Physiology-Acid Base Balance

Question 1

In what sequence do the body’s acid-base defenses normally get rid of excess H+ in the body?

Answer 1

First the bicarbonate buffer grabs H+, second the lungs help via expiration of CO2 and finally the kidneys generate new bicarbonate and secrete H+ bound as ammonia and phosphate in the urine.

Question 2

Use the Hendersen-Hasselbach equation to show how the kidney and the lung can both cause acidosis (respiratory acidosis & metabolic acidosis).

Answer 2

The lung can increase PCO2 by decrease expiration. This will cause a decrease in the pH as seen below. The kidney can increase HCO3 excretion, which will also decrease pH as seen below.

Question 3

The pKa of bicarbonate is 6.1, but the physiological pH is 7.4. How is bicarbonate such a good buffer in a system with a pH so far from its pKa?

Answer 3

The pKa of 6.1 is measured in a closed system. However, in our bodies the system is open and we can continually exhale CO2 from our bodies.

Question 4

What portion of buffering power in the blood is represented by bicarbonate?

Answer 4

2/3 is bicarbonate and 1/3 non-bicarbonate.

Question 5

Where in this diagram would you find respiratory acidosis and alkalosis? Metabolic acidosis and alkalosis?

Answer 5

Respiratory acidosis = low pH and high CO2. Metabolic acidosis = low pH and low CO2. Respiratory alkalosis = high pH and low CO2. Metabolic alkalosis = high pH and high CO2.

Question 6

What would happen to a blood pH of 7.4 in response to increasing PCO2 if non-bicarbonate buffer concentration was unlimited? What if it was 0?

Answer 6

Unlimited = no change in pH. 0 non-bicarbonate buffer = new blood pH of 7.1 instead of 7.2 as seen below.

Question 7

You have an old patient with emphysema in your clinic. Blood work reveals that the patient is in respiratory acidosis. How do the kidneys function to return this patient back to a healthy physiological pH?

Answer 7

By making more bicarbonate and secreting more acid, the pH will raise back to 7.4ish.

Question 8

How do the lungs function to recover from metabolic alkalosis?

Answer 8

Metabolic alkalosis happens when HCO3 levels are high. The lungs increase PCO2 and return the pH back to normal.

Question 9

HCO3 doesn’t just get willy nilly reabsorbed in the kidney anywhere it wants. What does HCO3 reabsorption depend on?

Answer 9

H+ secreting via either the Na+/H+ exchange pump or the H+ATPase. Once H+ is in the lumen, it finds bicarbonate and forms carbonic acid. Carbonic anhydrase then converts the carbonic acid to water and CO2 which can be reabsorbed into the PCT epithelium. Inside the cell, carbonic anhydrase converts H2O and CO2 back to HCO3 and it exits via the (3)HCO3-Na exchanger.

Question 10

Where is most bicarbonate reabsorbed in the kidney?

Answer 10

PCT

Question 11

How is bicarbonate reabsorbed in the collecting duct?

Answer 11

It occurs in the intercalated cells. It is the exact same as the PCT except the lumen exchanger is a K+/H+ exchange pump instead of a Na+/H+ exchange pump. The basal exchanger is a Cl/HCO3 exchange pump.

Question 12

How do the intercalated cells of the collecting duct adjust to alkalosis?

Answer 12

In beta-intercalated cells, the HCO3/Cl exchanger switches from the blood side to the lumen side.

Question 13

How do the cells in the nephron compensate for acidosis with bicarbonate?

Answer 13

This is different from normal bicarbonate reabsorption because carbonic acid is not reabsorbed and the H+ is secreted where it binds phosphate and leaves in the urine.

Question 14

How do the cells in the nephron compensate for acidosis with H+ handling?

Answer 14

Ammonia produced from protein metabolism can bind up H+ and be secreted as NH4+ in the PCT. As NH4+ moves down the loop of Henle, it is taken out of the tubule and into the interstitium where it generates an osmotic gradient for water retention. It is then taken back up by the cells of the collecting duct and moved back into the tubule so it can be excreted in the urine. Additionally, new bicarbonate is released from protein metabolism.

Question 15

How does NH4+ get into the proximal convoluted tubule?

Answer 15

Diffusion of gaseous NH3 and then it combines with H+ to make NH4+ or passage of liquid NH4+ through a Na+ exchange pump.

Question 16

How much H+ do we typically excrete in a day? How is that H+ distributed? How do these values differ in acidosis and alkalosis?

Answer 16

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Question 17

The kidney excretes about 70mmol H+/day. How much “new” HCO3 is created by the kidney each day?

Answer 17

70mmol. Any formation of acid = loss of HCO3 which must be produced again by the kidney.

Question 18

You are in the field working as a doctor and one of your patients is acidotic. You don’t have any lactated Ringer solution, what food can you give this patient?

Answer 18

Oranges. The citrate in oranges is metabolized to form bicarbonate and will alkalize the blood.

Question 19

How would you expect plasma bicarbonate to change in a patient with COPD?

Answer 19

Respiratory acidosis. Due to decreased ventilation they will have an increased PCO2. This causes a decreased plasma pH. The kidney responds by increasing HCO3 production to bind up acid and raise the pH.

Question 20

How would you expect plasma bicarbonate to change in a patient having a panic attack?

Answer 20

Respiratory alkalosis. Due to increased ventilation they will have a decreased PCO2. This causes an increase in plasma pH. The kidney responds by decreasing HCO3 reabsorption and decrease new HCO3 production to increase free acid and lower the pH.

