Na+ Homeostasis
Sodium reabsorption by the nephron
1%
3%
6%65%
25%
Percentages give the proportionfrom filtered load reabsorbed
Normally, only 1% or less is excreted
But this 1% is a significant quantity
Na+ Balance
Na+ balance will be maintained if the following is true
Intake = Excretion
OR
If Na+ content is high, increase excretionIf Na+ content is low, increase reabsorption
Na+ Content of the Body
Na+ content of the body can not be detected
What can be measured are: Concentration of in ECF, strongly correlated to ECF osmolality
as it is the main solute in ECF ECF volume, which correlated with blood volume and
arterial blood pressure
Na+ Content of the Body
Would changes in Na+ content of the body primarily result in changes in:
? Osmolality of ECF
? Volume of ECF
Mechanisms of Na+ Homeostasis
These mechanisms are responsive mostly to Change in ECF volume and its consequences which are Change in blood volume Change in blood pressure
Response of Na+ regulatory mechanisms to change is osmolality is minimalNa+ Homeostatic are directed towards regulateing blood volume and arterial blood pressure
Total Vs Effective Blood Volume
Effective blood volume – volume of blood available for perfusion, the blood volume contained in the arterial system
Total blood volume – volume of blood in the whole circulatory system
Na+ homeostatic mechanisms are linked to the effective blood volume
Total Vs Effective Blood Volume
Can the effective blood volume change when the total blood volume remains the same?
Yes, due to changes in venous capacityExample: “septic shock” blood vessels are dilated, effective blood volume
decreased but total blood volume is normal
Glomerular Mechanisms
Raised Blood Volume Raised Blood Pressure
Increased Renal Blood Flow
Increased Glom CapillaryHydrostatic Pressure
Increased GFR
Trend Towards Increased Na+ and H2O excretion
Increased Glom CapillaryOncotic Pressure,
Glomerular Mechanisms
Raised Blood Volume Raised Blood Pressure
Increased Renal Blood Flow
Increased Glomerular CapillaryHydrostatic Pressure
Increased GFR
Trend Towards Increased Na+ and H2O excretion
Baroreceptor mechanism
Glomerular arteriolar dilatation (mostly afferent)
Decreased Glomerular CapillaryOncotic Pressure
Glomerular Mechanisms
Raised Blood Volume
Atrial Distention
Atrial Natriuretic Peptide
Increased GFR – dilatation of afferent arteriole
Proximal Tubular Mechanisms
1. Increased delivery of Na+ to the proximal tubule
Proximal tubular reabsorption of Na+ is on a percentage basis Total reabsorbed in increased but the total unabsorbed by
the proximal tubule is also increased
More Na leaves the proximal tubule and enters the loop of Henle
Proximal Tubular Mechanisms
2. Status of the peritubular capillaries
Higher hydrostatic pressure in peritubular capillaries Lower oncotic pressure in the peritubular capillariesReabsorption is inhibited
Minor decrease in the percentage of Na+ absorbed in the proximal tubuleBut constitutes a significant decrease in quantity if Na+ reabsorbed
Proximal Tubular Mechanisms
3. Increased interstitial pressure
Higher arterial pressure results in higher medullary interstitial pressure
Reabsorption is inhibited – mediated by a reduction of Na+K+ATPase activity
Proximal Tubular Mechanisms
4. Flow rate (minor effect only)
Higher GFR will increase flow rate in the tubule Less Na+ reabsorption takes place
Loop of Henle
No significant Na+ regulatory mechanism in Loop of Henle
JGA & Distal Tubule
Major contributor to Na+ homeostasis1. JUXTAGLOMERULAR APPARATUS Sensor function
renal arteriolar pressure distal tubular flow
Secretion of renin
2. DISTAL TUBULE Modulation of Na+ reabsorption
Juxtaglomerular Apparatus
Sensory function1. Granular cells of afferent arteriole – sensitive to pressure
within the arteriole High pressure – renin secretion is inhibited
2. Macula Densa – sensitive to fluid flow rateNaCl flow rate
High flow rate – renin secretion is reduced
Renin Secretion
Renin secretion is by afferent arteriolar granular cells
STIMULI1. Granular cells themselves2. Macula densa3. Renal sympathetic nerves (through beta 1 receptors)
Renin Secretion
High blood volume High blood pressure
Distension of afferent arteriole
Raised GFR & reduced proximal
tubular reabsorption
High distal tubular flow rate
Inhibition of sympathetic nerves
Inhibition of renin secretion
Renin Secretion
Renin secretion is
Inhibited by high effective blood volume and high blood pressure
Increased when effective blood volume and blood pressure are low
Renin Angiotensin Aldosterone System
Angiotensinogen
Angiotensin I
Angiotensin II
Arteriolar constriction
Aldosterone secretion
Renin
Increase distal tubularNa+ reabsorption
Decrease GFR
Renin activates mechanisms that increase Na+ and water retention
ADH secretion
Angiotensin converting
enzyme
Overview of Na+ Homeostasis
Changes in effective blood volume and blood pressure
Arteriolar changes, hydrostatic & oncotic pressure changes, endocrine responses
Responses by glomerulus, proximal tubule and distal tubule
Modulation of Na+ excretion
Compensation of changes in effective blood volume and blood pressure