Acid base physiology Flashcards
Hydrogen ion
Single free proton released from a hydrogen atom
- H+ exists in solution bound to water (H2O)
Acid
Molecule that releases hydrogen ions in solution
- HCl
Base
Molecule that can accept H ions in solution
- calcium carbonate
Hydrogen ions are very _____
Reactive!!
- alter structure and function of virtually every protein and enzyme in the body
Homeostasis requires control of _____
H+
Metabolic acidosis causes
- decreased myocardial contractility
- vasodilation
- reduced responsiveness to catecholamines
- impaired coagulation
- decreased cell function
Respiratory acidosis causes
Cerebral edema via vasodilation
H+ concentration is kept ____ relative to other ions
Very low
- 0.00004 mEq/L (pH 7.4)
pH formula
log (1/[H+]) = -log[H+]
- high [H+] = low pH
- low [H+] = high pH
Normal blood pH is ____
- 4 (7.35=7.45)
- arterial blood is 7.4
- venous blood is 7.35 (contains more CO2, which acts as an acid)
Physiologic limits of blood pH
6.8-8.0
Intracellular fluid pH is ____ than blood
Lower (6.0-7.4
- cellular metabolism produces H+
Urine pH
4.5-8.5
What are the 3 primary systems that regulate changes in [H+]?
- buffers: seconds (weakest)
- lungs (ventilation): minutes (stronger)
- kidneys: hours to days (strongest)
Is the body more efficient at dealing with acids or bases?
Acids
Buffer
Substance that can reversibly bind with H+
- accept or donate H+ to minimize changes in [H+] until balance can be restored
buffer + H+ Hbuffer
What are the 4 main buffers in the body?
- bicarb (extracellular)
- proteins (intracellular)
- phosphate (intracellular and renal tubules)
- ammonia (renal tubules) most important in removing bicarb
Bicarb buffer system
Consists of carbonic acid (H2CO3) and bicarb salt (NaHCO3)
- carbonic acid weakly dissociates
- bicarb salt almost completely dissociates
- carbonic anhydrase is abundant in lungs and kidney
Bicarb buffer system formula
CO2 + H2O H2CO3 H+ -HCO3
What happens if an acid is added to the bicarb buffer system?
Consumed by bicarbonate, driving the equation left
- carbon dioxide is produced (and exhaled)
What happens if a base is added to the system?
Consumed by carbonic acid, driving the equation to the right
- carbon dioxide is consume (and ventilation decreases)
The bicarb system is connected to the _____
Lungs
- both are dependent on each other
- ventilation (CO2) is required to keep the bicarb system working at max efficiency
Abnormalities in ventilation affect the _____
pH
- hypoventilation –> increased CO2 –> increased carbonic acid
The lungs thru ventilation, control extracellular [H+] and provides a second means to respond to changes in ____
[H+]
Minute ventilation
Tidal volume x respiratory rate
Ventilation has 2 main stimuli
- changes in blood carbon dioxide
- changes in blood oxygen
The body favors ___ control over ___
CO2; O2 control
The lungs are very effective at compensating for ____
Acidemia
- ventilation can be easily increased
- overall buffering capacity is 1-2x that of all other chemical buffers combined
The lungs are less effective at compensating for _____
Alkalemia
- if ventilation decreases too much, hypoxemia occurs
- the body won’t starve itself of oxygen
Why are diseases that suppress ventilation life threatening?
The body has limited ways to immediately respond to the acid base effects of hypercapnea
What are the cons of the buffer and lung systems?
- in capable of returning pH back to normal
- in capable of permanently changing the hydrogen ion or bicarbonate concentrations in the body
How to do the kidneys control the acid-base balance of the system?
Excrete acidic or basic urine
4 processes that allow urine and blood pH to be controlled
- bicarb is filtered continuously and reabsorbed
- hydrogen ions are secreted
- new bicarbonate can be made
- non-volatile acids are filtered (by products of protein metabolism)
How does the kidney respond to alkalosis?
Fails to reabsorb HCO3
- helps retain hydrogen ions due to a lack of buffering
How does the kidney respond to acidosis?
Reabsorbs all bicarb, actively secretes H+, makes new bicarb
Where does bicarb reabsorption and H secretion occur?
In all segments except the thin loop of Henle
___ must be secreted to reabsorb ____
H+; HCO3
How is H+ secreted?
- Na/H counter transport
- Na/K on basolateral membrane establishes concentration gradient
- H combines with bicarb to form CO2
- CO2 enters cell
- CO2 combines with water to form H and bicarb
- bicarb re-enters the blood
Proximal tubule
Does not secrete much H, just uses it to reabsorb bicarb
- H and bicarb completely titrate each other
Incomplete titration
- acidosis: bicarb is completely reabsorbed
- alkalosis: excess bicarb is excreted because it doesn’t have a H to help it re-enter the cell
Intercalated cells of the distal tubule and collecting ducts
H is actively secreted
- only secretes 5% of total H
- concentrates H 900x that of the proximal tubules
- is the segment that maximally acidifies urine*
____ is reciprocal to bicarb
Chloride
Intercalated cells can not make urine pH below
4.5
How does the body bind H+?
Utilizes buffers to trap H+ and excrete them in the bound form
- keeps concentration gradient low enough for active transport to keep working
Buffering systems
- phosphate
- ammonium: most important quantitatively
- urate and citrate (to a lesser extent)
What are the 2 main jobs of the buffers?
- keep hydrogen ion concentration in the urine low
- helps form new bicarb
What happens to hydrogen once all of the bicarb is reabsorbed?
