Renal V

Question 1

What is the most abundant intracellular ion in the renal system?

Answer 1

Potassium

Question 2

What is the role of the K concentration in extracellular fluid?

Answer 2

It is important for the function of excitable tissues (nerve and muscle), as their resting potentials are related to relative intracellular and extracellular K concentrations.

Question 3

What is the typical range of extracellular potassium?

Answer 3

3.5 - 5 mEq/L

Question 4

Too much K in the extracellular fluid is called […], which occurs at a K concentration of […]

Answer 4

Hyperkalemia, >5 mEq/L

Question 5

Not enough K in the extracellular fluid is called […], which occurs at a K concentration of […]

Answer 5

Hypokalemia, <3.5 mEq/L

Question 6

Describe how hyperkalemia affects the electrocardiogram.

Answer 6

When potassium gets a little high, the T-wave gets higher. This is called tent T or tall T. As potassium increases byeond 8.0 mEq/L, the QRS complex starts looking almost sinosoidal - this is the pre-arrest arrythmia.

Question 7

Describe our sources of potassium intake.

Answer 7

It comes from dietary intake only.

Question 8

Name 6 examples of potassium-rich foods.

Answer 8

Bananas, tropical fruits, orange juice, nuts, potatoes, tomatoes

Question 9

Describe how potassium leaves the body. Which ways are regulatable?

Answer 9

90% excreted into urine and 10% excreted into feces and sweat. Only the urine is regulatable.

Question 10

Describe K in terms of filtration, reabsorption, and secretion in the tubules.

Answer 10

• Freely filtered at the glomerulus
• Tubules absorb most filtered K
• K can also be secreted at the cortical collecting ducts

Question 11

Where and how do changes in K excretion amounts take place?

Answer 11

Changes in K secretion are due mainly to changes in K secretion in the cortical collecting duct (some in the distal convoluted tubule).

Question 12

Is potassium net reabsorbed or net secreted? What is the typical range of values?

Answer 12

It is net reabsorbed. The range varies between 15% and 99%, but the typical value is around 86%.

Question 13

Explain how K+ gets secreted in the cortical collecting ducts.

Answer 13

It is coupled with Na+ reabsorption. On the basolateral membrane the tubular cell, K+ is transported into the cell via Na+/K+-ATPase. It then diffuses into the tubule via K+ ion channel.

Question 14

Potassium secretion is regulated by what 2 mechanisms?

Answer 14

1. Dietary intake of potassium
2. Aldosterone

Question 15

Describe the mechanism of K+ secretion by dietary intake and aldosterone.

Answer 15

• Increased potassium intake
• Increased plasma potassium
• Increased aldosterone secretion in adrenal cortex
• Increased plasma aldosterone
• Increased potassium secretion in cortical collecting ducts
• Increased potassium excretion

Question 16

Describe the regulation of K+ secretion by the RAA system.

Answer 16

• Decreased plasma volume
• Increased plasma angiotensin II
• Increased aldosterone secretion from adrenal cortex
• Increased plasma aldosterone
• Increased sodium reabsorption and increased potassium secretion by cortical collecting ducts
• Decreased sodium excretion and increased potassium excretion

Question 17

What is hyperaldosteronism?

Answer 17

When adrenal hormone aldosterone is released in excess.

Question 18

What is the most common cause of hyperaldosteronism?

Answer 18

Adenoma (benign tumour) of the adrenal gland that produces aldosterone autonomously. Here, there is no change in volume involved.

Question 19

What are the major symptoms of hyperaldosteronism?

Answer 19

Increased fluid volume, hypertension, hypokalemia, suppressed renin, metabolic alkalosis

Question 20

To what extent are hydrogen ions regulated in the renal system?

Answer 20

The hydrogen ion concentration is very tightly regulated.

Question 21

What is the typical H+ concentration (and pH) of the extracellular fluid?

Answer 21

40 nmol/L (7.4) (note very small quantity)

Question 22

Describe the important mass reaction involved in H+ regulation.

Answer 22

CO2 + H2O <–> H2CO3 <–> HCO3- + H+
First part of reaction done by carbonic anhydrase.

Question 23

When the body loses a bicarbonate ion, it is equivalent to […]

Answer 23

Gaining a hydrogen ion, because the mass reaction will shift to the right.

Question 24

When the body gains a bicarbonate ion, it is equivalent to […]

Answer 24

Losing a hydrogen ion, because the mass reaction will shift to the left.

Question 25

Name 4 sources of hydrogen ion gain.

Answer 25

1. Generation of hydrogen ions from CO2
2. Production of nonvolatile acids from the metabolism of protein and other organic molecules
3. Gain of hydrogen ions due to loss of bicarbonate in diarrhea or other nongastric GI fluids
4. Gain of hydrogen ions due to loss of bicarbonate in the urine

Question 26

Name 4 sources of hydrogen ion loss.

