– Too much base
– Too little acid
Compensation for deviation
• Lungs (only if not a respiratory problem)
– If too much acid (low pH)—respiratory
system will ventilate more (remove CO2) and
this will raise pH back toward set point
– If too little acid (high pH)—respiratory will
ventilate less (trap CO2 in body) and this will
lower pH back toward set point
• Kidneys
– If too much acid (low pH)—intercalated cells
will secrete more acid into tubular lumen
and make NEW bicarbonate (more base)
and raise pH back to set point.
– If too little acid/excessive base (high pH)proximal convoluted cells will NOT reabsorb
filtered bicarbonate (base) and will eliminate
it from the body to lower pH back toward
normal.
Acid-Base Balance
• How would your ventilation change if you
had excessive acid?
– You would hyperventilate
• How would your ventilation change if you
had excessive alkalosis?
– Your breathing would become shallow
How can the kidneys control acids
and bases?
• Bicarbonate is filtered
and enters nephron at
Bowman’s capsule
• Proximal convoluted
tubule
– Can reabsorb all
bicarbonate (say, when you
need it to neutralize
excessive acids in body)
OR
– Can reabsorb some or
NONE of the bicarbonate
(maybe you have too much
base in body and it needs
to be eliminated)
How can the kidneys control acids
and bases?
• Acidosis
• Intercalated cells
– Secrete excessive
hydrogen
– Secreted hydrogen
binds to buffers in the
lumen (ammonia and
phosphate bases)
– Secretion of hydrogen
leads to formation of
bicarbonate
HPO4NH3
What would happen if the respiratory
system had a problem with ventilation?
Respiratory Acidosis and Alkalosis
Normal PCO2 fluctuates between 35 and 45 mmHg
• Respiratory Acidosis (elevated
CO2 greater than 45mmHg)
• Depression of respiratory centers
via narcotic, drugs, anesthetics
• CNS disease and depression,
trauma (brain damage)
• Interference with respiratory
muscles by disease, drugs,
toxins
• Restrictive, obstructive lung
disease (pneumonia,
emphysema)
• Respiratory Alkalosis (less
than 35mmHg- lowered CO2)
• Hyperventilation syndrome/
psychological (fear, pain)
• Overventilation on mechanical
respirator
• Ascent to high altitudes
• Fever
What if your metabolism changed?
• Metabolic acidosis
• Bicarbonate levels
below normal (22
mEq/L)
• Ingestion, infusion or
production of more
acids (alcohol)
• Salicylate overdose
(aspirin)
• Diarrhea (loss of
intestinal bicarbonate)
• Accumulation of lactic
acid in severe Diabetic
ketoacidosis
• starvation
• Metabolic alkalosis
• bicarbonate ion levels
higher (greater than
26mEq/L)
• Excessive loss of acids
due to ingestion,
infusion, or renal
reabsorption of bases
• Loss of gastric juice
during vomiting
• Intake of stomach
antacids
• Diuretic abuse (loss of
H+ ions)
• Severe potassium
depletion
• Steroid therapy
How can you tell if the acid-base
balance is from a kidney disorder
or a lung disorder?
Acidosis: pH < 7.4
- Metabolic:
- respiratory:
HCO3 pCO2
Alkalosis: pH > 7.4
- Metabolic: HCO3 - respiratory: pCO2
Compensation
• If the kidneys are the problem, the
respiratory system can compensate.
• If the kidneys are secreting too much
H+(which makes too much bicarbonate,
causing metabolic alkalosis), breathing will
become slower so that less CO2 (an acid)
is lost.
• If the kidneys are reabsorbing too much
H+(metabolic acidosis), breathing will
become faster.
Compensation
• If the respiratory system is the problem,
the kidneys can compensate.
• If breathing is too rapid (too much CO2, an
acid, is lost, leaving the blood in
respiratory alkalosis), Kidneys respond by
reabsorbing more H+.
• If breathing is too shallow (not enough
CO2 is lost, leaving the blood in respiratory
acidosis), Kidneys respond by secreting
more H+.
Summary
• Let’s summarize so we can
apply this to clinical conditions!
• Acidosis
–Can be metabolic or respiratory
• Alkalosis
• Can be metabolic or respiratory
Acidosis
• Acidosis is excessive blood acidity caused
by an overabundance of acid in the blood
or a loss of bicarbonate from the blood
(metabolic acidosis), or by a buildup of
carbon dioxide in the blood that results
from poor lung function or slow breathing
(respiratory acidosis).
