Lecture 6 : Acid-Base Balance Flashcards
___ is a problem for the body
Acidity
Our bodies are challenged by intake and production of ___
acids (more than bases)
How we get acid:
Diet - fatty acids and amino acids
Metabolism - CO2 (+H2O), lactic acid, ketoacids
How we get rid of acid:
Ventilation - CO2 (+H2O)
Renal - H+
pH is a measure of ____
free H+
Normal arterial blood pH is
~7.4 (range 7.35-7.45)
Alkalosis =
arterial blood pH > 7.45
Acidosis =
arterial blood pH < 7.35
If pH falls below 6.8:
CNS depression, coma, death
If pH rises above 7.8:
Overexcitation of nervous system, muscle tetany, convulsions, respiratory arrest
3 Systems Regulate Acid-Base Balance
Chemical buffers
Respiratory mechanisms
Renal mechanisms
Chemical buffers
1st line of defense – very fast, act in fraction of second
resist pH changes by binding H+ ions when pH drops and releasing them when pH rises
Respiratory mechanisms
Brainstem respiratory centers change respiratory rate and depth to compensate for pH changes in 1-3 min (control CO2 levels)
Renal mechanisms
Body’s most powerful acid-base regulatory system, but may require hours to days to correct blood pH (control H+/bicarbonate levels)
Respiratory and renal systems form -
“physiological buffering systems”
control pH by changing the amount of acid or base in the body
slower than chemical buffering systems, they are most powerful
Only____ contribute to the acidity of a solution
free H+
Strong acids/bases can produce ____ changes in pH
large -
because they dissociate completely
Ex: Strong base completely dissociates: Na+ and OH-
Weak acids/bases dissociate ___, so produce ___ change in pH; these help prevent pH changes by binding H+ or releasing H+
only partially
little
Acids are proton ____
Bases are proton ____
donors
acceptors
Chemical Buffers
a system of 1 or more compounds that resist pH change in the presence of a strong acid or base
Chemical buffer systems –
first line of defense to control pH of ICF and ECF
3 chemical buffer systems
Carbonic acid-Bicarbonate buffer system – extracellular fluid compartment (ECF)
Phosphate buffer system – intracellular fluid compartment (ICF) and urine
Protein buffers – both ICF and ECF
Chemical buffer systems have a ___ acid and a ___ base
weak
weak
The buffering system resists changes in pH by :
binding H+ ions when pH drops (the weak base does this)
releasing H+ ions when pH rises (the weak acid does this)
Bicarbonate Buffer System
mixture of H2CO3 (weak acid) and salts of HCO3- (weak base)
maintain ECP buffer; also operates in ICF
Phosphate Buffer System
salts of H2PO4- (weak acid) and HPO4 2- (weak base)
important buffer in urine and ICF
Protein Buffer System
some amino acid side chains act as weak acids (-COOH) or weak bases (-NH2)
most important buffer in ICF; also in blood plasma
CARBONIC ACID-BICARBONATE BUFFER SYSTEM
most important buffer of the ECF
Bicarbonate buffer system:
Mixture of carbonic acid (H2CO3) and the salt sodium bicarbonate (NaHCO3)
Large amounts of plasma bicarbonate (HCO3-) produced from metabolic CO2 create the most important extracellular buffer system of the body.
Bicarbonate ions enter the plasma and form the _____ which will help buffer acids entering the blood.
alkaline reserve
______ ⇌ ____ ⇌ ______
CO2 + H2O
(carbon dioxide plus water)
H2CO3
(carbonic acid)
HCO3- + H+
(bicarbonate ion plus hydrogen ion)
Plasma HCO3- concentration averages ____ which is hundreds of thousands of times as concentrated as ____
~24 mEq/L
plasma H+
Although H+ and HCO3- are both created from CO2 and H2O, the majority of H+ is buffered by _____
hemoglobin
The HCO3- in plasma is then available to buffer H+ from ___ sources, such as metabolism.
nonrespiratory
Weak acid =
Weak base =
carbonic acid (H2CO3)
sodium bicarbonate (NaHCO3)
Buffers strong acids
______ → ______ + ______
HCl + NaHCO3 → H2CO3 + NaCl
(strong acid) + (weak base) → (weak acid) + (salt)
When a strong acid is added to buffering system =
the sodium bicarbonate acts as a weak base to tie up the H+ released by the strong acid
The net effect is that ______
the strong acid is converted to a weak acid so that arterial pH goes down very little.
