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Define pH?

It is a measure of hydrogen ions in concentration in a solution. Use -log [H+].


What is the plasma pH range?

7.35-7.45 (7.4). The range is narrow because enzymes, cell structures, all the things that proteins rely on - so pH needs to be tightly regulated. It corresponds to [H+] = 40nmol/L.

pH is lower in intracellular than extracellular.


How do you describe pH of the blood when it is abnormal?

Acidaemia - low pH.
Alkalaemia - high pH.

Acidosis - the process what causes low pH.
Alkalosis - the process what causes high pH.


Define acidosis?

When the pH <7.35. Severe acidosis the pH is <7.2 and a pH <6.9 means incompatible with life.


Define alkalosis?

When the pH >7.45. Severe alkalosis pH >7.6 and a pH >7.9 means incompatible with life.


Describe the concept of a buffer?

This is the first step as to how your body protects you against hydrogen ion changes. A buffer is a weak acid/base that minimises pH changes by removal/addition of hydrogen ions. The buffer removes hydrogen ions temporarily.


What is pK?

pK is the pH at which the concentrations of a base and acid are equal.


What is an important buffer in blood?

Bicarbonate - HCO3-. Also proteins such as albumin and haemoglobin.


Describe the bicarbonate buffer system?

If you add H+ ions you shift the equilibrium to the right to correct normal pH. The ratio of bicarbonate and the pressure of carbon dioxide determines the pH.


What is the Henderson-Hasselback equation?

pH = 6.74 + log (HCO3-/pCO2).
Can calculate the pH of a solution if you know the pK and the bicarbonate.
1kPa = 7.5mmHg.


What does body metabolism produce?

It produces a large amount of carbon dioxide - 15,000mmol/day. The body needs a mechanism to get rid of the CO2.


How do you get rid of carbon dioxide?

Your body breathes out CO2. You can get rid of a large amount of potential H+ ions through breathing.


What happens to respiratory control when you have low pH?

Low pH indicates an increase in H+ ions, this will cause a shift in production of CO2 (due to the drive to keep equilibrium) because CO2 has increased ventilation rate will increase.

Increased pCO2 = acidosis.
Decreased pCO2 = alkalosis.


How do you measure gas exchange and pH?

Take venous or arterial blood collection to measure:
1. pH.
2. pCO2.
3. pO2.

We use this to calculate bicarbonate and base excess.


How do respiratory disturbances affect pH?

If you increase pCO2 (retain CO2) you will cause a drive to increase H+ ions. Thus cause a low pH - RESPIRATORY ACIDOSIS.
If you have hyperventilation, you will decrease H+ ions and pH will increase - RESPIRATORY ALKALOSIS.


Describe a clinical example: acute asthma?

15yo girl was admitted to the emergency department with an acute asthma attack.
pH = 7.31 (low).
pCO2 = 7.0kPA (high).
pO2 = 8.0kPa (low).
bicarbonate = 24mmol/L (normal).
She is unable to ventilate properly due to bronchiolar constriction, leading to a respiratory acidosis.


What occurs when there is sudden hyperventilation?

pH increases, pCO2 will decrease. The person will experience the following symptoms:
1. Light-headed.
2. Pins and needles (parasthesiae) in his feet and hands.
3. Cramps in his hand muscles (tetany).
This is known as respiratory alkalosis - breathed into a bag to increase pCO2.


How do metabolic disturbances affect pH?

When bicarbonate decreases this will lead to a lower pH (causes drive to increase bicarbonate which in turn increases H+) - METABOLIC ACIDOSIS.
When bicarbonate increases this will lead to a higher pH - METABOLIC ALKALOSIS.


Describe metabolic acidosis?

An example is increased acid production e.g. diabetic ketoacidosis. Because there is a lack of/no insulin to take up glucose, people have to undergo anaerobic glycolysis - generate ketone bodies. pH now increases, and there is low bicarbonate. The body will respond by trying to increase renal excretion, however, the excess H+ ions will exceed H+ excretion. The bicarbonate is used up by binding to the H+ ions. The kidneys get rid of the H+ by using urinary buffers - phosphoric acid and ammonium ions. The urine's pH will be between 5 and 8.


What are the causes of metabolic acidosis?

1. Increased acid production: lactic acidosis - hypoxia, poor tissue perfusion (hypotension, blood loss, heart failure, ischaemia), CO or cyanide poisoning (pink but unable to utilise oxygen).
2. Decreased acid excretion: renal failure, renal tubular acidosis (specific defects in renal tubular acid secretion).
3. Bicarbonate loss.


What are the rare causes of metabolic acidosis?

1. Methanol and ethylene glycol poisoning: methanol metabolised to formic acid and EG metabolised to glycolic and oxalic acids.
2. Glue and paint sniffing: toluene metabolised to hippuric acid.
3. Alcoholic ketoacidosis: complex mechanism.
4. Genetic metabolic disorders ("organic acidemias"): propionic acidemia.


Describe metabolic alkalosis?

If you were to increase sodium bicarbonate, this will lead to an increase in pH.


What is the most common cause of metabolic alkalosis?

Loss of acid - mostly due to vomiting as there is the loss of stomach juice (HCl).


Describe respiratory compensation in metabolic acidosis?

This is how your body deals with abnormal pH. Uncompensated is when there is a low pH, low bicarbonate but normal (slightly low) pCO2. Compensation is when low pH stimulates ventilation, lowering pCO2, which will increase the pH. This is called acidotic breathing. It is not possible to fully compensate.


How does the kidney affect compensation?

The kidney reabsorbs bicarbonate - proximal tubule. The kidney also regenerates new bicarbonate in the proximal tubule. It also secretes H+ ions in the distal tubule.


What drug inhibits carbonic anhydrase?

Acetazolamide causes metabolic acidosis as it inhibits/blocks carbonic anhydrase. It is used in mountaineering to counteract alkalosis due to hyperventilation.


Describe the ENaC channel?

It is an epithelial Na channel - absorbs sodium which causes the drive of H+ and K+ to be secreted into the lumen.


Describe renal compensation?

By increasing bicarbonate uptake, whilst still having the same pCO2 (at high levels - respiratory acidosis) you can increase the pH (return the ratio towards normal).