Lecture #8: Physiology: Transport of Sodium & Chloride

What is the organizing principle of renal fluid balance?

Sodium sets extracellular fluid volume, while water sets plasma concentration/osmolality; the kidneys regulate these two variables independently through sodium and water excretion.

How does total body sodium primarily affect the body?

Because Na⁺ is the major extracellular osmole, changes in total body sodium primarily change ECF volume; where sodium goes, water follows.

How does total body water primarily affect serum sodium concentration?

Water determines plasma osmolality and serum [Na⁺]; excess retained water dilutes serum sodium, while water loss concentrates it.

How is total body water distributed?

About 60% of body weight is water; approximately 2/3 is intracellular fluid and 1/3 is extracellular fluid.

How is extracellular fluid divided?

About 3/4 of ECF is interstitial fluid and 1/4 is plasma, with a small fraction in transcellular fluids such as CSF and synovial fluid.

Why do changes in total body sodium mainly affect the ECF?

Intracellular Na⁺ is kept relatively stable by the Na⁺/K⁺-ATPase, so body Na⁺ gains and losses occur primarily in the extracellular compartment.

What is the relationship between ECF and ICF osmolality?

ECF and ICF remain in osmotic equilibrium because water shifts between compartments to equalize osmolality.

What is the difference between osmolality and tonicity?

Osmolality is total solute concentration, whereas tonicity describes the effect of solutes on water movement across cell membranes.

How is sodium reabsorbed across renal tubular cells?

Sodium enters through segment-specific apical transporters, exits across the basolateral membrane through the Na⁺/K⁺-ATPase, enters the interstitium, then returns to blood through peritubular capillaries or vasa recta.

Which Na⁺ transporters are important in the proximal tubule?

NHE3, SGLT2, and Na⁺-amino acid cotransporters mediate bulk isosmotic sodium reabsorption in the proximal tubule.

How much filtered Na⁺ and water are reabsorbed in the proximal tubule?

Approximately 65% of filtered sodium and 65% of filtered water are reabsorbed isosmotically in the proximal tubule.

What occurs in the thin descending limb of Henle?

There is essentially no sodium reabsorption; the segment is highly water-permeable through AQP1, so water leaves and tubular fluid becomes concentrated.

What is the major sodium transporter in the thick ascending limb?

NKCC2, the Na⁺-K⁺-2Cl⁻ cotransporter, reabsorbs about 25% of filtered NaCl.

Why is the thick ascending limb called a diluting segment?

It reabsorbs NaCl but is impermeable to water, so salt leaves the tubular fluid while water remains.

Which drugs act on NKCC2?

Loop diuretics inhibit NKCC2 in the thick ascending limb.

What is the major sodium transporter in the distal convoluted tubule?

NCC, the Na⁺-Cl⁻ cotransporter, which reabsorbs about 5% of filtered NaCl.

Which drugs target NCC?

Thiazide diuretics inhibit the Na⁺-Cl⁻ cotransporter in the distal convoluted tubule.

How is Na⁺ reabsorption fine-tuned in the collecting duct?

Aldosterone increases ENaC and basolateral Na⁺/K⁺-ATPase activity in principal cells, increasing Na⁺ reabsorption.

How is water reabsorbed throughout the nephron?

Water moves passively by osmosis only through nephron segments that are water-permeable; it is never actively pumped.

Which aquaporin mediates proximal tubular water reabsorption?

AQP1 allows free water permeability in the proximal tubule.

How much water is reabsorbed in the thin descending limb?

Approximately 10–15%, as water moves through AQP1 into the hyperosmotic medullary interstitium.

Which nephron segments are water-impermeable?

The thick ascending limb and early distal convoluted tubule are water-impermeable.

How is collecting duct water permeability regulated?

ADH controls collecting duct water permeability by causing insertion of AQP2 channels into the apical membrane of principal cells.

What happens to urine when ADH is present?

AQP2 channels are inserted into the collecting duct, water follows the medullary osmotic gradient into the interstitium, and urine becomes concentrated.

What happens to urine when ADH is absent?

The collecting duct remains relatively water-impermeable, so water stays in the tubular lumen and urine becomes dilute.

How is the medullary osmotic gradient generated?

The water-impermeable thick ascending limb pumps NaCl into the medulla, creating a hyperosmotic gradient through countercurrent multiplication.

Why are loop diuretics powerful diuretics?

By blocking NKCC2 in the thick ascending limb, they reduce the medullary osmotic gradient and therefore impair the kidney's ability to reabsorb water and concentrate urine.

What is the long-term relationship between sodium retention and blood pressure?

More Na⁺ retained → more water retained → increased ECF and plasma volume → increased venous return → increased preload → increased stroke volume and cardiac output → increased blood pressure.

What is mean arterial pressure determined by?

MAP = cardiac output × total peripheral resistance.

What determines cardiac output?

Cardiac output = heart rate × stroke volume.

