Week 3 - H - Physiology 8 - Arterial blood gases (acidosis&alkalosis)

Question 1

What is a person’s normal plasma pH? When is it acidic and when is it alkali?

Answer 1

pH - 7.35 to 7.45 Acidic - below 7.35 and lkali is above 7.45

Question 2

What is the normal bicarbonate and CO2 range for a patient?

Answer 2

Bicarbonate (HCO3-) - 23 to 27 mmol/l CO2 - 35-45 mmol/l

Question 3

If normal acid-base balance is disrupted, the first priority is to restore pH to 7.4 as soon as possible What is this known as?

Answer 3

This is known as compensation

Question 4

What is the correction of the acid base disturbance then?

Answer 4

This is the stage of returning the HCO3-, CO2 and pH levels to normal Compensation is the restoration of pH irrespective of what happens to [HCO3-]p and PCO2

Question 5

When an acidosis occurs, what happens to the CO2 levels and pH levles?

Answer 5

CO2 levels increase causing a fall in pH in acidotic conditions (CO2 becomes an acid in solution)

Question 6

What is the main buffer system in the human body?

Answer 6

The bicarbonate buffer system

Question 7

If there is a riise in H+ ion concentration therefore causing a decrease in pH, what in the blood quickly binds to the increase [H+] to buffer the blood? What regenerates this ion?

Answer 7

HCO3- binds to the increased hydrogen ions The kidney then works to regenerate the bicarbonate used

Question 8

How is the bicarbonate used to monitor the H+ levels reasborbed?

Answer 8

Once it binds to H+ it becomes H2CO3 (carbonic acid) filtered into the tubular lumen, this then dissociates to become CO2 and HCO3- and reenters the tubualr cells/interstitial fluid

Question 9

What is the Henderson Hasselbach equation?

Answer 9

pH = pK + log [A-/HA]

Question 10

In the henderson hasselbach equation, if regarding the bicarbonate and CO2 levels, what is the pH proportional to?

Answer 10

pH is proportional to the HCO3- levels and inversely proportional to the levels of CO2 in the blood

Question 11

A blood-gas analyser can measure pH and PCO2 [HCO3-] can be calculated What type of diagram can these variables be plotted on?

Answer 11

Can be plotted on a Davenport diagram

Question 12

What type of arterial blood gas does the green circle represent?

Answer 12

Represents the normal value

Question 13

What are some cause of CO2 retention in the body?

Answer 13

Type II repsiratory failure - chronic bronhcitis, emphsyema Respiratory depression

Question 14

What is the sydnrome characterised by obesity and hypoventialtion which drives a person into hypercapnia?

Answer 14

This is Pickwickian Syndrome

Question 15

An increase in carbon dioxide levels drives the equilibirum in which direction?

Answer 15

Drives the equilibrium to the right therefore accounting for the increased hydrogen ions

Question 16

When there is a rise in CO2 this causes an increase in both HCO3- and H+ ions Why is it that the pH will fall? The pH is proportional to HCO3- and inversely to pCO2, what does this mean for respiratory acidosis?

Answer 16

There are many many times more bicarb ions than there is hydrogen. You see a bigger overall increase in hydrogen than bicarb because there was so many bicarb ions to start off with. This means that when the carbon dioxide levels rise, the pH falls

Question 17

What does the purple dot represent?

Answer 17

A low pH - since HCO3- is high then cannot be metabolic acidosis SO respiratory acidosis

Question 18

What organ needs to compensate for the respiratory acidosis? Recall that blood PCO2 drives H+ secretion by this organ

Answer 18

The kidneys compensate as the increased pCO2 will increase H+ secretion by the kidneys

Question 19

H+ secretion is stimulated All filtered HCO3- is reabsorbed (i.e. no HCO3- excretion) What does the increased secretion of H+ generate in the urine (previous lecture)?

Answer 19

The increased H+ secretion stimulates the production of titratable acid (H2PO4) and NH4+ [HCO3-]p rises; (a) as a result of the disorder & (b) as a result of the renal compensation

Question 20

How are the normal pressures of carbon dioxide brought back in the process of correction?