Buffering capacity is dependent on the concentration of _____ This is closely regulated by the ____.
bicarbonate ions (alkaline reserve)
kidneys
Buffers strong bases
______ → ______ + ______
NaOH + H2CO3 → NaHCO3 + H2O
(strong base) + (weak acid) → (weak base) + (water)
When a strong base such as NaOH is added to buffering system =
the carbonic acid dissociates further, donating a H+ to tie up the OH- released by the strong base (and forming water)
The net effect is that _____
the strong base is converted to a weak base so that arterial pH goes up very little
buffering capacity is dependent on the concentration of ______
carbonic acid (essentially limitless)
Phosphate BUFFER SYSTEM
most important buffer of the ICF and urine
Phosphate buffer system
Weak acid =
Weak base =
dihydrogen phosphate (H2PO4-)
monohydrogen phosphate (HPO42-)
In the presence of a strong acid, like hydrogen chloride (HCl):
HCl + Na2HPO4 → NaH2PO4 + Nacl
(strong acid) + (weak base) → (weak acid) + (salt)
In the presence of a strong base, like sodium chloride (NaOH):
NaOH + Na2HPO4 → NaH2PO4 + H2O
(strong base) + (weak acid) → (weak base) + (water)
PROTEIN BUFFER SYSTEM
buffers ICF and plasma
Individual amino acids on a protein can donate or accept H+ allowing it to function as a weak acid or base as needed
PROTEIN BUFFER SYSTEM
When the pH begins to rise:
R-COOH → R-COO- + H+
(carboxyl group)
PROTEIN BUFFER SYSTEM
In the presence of an acid (pH drops):
R-NH2 + H+ → R-NH3+
(amine group)
A large percentage of the body’s total buffering capacity is through the protein buffer system inside :
cells
(ex. Hemoglobin in RBC)
but this also works with the proteins in plasma.
Respiratory and Renal systems comprise the _____
Physiological Buffering System
slower than chemical buffer systems, but more powerful
Respiratory System :
lungs work to control pH by controlling CO2 levels
(works in minutes)
Kidneys :
control the levels of H+ and HCO3- levels in the body
(long term)
The respiratory system helps maintain pH by:
eliminating CO2 from the lungs
How respiratory system makes adjustments to pH:
Central respiratory centers are most sensitive to CO2 levels
Buildup of CO2 or H+ in blood activates respiratory centers to increase respiratory rate and depth: CO2 is “blown off” so H+ concentration is reduced
If blood pH rises, respiratory rate slowed: CO2 accumulates, pushing equation to right so H+ concentration increases and blood pH is restored to normal
Acid-base imbalances may be due to respiratory system dysfunction:
respiratory acidosis
respiratory alkalosis
Hypoventilation → CO2 retention → build up of H+ → respiratory acidosis
Hyperventilation → CO2 elimination → reduce H+ → respiratory alkalosis
____ is a long-term mechanism for controlling acid-base balance
Renal regulation
Kidneys regulate acid-base balance by adjusting the amount of H+ or bicarbonate in the blood:
Directly by excreting or reabsorbing H+
Indirectly by controlling the concentration of bicarbonate ions
Since acidity is usually a bigger problem than alkalinity, the kidney mostly eliminates H+ by secretion and retains HCO3- by ____________
reabsorption at the nephron
Renal regulation of pH
H+ is secreted while HCO3- is reabsorbed (maintains bicarbonate reserve)
Phosphate buffer in filtrate ensures excretion of H+ (“trapping” H+ in filtrate)
Glutamine metabolism and NH4+ (ammonium) secretion and excretion
Acidosis and Alkalosis can be classified as :
Respiratory or Metabolic
Acid-base imbalances classified according to cause:
Respiratory system ventilation issues:
Metabolic imbalances:
Respiratory system ventilation issues:
(normal PCO2 ~40 mm Hg)
Respiratory acidosis: Hypoventilation → PCO2 > 45 mm Hg → H+ rises; pH < 7.35
can be caused by shallow breathing, diseases that affect gas exchange such as pneumonia, cystic fibrosis, emphysema
Respiratory alkalosis:
Hyperventilation → PCO2 < 35 mm Hg → H+ falls; pH > 7.45
can be caused by stress, anxiety, pain
Metabolic imbalances:
(anything NOT caused by abnormal PCO2 levels, not respiratory problem) (normal bicarbonate levels ~24 mEq/L)
Metabolic acidosis:
arterial pH < 7.35 (low)
HCO3- levels < 22 mEq/L (low)
can be caused by diarrhea, lactic acid buildup, ketosis, excessive alcohol
Metabolic alkalosis:
arterial pH > 7.45 (high)
HCO3- levels > 26 mEq/L (high)
can be caused by vomiting, loss of stomach acid, excessive antacids
If a pH imbalance is caused by a respiratory disorder, it is a respiratory pH imbalance:
respiratory acidosis or respiratory alkalosis
kidney will try to compensate
If the pH imbalance is not due to hyperventilation or hypoventilation, it is considered a metabolic pH imbalance:
metabolic acidosis or metabolic alkalosis
lungs will try to compensate
