Acid base balance

1. MODERATOR – Dr. P.C. Attri
Presemted by dr. atul jain]

2. Terms
 Acid
 Any substance that can yield a hydrogen ion (H+) or
hydronium ion when dissolved in water
 Release of proton or H+
 Base
 Substance that can yield hydroxyl ions (OH-)
 Accept protons or H+

3. Terms
 pK/ pKa
 Negative log of the ionization constant of an acid
 Strong acids would have a pK <3
 Strong base would have a pK >9
 pH
 Negative log of the hydrogen ion concentration
 pH= pK + log([base]/[acid])
 Represents the hydrogen concentration

4. Terms
 Buffer
 Combination of a weak acid and /or a weak base and its
salt
 What does it do?
 Resists changes in pH
 Effectiveness depends on
 pK of buffering system
 pH of environment in which it is placed

5. Terms
 Acidosis
 pH less than 7.35
 Alkalosis
 pH greater than 7.45
 Note: Normal pH is 7.35-7.45

6. Acid-Base Balance
 Function
 Maintains pH homeostasis
 Maintenance of H+ concentration
 Potential Problems of Acid-Base balance
 Increased H+ concentration yields decreased pH
 Decreased H+ concentration yields increased pH

7. Regulation of pH
 Direct relation of the production and retention of acids and bases
 Systems
 Respiratory Center and Lungs
 Kidneys
 Buffers
 Found in all body fluids
 Weak acids good buffers since they can tilt a reaction in the other
direction
 Strong acids are poor buffers because they make the system more
acid

8. 8

9. Blood Buffer Systems
 Why do we need them?
 If the acids produced in the body from the catabolism of
food and other cellular processes are not removed or
buffered, the body’s pH would drop
 Significant drops in pH interferes with cell enzyme
systems.

10. Blood Buffer Systems
 Four Major Buffer Systems
 Protein Buffer systems
 Amino acids
 Hemoglobin Buffer system
 Phosphate Buffer system
 Bicarbonate-carbonic acid Buffer system

11. Blood Buffer Systems
 Protein Buffer System
 Originates from amino acids
 ALBUMIN- primary protein due to high concentration in
plasma
 Buffer both hydrogen ions and carbon dioxide

12. Blood Buffering Systems
 Hemoglobin Buffer System
 Roles
 Binds CO2
 Binds and transports hydrogen and oxygen
 Participates in the chloride shift
 Maintains blood pH as hemoglobin changes
from oxyhemoglobin to deoxyhemoglobin

13. Oxygen Dissociation Curve
Curve B: Normal
curve
Curve A: Increased
affinity for hgb, so
oxygen keep close
Curve C: Decreased
affinity for hgb, so
oxygen released to
tissues

14. Bohr Effect
 It all about
oxygen affinity!

15. Blood Buffer Systems
• Phosphate Buffer System
• Has a major role in the elimination of H+ via the kidney
• Assists in the exchange of sodium for
hydrogen
• It participates in the following reaction
• HPO-2
4 + H+ H2PO –
4
• Essential within the erythrocytes

16. Blood Buffer Systems
 Bicarbonate/carbonic acid buffer system
 Function almost instantaneously
 Cells that are utilizing O2, produce CO2, which builds
up. Thus, more CO2 is found in the tissue cells than in
nearby blood cells. This results in a pressure (pCO2).
 Diffusion occurs, the CO2 leaves the tissue through the
interstitial fluid into the capillary blood

17. Bicarbonate/Carbonic Acid Buffer
Carbonic
acid
Bicarbonate
Conjugate
base
Excreted in
urine
Excreted
by lungs

18. Bicarbonate/carbonic acid buffer system
 How is CO2 transported?
 5-8% transported in dissolved form
 A small amount of the CO2 combines directly with
the hemoglobin to form carbaminohemoglobin
 92-95% of CO2 will enter the RBC, and under the
following reaction
 CO2 + H20 H+ + HCO3
-
 Once bicarbonate formed, exchanged for chloride

19. Henderson-Hasselbalch Equation
 Relationship between pH and the bicarbonate-
carbonic acid buffer system in plasma
 Allows us to calculate pH

20. Henderson-Hasselbalch Equation
 General Equation
 pH = pK + log A-
HA
 Bicarbonate/Carbonic Acid system
o pH= pK + log HCO3
H2CO3 ( PCO2 x 0.0301)

