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
What 5 factors decrease H+ secretion and bicarbonate reabsorption?
- ↓ CO2
- ↓ angiotensin II
- ↓ aldosterone
- hyperkalaemia
- ↑ ECF volume
What are the effects of high CO2 on acid secretion?
high CO2 increases carbonic acid which increases H+ secretion
What does ECF depletion do?
activate the renin-angiotensin aldosterone axis which leads to increased H+ secretion and alkalosis
What is the lung response to acidosis?
they breathe harder and faster
How can the lungs increase and decrease H+ concentration respectively?
- increase = decreased ventilation → reduced elimination of CO2
- decrease = increased ventilation → elimination of CO2
What are the 4 categories of acid-base disorders?
- metabolic acidosis
- respiratory acidosis
- metabolic alkalosis
- respiratory alkalosis
What are the causes of metabolic acidosis?
- loss of bicarbonate leading to diarrhoea and type 2 RTA
- ↓ excretion of H+ leading to renal failure and type 1 RTA
- too much acid in the diet e.g. acidifying salts
- ↑ H+ production (anaerobic respiration) leading to diabetic ketosis, lactic acid generation, tissue ischemia
What is RTA?
renal tubular acidosis
What are the causes of metabolic alkalosis?
- ↑ ingestion of bicarbonate e.g. antacids
- loss of H+ due to vomiting or diuretics
What are the causes of respiratory acidosis?
- ↓ ventilation so increased CO2 and ↑ production of H+- lung diseases e.g. severe pneumonia or asthma
- neuromuscular problems leading to respiratory muscle weakness
What are the causes of respiratory alkalosis?
- ↑ ventilation so ↓ CO2 and ↓ production of H+
- physiological causes e.g. high altitudes (low O2 levels stimulate respiration)
- hyperventilation
What will happen with a patient with severe pneumonia respiratory failure?
- ↑ CO2 so respiratory acidosis
- ↑ H+
- kidneys will excrete more H+
What are the 2 compensation methods and when are they used?
- metabolic = respiratory compensation
- respiratory = renal compensation
What is the renal compensation of respiratory acidosis?
increased renal H+ secretion and bicarbonate reabsorption
What is the renal compensation of respiratory alkalosis?
decreased renal H+ secretion and bicarbonate reabsorption
What is the compensation of metabolic acidosis?
- ↑ H+ → heavy breathing (activates bicarbonate buffer system) → CO2 excreted by lungs
- ↓ glomerular filtration of bicarbonate
What happens with chronic acidosis?
there is increased production of NH4+ which further contributes to excretion of H+ and addition of new bicarbonate to the ECF
What is the compensation of metabolic alkalosis?
- ↓ H+ → reduced ventilation drive → increased PCO2
- ↓ renal H+ secretion
- reduced bicarbonate reabsorption
What is the magnitude of compensation of metabolic alkalosis limited by?
the carotid and aortic chemoreceptor mechanisms, as pO2 should be maintained
When will compensation happen most efficiently?
when the acid base disorder slowly develops e.g. chronic lung disease compensates better than acute lung disease
What are the 4 common kidney diseases?
- acute kidney injury
- chronic kidney disease
- urinary tract infections
- renal stones
What are the symptoms of kidney failure?
- water retention
- electrolyte derangements
- acidosis
- high blood pressure
- anaemia
- low calcium levels
- low blood sugar
What is treatment of kidney failure?
- dietary modifications; low protein, low sodium
- water restriction
- renal replacement therapy
Give examples of renal replacement therapy
dialysis and renal transplant
What do peristaltic movements of the ureters do?
facilitate the flow of urine from kidneys to bladder
Why do muscles fibres extend in all directions in the bladder?
so an action potential can spread throughout the detrusor muscle to cause contraction of the entire bladder at once
What are the 2 structures that hold in contents in the bladder?
the internal and external sphincters
What is the internal sphincter made up of?
smooth muscle (not under conscious control)
What is the external sphincter made up of?
skeletal muscle (under conscious control)
What is parasympathetic activation of bladder?
- detrusor contraction and inhibition of the internal sphincter
- bladder emptying
What is sympathetic activation of the bladder?
- detrusor relaxation and contraction of the internal sphincter
- bladder filling
What is a cytsometrogram?
a plot of intravesical pressure against the volume of fluid in the bladder
Describe a cystometrogram plot
- initial slight rise in pressure with the first increments in volume
- further increments in volume don’t cause a significant rise in pressure
- sudden sharp rise in pressure when the bladder is full
Describe the micturition reflex
once the reflex is powerful enough, the pudendal nerve impulses are inhibited which causes external sphincter relaxation and the detrusor muscles contract while the internal sphincter relaxes, which together lead to micturition
What are causes of urinary incontinence?
- distruption/damage to the neuronal pathways mediating voluntary control i.e. brain spinal cord, pudendal nerves
- damage to the external sphincter or impaired sphincter function (usually cause incontinence in sudden increase in bladder pressure such as coughing or sneezing)
