How does the body increase sodium reabsorption in times of salt deficit?
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.
When extracellular fluid volume is altered, what are the functions of the components of the RAAS system to correct the issue?
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.
What is the function of the cardiopulmonary baroreceptors?
They affect the brainstem’s vasomotor center in the same way as the arterial baroreceptors, but also influence the hypothalamus to produce ADH.
What percentage decrease in blood pressure is necessary to instigate ADH secretion greater than what is necessary to maintain osmolality?
A drop of 20% or greater
What three systems are inhibited by atrial natriuretic peptide?
The RAAS system
The ADH system
The SNS system
What is the function of the intrarenal baroreceptors?
Major role in the RAAS system
GFR modification
Salt and water reabsorption
When an increase in extracellular fluid volume occurs, what are the effects of the natriuretic peptides to correct the issue?
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
What is the effect of the body’s response to an increase/decrease in effective circulating volume?
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.
How does the body eject salt in times of salt excess?
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.
What is actually sensed by the body in times of sodium imbalance?
Effective circulating volume
In times of increased/decreased body water content:
What is sensed?
What is the sensor?
What is the effector?
What is affected?
Sensed: Increase/decrease in osmolality.
Sensor: Hypothalmic osmoreceptors.
Effector: ADH.
Affected: Urine osmolality (H2O output) & Thirst
Where are the arterial baroreceptors located?
In the aorta and carotid arteries
What four things (listed in this lecture) are modified by the brainstem vasomotor center?
Total peripheral resistance
Cardiac performance
Sympathetic “drive” to the kidney
Venous compliance
How does the secretion of ADH change with an increase or decrease in sensed blood volume?
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.
What percentage of the extracellular fluid (ECF) makes up the effective circulatory volume (ECV)?
About what volume?
5%
(About .7 L)
What is the function of the arterial baroreceptors?
Activation of the brainstem vasomotor center only – does not directly affect the hypothalamus
What is effective circulatory volume?
What proportion of what compartment is it?
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)
Where are the cardiopulmonary baroreceptors located?
In the cardiac atria and pulmonary arteries
When a change in the effective circulating volume is detected by the arterial and cardiac baroreceptors, what effectors will cause the body’s response?
RAAS system
Sympathetic nervous system
ADH system
Increased renal fluid retention via Starling’s forces
What is the numerical definition of hyponatremia?
Serum sodium of less than 135 mEq/L
AND
Serum osmolality of less than 280 mOsm/kg
What three major mechanisms increase renin production?
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)
When extracellular fluid volume is decreased, what is the function of the renal sympathetic nerves to correct the problem?
Decrease GFR
Increase renin secretion
Increase sodium reabsorption along the nephron
The sympathetic nervous system decreases urination and expands extracellular fluid volume
