Chapter 15 - Acid/Base Balance Flashcards
What does acid-base balance refer to?
the regulation of free H+ concentration in body fluids
Acids ______ hydrogen ions
liberate (separate)
Bases ____ hydrogen ions
accept (combine with free H+)
What do acids do in solution?
dissociate to release free H+ and anions
What do bases do in solution?
combine with H+ and remove it from the solution
Why are other substances that contain H+ not considered acids? (ie. carbohydrates)
the hydrogen is tightly bound within their structure and never liberated as free H+
A ______ acid has a greater tendency to dissociate than a ____ acid
strong; weak
What is an example of a strong acid?
HCl
What is an example of a weak acid?
Carbonic acid H2CO3
Dissociation Constant
-K
-extent of dissociating in a given acid is constant
The _____ the base, the more readily it can bind to free H+ in a solution.
stronger
Dissociation Constant Equation
[H+][HCO3]/[H2CO3]=K
What is pH used to express?
the concentration of H+
What is a pH of 7?
neutral
What is a pH less than 7?
acidic
What is a pH greater than 7?
basic
pH equation
-log[H+]=pH
A high pH corresponds to a _____ H+ concentration
low
What is the arterial pH range that is compatible with life?
between 6.8 and 8.0
Acidosis
when general body pH falls below 7.35
Alkalosis
when general body pH is above 7.45
Why is venous blood slightly more acidic than arterial blood?
it contains more CO2 which reacts with H2O to generate more carbonic acid
Acidaemia
when plasma pH falls below 7.35
Alkalaemia
when plasma pH rises above 7.45
What are the 3 main consequences of pH fluctuations in the body?
- changes in excitability of nerve and muscle cells
- influence enzyme activity
- influence on K+ levels
A state of acidosis leads to an overall ______ of the CNS
depression
A state of alkalosis leads to an overall ______ of the nervous system
hyper-excitability; sensory division of PNS usually affected first, then motor division, then CNS
How do changes in pH affect enzyme activity?
-may go outside the optimal range and denature the enzyme
How are K+ levels influenced by pH changes?
-H+ and K+ secretion in renal tubule is is an indirect relationship
-ie. if more H+ is eliminated then less K+ is excreted
-may affect cardiac function
What are the 3 lines of defence against pH changes?
- Buffer systems
- Respiratory mechanisms
- Kidneys
What is a chemical buffer system?
a mixture of two chemical compounds in a solution that minimize pH changes when either an acid or base is added or removed
Chemical buffer system is the ____ line of defence.
first
In a buffer system, one substance must ____ with free H+ if the [H+] falls, and the other must ____ with H+ if the [H+] rises
yield; bind
The body has ___ types of buffer systems.
4
Carbonic Acid:Bicarbonate Buffer System Role…
primary ECF buffer against non-carbonic acid changes
Protein Buffer System Role…
Primary ICF buffer; also buffers ECF
Hemoglobin Buffer System
Primary buffer against carbonic acid changes
Phosphate Buffer System
Important urinary buffer; also buffers ICF
Why is the H₂CO₃:HCO₃⁻ buffer system so effective in the ECF?
- both are readily available in the ECF
- both these components are closely regulated by respiratory system and kidneys
H₂CO₃ is regulated by the ______ system
respiratory system via CO2
HCO₃⁻ is regulated by the ______ system
renal
H₂CO₃:HCO₃⁻ Buffer System Reaction
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻
H₂CO₃:HCO₃⁻ Buffer System: What occurs when H+ is added to the plasma (not via CO2)
-ie. exercise; lactic acid release
-rxn. is driven to the left side
-extra H+ binds with HCO₃⁻
H₂CO₃:HCO₃⁻ Buffer System: What occurs when there is a decrease in H+ (not via CO2)
-ie. loss of HCl during vomiting
-rxn. is driven toward the right side
-dissolved CO2 and H2O form H2CO3 to generate additional H+
What does the Henderson-Hasselbalch Equation explain?
the relationship between H+ and the members of a buffer pair
What is the Henderson-Hasselbalch Equation for the H₂CO₃:HCO₃⁻ Buffer System?
pH = pK + log[HCO₃⁻]/[H₂CO₃]
What is the enzyme needed for CO2 and H2O to make H2CO3 (carbonic acid)?
carbonic anhydrase
What is the pK constant for H2CO3?
6.1
What is the H₂CO₃:HCO₃⁻ in ECF?
20:1
What is the pH of plasma?
7.4 (6.1 + log[20]/[1] = 7.4)
What buffering system in the most important in plasma (ECF)?
H₂CO₃:HCO₃⁻ Buffer System
What happens when the [H₂CO₃] increases (ie. 40:1)?