Answer 20

Correction requires lowering PCO2 by restoration of normal ventilation

Question 21

What is respiratory alkalosis known as?

Answer 21

Excessive removal of CO2 from the body resulting in a rise in the pH

Question 22

Give examples of when a respiratory alkalosis may occur?

Answer 22

During hyperventialation Hypoxia causing a subsequent hyperventilation: pulmonary embolism, high altitude During hysterical overbreathing

Question 23

What does the blue circle represent?

Answer 23

This represents the respiratory alkalosis -seen by the high pH with a low HCO3 showing this is not the cause

Question 24

Since the respiratory system is the cause of the disturbance the renal system must compensate Recall that it is blood PCO2 that drives H+ secretion by the kidney  Excessive removal of CO2 reduces H+ secretion into the tubule What does the renal system do to compensate for the respiratory alkalosis? What does this do to [HCO3-]p and what does it do to urine pH?

Answer 24

Compensates by lowering [HCO3-] plasma (bicarbonate plasma concentration) This causes an increase in pH of the urine

Question 25

Is any titratable acid or NH4+ produced in respiratory alkalosis?

Answer 25

No as the kidneys are trying to save H+ ions to restore blood pH

Question 26

Again what does correction require? (for respiratory alkalosis this time)

Answer 26

Correction requires the restoration of normal ventilation Can see the compensation restoring the pH irrespective of the what is happening to the ion concentration

Question 27

What is Excess H+ from any source other than CO2 ? Give two examples when this may occur? (both are to do with diabetes)

Answer 27

This is metabolic acidosis My occur in lactic acidosis and DKA (diabetic ketoacidosis)

Question 28

[HCO3-]p is depleted as a result of buffering excess H+ or loss of HCO3- from the body What do the ABGs for [HCO3-]p and pH show in metabolic acidosis?

Answer 28

Bicarbonate concentration is low pH is also low

Question 29

How does the respiratory system compensate for the decrease in the HCO3- leading to the metabolic acidosis? What does it do to the man buffering system equilibrium?

Answer 29

It increases the ventilation of CO2 therefore reducing H+ ion levels and raising the pH Shifts the buffering system equilibrium to the left therefore decreasing both H+ and HCO3- but in doing so raises the pH

Question 30

H+ secretion continues and produces TA & NH4+ to generate more what?

Answer 30

Generates more new HCO3- ions Acid load cannot be excreted immediately therefore respiratory compensation is essential

Question 31

Metabolic alkalosis is less common than metabolic acidosis and involves the increased excessive loss of H+ ions from the body One cause of this is vomiting as it can remove HCL from the body How does aldosterone hypersecretion cause this? (aka conn's syndrome)

Answer 31

Aldsoterone hypersecretion causes the tubular reabsorption of Na+ via the H+ and K+ exchangers The excess loss of H+ leads to acidic urine but a metabolic alkalosis

Question 32

Metabolic alkalosis occurs a result of loss of H+ or addition of base, [HCO3-]p rises HCO3- rises because the buffer system is trying to restore H+ ions and therefore also HCO3- ions What is the light blue circle?

Answer 32

Metabolic alkalosis

Question 33

How does the respiratory system compensate for a metabolic alkalosis? How do the kidneys carry out correction?

Answer 33

Respiratory system slows ventilation to keep CO2 to rise pH Filtered HCO3- load is so large compared to normal that not all of the filtered HCO3- is reabsorbed No TA or NH4+ is generated , HCO3- is excreted (urine is alkaline) and the [HCO3-]p falls back towards normal

Question 34

When the respiratory system retains or loses CO2 , why does this cause a decrease or rise in [HCO3-] levels?

Answer 34

This is due to the CO2 shifting the equilibrium of the buffer equation causing an effect on both H+ and HCO3- levels It is only becuase there is less H+ that a change will have a bigger effect than HCO3

Question 35

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Answer 35

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Question 36

Name the acid base disturbance for each circle

Answer 36

Purpel - respiratory acidosis Light blue - metabolic alkalosis Dark blue - respiratory alkalosis Red - metabolic acidosis