Physiology | Lesson 40: Renal Sodium and Potassium Balance

Under normal conditions, salt and water losses from the body exactly ________ salt and water gains to the body

Under normal conditions, salt and water losses from the body exactly equal salt and water gains to the body

• NaCl intake = NaCl loss from the body

Na⁺ is reabsorbed by ____________________________ across the basolateral membrane throughout the tubule

Na⁺ is reabsorbed by primary active transport across the basolateral membrane throughout the tubule

Primary active transport creates the ____________________________

Primary active transport creates the ion gradient that powers secondary active transport

Primary active transport (Na⁺/K⁺-ATPase) uses _________________, which pumps ____________ out and _______________ into the cell.

Primary active transport (Na⁺/K⁺-ATPase) uses ATP directly to pump ions against their concentration gradient, which pumps Na⁺ out and K⁺ into the cell.

What is meant by “downhill” entry of Na⁺ into tubular cells?

Sodium moves passively from the tubular lumen into the cell, following its electrochemical gradient created by the Na⁺/K⁺ ATPase pump on the basolateral membrane.

Describe sodium (Na⁺) transport in the proximal tubule

Na⁺ entry is coupled with glucose and amino acids, and H⁺ (via co-transport or counter-transport).

Describe sodium (Na⁺) transport in the loop of Henle

Na⁺ enters with K⁺ and Cl⁻ through the Na⁺–K⁺–2Cl⁻ cotransporter.

Describe sodium (Na⁺) transport in the distal tubule

Na⁺ entry occurs with Cl⁻ via the Na⁺–Cl⁻ cotransporter.

Describe sodium (Na⁺) transport in the collecting duct

Na⁺ enters through epithelial Na⁺ channels (ENaC), regulated by aldosterone.

What are the two most important hormones for regulating Na⁺?

• Aldosterone

• Atrial natriuretic peptide

Aldosterone

• released from the _________________ when _______ is low or __________ is high

• ___________ salt and water reabsorption to __________________ ECFV

• ___________ salt and water excretion to _______________ ECFV

• Causes systemic and renal arteriole ____________ to increase MABP

Aldosterone

• released from the adrenal cortex when Na⁺ is low or K⁺ is high

• increases salt and water reabsorption to increase ECFV

• decreases salt and water excretion to increase ECFV

• Causes systemic and renal arteriole constriction to increase MABP

Atrial Natriuretic Peptide

• released from the ____________ when blood volume or pressure is _________

• _____________ salt and water reabsorption to ___________ ECFV

• _____________ salt and water excretion to ______________ ECFV

• causes systemic and renal arteriole ____________ to decrease MABP

Atrial Natriuretic Peptide

• released from the atria of the heart when blood volume or pressure is high

• decreases salt and water reabsorption to decrease ECFV

• increases salt and water excretion to decrease ECFV

• causes systemic and renal arteriole dialation to decrease MABP

Aldosterone _____________ and ANP _____________ renal Na⁺ reabsorption

Aldosterone increases and ANP decreases renal Na⁺ reabsorption

Steps of Regulation of Aldosterone secretion by the renin-angiotensin system (RAS)

(1) When blood pressure, blood volume, or Na⁺ levels drop, the kidney’s juxtaglomerular (JG) cells release renin.

(2) Renin converts angiotensinogen (from the liver) into angiotensin I.

(3) Angiotensin-converting enzyme (ACE) (mainly in the lungs) converts angiotensin I into angiotensin II.

(4) Angiotensin II stimulates the adrenal cortex to release aldosterone.

(5) Aldosterone increases Na⁺ reabsorption (and K⁺ secretion) in the distal tubule and collecting duct, which raises blood volume and pressure.

What are the stimuli for renin secretion?

o   ­↑  atrial distention & ↑ plasma volume

o   ↓ Blood pressure (detected by renal baroreceptors)

o   ↓ Na⁺ or Cl⁻ concentration at the macula densa

o   ↑ Sympathetic nervous activity (β₁-adrenergic stimulation of JG cells)

When MABP decreases, what reflexes are initiated that lead to changes in Na⁺ reabsorption and ultimately blood volume?

• Baroreceptors (in carotid sinus and aortic arch) sense the drop.

• This activates the sympathetic nervous system, causing:

  • Constriction of afferent arterioles, lowering GFR and reducing Na⁺ and water loss.

  • Increased renin release, activating the renin–angiotensin–aldosterone system (RAAS).

  • Aldosterone then increases Na⁺ reabsorption in the distal nephron.

• Result: Na⁺ and water are retained, raising blood volume and pressure.

When MABP increases, what reflexes are initiated that lead to changes in Na⁺ reabsorption and ultimately blood volume?

• Baroreceptors reduce sympathetic output.

• Afferent arterioles dilate, GFR rises, and Na⁺ excretion increases.

• Atrial natriuretic peptide (ANP) is released, which inhibits Na⁺ reabsorption and suppresses renin and aldosterone.

• Result: Na⁺ and water excretion increase, lowering blood volume and pressure.

