Acid-Base Balance Flashcards
Why is the acid-base balance important
The activity of enzymes is dependent on H+ concentration and only function within a narrow range
What is the equation for pH
pH = -log[H+]
What is the normal pH range
7.36-7.44
What does the metabolism of carbs and fats produce
Large amounts of CO2 and volatile acids
What kind of acids does the metabolism of proteins produce
Non-volatile acids (AAs, uric acid etc.) that cannot be excreted from the lungs
Why do non-volatile acids need to be excreted
The kidneys excrete non volatile acids to maintain acid-base balance
Why don’t volatile acids need to be excreted by the kidneys
They can be excreted by the lungs as they can be turned into gases
What is the capacity of buffer systems determined by
- dissociation constant (pK), the relationship between pK and pH is determined using the Henderson-Hasselbach equation
- quantity of buffer present
Why is the bicarbonate buffer unique
It remains in equilibrium with atmospheric air (it acts on CO2)
What is the concentration of bicarbonate ions controlled by
Kidneys
What buffer system is used in erythrocytes
Haemoglobin
H+ + Hb HHb
What buffer system is used in the ICF and ECF
Phoshphate
H+ + HPO4^2- H2PO4-
What buffer system is used in the kidney
Ammonia
H+ + NH3- NH4+
What organs and parts of the blood contribute to the acid-base balance
Lungs
Kidneys
Plasma/erythrocytes
Describe how CO2 forms H+ and how these are buffered
CO2 from tissues is transported in the plasma as bicarbonate and haemoglobin in erythrocytes buffers the H+ derived from carbonic acid.
Where in the nephron is filtered bicarbonate reabsorbed
In the PCT and DCT, where there is a net secretion of hydrogen ions
What is the site of bicarbonate reabsorption and hydrogen ion excretion in the nephron
The carbonic anhydrase in the the PCT and DCT
Describe the mechanism of Bicarbonate reabsorption in the PCT
- HCO3- and H+ = H2CO3
- Carbonic anhydrase in the brush border catalyses the dehydration of H2CO3 to produce CO2 and H2O
- Within cell H+ and HCO3- produced by reaction catalysed with carbonic anhydrase
- H+ secreted and HCO3- diffuses into blood
- Majority of HCO3- exits via symporter with Na+ and some exits in exchange with Cl-
Describe the mechanism of Bicarbonate generation and hydrogen ion excretion in the DCT
- majority of bicarbonate is reabsorbed in the PCT, DCT gets small amount remaining
- In cell H+ and HCO3- produced by H2O and CO2 reaction catalysed by carbonic anhydrase
- H+ are secreted into the tubular fluid via H+/ATPase and K+/H+/ATPase
- The HCO3- exits cell across basolateral membrane in exchange for Cl-
What is acidosis
An abnormal increase in the H+ concentration of the blood that lowers the arterial pH below 7.35
What is alkalosis
An abnormally high alkalinity of the blood and body fluids, pH above 7.45
What mechanisms are in place to minimise changes in pH of body fluid
- extracellular and intracellular buffering
- adjustments of blood PaCO2 (ventilation)
- adjustments in renal [H+/HCO3-] secretion
What might be the primary regulatory problems that lead to a disturbance in acid-base balance
Problems affecting regulation of PaCO2 or bicarbonate concentration
What is respiratory acidosis
Reduction in ventilation due to the action of drugs or lung disease resulting in reduced pH and raised PaCO2
How does the body carry out renal compensation for respiratory acidosis
Increased acid excretion and enhanced reabsorption of HCO3- to buffer the H+
How does metabolic acidosis occur
The addition of non-volatile acid to the body e.g. diabetic ketoacidosis or in kidney failure
What is the respiratory response to metabolic acidosis
Decrease in pH stimulates respiratory centres and increases ventilation rate as a reduction in PaCO2 minimises fall in plasma pH
What is the renal response to metabolic acidosis
Increase in acid secretion and enhanced reabsorption of HCO3-
What is and what causes metabolic alkalosis
Caused by the addition of non-volatile alkalis (ingestion of antacids) to the body or loss of non-volatile acid (vomiting/gastric HCl)
Shows high pH and high plasma HCO3-
What is the respiratory response to metabolic alkalosis
Increase in pH inhibits respiratory centres and decreases ventilation rate, leading to an increase of PaCO2
What is the renal response to metabolic alkalosis
Increase in HCO3- excretion (reduced reabsorption)
What is and what causes respiratory alkalosis
Caused by increased ventilation (stimulation of respiratory centres e.g. drugs) or hyperventilation through anxiety
Shows high pH and reduced PaCO2
What is the renal response to respiratory alkalosis
Reduced acid excretion and reduced reabsorption of HCO3-
