Acid-base Balance and Ca2+ Homeostasis

Question 1

If the pH is less than ___ or greater than ___ life cannot be sustained because proteins will stop working.

Answer 1

6.8; 7.8

Question 2

Why is too much acid the greatest concern?

Answer 2

because the body takes in (and/or produces) more acid than base

Question 3

What two forms of acid should we be concerned about?

Answer 3

1. volatile acids - these by-products of metabolism can be exhaled or dissipated by the lungs (eg CO2 is an acid because it produces H+ in the water of body fluids)
2. nonvolatile acids - arise from metabolism or the diet (phosporic, latic, and sulfuric acids and ketones)

Question 4

Nonvolatile acids must be neutralized with ___, are regulated by the ___ system, and neutralization requires continual replenishment of ___

Answer 4

HCO3-; renal; HCO3-

Question 5

The amount of acid in the urine should equal the nonvolatile acid load. How does the kidney accomplish this?

Answer 5

By reabsorbing all of the filtered bicarbonate, and then producing enough new bicarbonate to neutralize acids produced by the body.
However, the lowest possible pH for the urine is ~4.4, which represents very little free H+ and this pH of the urine is not sufficient to excrete the nonvolatile acid load. Therefore, the kidney must excrete more acid than can be held in a solution with pH = 4.4.

Question 6

How do buffers function in the urine?

Answer 6

They bind to excess of free H+ to increase acid carried in urine without decreasing pH

Question 7

The kidney can produce ___ to act as a buffer, but it also adds ___ and ___ to the filtrate to increase the amount of H+ excreted

Answer 7

HCO3-; phosphate; ammonium

Phosphate is used first but its supply is limited. Ammonium production within tubular cells eliminates H+ and produces an HCO3- that is reabsorbed

Question 8

___ ___ ___ (NAE) should remove all nonvolatile acid. What are the three components or forms excreted?

Answer 8

free H+, HPO4-; and NH4+

Question 9

What does urine contain?

Answer 9

both titratable acids (H+ and HPO4-) and NH4+.

Question 10

T/F. H+ and HPO4- are not titratable (pKa ~9), however NH4+ can be measured by titration with a base to a pH=7.4.

Answer 10

False, NH4+ is not titratable (pKa ~9), however H+ and HPO4- can be measured by titration with a base to a pH=7.4.

Question 11

T/F. The kidney regulates the acidity of the urine by regulating HCO3- reabsorption. Under normal conditions, the kidney excretes acid equal to the nonvolatile load and replenishes the HCO3- lost due to neutralizing the nonvolatile acids.

Answer 11

True.

Question 12

What are the three options for renal regulation of body pH that will produce urine with different pH?

Answer 12

1. decrease body pH by not reabsorbing all HCO3- to produce an alkaline urine and acidifies body fluids
2. no effect on body pH by reabsorbing all HCO3- (not common)
3. increase body pH by reabsorbing all and producing more HCO3- to produce an acidic urine and alkalinizes body fluids (most common)

Question 13

___ is not directly transported from tubular fluid into the blood so its production and reabsorption results in ___ secretion.

Answer 13

HCO3-; H+

Question 14

Bicarbonate reabsorption begins in the ___ ___ and ___% of filtered HCO3- is reabsorbed here. How?

Answer 14

proximal tubule; 80

1. CA activity in the tubular epithelium produces H+ and HCO3-.
2. H+ is secreted via Na+/H+ antiporter. There is also the H+ ATPase pumps (apical) and H+/K+ ATPase pumps operating to secrete H+
3. HCO3- is transported/reabsorbed across the basolateral membrane

Question 15

T/F. Some HCO3- is reabsorbed in the thick ascending limb of the loop of Henle in a manner similar to the mechanism used in the proximal tubule.

Answer 15

True.

Question 16

T/F. In the late distal tubule, and the collecting duct, CA activity in the tubular epithelium produces H+ and HCO3-

Answer 16

False, In the late distal tubule, and the collecting duct, CA activity in the INTERCALATED CELLS produces H+ and HCO3-.

CA is found in tubular epithelium of the proximal tubule

Question 17

How is secreted H+ buffered in the distal tubule or collecting duct?

Answer 17

by HCO3-, HPO4-; or ammonia (NH3)

Question 18

A less common type of ___ ___ reverses the position of the H+ and HCO3- transporters to the ___ and ___ membranes, respectively. These cells ___ H+ and ___ HCO3-.

Answer 18

intercalated cell; basolateral; apical; reabsorb; secrete

activity of this cell type is normally very low

Question 19

If the body is alkaline, what does the kidney do?

Answer 19

it produces alkaline urine which requires incomplete reabsorption of HCO3- by not neutralizing all the HCO3- that is in the tubular fluid

Question 20

Cells in the ___ tubule can detect intracellular pH and can alter ___ activity accordingly.

Answer 20

proximal; CA

increase pH - decrease CA activity, not reabsorbing HCO3- therefore more excreted

decreased pH - increase CA activity, more HCO3- reabsorbed and less excreted.

Question 21

If the body is acidic, use ___ as an additional buffer to increase excretion of ___.

Answer 21

phosphate (HPO4-); H+

Question 22

After all the HCO3- has been reabsorbed and HPO4- is depleted, what does the kidney do?

Answer 22

it will produce ammonium as a third buffer to increase the amount of H+ that can be excreted.
Note - it is important to keep NH4+ from entering body where it would be converted to urea and produce H+ as a result

Question 23

NH4+ is produced in the ___ tubule by metabolism of ___. The NH4+ that is produced is transported into the tubular fluid and the ___ moves into blood.

Answer 23

proximal; glutamine; HCO3-

Question 24

How is the production of NH4+ produced in the proximal tubular cells regulated?