Describe the mechanism of potassium (K⁺) secretion in the nephron

K⁺ secretion occurs in the distal tubule and collecting duct, driven by the Na⁺/K⁺ ATPase and K⁺ channels, and is increased by aldosterone, high plasma K⁺, and rapid tubular flow

High plasma K⁺ causes ___________ aldosterone and ____________ K⁺ secretion into urine

High plasma K⁺ causes increased aldosterone and increased K⁺ secretion into urine

Low plasma K⁺ causes ___________ aldosterone and ____________ K⁺ secretion into urine

Low plasma K⁺ causes decreased aldosterone and decreased K⁺ secretion into urine

Kidneys maintain ECFV by regulating the amount of ________ in urine

Kidneys maintain ECFV by regulating the amount of Na⁺ in urine

• ______________ → rate of urinary Na⁺ excretion by kidneys is above usual levels

• ______________ → rate of urinary Na⁺ excretion by kidney is below usual levels

• Natriuresis → rate of urinary Na⁺ excretion by kidneys is above usual levels

• Antinatiuresis → rate of urinary Na⁺ excretion by kidney is below usual levels

The Renin-angiotensin-aldosterone system (RAAS) is activated when _______________

The Renin-angiotensin-aldosterone system (RAAS) is activated when BP or blood volume is low

The Renin-angiotensin-aldosterone system (RAAS) causes ______________________________________

The Renin-angiotensin-aldosterone system (RAAS) causes vasoconstriction and aldosterone release, which increases sodium and water reabsorption → raises blood volume → raises BP

T/F: Kidneys have parasympathetic nervous system innervation

False; kidneys only have sympathetic nervous system innervation

Where does secondary active transport get its energy from?

Secondary active transport uses stored energy from the Na⁺ gradient created by the Na⁺/K⁺ ATPase pump

Ion channels allow _______ diffsuion of ions down their concentration gradient

• Na⁺ channels, K⁺ channels, Cl⁻ channels, Ca²⁺ channels

Ion channels allow passive diffsuion of ions down their concentration gradient

• Na⁺ channels, K⁺ channels, Cl⁻ channels, Ca²⁺ channels

Facilitated diffusion uses _____________, but no _____________

• (i.e. glucose)

Facilitated diffusion uses carrier proteins, but no energy (ATP)

• (i.e. glucose)

In the proximal tubule, Na⁺ moves into the cell ________________ amino acids (secondary active transport)

In the proximal tubule, Na⁺ moves into the cell together with amino acids (secondary active transport)

In the proximal tubule, Na⁺ enters the cell and H⁺ is ______________ into the tubular fluid

In the proximal tubule, Na⁺ enters the cell and H⁺ is secreted into the tubular fluid (antiport)

The ATPase pump uses ATP and maintains _______ Na⁺ inside the cell to allow sodium to keep entering from the luminal side

The ATPase pump uses ATP and maintains low Na⁺ inside the cell to allow sodium to keep entering from the luminal side

T/F: Sodium is never secreted by the nephron epithelial cells

True, sodium only undergoes reabsorption

In the proximal tubule:

• ______________ is reabsorbed

• ______________ is reabsorbed

• ______________ is secreted

In the proximal tubule:

• sodium is reabsorbed

• glucose is reabsorbed

• hydrogen ion is secreted

• _________ membrane → side of the cell facing the tubular fluid

• _________ membrane → side of the cell facing the blood

• luminal membrane → side of the cell facing the tubular fluid

• basolateral membrane → side of the cell facing the blood

__________ and __________ are co-transported with sodium, while __________ is exchanged with sodium

glucose and amino-acids are co-transported with sodium, while hydrogen is exchanged with sodium

Counter-Transport of Sodium and Hydrogen (Na⁺/H⁺ Exchanger)

• H⁺ is secreted into the lumen __________ its concentration gradient

Counter-Transport of Sodium and Hydrogen (Na⁺/H⁺ Exchanger)

• H⁺ is secreted into the lumen against its concentration gradient

__________ and __________ channels are on the luminal membrane of the collecting duct, whereas there are no ______________ channels on the luminal membrane of the proximal tubule

sodium and potassium channels are on the luminal membrane of the collecting duct, whereas there are no sodium channels on the luminal membrane of the proximal tubule

Na⁺ reabsorption is an __________ process occuring in all tubular segments except __________________ (b/c they don’t have any proteins in the luminal membrane that will transport sodium)

Na⁺ reabsorption is an active process occuring in all tubular segments except descending limb of loop of Henle (b/c they don’t have any proteins in the luminal membrane that will transport sodium)

Water reabsorption happens via ___________, but is dependant upon __________________

Water reabsorption happens via osmosis, but is dependant upon Na⁺ reabsorption

Water moves through ______________

• highly expressed in ____________________

• absent in luminal membrane of collecting ducts unless __________ is present

Water moves through aquaporins

• highly expressed in proximal tubule 

• absent in luminal membrane of collecting ducts unless ADH is present

• AQP1 → only found in _________________

• AQP2, AQP3, AQP4 → only found in ______________

• AQP1 → only found in proximal tubule

• AQP2, AQP3, AQP4 → only found in collecting duct

Angiotensinogen circulates in the blood and does noting untul it comes in contact with the protein hormone _____________

Angiotensinogen circulates in the blood and does noting untul it comes in contact with the protein hormone renin

Steps of the Renin-Angiotensin Hormone System

1. Angiotensinogen → does nothing until it comes in contact with renin

2. Renin clips off a 10 amino acid segment called Angiotensin I

3. Angiotensin I does nothing until it comes in contact with the angiotensin converting enzyme

4. Angiotensin converting enzyme will clip off 2 amino acids off of Angiotensin I, creating Angiotensin II

5. Angiotensin II binds to it receptor and increases aldosterone secretion and vasoconstriction, which then causes increased rate of sodium reabsorption

Aldosterone increases _______________ in the collecting duct

Aldosterone increases Na⁺ reabsorption, as well an increase K⁺ secretion and excretion n the collecting duct

____________ secrete renin

Juxtaglomerular cells secrete renin

What does an decrease in plasma volume result in?

What does an increase in plasma volume result in?