21. Henderson-Hasselbalch Equation
1. pH= pK+ log H
HA
2. The pCO2 and the HCO3 are read or derived from the blood gas analyzer
pCO2= 40 mmHg
HCO3
-= 24 mEq/L
3. Convert the pCO2 to make the units the same
pCO2= 40 mmHg * 0.03= 1.2 mEq/L
3. Lets determine the pH:
4. Plug in pK of 6.1
5. Put the data in the formula
pH = pK + log 24 mEq/L
1.2 mEq/L
pH = pK + log 20
pH= pK+ 1.30
pH= 6.1+1.30
pH= 7.40

22. The Ratio….
Normal is : 20 = Kidney = metabolic
1 Lungs respiratory
 The ratio of HCO3
- (salt) to H2CO3 ( acid) is normally 20:1
 Allows blood pH of 7.40
 The pH falls (acidosis) as bicarbonate decreases in
relation to carbonic acid
 The pH rises (alkalosis) as bicarbonate increases in
relation to carbonic acid

23. Physiologic Buffer Systems
 Lungs/respiratory
 Quickest way to respond, takes minutes to hours to
correct pH
 Eliminate volatile respiratory acids such as CO2
 Doesn’t affect fixed acids like lactic acid
 Body pH can be adjusted by changing rate and depth of
breathing “blowing off”
 Provide O2 to cells and remove CO2

24. Physiologic Buffer Systems
 Kidney/Metabolic
 Can eliminate large amounts of acid
 Can excrete base as well
 Can take several hours to days to correct pH
 Most effective regulator of pH
 If kidney fails, pH balance fails

25. 25

26. Respiratory Compensation
 Used to compensate for metabolic imbalances only
 Chemoreceptors respond to changes in H+
concentrations  alters respiratory rate and depth
 Remember CO2 is an acid

27. Respiratory Compensation
 Respiratory Rate will…
 Increase when blood H+ is increased (acidic pH)
 CO2 is “blown off”
 Amount of acid in blood is decreased
 Decrease when H+ is decreased (alkaline pH)
 CO2 is retained
 Amount of acid in blood is increased

28. Respiratory Compensation
 This means
 Metabolic acidosis causes an increase in rate and depth
of ventilation as the body attempts to get rid of acid
(CO2)
 Metabolic alkalosis causes a decrease in rate and depth
of ventilation as the body attempts to retain acid (CO2)

29. Renal Compensation
 Used to compensate for respiratory imbalances
 Remember: HCO3
- is a base
 Kidneys respond to changes in blood pH
 Excrete H+ and retain HCO3
- when acidemia is present
(1:1 ratio)
 Retain H+ and excrete HCO3
- when alkalemia is present
(1:1 ratio)

30. Renal Compensation
 This means
 A respiratory acidosis will make the kidneys excrete acid
(H+) and retain base (HCO3
-)
 A respiratory alkalosis will make the kidneys excrete
base (HCO3
-) and retain acid (H+)

31. Renal Compensation
 This is the slowest compensation
 May take hours to days
 Most powerful method of compensation
 Ineffective in patients with renal failure

32. Degrees of Compensation
 An acid-base imbalance will be compensated for in
one of three ways
 Uncompensated
 Partially compensated
 Fully compensated

33. Degrees of Compensation
 Uncompensated
 Body has made no attempt to correct the acid-base
imbalance
 Partially compensated
 Body is attempting to correct the imbalance
 Blood pH remains abnormal in spite of the attempt

34. Degrees of Compensation
 Fully compensated
 The body is correcting the imbalance
 Blood pH is normal
 Other blood gas values remain abnormal until the root
cause is treated and corrected

36. Uncompensated Imbalance
 pH abnormal
 Either PaCO2 OR HCO3
- abnormal
 All other values normal
 If PaCO2 is abnormal
 Problem is respiratory
 If HCO3
- is abnormal
 Problem is metabolic

37. Uncompensated
Imbalance
 Uncompensated
respiratory acidosis
 pH < 7.35
 PaCO2 > 45
 HCO3
- WNL
 Uncompensated
respiratory alkalosis
 pH > 7.45
 PaCO2 < 35
 HCO3
- WNL
Remember that CO2 is an acid and that the more of it there is the worse is
the acidemia. Notice that with uncompensated respiratory, the HCO3 is
normal – this is because the body has not began to compensate for the
alterations in CO2

38. Uncompensated
Imbalance
 Uncompensated
metabolic acidosis
 pH < 7.35
 PaCO2 WNL
 HCO3
- < 22
 Uncompensated
metabolic alkalosis
 pH > 7.45
 PaCO2 WNL
 HCO3
- > 26
Remember that HCO3 is a base and that the more of it there is the more
alkalotic you will be. Notice that in the case of uncompensated metabolic
the PaCO2 is normal indicating that the body has not began to compensate.