Answer 24

by systemic pH. Acidosis upregulates synthesis of enzymes for glutamine metabolism, but not that this takes time (as does all protein synthesis). This causes a drop in pH and stimulates reactions to produce more NH4+

Question 25

More NH4+ can be added to the urine by ___ ___.

Answer 25

diffusion trapping

Question 26

T/F. Much of the NH4+ leaving the proximal tubule is reabsorbed by the descending limb of the loop of Henle.

Answer 26

False, Much of the NH4+ leaving the proximal tubule is reabsorbed by the ASCENDING limb of the loop of Henle.

Question 27

NH4+ substitutes for ___ in the Na+/K+/Cl- symporter and enters the ___ fluid in the ___ where it is in equilbrium with NH3.

Answer 27

K+; interstitial; medulla

Question 28

Because it is a ___ molecule, NH4+ is "trapped" in the ___ fluid but NH3 (being ___) can diffuse into the lumen of nearby collecting ducts.

Answer 28

polar; interstitial; nonpolar

Question 29

T/F. Because there is excess H+ in fluid, NH3 in lumen is protonated to NH4+, which is once again trapped and gets eliminated with the urine.

Answer 29

True.

Question 30

T/F. Respiratory system keeps PCO2 constant (takes care of volatile acids) and the kidneys regulate bicarbonate (takes care of nonvolatile acids).

Answer 30

True.

Question 31

T/F. The kidneys typically produce an acidic urine.

Answer 31

True.

Question 32

T/F. Net acid excretion occurs before all filtered bicarbonate is reabsorbed.

Answer 32

False, Net acid excretion occurs AFTER all filtered bicarbonate is reabsorbed.

Question 33

T/F. Additional H+ must be buffered to remove significant amounts of it.

Answer 33

True.

Question 34

T/F. New bicarbonate can be produced in the distal tubule by the production of NH4+ from alanine.

Answer 34

False, New bicarbonate can be produced in the PROXIMAL tubule by the production of NH4+ from GLUTAMINE.

Question 35

T/F. NH4+ produced in the proximal tubule facilitates H+ excretion in the collecting duct as a consequence of diffusion trapping.

Answer 35

True.

Question 36

What condition has a low pH due to CO2 buildup in the lungs due to impaired pulmonary function? How does the kidney compensate?

Answer 36

respiratory acidosis renal produces new HCO3- to increase reabsorption of HCO3-

Question 37

What condition has a high pH due to low PCO2 in blood due to hyperventilation, anxiety, altitude, fever or drugs? How does the kidney compensate?

Answer 37

respiratory alkalosis excrete HCO3-

Question 38

T/F. Metabolic acidosis and alkalosis both involve CO2.

Answer 38

False, they have nothing to do with CO2

Question 39

Metabolic ___ occurs with low pH due to low HCO3- due to diabetic ketosis, diarrhea, and renal failure. How does the respiratory and renal systems respond?

Answer 39

acidosis respiratory: hyperventilate to decrease PCO2, trying to blow off CO2 renal: produce new HCO3-

Question 40

Metabolic ___ has a high pH due to excess HCO3- due to vomiting, antacids, and hemorrhage. How does the respiratory and renal systems compensate?

Answer 40

alkalosis respiratory: hypoventilate to increase PCO2 in the blood to maintain the pH renal: excrete HCO3-

Question 41

What three places and in what capacity is calcium found in the body?

Answer 41

1. intracellular fluid (1%) 2. extracellular fluid (0.1%) - a little more that 1/2 of ECF calcium is free or complexed with anions, and therefore, can be filtered in the glomerulus 3. bone (99%)

Question 42

T/F. Calcium (and phosphate) are important for many cellular processes and are regulated by renal and digestive systems so that gastrointestinal absorption is balanced by renal excretion.

Answer 42

true

Question 43

Why is it important to regulate calcium?

Answer 43

hypocalcemia increases excitability of neural and muscle tissues (tetany) - decrease Ca2+ = decrease threshold of channels hypercalcemia can cause cardiac arrhythmia and disorientation that can lead to death - harder to open voltage gated Na+ channels

Question 44

A ___ in plasma pH will increase the amount of free ___ which can be filtered and excreted, so alkalosis can lead to ___.

Answer 44

decrease; Ca2+; hypocalcemia

Question 45

Calcium is regulated by ___ hormones all of which are regulated by a ___ ___ ___ found in plasma membrane of what cells?

Answer 45

three; calcium sensing receptor (CaSR) 1. parathyroid gland 2. thyroid parafollicular cells 3. cells of the proximal tubule

Question 46

The parathyroid hormone is released in response to ___. What does it do?

Answer 46

hypocalcemia 1. increases bone resorption (liberate Ca2+) 2. increases renal Ca2+ reabsorption 3. stimulates calcitriol production

Question 47

___ (___)'s metabolism is stimulated by hypocalcemia and/or hypophosphatemia. What does it do?

Answer 47

Calcitriol (1,25 dihydroxycitamin D) 1. stimulates active transport mechanism for Ca2+ absorption in the small intestine 2. facilitates action of PTH and increases renal Ca2+ transport

Question 48

When is calcitonin released? What does it do?

Answer 48

It is released in response to hypercalcemia increases bone deposition

Question 49

Describe where are how calcium is reabsorbed in the nephron.

Answer 49

1. proximal tubule - by paracellular transport/solvent drag (70%) 2. thick ascending limb - transcellular and paracellular transport by paracellular NOT solvent drag (20%) 3. distal tubule by transcellular reabsorption, which is regulated because expression of Ca2+ transporters is regulated by PTH. If no PTH, there will be no activated channels to excrete Ca2+. 4. collecting duct - not involved in Ca2+ reabsorption