H+ is free to interact with other buffers
- H+ is excreted with phosphate as a salt
- net effect is generation of new bicarb, whenever H+ combines with a buffer other than bicarb
Ammonia buffer system in the proximal tubules
- glutamine is an amino acid by product made in the liver
- when acidosis occurs, the kidney uses it to make ammonium (NH4) and bicarb = generates 2 new bicarb molecules
- ammonium is excreted in exchange for sodium
Ammonia buffer system in the collecting ducts
- ammonia (NH3) is able to freely pass thru the membrane into the lumen
- combines with H+to form ammonium ions, which are less permeable and excreted
- one bicarb is made/kept for every H+ that is secreted
Ammonia buffer system control
With chronic acidosis, this system becomes the dominant means by which acid is secreted
- an increase in extracellular H+ concentration stimulates renal glutamine metabolism
- generates more ammonium to act as a buffer
- new bicarb is made
- opposite occurs when H+ concentrations drop
Factors that affect H+ secretion
- hydrogen ion concentration
- carbon dioxide concentration
- processes that control sodium (indirectly)
Factors that increase H+ secretion and bicarb reabsorption
- respiratory acidosis: increases CO2
- metabolic acidosis: increase H, decrease bicarb
- decrease ECF
- increase angiotensin 2
- increase aldosterone
- hypokalemia
Factors that decrease H+ secretion and bicarb reabsorption
- respiratory alkalosis: decrease CO2
- metabolic alkalosis: decrease H, increase bicarb
- increase ECF
- decrease angiotensin 2
- decrease aldosterone
- hyperkalemia
CO2
Behaves as an acid
- forms H+
- respiratory
HCO3
Behaves as a base
- binds H+
- metabolic
How to cause acidosis
- increase CO2 (hypoventilation)
- decrease bicarb
- add acid
How to cause alkalosis
- decrease CO2 (hyperventilation)
- increase bicarb
- lose acid
Traditional blood gas analysis
Measurement of blood gases (O2 and CO2) and other parameters to evaluate acid-base status
- respiratory system (PCO2)
- metabolic system (HCO3)
- when an abnormality occurs in one system, the other compensates
Arterial blood sampling
- acid base (more accurate)
- ventilation
- oxygenation (lung function)
Venous blood sampling
- acid base
- ventilation
- oxygen extraction (oxygen use by tissues)
Pre analytical errors
- sampling site
- inappropriate blood tube
- inappropriate blood to anti-coagulant ratio
- air bubbles or prolonged exposure to room air
- delay in performing the analysis
Traditional blood gas analysis
Based on bicarb buffer system
- ratio between CO2 and HCO3 determines pH
- alterations in pH are explained by an abnormality of CO2 or HCO3
Acidosis involves an increase in ___ and a decrease in _____
CO2; HCO3
Alkalosis involves a decrease in ____ and an increase in ______
CO2; HCO3
Respiratory analysis
Acidosis - CO2 increases = acidifying - HCO3 increases to compensate Alkalosis - CO2 decreases = alkalinizing - HCO3 decreases to compensate
Metabolic analysis
Alkalosis - HCO3 increases = alkalinizing - Co2 increases to compensate Acidosis - HCO3 decreases = acidifying - CO2 decreases to compensate
Compensation
Secondary change in the opposite system to oppose the primary acid base disturbance
Respiratory system compensation
- responds to metabolic disturbance
- rapid time of onset and complete within hours
Metabolic system compensation
Responds to respiratory disturbance
- slower time of onset (hours) and takes 2-5 days to complete
Acute compensation will ____ return the pH to a normal value
NOT
- a normal pH with an abnormal CO2 and bicarb implies a mixed acid-base disturbance
What 3 components are needed to evaluate the acid base status?
- pH
- respiratory: PCO2
- metabolic: HCO3
(anion gap is helpful)
Base excess
The titratable acid or base in the blood
- the amount of acid or base that must be added to a sample of oxygenated whole blood to restore the pH to 7.4 at 37 C at a PCO2 of 40
BE is the metabolic component _____ of CO2
Independent
- when CO2 levels are normal, BE and HCO3 correlate well
- when CO2 is high, use BE
Total CO2
Evaluates metabolic component
- has nothing to do with respiratory component
- is not a blood gas!!
____ of all the CO2 in the body exists as bicarb
99%
- total CO2 and HCO3 will always be similar
Anion gap
Developed to better characterize metabolic acidosis
- rule of electroneutrality means there is no anion gap within the body
- AG represents anions that aren’t readily measured (mostly neg plasma proteins)
Normal (decreased) AG
- bicarb is lost via kidneys or GIT
- HCO3 and Cl are reciprocal, so chloride increases as bicarb decreases
- hyperchloremic metabolic acidosis
Increased AG
- acid (unmeasured anions) is added to the system
- AG increases because bicarb decreases as Cl stays the same
What are the 4 main causes of an increased anion gap metabolic acidosis
- ketones
- uremic acids (azotemia)
- lactic acidosis
- ethylene glycol
Criticisms of traditional blood gas
- doesn’t fully explain complexity of patient’s blood gas abnormalities
- fails to provide direction on how best to treat complex blood gas abnormalities
Semi-quantitative approach
Recognizes:
- strong ions that fully dissociate in water
- weak acids that can buffer
The semi-quantitative approach depends on:
- law of electroneutrality
- law of conservation of mass
What contributes to the metabolic portion of the patient’s pH?
Strong ion - changes in free water - changes in chloride concentration - increase in unmeasured anions Weak acid - decrease in albumin concentration - increase in phosphorous concentration
