Blood Gas Physiology

Question 1

At what pH does the body’s physiological processes work best at?

Answer 1

7.35 - 7.45

Question 2

What is the most important extracellular buffer system?

Answer 2

CO2 + H2O <-> HC3O + H^+

Question 3

CO2 is a waste product from respiration that acts like an acid.

How is the concentration of CO2 in the body controlled?

Answer 3

Retention/excretion controlled by the lungs (hypoventilation/hyperventilation)
This can be changed rapidly - by breathing faster or slower

Question 4

Bicarbonate (HCO3) is a base/alkali, the conc. changes in the body tend to be slower than CO2.

What controls the conc.?

Answer 4

Kidneys: excreted to resorbed

Question 5

How does the body correct a pH imbalance - compensate

Answer 5

Body can rapidly lose CO2 or more slowly gain or lose HCO3

can alter pH to right direction but will not return to normal -> not possible to overcompensate

Question 6

When does metabolic acidosis come about?

Answer 6

When there is an excess of H+ produced by the body due to disease e.g. lactic acidosis/renal failure/sepsis

Question 7

What is respiratory acidosis?

Answer 7

An accumulation of CO2 due to disease of the lungs or a problem with the mechanics of respiration

Question 8

How does the body counteract accumulation of acid (CO2) in the body?

Answer 8

• Counteract respiratory acidosis: Body holds on to alkali by resorbing bicarbonates from the kidneys (remember this is a slow process - creating metabolic alkalosis)
• counteracting metabolic acidosis: Body will try compensate metabolic acidosis by lowering PaCO2 by hyperventilating

Question 9

What is metabolic alkalosis usually due to?

Answer 9

Due to a net loss of acid from the body leaving an alkali surplus

Question 10

How does the body counteract against metabolic alkalosis?

Answer 10

Body tries to hold onto acid by hypoventilating and retaining CO2 to counteract relative surplus of alkali (PaCO2 is raised)

Question 11

What is respiratory alkalosis due to?

Answer 11

Due to a relative lack of acid because of the loss of CO2. This arises due to hyperventilation from causes such as anxiety, pain, early sepsis

Question 12

What is the appropriate response to respiratory alkalosis but why is this rarely ever seen?

Answer 12

Appropriate response to excrete HCO3 (creating a metabolic acidosis) but this is a slow process and the causes of respiratory alkalosis is usually short lived so rarely seen

Question 13

What is PaO2?

Answer 13

The partial pressure of oxygen in arterial blood. Generally abbreviated to PO2 in blood gas results

Question 14

What is the normal range breathing air in kPa?

Answer 14

9.3-13.3 kPa

Question 15

Why must PaO2 be interpreted with the inspired oxygen concentration?

Answer 15

Because for example, a patient with pa02 of 8.9 breathing air is much better than a patient with a PaO2 of 8.9 breathing 100% oxygen

Question 16

What is a normal range of CO2 in kPa?

Answer 16

4.7-6

• high = respiratory acidosis
• low = respiratory alkalosis

Question 17

What is a normal range of base excess in mmol/L

Answer 17

-2 to +2 mmol/L

Question 18

What is base excess (BE)?

Answer 18

A measure of how much base has to be removed or added to the system to correct the pH in standard conditions (i.e. is assumes a normal CO2). Therefore a measure of metabolic acidosis or alkalosis

Question 19

What does a positive and what does a negative BE indicate

Answer 19

• negatives BE = metabolic acidosis
• positive BE = metabolic acidosis