Please use the Henderson-Hasselbalch (HH) equation to answer Q2a-c.
In the HH equation, there are 3 terms which can be variable: pH, PCO2 and HCO3.
Since there are 3 variables, it means that if you know the values for 2 variables, you can work out the unknown variable.
pH - is controlled by HCO3 and PCO2
PCO2 - is controlled by pH and HCO3
HCO3 - is controlled by pH and PCO2
WHAT YOU MUST KNOW
For learning, assessment and exam purposes, you must remember that these 3 variables always work together. Disturbing any one variable will disturb the other 2, which will respond to the change. Thus, you have what is called compensation coming into play.
EXAMPLES OF CASES
In the conditions we get at the hospital (vomiting, diarrhoea, emphysema, coma, hysteria, ketoacidosis, poisoning, etc), usually pH is altered by the alteration of HCO3 or PCO2 (simple acid base disorders). We look at the physical state of the patient and we should know whether the respiratory system is held up or not.
HOW TO UNDERSTAND AND INTERPRET
If the main change (lesion) is due to respiratory causes, then it is more likely that the PCO2 changed, causing blood pH to change, and the compensation will be the corresponding change in the HCO3, which needs to correct and bring back pH towards normal.
If the main change is due to metabolic causes, then it is more likely that the HCO3 changed, causing blood pH to change, and the compensation will be the corresponding change in the PCO2, which needs to correct and bring back pH towards normal.
IS IT ACIDOSIS OR ALKALOSIS?
Whether it is acidosis or alkalosis depends on what the causative agent was and what the initial effect was on blood pH.
If a patient ingested (makan) a whole bottle of aspirin, that gives rise to metabolic acidosis.
If a patient has diarrhoea (cirit-birit) for 3 days, that gives rise to metabolic alkalosis.
If a chronic smoker has difficulty breathing, coughs a lot and has water in his lungs (emphysema), that is respiratory acidosis.
If a patient suffers from Duchenne Muscular Dystrophy (DSD), the breathing muscles are affected (usually the muscles for expiration), and if his respirator fails to operate, he should have respiratory acidosis.
BREATHING CHANGES
Breathing has significant effects on blood pH. Breathing rate affects blood pH. Breathing rate can be normal, fast or slow, depending on the state or condition a person is in.
Breathing fast is hyperventilation.
For example, this occurs after you dashed from your car into your house because you saw a burglar with a knife in your compound. When you are immediately inside the safe confines of your house, you are still panting and hyperventilating but that should cease within some 20 minutes when you can think properly.
Breathing slowly is hypoventilation.
For example, you are so full after eating 2 plates of nasi biryani from your favourite restaurant
tepi jalan. The only thing you can do next is to sit back and breathe slowly cos even your lungs cannot expand properly cos your tummy is so tight and full, filled to the limits, that your lungs are slightly compressed. This slow breathing is dangerous as you tend to retain a lot of CO2 in your lungs and in the blood. Your blood pH drops as a result of high PCO2 in the blood. Now, when you have a lot of CO2 in blood, what comes next? What is the special property of CO2? I have told you in class that CO2 is a special gas that has a special property and that it can penetrate all parts of the body, diffuse everywhere and anywhere in the body but there is one location in the body that it likes to go to. Where is that? Yes, CO2 prefers to go to the brain. What does CO2 do to the brain? There are chemoreceptors in the brain that react to blood pH (from the reaction of CO2 with H2O). Essentially high CO2 tells the brain to go to sleep. And you will fall asleep after your nasi biryani feast, and also suffer from respiratory acidosis cos you can't breathe properly until that nasi biryani empties out of your tummy in 4 hours and your lungs can breath again.
RULES ABOUT BREATHING
Alveolar ventilation - hyperventilation reduces blood PCO2; hypoventilation increases blood PCO2
If the lungs are damaged (crushed or excised) - you can still live but with assisted respiratory support.
WHAT ABOUT BICARBONATE?
Bicarbonate is bicarbonate and never mention bicarbonate
ion unless you failed chemistry in Form 5.
Bicarbonate is a negative ion or an anion with a negative one charge (-1).
You can write bicarbonate in full on exams but you can also just write HCO3 or HCO3-. Nobody cares whether you put a negative one for bicarbonate or not. It is clearly understood that bicarbonate has a negative one charge whether you write it or not.
What is special about bicarbonate?
The bicarbonate balance in blood deals with 2 things - regeneration and reabsorption. What is what?
Regeneration of bicarbonate means bicarbonate is generated once more. You start visualising at the glomerulus. I told you in class that blood bicarbonate concentration is the same as that in the glomerular filtrate because bicarbonate diffuses freely through the glomerulus. Now the problem is how to get back the bicarbonate from the renal tubules into blood. First you need to regenerate bicarbonate inside the tubular cells by combining bicarbonate with secreted protons to form carbonic acid, which instantaneously breakdown to H2O and CO2. Both H2O and CO2 diffuse and get inside the tubular cells. H2O goes to combine with cellular CO2 to form carbonic acid which dissociates to form proton and bicarbonate ... which can then exit the tubular cells into interstitial fluid and re-enters blood. This reabsorption is accompanied by sodium and water.
Reabsorption of bicarbonate means bicarbonate is absorbed and that the absorption process goes on until bicarbonate goes back where it belongs (blood). I have told you in class that of the many blood buffers, 70% of CO2 exists as bicarbonate in blood plasma. That process of bringing bicarbonate from the renal tubules back into blood plasma is referred to as reabsorption (serapan balik ke dalam darah semula).
If you study carefully you will see that the regeneration is the first part and reabsorption is the second part. Both processes are linked, and must occur in tandem, before bicarbonate reappears in blood.
EXCRETION
What is excreted and when does excretion occur?
Acidosis - excess protons are excreted in urine.
Alkalosis - excess bicarbonate is excreted in urine.
In acidosis, the urine is more acidic than normal but there is a limit to the acidity of urine. The acidity of urine stops at pH4.5 (round about that #) and it cannot be any lower. If you get a urine specimen and the pH says 3, I think there's something funny with that urine specimen. Probably it is a ghost urine (
hantu jembalang).
In alkalosis, the urine is slightly more alkaline than normal and there is no limit to the alkalinity of urine, but of course the patient may pass out first. If the upper limit of blood pH is 7.45, you would expect urine from a dead person to be more than pH8. Next time you see a urine specimen with pH more than 8, let me know, I will bring
kain kafan too.
