Respiratory acid-base balance Flashcards
Distinguish between respiratory and metabolic pH disturbances
Respiratory
- A result of abnormal Pco2
- Lung disease, hypoventilation, hyperventilation
- A result of something other than abnormal Pco2
- A high-protein diet, a high-fat diet, heavy exercise, excessive vomiting, severe diarrhea
Briefly describe chemical buffering
- Chemical buffers are the first line of defense against changes in blood pH.
- Largest buffer pool in the body is in extracellular fluid (HCO3-/CO2, Inorganic H2PO4-, Plasma Proteins).
- Intracellular fluid contains cellular proteins (e.g., Hb), organic HPO4-, HCO3-/CO2.
- Bone serves as a buffer with mineral H2PO4- and mineral HCO3-.
- Chemical buffers minimize pH changes **but don’t remove acid or base from the body.
- Buffering power varies: ECF rapid (minutes), ICF/bone (hours).
How good a buffer depends on:
- its abundance
- mineral H2po4 found in large stores in bone
- its pKa (i.e. if close to optimal pH)
- H2PO4 = 6.8, close, not the best
- HPPO4 = pK ~ pHi
N.B. proteins largest buffer pool in body, in both ECF and ICF
Describe how Hb functions as a buffer
~bohr~
- Hemoglobin (Hb) can bind and release H+, acting as a buffer.
- CO2 produced by tissues is quickly converted into HCO3- and H+.
- Hb.O2 and Hb.H participate in reactions in tissues and lungs.
- H binds Hb and displaces oxygen in tissues; H increases as cells are metabolically active – conditions optimised for release
- the reverse occurs in lungs (in order to deliver oxygen from lungs in tissues)
- The Bohr effect describes how pH affects the Hb dissociation curve.
- This process is streamlined: Hb affinity for oxygen decreases, so can bind H as pH decreases
Describe the Bohr effect
Note the reaction is bi-directional/reversible
- depends on context
- acidity decreases affinity for oxygen (right shift)
- alkalinity increases it (left shift)
Recall from [[Physiology Lecture 10]] that temperature and CO2 also affect affinity of Hb for oxygen.
Describe how effective the respiratory system is at controlling pH
- If pH disturbance is outside the respiratory system, it can’t fully return pH to normal.
- Effectiveness ranges from 50-75% ^[i.e. of change]
- Example: pH falls from 7.4 to 7.0, respiratory system can return pH to ~7.2 - 7.3 (within 3 to 12 minutes).
- Note: this process happens quickly
Describe the buffering power of the respiratory system
- Respiratory system acts as a **physiologic buffer.
- Acts rapidly and prevents severe [H+] changes (until kidneys help with remaining imbalance).
- Buffering power **1-2 times that of all extracellular fluid chemical buffers combined.
- demonstrates essential role in maintaining balance
Describe the reasons why bicarbonate is a major buffer in the blood
- Bicarbonate is a major buffer in the blood. Why is it so important?
- Components are abundant: [HCO3-] = 24 mM, [CO2] = 1.2 mM (constant generation, despite low concentration).
- System is open
- regulated by both lungs and kidneys.
- Bicarbonate participates in reactions converting H+ to CO2 and vice versa.
- when H is high, it bind bicarbonates ions
- if CO2 concentration increases, more H and HCO3 are formed
- Acidosis occurs if CO2 levels build up.
Describe the Henderson Hasselbalch equation
- Henderson-Hasselbalch equation relates pH, [HCO3-], and [H2CO3].
- CO2 is expressed as Pco2, so equation relates pH, [HCO3-], and Pco2.
- Relationship between CO2 and acidity described by the equation.
- pH = 6.1 + log ([HCO3-] / (0.03) * Pco2)
- Strong acid, low concentration in blood
- CO2 can be substituted due to its conversion with H2O: [H2CO3] = [CO2] / 400
- Normal Pco2 = 40 mm Hg, [HCO3-] = 24 mM
- Henderson-Hasselbalch equation:
- pH = 6.1 + log ([HCO3-] / Pco2) ^[this demonstrates that pH is influenced by CO2 and bicarbonate]
- Ratio must be kept constant: [HCO3-] / [CO2] = 20:1 to maintain pH of 7.4
- Kidneys regulate [HCO3-], lungs regulate [CO2]
Partial pressure and concentration
Blood CO2 is typically expressed as partial pressures. Therefore, converting CO2 concentration can be converted into PCO2 = 40 mmHg- which gives 1.2 mM.
Therefore normal bicarbonate concentration should be equal to 24 mM.
Describe the signficance of bicarbonate buffer as an open system
Note that the pK is 6.1.
This is not close to the optimal pH.
But bicarbonate buffer operates in an open system.
Other mechanisms work in tandem with buffering to maintain 20:1 ratio and thus optimal pH.
Mechanisms include actions of lung: breathing and hyperventilating, kidney: synthesising HCO3 and excreting H+.
Describe gas exchange in pulmonary capillaries
The capillary wall is permeable to CO2 and O2.
O2 diffuses across wall into red blood well. It displaces proton binding to Hb.
Proton participates in CA reaction, dissociates to CO2 and H2O. CO2 diffuses out.
Note carbaminohaemoglobin, which dissociates.
Reverse chloride shift: Cl out and HCO3 in
Describe gas exchange in systemic capillaries
The reverse process occurs in systemic capillaries.
The capillary wall is permeable to CO2 and O2.
CO2 diffuses across wall into red blood well. It displaces proton binding to Hb.It also participates in CA reaction, dissociates H+ and HCO3. H displaces O2 from Hb, O2 diffuses out
Note oxygenated haemoglobin, which dissociates and takes up CO2.
chloride shift: Cl in and HCO3 out
Describe the ways in which CO2 is transported in blood
In the red cell:
- 63% CA reaction
- 21% Hb
- 5% dissolved
In plasma:
- 1% Hb
- 5% CA reaction ^[slow reaction]
- 5% dissolved
Describe the effect of ventilation of ECF pH
- more ventilation leads to positive change: more CO2 out, acid elimination
- less ventilation leads to negative change: lessCO2 out, acid retention
- normal: no pH change
Describe the effect of blood pH on ventilation rate
- acidic pH leads to dramatic increase in ventilation rate
- alkaline pH leads to decrease in ventilation rate, though not as dramatic as acid, to maintain breathing ^[hypoventilation drive is less]
Describe the relationship between ventilation rate and paCO2
- exercise not considered hyperventilation, though ventilation increases
- generate more CO2, need to breathe more to reach isometabolic line