Question 21

How would you expect plasma bicarbonate to change in a patient with diabetic ketoacidosis?

Answer 21

Metabolic acidosis. Due to low plasma bicarbonate levels, plasma pH will decrease. The lungs respond by decreasing PCO2 and increasing the pH.

Question 22

How would you expect plasma bicarbonate to change in a patient who has experienced excessive vomiting recently?

Answer 22

Metabolic alkalosis. Due to high plasma bicarbonate levels, plasma pH will increase. The lungs respond by increasing PCO2 and decreasing the pH.

Question 23

How do you know what type of acid-base problem this patient has?

Answer 23

Increased levels of HCO3 = renal compensation for acidosis. Decreased levels of HCO3 = renal compensation for alkalosis.

Question 24

What happens to urea metabolism when a patient is acidotic?

Answer 24

It is increased and NH3 from protein metabolism grabs H+ and is excreted in the urine.

Question 25

What two factors govern the regulation of hydrogen ion release from glomerular tubules?

Answer 25

PCO2 and local pH. (increased H+ secretion is a result of increased PCO2 or decreased pH).

Question 26

What regulatory mechanisms take place at the proximal tubule of the nephron in response to acidosis? What about on the distal end?

Answer 26

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Question 27

How do you determine acid-base status of a patient in the clinic?

Answer 27

pH [7.4], HCO3 [24 mEq/L] and PCO2 [40 mmHg]

Question 28

A patient comes to the ED while having an asthma attack. You get her stabilized and run some labs: pH 7.25, PCO2 62mmHg, HCO3 26 mEq/L. What is the acid-base state of this patient?

Answer 28

The patient has acute respiratory acidosis with beginnings go renal compensation.

Question 29

A patient comes to the ED while having a COPD exacerbation. Labs show a pH of 7.35, PCO2 of 60mmHg and HCO3 32 mEq/L. What is the acid-base state of this patient?

Answer 29

This patient has chronic respiratory acidosis with renal compensation.

Question 30

A patient comes to the ED. She has type I diabetes and has not had her insulin shot today. Labs reveal pH 7.29, PCO2 26mmHg and HCO3 12 mEq/L. What is the acid-base state of this patient?

Answer 30

This patient has metabolic acidosis with renal compensation.

Question 31

A patient comes to the ED. He has been vomiting all day. Labs reveal pH 7.5, PCO2 48mmHg and HCO3 47 mEq/L. What is the acid-base state of this patient?

Answer 31

This patient has metabolic alkalosis with respiratory compensation.

Question 32

A patient comes to the ED. She is a diabetic and hasn't taken her insulin in a few days. Labs reveal pH 7.31, PCO2 30 mmHg and HCO3 16 mEq/L. What is the acid-base state of this patient?

Answer 32

This patient has metabolic acidosis with respiratory compensation.

Question 33

A patient presents to the ED with abnormal acid-base labs that do not show appropriate compensatory response for alkalosis or acidosis. What should you suspect in this patient?

Answer 33

Presence of two or more underlying causes creating a mixed acid-base disorder.

Question 34

A 55 year old male presents to the ED with sever diarrhea. History only reveals tobacco us. Labs reveal a pH of 7.15, PCO2 of 50 mmHg and HCO3 of 7 mEq/L. What is the acid-base status of this patient?

Answer 34

Note that the patient has acidosis, but the bicarbonate is not coming up and the PCO2 is not coming down. This indicates a mixed acidosis with a metabolic component (loss of HCO3 in diarrhea) and failure of respiratory compensation (emphysema from tobacco use).

Question 35

How can ingestion of a large quantity of aspirin cause a mixed acid-base disorder? How will labs present in these patients?

Answer 35

Salicylic acid = metabolic acidosis. Aspirin stimulates hyperventilation = respiratory alkalosis. The patient may have a normal pH with abnormal PCO2 and HCO3 levels.

Question 36

What are the clinical causes of metabolic acidosis?

Answer 36

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Question 37

What are the clinical causes of metabolic alkalosis?

Answer 37

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Question 38

What lab value is used to distinguish between different types of metabolic acidosis? What does it measure?

Answer 38

Anion gap [12]. [Na+]-[HCO3-]-[Cl-]. It is essentially measuring the gap between the cation (Na) and the anions (Cl & HCO3). The gap will increase as the amount of unmeasured anions increase. The gap will decrease as the amount of unmeasured anions decreases.

Question 39

How might the anion gap present in patients with diabetes? What other conditions may present in this way?

Answer 39

With diabetes you have increased organic acids (ketoacids) and will cause a decrease in Cl. This will cause an increased anion gap. "MUDPILES": Methanol, Uremia, Diabetes, Propylene glycol, Infection, Lactic acidosis, Ethylene glycol, Salicylates

Question 40

What types of metabolic acidosis will present with a normal anion gap?

Answer 40

Diarrhea and renal tubular acidosis.

Question 41

How do you differentiate between acute and chronic respiratory acidosis and alkalosis?

Answer 41

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Question 42

You see a body builder in clinic and he tells you that he is not feeling well lately. History reveals that he has been overusing diuretics to get into a particular weight loss. How might this affect the acid-base status of this patient?

Answer 42

Extensive use of diuretics causes increased plasma aldosterone and potassium depletion. Both of these things cause H+ secretion at the distal tubule, increased HCO3 reabsorption and will cause metabolic alkalosis.