Answer 26

1. Utilization of hydrogen ions in the metabolism of various organic anions
2. Loss of hydrogen ions in vomitus
3. Loss of hydrogen ions in the urine
4. Hyperventilation (loss of CO2)

Question 27

Explain how the addition of CO2 causes hydrogen ion gain.

Answer 27

If you add CO2, the mass reaction shifts to the right, which adds H+ to the body.

Question 28

Give 3 examples of nonvolatile acids.

Answer 28

Phosphoric acid, sulfuric acid, lactic acid

Question 29

How much H+ do nonvolatile acids add per day? What effect does this have?

Answer 29

Around 40-80 mmol of H+ per day. This is much more than the nmol range that H+ sits in, so the kidney has to buffer these nonvolatile acids.

Question 30

Name the major extracellular and intracellular buffer of H+.

Answer 30

Extracellular: CO2/HCO3- system
Intracellular: phosphates and proteins

Question 31

The ultimate balance of hydrogen ions is determined by […]

Answer 31

The respiratory system (by controlling CO2) and the kidneys (by controlling HCO3-)

Question 32

What is the difference between alkalosis and acidosis?

Answer 32

Alkalosis = low H+, high pH in the blood
Acidosis = high H+, low pH in the blood

Question 33

The renal mechanism of hydrogen ion control occurs via control of […]

Answer 33

HCO3-

Question 34

What is the renal response to alkalosis?

Answer 34

Kidneys excrete HCO3- (same effect as adding H+ to the system)

Question 35

What is the renal response to acidosis?

Answer 35

Kidneys produce new HCO3- and add to to he plasma (same effect as removing H+ from the system)

Question 36

Describe the purpose of the Henderson-Hasselbalch equation.

Answer 36

It depicts the relationship between HCO3- and CO2: pH = [HCO3-]/[CO2]. To maintain the pH constant, they therefore both have to move in the same direction.

Question 37

What is the formula for HCO3- excretion?

Answer 37

HCO3- filtered - HCO3 reabsorbed (+ HCO3 secreted)
The secretion is in parentheses because it only happens in extreme cases of alkalosis.

Question 38

Describe the breakdown between HCO3- reabsorption and secretion.

Answer 38

Normally, the kidneys reabsorb all filtered HCO3-. The exception is in extreme alkalosis, where it can be secreted.

Question 39

Explain how bicarbonate reabsorption works in the proximal tubule.

Answer 39

In the tubular epithelial cell, H2O and CO2 combine via carbonic anhydrase to form H2CO3. This then dissociates into H+ and HCO3-.

The H+ flow into the tubular via H+ ion channel, H+/K+-ATPase, or Na+/H+ antiporter. It combines with filtered HCO3- from the glomerulus to form H2O and CO2.

The HCO- goes through an ion channel into the interstitial fluid.

The end result is that HCO3- left the tubular lumen and one HCO3- entered the interstitium, hence reabsorption.

Question 40

Describe the breakdown of where bicarbonate reabsorption occurs in the tubule.

Answer 40

Proximal tubule: 80%
Thick ascending limb of Henle’s loop: 15%
Distal tubule and CCD: 5%
After CCD: little to none

Question 41

How much HCO3- is typically present in the urine by the time it reaches the ureter?

Answer 41

Normally none unless the body has alkalosis.

Question 42

Besides HCO3- reabsorption, name 2 ways in which addition of new HCO3- to the plasma occurs.

Answer 42

1. H+ secretion and excretion on nonbicarbonate buffers (such as phosphate).
2. By glutamine metabolism with NH4+ excretion

Both of these processes can be viewed as H+ excretion by the kidney.

Question 43

When does H+ secretion and excretion by nonbicarbonate buffers occur?

Answer 43

This occurs only when all the HCO3- has been reabsorbed and is no longer available in the lumen.

Question 44

Where does H+ secretion and excretion by nonbicarbonate buffers occur?

Answer 44

It starts towards the end of the proximal tubule.

Question 45

Explain how H+ secretion and excretion by nonbicarbonate buffers occur.

Answer 45

H2O and CO2 in the tubular epithelial cell get converted to H2CO3 by carbonic anhydrase, which then dissociates into H+ and HCO3-.

The HCO3- goes into the interstitium.

The H+ goes into the tubular lumen and combines with monohydrogen phosphate (HPO42-) to form H2PO4-. This H2PO4- is excreted.

Question 46

Where does glutamine metabolism with NH4+ excretion occur?

Answer 46

It occurs mainly in the proximal tubule.

Question 47

Explain how glutamine metabolism with NH4+ excretion works in the proximal tubule.

Answer 47

Glutamine enters the tubular epithelial cell from the blood (channel) and from the tubular lumen (cotransporter with Na+). Once in the cell, glutamine splits into NH4+ and HCO3-.

The HCO- goes into the interstitial fluid.

The NH4+ goes into the tubular lumen via an antiporter with Na+ on the apical side.

The end result is the addition of more HCO3- to the blood.