Kaplan Prep - Fluid and Electrolyte Balance - Problem Set 2

Question 1

What are the salts which are of greatest physiological importance?

Answer 1

The salts which are of greatest physiological importance are Na+, K+, Ca+2 and Mg+2.

Question 2

How are these salts gained by the body?

Answer 2

These salts are obtained from:

ingested foods and fluids.

Question 3

How are these salts lost?

Answer 3

These salts are lost by way of:

defecation, sweating and urination.

Question 4

What is the predominant positively-charged ion in extracellular fluid?

Answer 4

Na+ is the predominant positive ion of the plasma and interstitial fluid (extracellular) compartments

Question 5

What does the statement:

“water follows salt” mean?

Answer 5

When salt is absorbed, water must also be absorbed to maintain osmotic pressure from which comes the familiar statement “water follows salt”.

Question 6

What are the 3 main ways in which sodium balance (and water) is maintained?

Answer 6

There are 3 main ways in which sodium balance (and water) is maintained: (1) the effect of the hormone aldosterone, (2) the cardiovascular baroceptors and (3) the antidiuretic hormone (ADH).

Question 7

Explain, in detail, how the cardiovascular baroceptors work to maintain blood pressure.

Answer 7

The cardiovascular baroceptors monitor and regulate blood volume (which is influenced directly by Na+ ion concentration) to maintain blood pressure. If blood volume (and consequently blood pressure) rises, the baroceptors signal the kidney causing a dramatic increase in the filtration rate, increasing the output of water and Na+ which reduces blood volume to quickly normalize the pressure.

Question 8

Explain, in detail, how Antidiuretic hormone (ADH) maintains Na+ concentration (and water volume) in the extracellular fluid compartment.

Answer 8

Antidiuretic hormone (ADH) is released by the posterior pituitary in response to triggers by the hypothalamus osmoreceptors which have sensed higher Na+ concentration (likely due to a lower water volume) in the extracellular fluid compartment. ADH binds to receptors cells of the collecting ducts of the kidney and promotes reabsorption of water into the blood system. In the absence of ADH (likely due to higher water volume in the extracellular compartment, low Na+ concentration), the collecting ducts allow all water to be freely excreted as very dilute urine.

Question 9

What is hypernatremia?

Answer 9

Sodium ion excess is called hypernatremia

Question 10

What is the one cause of hypernatremia?

Answer 10

Hypernatremia is usually caused by dehydration. (Too much salt, not enough water)

Question 11

What is hyponatremia?

Answer 11

Sodium ion deficiency is called hyponatremia (too low sodium)

Question 12

What are the six causes of hyponatremia?

Answer 12

Hyponatremia can be caused by solute loss and/or water retention due to:
skin burns
excessive sweating
vomiting
diarrhea
renal disease
Addison’s disease (aldosterone deficiency).

Question 13

What are the 2 main ways in which potassium balance is maintained?

Answer 13

The 2 main ways in which potassium balance is maintained are:

1. the collecting ducts
   and
2. the hormone aldosterone.

Question 14

Explain, in detail, how the renal cortical collecting ducts act to maintain potassium balance.

Answer 14

When K+ is high in blood plasma because of high K+ in the diet, the renal cortical collecting ducts reabsorb 90% of the ion, allowing the remainder to be excreted in the urine. When K+ is low in blood plasma because of low K+ in the diet, the intercalated cells of the renal cortical collecting ducts reabsorb greater than 90% of the ion, allowing a smaller amount to be excreted in the urine.

Question 15

Explain in detail, how aldosterone acts to maintain potassium balance.

Answer 15

The adrenal cortex cells which secrete aldosterone are triggered by higher K+ levels to increase the excretion of K+ to maintain potassium balance.

Question 16

What is hyperkalemia?

Answer 16

Hyperkalemia is potassium ion excess

Question 17

Hyperkalemia is potassium ion excess

Answer 17

Hyperkalemia can be due to renal or adrenal gland disease or potassium leaking out of cells into the blood circulation due to severe tissue damage.

Question 18

What is hypokalemia?

Answer 18

Hypokalmia is potassium ion deficiency

Question 19

What are the causes of hypokalemia?

Answer 19

Hypokalemia can be due to diarrhea, vomiting or hyperaldosteronism (over-secretion of aldosterone).

Question 20

What are the 2 main ways in which calcium balance is maintained?

Answer 20

Calcium balance is maintained by the action of calcitonin and parathyroid hormones which cause the reabsorption of 98% of calcium by the kidney, small intestines and bones.

Question 21

Explain, in detail, how calcitonin and parathyroid hormones act to maintain calcium balance.

Answer 21

If plasma calcium levels decrease, the parathyroid glands secrete parathyroid hormone (PTH) which causes a subsequent increase in calcium levels by: (1) activating bone-digesting osteoclasts which release Ca+2 from the bones into the blood, (2) stimulating the small intestine to absorb Ca+2 and (3) increasing the reabsorption of Ca+2 by the renal tubules. In response to increasing plasma calcium levels, calcitonin is released by the thyroid, inhibiting the bone reabsorption action of osteoclasts.

Question 22

What do you call calcium ion excess?

Answer 22

hypercalcemia

Question 23

What do you call calcium ion deficiency?

Answer 23

hypocalcemia

Question 24

What is the cause of hypercalcemia?

Answer 24

Hypercalcemia (Calcium ion excess) can be due to hyperparathyroidism or renal disease.

Question 25

What is the cause of hypocalcemia?

Answer 25

Hypocalcemia (Calcium ion deficiency) can be due to diarrhea, burns, Vitamin D deficiency and alkalosis.

Question 26

Where is a majority of Mg+2 found in the body?

Answer 26

About 60% of all body Mg+2 is found in the skeleton (in the bones)