Acidosis
• Blood acidity increases when people ingest substances
that contain or produce acid or when the lungs do not
expel enough carbon dioxide.
• People with metabolic acidosis have nausea, vomiting,
and fatigue and may breathe faster and deeper than
normal.
• People with respiratory acidosis have headache and
confusion, and breathing may appear shallow, slow or
both.
• Tests on blood samples show there is too much acid.
• Doctors treat the cause of the acidosis.
Metabolic acidosis
• Can be caused by ingestion of an acid
(aspirin, ethanol, or antifreeze) or too
many acidic waste products have built up
(such as from untreated diabetes mellitus
or eating too much protein that the kidneys
cannot keep up with excreting the acid ),
or it could be from loss of bicarbonate from
diarrhea.
• Treatment is give i.v. of sodium
bicarbonate.
Respiratory acidosis
• Caused from an increase in CO2 in the
blood because the lungs are
hypoventilating (seen in asthma, COPD,
and overuse of sedatives or narcotics such
as valium, heroin, or other drugs which
make you sleepy).
• Treatment is to increase ventilation
(oxygen mask).
Respiratory acidosis
• May have no symptoms but usually experience
headache, nausea, vomiting, and fatigue.
• Breathing becomes deeper and slightly faster
(as the body tries to correct the acidosis by
expelling more carbon dioxide).
• As the acidosis worsens, people begin to feel
extremely weak and drowsy and may feel
confused and increasingly nauseated.
• Eventually, blood pressure can fall, leading to
shock, coma, and death.
Alkalosis
• Alkalosis is excessive blood alkalinity
caused by an overabundance of
bicarbonate in the blood or a loss of acid
from the blood (metabolic alkalosis), or by
a low level of carbon dioxide in the blood
that results from rapid or deep breathing
(respiratory alkalosis).
Alkalosis
• People may have irritability, muscle
twitching, or muscle cramps, or even
muscle spasms.
• Blood is tested to diagnose alkalosis.
• Metabolic alkalosis is treated by replacing
water and electrolytes.
• Respiratory alkalosis is treated by slowing
breathing.
Metabolic alkalosis
• Caused from an increase in bicarbonate in
the blood because of ingestion of excess
antacid (Tums), eating excess fruits
(vegetarian diets and fad diets*), loss of acid
from vomiting, or loss of potassium from
diuretics.
• Treatment is to give an anti-emetic if the
problem is from vomiting. If not, give an i.v. of
normal saline to increase the blood volume.
• If potassium is also low, would have to add
that to the i.v.
FYI
• *Fruits are the normal source of alkali in
the diet. They contain the potassium salts
of weak organic acids. When the anions
are metabolized to CO2 and removed from
the body, alkaline potassium bicarbonate
and sodium bicarbonate remain. Metabolic
alkalosis may be found in vegetarians and
fad dieters who are ingesting a lowprotein, high fruit diet.
Respiratory alkalosis
• Caused from a decrease in CO2 in the blood
because the lungs are hyperventilating (anxiety,
but not panting).
• Symptoms include irritability, muscle twitching,
muscle cramps.
• Treatment for hyperventilation is to breathe into
a paper bag for a while, as the person breathes
carbon dioxide back in after breathing it out.
• For severe cases, need to replace the water and
electrolytes (sodium and potassium).
Respiratory alkalosis
• Caused from a decrease in CO2 in the
blood because the lungs are
hyperventilating (anxiety, but not panting).
• Treatment is to breathe into a paper bag
for a while.
Clinical Problem 1a
• Your patient’s blood pH is too low (acidosis).
• This can be caused by either respiratory acidosis or
metabolic acidosis. Let’s say it was respiratory acidosis
(abnormal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If blood carbon dioxide levels are elevated (too much blood
acid) and urine bicarbonate levels are elevated (H+ is being
secreted to make bicarbonate). That means that the original
problem of elevated carbon dioxide (pCO2) is being
compensated for by the kidneys, which are secreting more
H+ to make more bicarbonate.
• Since this patient’s bicarbonate levels are also elevated,
they are compensating already.
Clinical Problem 1b
• Your patient’s blood pH is too low (acidosis).
• This can be caused by either respiratory acidosis or
metabolic acidosis. Let’s say it was respiratory acidosis
(abnormal breathing rate).
• We need to look at the patient’s partial pressures of
carbon dioxide and bicarbonate to see if they are
compensating.