-alkalosis state
-pH 7.7
What happens when the [H₂CO₃] decreases (ie. 10:1)?
-acidosis state
-pH 7.1
How does the hemoglobin buffer system work?
-buffers H+ that is produced by CO2 between tissues and lungs
-free H+ binds to Hb
What is the second line of defence against pH changes?
the respiratory system
How does the respiratory system regulate pH?
by controlling the rate of CO2 removal
Changes in _____ pH can change ventilation patterns
arterial
Peripheral H+ detection
-chapter 13
-carotid and aortic bodies are chemoreceptors
When is peripheral H+ detection important?
when H+ comes from non-CO2 sources
**??What source of H+ can the respiratory system help correct?
metabolic; not respiratory, ironically
What do central chemoreceptors respond to?
a rise in CO2
Ventilation/CO2 removal _______ during metabolic acidosis?
increases
Ventilation/CO2 removal _______ during metabolic alkalosis?
decreases
H2CO3/H+ generation ______ during metabolic acidosis?
decreases
H2CO3/H+ generation ______ during metabolic alkalosis?
increases
If pH changes due to _____ ______, respiratory system cannot contribute.
respiratory failures
What is the third line of defence against changes in pH?
the kidneys (H+ secretion)
How long do the kidneys take to compensate for pH changes?
hours to days
How do kidneys control pH?
- H+ excretion
- HCO₃⁻ reabsorption/excretion
- ammonia secretion
Lungs can only remove ____ generated H+; whereas the kidneys can remove H+ generated from other acids.
CO2
There is very little H+ in the initial _____, most is secreted into tubular fluid via __, __, and ___.
filtrate; PT, DT, CD
What is the usual pH of urine?
6, acidic
What does HCO₃⁻ reabsorption depend on?
tubular secretion of H+ that combines with filtered HCO₃⁻ in the lumen
H+ secretion is linked to ____ handling
HCO₃⁻
HCO₃⁻ excretion in urine _____ H+ in plasma
increases
HCO₃⁻ addition to plasma ____ H+ in plasma
decreases
_____ HCO₃⁻ is generated in a H+ excess
new; generated to buffer the excess H+
What decreases H+ secretion?
-when filtered load of HCO₃⁻ is increased
-less H+ is available to combine with
HCO₃⁻ cannot be reabsorbed without ___ secretion
H+; thus more HCO₃⁻ is excreted in urine making it more basic
What do the kidneys do to H+ when plasma pH falls below normal (acidosis)?
-increased H+ secretion and excretion
-decreased plasma [H+]
What do the kidneys do to HCO₃⁻ when plasma pH falls below normal (acidosis)?
-reabsorption of all the filtered HCO₃⁻ and addition of NEW HCO₃⁻ to the plasma
-results in increased plasma [HCO₃⁻]
What do the kidneys do to H+ when plasma pH rises above normal (alkalosis)?
-decreased H+ secretion and excretion
-conserves H+
-increases plasma [H+]
What do the kidneys do to HCO₃⁻ when plasma pH rises above normal (alkalosis)?
-incomplete reabsorption of HCO₃⁻
-increased excretion of HCO₃⁻
-results in decreased plasma [HCO₃⁻]
What is normal HCO₃⁻ excretion and when does it occur?
-zero is excreted, all is reabsorbed
-occurs when plasma is in acidosis and thus urine is acidic
In states of acidosis kidneys ______ urine and _____ plasma to bring pH back to normal.
acidify; alkalinize
In states of alkalosis kidneys ______ urine and _____ plasma to bring pH back to normal.
alkalinize; acidify
How do kidneys acidify urine?
-H+ secretion
How do kidneys alkalinize plasma?
-HCO₃⁻ conservation
How do kidneys alkalinize urine?
-HCO₃⁻ elimination
How do kidneys acidify plasma?
-H+ conservation
What stops the tubular cells from excreting H+ against the concentration gradient?
-when tubular fluid becomes 800x as acidic as plasma
~pH 4.5
What pH do the kidneys cease to secrete H+ into tubular fluid?
4.5
What is used to prevent the concentration gradient from becoming too high?
buffering to reduce free H+; filtered phosphate buffers and secreted ammonia
Why can’t HCO₃⁻ buffer urinary H+ like it can in the ECF?
it is not secreted simultaneously with H+ (only the excess substance is excreted ie. H+ over HCO₃⁻)
What is secreted H+ first buffered by?
phosphate buffer system
Why is phosphate present in tubular fluid?
dietary excess
How does the phosphate buffer system work?
by absorbing H+ ions by binding with them
Why is the additional ammonia buffer system needed?
-high H+ secretion the phosphate capacity is reached
-acidity rises as more H+ is secreted
-phosphate system would be overpowered on its own
What is metabolized to produce ammonia (NH₃)?
glutamine; in the proximal tubular cells
How does the ammonia buffer system act?