What determines total peripheral resistance?

Primarily systemic arteriolar tone; vasoconstriction raises resistance and pressure, while vasodilation lowers resistance and pressure.

What is effective circulating volume (ECV)?

The portion of the arterial circulation that is physically able to perfuse tissues and is sensed by the body's volume and pressure receptors.

How does effective circulating volume differ from total ECF volume?

They are not the same; a patient can have increased total ECF but decreased effective circulating volume if tissue perfusion is poor, as in heart failure.

What is the main physiologic purpose of RAAS?

RAAS activates when the body senses underfilling or underperfusion and defends effective circulating volume and arterial pressure by retaining Na⁺/water and increasing vascular tone.

What are the three major triggers for renin release?

Decreased afferent arteriolar pressure, decreased NaCl delivery to the macula densa, and increased sympathetic β1 stimulation of juxtaglomerular cells.

Where is renin produced?

Juxtaglomerular/granular cells of the afferent arteriole.

What is the rate-limiting step of RAAS?

Renin release is the rate-limiting step.

What is the sequence of the RAAS cascade?

Liver-derived angiotensinogen → renin converts it to angiotensin I → ACE converts angiotensin I to angiotensin II → angiotensin II produces vascular effects and stimulates aldosterone release.

Where is ACE emphasized as being located in the lecture?

ACE is located primarily on the endothelium of pulmonary vessels.

What is the main active effector hormone of RAAS?

Angiotensin II.

How does angiotensin II help restore blood pressure?

It produces vasoconstriction, increasing total peripheral resistance and arterial pressure, while also stimulating sodium-retaining mechanisms.

How does aldosterone increase sodium retention?

It binds intracellular mineralocorticoid receptors and increases ENaC and basolateral Na⁺/K⁺-ATPase expression in distal nephron principal cells.

What are the major electrolyte effects of aldosterone?

Increased Na⁺ reabsorption with water retention and increased K⁺ excretion; it also increases H⁺ secretion by α-intercalated cells.

How does RAAS turn itself off?

Angiotensin II directly inhibits renin release in a short feedback loop, while restoration of volume and pressure removes the original renin triggers in a long feedback loop.

What are the three timescales of blood-pressure control?

Short-term: arterial baroreflex over seconds to minutes; intermediate: angiotensin II/RAAS over minutes to hours; long-term: renal Na⁺ and water handling over hours to days.

What does the short-term baroreflex regulate?

Carotid sinus and aortic arch stretch receptors rapidly alter sympathetic output to adjust heart rate, contractility, and vascular tone.

Why can the arterial baroreflex not provide long-term blood-pressure control?

It adapts or resets within approximately a day or two and therefore cannot correct a sustained volume problem.

What provides intermediate blood-pressure control?

Angiotensin II raises pressure over minutes to hours mainly through vasoconstriction and increased vascular resistance.

What provides long-term blood-pressure control?

The kidney, by changing actual ECF and blood volume through adjustment of sodium and water excretion.

Why is renal control essential for chronic blood-pressure regulation?

Only the kidney can change how much fluid is actually present in the vascular system, making renal sodium and water balance the ultimate long-term controller of blood pressure.

How do osmoreceptors regulate ADH under normal conditions?

Osmoreceptors sense plasma osmolality; increased osmolality stimulates ADH release, causing water retention and lowering osmolality toward normal.

How do baroreceptors affect ADH secretion?

Reduced pressure or effective circulating volume activates baroreceptor-mediated nonosmotic ADH release to preserve circulating volume and perfusion.

What happens to ADH regulation during a dangerous fall in effective circulating volume?

Baroreceptor input overrides osmoreceptor regulation, causing strong ADH secretion even if plasma is already dilute.

Why can severe volume depletion cause hyponatremia?

Low volume strongly stimulates ADH, causing water retention in excess of solute retention; this dilutes serum Na⁺ and causes hyponatremia.

What additional factors stimulate ADH?

Angiotensin II, pain, severe emotional stress, nausea, and vomiting can stimulate ADH; alcohol inhibits ADH.

Why does angiotensin II promote water retention as well as sodium retention?

Angiotensin II stimulates ADH secretion and thirst/salt appetite in addition to stimulating aldosterone.

Why does congestive heart failure activate RAAS despite total-body fluid overload?

Reduced cardiac output produces a low effective circulating volume, so renal pressure/volume sensors interpret the circulation as underfilled and continue activating RAAS.

Why does RAAS become maladaptive in congestive heart failure?

The low effective circulating volume persists despite increased total ECF, so the negative-feedback trigger never disappears; persistent RAAS activation causes additional Na⁺ and water retention, worsening edema and congestion.

Why can a patient with heart failure be both edematous and hyponatremic?

Heart failure lowers effective circulating volume, triggering RAAS and strong ADH release; sodium and water are retained, but ADH-mediated water retention can exceed sodium retention, causing dilutional hyponatremia despite total-body fluid overload.