SODIUM BALANCE

Dr.Mohammed Sharique Ahmed QuadriAssistant prof. Physiology

Al Maarefa College

OBJECTIVES • Describe the overall handling of Na+ by nephron.• Discriminate the mechanism of reabsorption of Na+ in

different parts of nephron • Describe the tubular reabsorption of water• Explain the concept of tubule glomerular feedback

and glomerulo tubular balance • Discuss how various drugs affect the reabsorption of

sodium (Diuretics).• Illustrate the renal mechanism of regulation of Na+

excretion.(effect of sympathetic , ANP, Starlings forces,& renin-angiotensin-aldosterone mechanism )

Sodium Balance

• Na+ is major cation in ECF

Amount of sodium in ECF

Volume of ECF

Plasma volume

Blood volume

Blood pressure

Sodium content VS Sodium concentration

• Na+ concentration is determined not only by the amount of Na+ present but also by the volume of water.

• For example, – a person can have an increased Na+ content but a

normal Na+ concentration (if water content is increased proportionately).

– Or, a person can have an increased Na+ concentration with a normal Na+ content (if water content is decreased).

OVERALL HANDLING OF NA+

Na+ Reabsorption• An active Na+ - K+ ATPase

pump in basolateral membrane is essential for Na+ reabsorption

• Of total energy spent by kidneys, 80% is used for Na+ transport

• Na+ is not reabsorbed in the descending limb of the loop of Henle

• Water follows reabsorbed sodium by osmosis which has a main effect on blood volume and blood pressure

Tubule area % of Na+

reabsorbed

Role of Na+ reabsorption

Proximal tubule 67% Plays role in reabsorbing glucose, amino acids, H2O, Cl-, and urea

Ascending limb of the loop of Henle

25% Plays critical role in kidneys’ ability to produce urine of varying concentrations

Distal and collecting tubules

8% Variable and subject to hormonal control; plays role in regulating ECF volume

Diuretics that act here:- a)Osmotic diuretics – Mannitol,Urea etc b)Carbonic Anhydrase Inhibitors - Azetazolamide

Cellular mechanisms of Na+ reabsorption in the early proximal tubule

Cellular mechanisms of Na+ reabsorption in the late proximal tubule

Isosmotic Reabsorption• Na+ enters the cell across the luminal membrane

water follows the solute to maintain isosmolarity.

• Na+ is pumped out of the cell by the Na+-K+ ATPase, at basolateral membranes, water again follows passively.

• Isosmotic fluid accumulates in lateral intercellular space between the proximal tubule cells.

• This isosmotic fluid is reabsorbed in peritubular capillary blood due to high oncotic pressure in peritubular capillaries (πc) .

Mechanism of isosmotic reabsorption in the proximal tubule

Regulation of Tubular Reabsorption

Glomerulotubular Balance

• Tubular reabsorption increases/decreases automatically as the filtered load increases/ decreases.

• Glomerulotubular balance ensures that a constant fraction of the filtered load is reabsorbed by the proximal tubule, even if the filtered load increases or decreases.

• Glomerulotubular balance ensures that normally 67% of the filtered Na+ and water is reabsorbed in the proximal tubule

Glomerulotubular Balance

Increase in GFR

Increase in filtration fraction Increase in concentration of protein in

peritubular capillaries

Increase in πc

Increase reabsorption in proximal tubule

Effects of ECF volume expansion & ECF volume contraction on isosmotic fluid reabsorption in the

proximal tubule

PROXIMAL TUBULEPoints to ponder

• Reabsorbs 67% of the filtered Na+. • Reabsorbs 67% of the filtered water. • The tight coupling between Na+ and water

reabsorption is called isosmotic reabsorption.• This bulk reabsorption of Na+ and water (the

major constituents of ECF) is critically important for maintaining ECF volume.

• The site of glomerulotubular balance, a mechanism for coupling reabsorption to the GFR.

Cellular mechanism of Na+ reabsorption in the early distal tubule

Thick Ascending Limb of Loop of Henle(points to ponder)

• Load dependent reabsorption

• Electrogenic– The role of lumen possitive potential is

driving the reabsorption of Mg & Ca

• Impermeable to water

Cellular mechanism of Na+ reabsorption in the early distal tubule

EARLY DISTAL TUBULE

• IMPERMEABLE TO WATER • CORTICAL DILUTING SEGMENT

Late distal tubule

• Principal cell :Na+ reabsorption & K+ secretion

• α- inetercalted cells K+ reabsorption & H+ secretion

Late distal tubule(points to ponder)

• Na+ reabsorption in these segments is hormonally regulated. Aldosterone is a steroid hormone that acts directly on the principal cells to increase Na+ reabsorption

• Water permeability of principal cells is controlled by ADH

Regulation of sodium balance

• EABV: Portion of ECF that is in arteries & is the volume effectively perfusing the tissues

• The renal mechanism to regulate the Na excretion are – Sympathetic nerve activity– Atriopeptin (ANP)– Starling’s forces in peritubular capillaries– Renin-angiotensin aldosterone system

– Sympathetic nerve activityIn response to dec arterial pressureV.C of aff arteriole & inc Na reabsorption in proximal

tubule

– Starlinf forcess in peritubular capillaries

– Renin-angiotensin aldosterone system – In respose to decrease in arterial pressure– Angiotensin II inc reabsorption of Na in proximal

tubule – Aldosterne inc reabsorption of Na in distal tubule

& collecting ducts

RAAS

– Atriopeptin (ANP)

• In rsponse to increase in ECF volume • V.D of aff & V.C OF eff arteriol• Incease GFR & dec reabsorption of Na

References

• Human physiology by Lauralee Sherwood, seventh edition

• Text book of physiology by Linda .s contanzo,third edition

• Text book physiology by Guyton &Hall,11th edition