39. Partially Compensated
Imbalances
 Occur when compensation mechanisms are activated,
but have not had sufficient time to normalize the
blood pH
 NOTE: Some people say that there is no such thing as
“partially” compensated – it is kind of like being “a
little pregnant” – but it is indicative of a part of the
process called compensation

40. Partially Compensated
Imbalances
 pH is abnormal
 Both PaCO2 and HCO3
- are abnormal in the same
direction (increased or decreased from normal)
 If PaCO2 is high (↑ acid), HCO3
- will also be high (↑
alkaline) to neutralize the environment
 If PaCO2 is low (↓ acid), HCO3
- will also be low (↓
alkaline) to neutralize the environment

41. Partially Compensated
Imbalances
 Partially Compensated
Respiratory Acidosis
 pH < 7.35
 PaCO2 > 45
 HCO3
- > 26
 Partially Compensated
Respiratory Alkalosis
 pH > 7.45
 PaCO2 < 35
 HCO3
- < 22
In the case of Partially Compensated Resp Acidosis, the pH is low, indicating an
acid environment…when you look at the PaCO2, it too is acidic, which is how you
know that you have a respiratory acidosis. With the HCO3 being high, you can
deduce that the body is raising its base to counteract the acid represented by the pH;
therefore, partially compensated respiratory acidosis.

42. Partially Compensated
Imbalances
 Partially Compensated
Metabolic Acidosis
 pH < 7.35
 PaCO2 < 35
 HCO3
- < 22
 Partially Compensated
Metabolic Alkalosis
 pH > 7.45
 PaCO2 > 45
 HCO3
- > 26
With partially compensated metabolic acidosis, you notice first that the pH is low (acidos
Ask yourself, which number is representative of an acid condition. In this case it is the low
base (HCO3), so you know you have a metabolic acidosis. You know it is partially compen
because the PaCO2 is low indicating that CO2 (an acid) is being lost from the body to cor
for the low pH.

43. Compensated Imbalances
Occur when compensatory mechanisms have been able
to fully normalize blood pH

44. Compensatory Mechanisms
 Both PaCO2 and HCO3
- are abnormal, but in the same
direction
 If PaCO2 is high (↑ acid), HCO3
- will also be high (↑
alkaline)
 If PaCO2 is low (↓ acid), HCO3
- will also be low
(↓alkaline)

45. Compensated Imbalances
 Compensated
Respiratory Acidosis
 pH WNL but
closer to
7.35
 PaCO2 > 45
 HCO3- > 26
 Compensated
Respiratory Alkalosis
 pH WNL but
closer to
7.45
 PaCO2 < 35
 HCO3- < 22
In compensated respiratory acidosis, the pH tends to range between 7.35 and 7.39 – still a
But in the normal pH range. When you look at the PaCO2, you notice that it is high (acid
The HCO3 is also high, indicating that the body has compensated and normalized the low

46. Compensated Imbalances
 Compensated Metabolic
Acidosis
 pH WNL but
closer to
7.35
 PaCO2 < 35
 HCO3- < 22
 Compensated Metabolic
Alkalosis
 pH WNL but
closer to
7.45
 PaCO2 > 45
 HCO3- > 26

47. Mixed Imbalances
 Occur when patient has both metabolic and
respiratory disorders that cause an acid-base
imbalance
 Examples:
 Diabetic KetoAcidosis (metabolic acidosis) with
decreased respiratory drive (respiratory acidosis)
 Severe vomiting (metabolic alkalosis) with high fever
(respiratory alkalosis)

48. Mixed Imbalances
 pH will be normal
 PaCO2 and HCO3
- will be abnormal
 PaCO2 will be high with low HCO3
- (both tend
toward acid side)
 PaCO2 will be low with high HCO3
- (both tend
toward base side)

49. Mixed Imbalances
 Mixed acidosis
 pH < 7.35
 PaCO2 > 45
 HCO3
- < 22
 Mixed alkalosis
 pH > 7.45
 PaCO2 < 35
 HCO3
- > 26
Notice with the mixed acidosis that you have an acidic pH (less than 7.35, with other
Parameters indicating an acid environment. High PaCO2 (too much acid). Low HCO3
(too little base – an acidic environment). This is classic mixed acidosis.

51. Analysis of simple acid base disorders and how they are compensated for by the body.

52. Assessment of A-B balance
Arterial blood Mixed venous blood
range range
pH 7.40 7.35-7.45 pH 7.33-7.43
pCO 40 mmHg 35 – 45 pCO2 41 – 51
pO2 95 mmHg 80 – 95 pO2 35 – 49
Saturation 95 % 80 – 95 Saturation 70 – 75
BE 2 BE
HCO3
- 24 mEq/l 22 - 26 HCO3
- 24 - 28

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