4. Regulation of Extracellular Fluid Volume and NaCl Balance

Question 1

How does the body increase sodium reabsorption in times of salt deficit?

Answer 1

Renal sympathetic nervous system activity increases.

Osmolality is decreased, so ADH decreases to compensate.

This causes water diuresis, which decreases blood volume and pressure.

Decreased blood volume / pressure – along with increased renal SNS activity – increases renin production.

Renin increases angiotensin II and aldosterone, which increases sodium reabsorption.

Question 2

When extracellular fluid volume is altered, what are the functions of the components of the RAAS system to correct the issue?

Answer 2

Angiotensin II stimulates sodium reabsorption along the nephron.

Angiotensin II also stimulates ADH release.

Aldosterone stimulates sodium reabsorption primarily in the distal convoluted tubule, the collecting duct, and to a lesser extent the thick ascending loop of Henle.

The RAAS system expands fluid volume.

Question 3

What is the function of the cardiopulmonary baroreceptors?

Answer 3

They affect the brainstem’s vasomotor center in the same way as the arterial baroreceptors, but also influence the hypothalamus to produce ADH.

Question 4

What percentage decrease in blood pressure is necessary to instigate ADH secretion greater than what is necessary to maintain osmolality?

Answer 4

A drop of 20% or greater

Question 5

What three systems are inhibited by atrial natriuretic peptide?

Answer 5

The RAAS system

The ADH system

The SNS system

Question 6

What is the function of the intrarenal baroreceptors?

Answer 6

Major role in the RAAS system

GFR modification

Salt and water reabsorption

Question 7

When an increase in extracellular fluid volume occurs, what are the effects of the natriuretic peptides to correct the issue?

Answer 7

Increase GFR

Decrease renin secretion

Decrease aldosterone secretion indirectly (decreased renin secretion)

Decrease aldosterone secretion directly (acting on the adrenal gland)

Decrease NaCl and water reabsorption by the collecting duct

Decrease ADH secretion, and inhibit ADH action on the distal tubule/collecting duct

Decrease activity of the renal sympathetic nervous system

Natriuretic peptides decrease extracellular fluid volume

Question 8

What is the effect of the body’s response to an increase/decrease in effective circulating volume?

Answer 8

Increased effective circulating volume will increase urine sodium excretion by inhibiting renin, and therefore aldosterone.

Decreased effective circulating volume will decrease urine sodium excretion by stimulating renin, and therefore aldosterone.

Question 9

How does the body eject salt in times of salt excess?

Answer 9

Osmolality is increased, so ADH increases to compensate and renal SNS activity decreases.

This results in reduced water diuresis, which means increased blood volume / blood pressure.

Increase blood volume / blood pressure - as well as decreased renal SNS activity - means decreased renin, and increased atrial natriuretic peptide.

Atrial natriuretic peptide promotes salt ejection, and the inhibition of aldosterone (by ANP) and renin (by BV/BP) inhibits sodium reabsorption.

Question 10

What is actually sensed by the body in times of sodium imbalance?

Answer 10

Effective circulating volume

Question 11

In times of increased/decreased body water content:

What is sensed?

What is the sensor?

What is the effector?

What is affected?

Answer 11

Sensed: Increase/decrease in osmolality.

Sensor: Hypothalmic osmoreceptors.

Effector: ADH.

Affected: Urine osmolality (H2O output) & Thirst

Question 12

Where are the arterial baroreceptors located?

Answer 12

In the aorta and carotid arteries

Question 13

What four things (listed in this lecture) are modified by the brainstem vasomotor center?

Answer 13

Total peripheral resistance

Cardiac performance

Sympathetic “drive” to the kidney

Venous compliance

Question 14

How does the secretion of ADH change with an increase or decrease in sensed blood volume?

Answer 14

An increase in sensed blood volume desensitizes the body to osmolality – requiring greater osmolar changes for the same ADH release.

A decrease in sensed blood volume sensitizes the system, such that the same osmolar change results in a greater ADH response.

Question 15

What percentage of the extracellular fluid (ECF) makes up the effective circulatory volume (ECV)?

About what volume?

Answer 15

5%

(About .7 L)

Question 16

What is the function of the arterial baroreceptors?

Answer 16

Activation of the brainstem vasomotor center only – does not directly affect the hypothalamus

Question 17

What is effective circulatory volume?

What proportion of what compartment is it?

Answer 17

Portion of ECF that is in the arterial system under high enough pressure to effectively perfuse the tissues.

20% of the plasma compartment (plasma = 25% of ECF)

Question 18

Where are the cardiopulmonary baroreceptors located?

Answer 18

In the cardiac atria and pulmonary arteries

Question 19

When a change in the effective circulating volume is detected by the arterial and cardiac baroreceptors, what effectors will cause the body’s response?

Answer 19

RAAS system

Sympathetic nervous system

ADH system

Increased renal fluid retention via Starling’s forces

Question 20

What is the numerical definition of hyponatremia?

Answer 20

Serum sodium of less than 135 mEq/L

AND

Serum osmolality of less than 280 mOsm/kg

Question 21

What three major mechanisms increase renin production?

Answer 21

Perfusion pressure (low perfusion in the afferent arterioles)

Sympathetic nerve activity (sympathetics increase renin)

NaCl delivery to the macula densa (less NaCl delivered means more renin produced)

Question 22

When extracellular fluid volume is decreased, what is the function of the renal sympathetic nerves to correct the problem?

Answer 22

Decrease GFR

Increase renin secretion

Increase sodium reabsorption along the nephron

The sympathetic nervous system decreases urination and expands extracellular fluid volume