• If blood carbon dioxide levels are elevated (too much
blood acid) and urine bicarbonate levels are decreased
(H+ is being absorbed). That means that the original
problem of elevated carbon dioxide (pCO2) is NOT being
compensated for by the kidneys, which should be
secreting more H+ to make more bicarbonate.
Clinical Problem 2a
• Your patient’s blood pH is too high (alkalosis).
• This can be caused by either respiratory alkalosis or metabolic
alkalosis. Let’s say it was respiratory alkalosis (abnormal breathing
rate).
• We need to look at the patient’s partial pressures of carbon dioxide and
bicarbonate to see if they are compensating.
• If blood carbon dioxide levels are low (too little acid) and bicarbonate
levels in the urine are low (too little base). That means that the original
problem of decreased carbon dioxide (pCO2) is being compensated for
by the kidneys, which need to start reabsorbing more H+ to make more
acid in the blood. If they are reabsorbing more H+, they are making less
bicarbonate in the urine.
• Since this patient’s bicarbonate levels in the urine are low, they are
compensating.
Clinical Problem 2b
• Your patient’s blood pH is too high (alkalosis).
• This can be caused by either respiratory alkalosis or
metabolic alkalosis. Let’s say it was respiratory alkalosis
(abnormal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If blood carbon dioxide levels are low (too little acid) and
bicarbonate levels in the urine are high (too much base).
That means that the original problem of decreased carbon
dioxide (pCO2) is NOT being compensated for by the
kidneys, which need to start reabsorbing more H+ to make
the blood more acidic. The bicarbonate levels would be low
if the kidneys were doing this.
Clinical Problem 3a
• Your patient’s blood pH is too low (acidosis).
• This can be caused by either respiratory acidosis or
metabolic acidosis. Let’s say it was metabolic acidosis
(normal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If urine bicarbonate levels are too low (H+ is being
reabsorbed) and blood carbon dioxide levels are too low
(too little blood acid), that means that the original problem
of a low bicarbonate level needs to be compensated for by
the lungs, which need to hyperventilate, expelling more CO2
(an acid).
• Since this patient’s pCO2 levels are also low, they are
compensating already.
Clinical Problem 3b
• Your patient’s blood pH is too low (acidosis).
• This can be caused by either respiratory acidosis or
metabolic acidosis. Let’s say it was metabolic acidosis
(normal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If urine bicarbonate levels are too low (H+ is being
reabsorbed) and blood carbon dioxide levels are too high
(too much blood acid), that means that the original problem
of a low bicarbonate level needs to be compensated for by
the lungs, which need to hyperventilate, expelling more CO2
(an acid).
• However, since this patient’s pCO2 levels are also high (not
expelling enough acid), they are NOT compensating.
Clinical Problem 4a
• Your patient’s blood pH is too high (alkalosis).
• This can be caused by either respiratory alkalosis or
metabolic alkalosis. Let’s say it was metabolic alkalosis
(normal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If bicarbonate levels in the urine are high (too much base)
and blood carbon dioxide levels are high (too much acid),
that means that the original problem of increased
bicarbonate levels need to be compensated by the lungs,
which should take shallower breaths so less acid is lost.
• Since this patient’s pCO2 levels are elevated, they are
compensating.
Clinical Problem 4b
• Your patient’s blood pH is too high (alkalosis).
• This can be caused by either respiratory alkalosis or
metabolic alkalosis. Let’s say it was metabolic alkalosis
(normal breathing rate).
• We need to look at the patient’s partial pressures of carbon
dioxide and bicarbonate to see if they are compensating.
• If bicarbonate levels in the urine are high (too much base)
and blood carbon dioxide levels are low (too little acid), that
means that the original problem of increased bicarbonate
levels need to be compensated by the lungs, which should
take shallower breaths so less acid is lost.
• Since this patient’s pCO2 levels are low, they are NOT
compensating.
Condition
pH
Resp
CO2
Bicarb
Compensating?
Resp acidosis
Low
Hypoventilating
High
High
Yes
Resp acidosis
Low
Hypoventilating
High
Low
No
Resp alkalosis
High
Hyperventilating
Low
Low
Yes
Resp alkalosis
High
Hyperventilating
Low
High
No
Metab acidosis
Low
Normal
Low
Low
Yes
Metab acidosis
Low
Normal
High
Low
No
Metab Alkalosis
High
Normal
High
high
Yes
Metab Alkalosis
High
Normal
Low
High
No