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Flashcards in Acid-Base Balance Deck (42)
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1

How is pH calculated?

pH = log101/[H+]

 

Increased [H+] reduces pH

Decreased [H+] increases pH

2

What is the pH of normal plasma?

~7.4

3

What is the pH of arterial blood?

~7.45

4

What is the pH of venous blood?

~7.35

5

How is the pH of plasma maintained? What is the normal range and what happens outside of this range?

Tight regulation of [H+] input and output.

pH outside 6.8-8.0 is fatal. It causes neuromuscular excitability, cardiac arrhythmias (changes in K+ induced) and enzyme denaturation.

Some H+ can be ingested, but major source is metabolism. 

6

What are the 3 elements which maintain acid-base balance?

  1. Buffering - the least important mechanism, but closely related to the lungs and kidneys which underlie life. 
  2. Lungs
  3. Kidneys

7

Define the bicarbonate buffer system.

CO2+H2O ⇌ H2CO3 ⇌ H+ HCO3-

 

In the presence of carbonic anhydrase, this will happen much faster.

To control pH, we must control free [H+].

H+ is produced by metabolism and added to the body through diet.

8

What are the relative proportions of CO2 and [HCO3-] in arterial blood under normophysiology?

  • PCO= 40mmHg
  • [HCO3-] = 24mM/L

9

Describe the Henderson-Hasselbalch equation.

pH = 6.1 + log [HCO3-] / PCO2 x 0.03

  • 6.1 = pKa for the reaction
  • 0.03 = solubility coefficient (mmol/mmHg/L)

10

What is the relationship of pH with the kidneys and lungs?

  • pH is proportional to the bicarbonate ion concentration divided by the partial pressure of carbon dioxide. 
  • The base is controlled by the kidneys and the acid is controlled by the lungs. 
  • This means we can control our pH by changes in the renal system and changes in the respiratory system.

11

Describe how buffering helps to maintain acid-base balance.

  • Buffering is the rapid response to change in pH (seconds - few hours):
    • Extracellular buffers
    • Intracellular buffers
    • Bone

12

Describe the role of extracellular buffers in the maintenance of acid-base balance.

  • The most important of these is the bicarbonate buffer system.
  • Quite weak - but ubiquitous and CO2 and H2CO3 can be controlled, so other buffering system are needed. 
  • Phosphate buffer system also, but this is much more important in the kidney and ICF.

13

Describe the role of intracellular buffers in the maintenance of acid-base balance and give an example.

  • CO2 rapidly diffuses from ECF to ICF of all cells and H+ increases (pH changes).
    • Hand HCO3- might also diffuse to a small extent (except in RBC where it is high).
    • Buffering by proteins e.g. Hb in RBCs 'mops-up' H+
      • H+ + Hb ⇌ HHb
      • Example: haemoglobin. 4 subunit protein. Like other proteins, it has the capacity to grab ions and take them out of solution. It can take H+ ions out of solution and hold onto them. If they are not free they are not acidic so in the pocket of haemoglobin they are not causing a solution to be acidic.

14

Describe the role of bone in maintaining acid-base balance.

  • In acidosis His buffered by, for example, PO43-, OH(short-term). 
  • Note also dissolution of bone mineral by osteoclasts in chronic acidosis. 
    • Osteoclasts dissolve bone and the products of this dissolution are free to be used in other metabolic processes.

15

Describe the role of the respiratory system in maintaining acid-base balance. 

CO2+H2O ⇌ H2CO3 ⇌ H++HCO3-

  • Assuming metabolic CO2 production is constant, the only thing that affects [CO2] is alveolar ventilation.
  • Assume normal alveolar ventilation = 1

  • By increasing alveolar ventilation or decreasing we can change the pH. 

16

Describe the role of the kidneys in maintaining acid-base balance. 

  • Kidneys excrete either acidic or basic urine. 
  • HCO3- is filtered continuously into tubules; if excreted in the urine, this will reduce base from the blood. 
  • His secreted into tubules, removing acid from the blood.
    • More H+ secreted than HCO3- filtered - net loss of acid from ECF.
    • More HCO3- filtered than H+ secreted - net loss of base from ECF.
  • The body produces ~80mEq of 'non-volatile acids' per day (not H2CO3 so can't be lost through ventilation). The kidney must remove these. 
  • Kidney must also coneserve bicarbonate levels.

17

Which 3 processes are used by the kidney to maintain acid-base balance?

  • Secretion of H+
  • Reabsorption of filtered HCO3-
  • Production of new HCO3-

18

What are the 4 categories of possible acid-base balance disturbance?

  • Respiratory acidosis
  • Respiratory alkalosis
  • Metabolic acidosis
  • Metabolic alkalosis

19

What causes the initial respiratory acid-base balance disturbance?

Initial disturbance results from increased or decreased PCO2

20

What causes the initial metabolic acid-base balance disturbance?

Initial disturbance results from increased or decreased [HCO3-]. 

This is unrelated to PCO2.

21

What happens to pH, H+ concentration, PCO2 and HCO3- concentration in respiratory acidosis?

  • Increased PCO2 is the primary event.
  • The secondary events are:
    • Decreased pH
    • Increased H+ concentration
    • Increased HCO3- concentration

22

What happens to pH, H+ concentration, PCO2 and HCO3- concentration in respiratory alkalosis?

  • Decreased PCO2 is the primary event.
  • The secondary events are:
    • Increased pH
    • Decreased H+ concentration
    • Decreased HCO3- concentration

23

What happens to pH, H+ concentration, PCO2 and HCO3- concentration in metabolic acidosis?

  • Decreased HCO3- is the primary event.
  • The secondary events are:
    • Decreased pH
    • Increased H+ concentration
    • Decreased PCO2

24

What happens to pH, H+ concentration, PCO2 and HCO3- concentration in metabolic alkalosis?

  • Increased HCO3- is the primary event.
  • The secondary events are:
    • Increased pH
    • Decreased Hconcentration
    • Increased PCO2

25

What are the common causes of respiratory acidosis?

  • Respiratory depression
  • Emphysema
  • Chest injury
  • Chronic bronchitis

26

What are the common causes of respiratory alkalosis?

  • Excessive ventilation (voluntary or 'panic')
    • Blows off CO2
  • High altitude - reduced PO2 (hypoxia) stimulates ventilation via peripheral chemoreceptor response.

27

What are the common causes of metabolic acidosis?

  • May be due to:
    • Failure of the kidneys to secrete metabolic acids normally fromed in the body.
    • Formation of excess metabolic acids.
    • Addition of excess acidss to th body via ingestion or infusion. 
    • Loss of base from the body.
  • Unmanaged type 1 diabetes mellitus.

28

Describe the mechanism by which type 1 diabetes mellitus can cause metabolic acidosis?

 

  • Type 1 diabetes is a common cause of matabolic acidosis if it is not managed.
  • In the absence of insulin, fats are metabolised instead of glucose.
  • Acetoacetic acid (ketoacid) is formed and blood levels rise.
    • This can be severe and the kidneys excrete a large amount in the urine. 

29

What are the common causes of metabolic alkalosis?

  • Excess retention of HCO3- or loss of H+ 
  • Vomiting - loss of H+ from the stomach results in an imbalance.
    • (Vomiting the contents of the duodenum which is rich in bicarbonate can result in metabolic acidosis)

30

How is acid-base balance maintained following an assault?

  1. Buffering 
  2. Compensation
  3. Correction