Acid-Base Disorders Flashcards
What is the pH of venous blood and interstitial fluid and why?
7.35 due to CO2 being released from tissues to form carbonic acid
What is the pH of arterial blood?
~7.4
What is the upper and lower limit of pH at which a person can live?
- upper = ~8.0
- lower = ~6.8
What is acidosis and alkalosis respectively?
- acidosis = arterial pH of below 7.4
- alkalosis = arterial pH of above 7.4
What is the pH range of urine?
4.5-8.0
Which 2 organs are important in regulating body pH?
kidneys and lungs
What is a buffer?
a substance that can bind or release H + to keep the pH of the solution relatively constant despite the addition of considerable quantities of acid or base
What are the main sources of acid in the body?
carbon dioxide and amino acid metabolism
What does the kidney use as a buffer?
bicarbonate
Where does H+ secretion and bicarbonate reabsorption not occur within the kidney?
the thin limbs of the loop of Henle
Describe pH control in the proximal tubules
- H+ is secreted by the Na-H cotransporter
- Na+ is removed by Na+/K+ATPase to maintain gradient
- H+ ions combine with bicarbonate to form carbonic acid then carbonic anhydrase catalyses the breakdown into CO2 and H2O
- CO2 and H2O enter tubular cells and intracellular carbonic anhydrase catalyses the formation of carbonic acid
- H+ is secreted and bicarbonate enters the interstitial fluid
What are the effects of inhibition of carbonic anhydrase?
less acid is secreted, more bicarbonate is secreted and less sodium is reabsorbed
What are carbonic anhydrase inhibitors?
diuretics
How is pH controlled in the distal tubules?
- Cl/bicarbonate exchangers that transport bicarbonate into the interstitium
- H+ secretion and bicarbonate reabsorption is mostly driven by an ATP proton pump in the type A intercalated cells
What does aldosterone do in the distal tubules?
- increase Na+ and H2O reabsorption
- increase activity of the ATP proton pump and consequent acid excretion
What is the limiting pH of the urine?
4.5
What is limiting pH?
the pH at which no more acid secretion is possible
What do the buffering systems in the urine do?
- bind H+
- allow more H+ secretion
- delay reaching the limiting pH
What are the 3 buffering systems in the urine and where are they most common?
- bicarbonate system - PCT
- phosphate system - distal tubules and collecting ducts
- ammonia system - proximal and collecting ducts
What is the most important buffer system for maintaining the pH homeostasis of blood?
bicarbonate
What happens in the phosphate buffer system?
a new bicarbonate is returned to the blood for each NaHPO4 that reacts with a secreted H+
What happens in the ammonia buffer system?
2 new bicarbonate ions are returned to the blood for each glutamine metabolised
What 5 factors increase H+ secretion and bicarbonate reabsorption?
- ↑ CO2
- ↑ angiotensin II
- ↑ aldosterone
- hypokalaemia
- ↓ ECF volume