-ammonia is secreted once phosphate buffers are saturated
-NH3 combines with H+ to form ammonium (NH4)
What does ammonia enable the kidneys to do?
continue secreting H+ to reduce acidosis
Ammonium Equation
NH₃ + H⁺ → NH₄⁺
How is NH₄⁺ removed from the tubules?
-lost in urine; tubular membranes not permeable to NH₄⁺
Do the kidneys deal with pH changes from respiratory dysfunction or metabolic disturbances?
both
A change in pH that has an underlying respiratory cause is associated with a change in [___].
[CO₂]
A change in pH that has an underlying metabolic cause is associated with a change in [___].
[HCO₃⁻]
Respiratory Acidosis has a ratio of ____ than 20:1 due to an _____ in [CO2]
less; elevation
Respiratory Alkalosis has a ratio of _____ than 20:1 due to a ________ in [CO2]
greater; decrease
Metabolic Acidosis has a ratio of _____ than 20:1 due to a ______ in [HCO₃⁻]
less; decrease
Metabolic Alkalosis has a ratio of _____ than 20:1 due to an ______ in [HCO₃⁻]
greater; increase
Respiratory acidosis occurs when CO2 builds up in the blood as a result of ________
hypoventilation ie. COPD (emphysema and chronic bronchitis), asthma, severe pneumonia
What does respiratory acidosis lead to?
increased H2CO3 production and therefore increased free H+
Uncompensated respiratory acidosis: [CO2] is _____ and [HCO₃⁻] is ______
elevated; normal
Why can’t respiratory measures compensate for respiratory acidosis?
-they don’t respond to respiratory abnormalities
How does the body compensate respiratory acidosis?
-increasing plasma HCO₃⁻ by kidney conservation and NEW HCO₃⁻
Metabolic acidosis encompasses all types of acidosis except for those caused by excess ___ in body fluids.
CO2
Uncompensated metabolic acidosis: [CO2] is _____ and [HCO₃⁻] is ______
normal; reduced
Possible causes of metabolic acidosis?
-severe diarrhea where HCO3- is lost from the body before it can be reabsorbed
-exercise
How is metabolic acidosis compensated for?
-lungs blow off H+ (generating CO2)
-chemical buffers taking up extra H+
-kidneys excreting more H+ and conserving HCO3-
Uraemic Acidosis
-serious and severe kidney failure
-kidneys cant get rid of excess H+ and it accumulates
-kidneys can’t restore pH
Respiratory alkalosis results from ______ and is when CO2 amounts in the blood are ______
hyperventilation; decreased
What are some causes of respiratory alkalosis?
-fever
-anxiety
-severe infection
-pregnancy
-high altitude
Uncompensated respiratory alkalosis: [CO2] is _____ and [HCO₃⁻] is ______
reduced; normal
What are the compensatory measures for respiratory alkalosis?
-kidneys: conserve H+ and excrete more HCO₃⁻
-chemical buffer systems liberate H+
What is metabolic alkalosis?
-a reduction in plasma H+ cause by a deficiency of non-carbonic acids
Uncompensated metabolic alkalosis: [CO2] is _____ and [HCO₃⁻] is ______
normal/unchanged; increased
What are some causes of metabolic alkalosis?
-vomiting (H+ lost in gastric juice elimination)
-ingestion of alkaline drugs
How is metabolic alkalosis compensated for?
-reduced ventilation to retain more CO2 and generate more H+
-excess HCO3- excretion by the kidneys to conserve H+
Uncompensated alkalosis ratio
40:1 (pH ~ 7.7)
Uncompensated acidosis ratio
10:1 (pH ~ 7.1)
Respiratory Acidosis Causes
-lung disease
-respiratory center depression
-nerve or muscle disorders that reduce respiratory activity
-holding breath
Metabolic Acidosis Causes
-diarrhea
-diabetes mellitus
-strenuous exercise
-uremic acidosis
Respiratory Alkalosis Causes
-fever
-anxiety
-asprin poisioning
-high altitude
What is the physiological response to acidosis?
-H+ secreted to reabsorb filtered HCO3-
-new HCO3- produced, H+ is excreted bound to urinary buffers (HPO4 and NH4)
Result of the physiological response to acidosis:
-more new HCO3- added to blood
-plasma bicarbonate increased compensating for acidosis
-urine is highly acidic
What is the physiological response to alkalosis?
-HCO3- is excreted in the urine
-little or no H+ excretion
-glutamite and ammonium excretion decreased so no new HCO3- contributed to plasma from this source
Result of the physiological response to alkalosis:
-plasma bicarbonate concentration is decreased
-urine is alkaline
Bicarbonate
HCO3-
