Kidney Function and Sodium Balance

Renal Handling of Sodium (Na+)

• Sodium (Na+) is the primary cation in the extracellular fluid, with only 11g present in the intracellular fluid.
• The body contains 92 grams of Na+.
• 35g are in a non-exchangeable form found in bone.
• 46g are in an exchangeable form in the extracellular fluid and are subject to strict homeostasis.

Sodium Intake and Excretion

• The typical dietary intake of Na+ ranges from 4-20 grams per day.
• Increased sodium chloride(NaCl) intake leads to increased sodium excretion in urine, and vice versa.
• Only a small amount of sodium, about 4%, is excreted in sweat and stool.

Functions of Sodium in the Body

• Sodium helps maintain constant volumes of extracellular fluid and blood, which maintains normal ABP (Arterial Blood Pressure).
• Sodium plays a role in the formation of resting membrane potential, action potential, and conduction of nerve impulses.
• Sodium is involved in skeletal and smooth muscle contraction by releasing Ca++ from the sarcoplasmic reticulum.
• Sodium controls the release of vital substances such as renin and aldosterone.
• Sodium is also involved bone formation.

Sodium Reabsorption in Renal Tubules

• Sodium reabsorption is associated with the transport of glucose, amino acids, H2O, H+, Cl-, HCO3-, and K+.
• Reabsorbing Sodium is a major kidney function.
• 80% of oxygen consumption by the kidney is used for both Na+ and Cl- reabsorption.

Sodium Reabsorption in Proximal Convoluted Tubule (PCT)

• 70% of Na+ is reabsorbed in the PCT.
• At the luminal border, Na+ is transported from the lumen to inside cells by facilitated diffusion which is affected by:
• Concentration gradient
• Electrical gradient (lumen -3 mv & inside cell -70 mv)
• Carrier proteins
• At the baso-lateral border, Na+ transport to interstitium fluid is characterized as:
• Primary active transport uses an active Na+-K+ pump.
• Movement against its electrochemical gradient, with 3 Na+ ions pumped out for every 2 K+ ions carried in.
• Maintains intracellular negativity, increasing Na+ entry to the cell, while K+ ions diffuse back to the interstitium, aided by the concentration gradient and cell membrane permeability.

Results of Sodium Reabsorption

• Passive water reabsorption accounts for 70% of water "obligatory water reabsorption" due to the high osmolality created by Na+.
• Passive Cl- diffusion occurs in the second half of the PCT because of increased Cl- concentration.
• Active co-transport supports glucose, amino acids, HCO3- and other organic acids, which are carried by the same carrier as Na+.

Sodium Reabsorption in the Loop of Henle (20%)

• In the thin ascending part, passive reabsorption of Na+ occurs.
• In the thick ascending part, active reabsorption of 20% of Na+ occurs via a co-transport protein carrier, transporting 1Na+, 1K+, and 2Cl-.
• This part is poorly permeable to water, and the fluid leaving this part is hypotonic

Sodium Reabsorption in Distal Convoluting Tubule (DCT) & Collecting Tubules

• 10% of Na+ is actively reabsorbed, in exchange with either H+ or K+, assisted by aldosterone.
• Sodium reabsorption is active along the nephron except in thin ascending part of the loop of Henle.
• The thin descending part is the only part in the nephron where Sodium is not reabsorbed.
• The thin descending part is freely permeable to H2O which results in hypertonic tubular fluid.

Factors Controlling Sodium Reabsorption

• The amount of Na+ excreted per day ranges from 1mEq/day to 400mEq/day.
• Amount of NaCL intake per day affects Na+ reabsorption and excretion
• An increase in intake leads to increase in Na+ reabsorption & excretion, and vice versa.

Hormonal Factors Affecting Sodium Reabsorption

- Aldosterone
- Glucocorticoids
- Sex hormones (estrogens).
- PGE2:
- Endothelins.
- Atrial naturetic peptide (ANP)

• Glomerulotubular balance is an intrinsic mechanism, independent of nervous or hormonal factors, that occurs mainly in the PCT and, to a lesser extent, in the loop of Henle.
• Increased GFR increases the tubular load of any substance, which increases its reabsorption and therefore prevents overloading of the distal tubules with these solutes.
• An ABP above 180mmHg increases Na+ excretion and urine output, known as "pressure diuresis" which is independent of nervous or hormonal effects.

Diuretics

• Osmotic diuretics such as mannitol decreases in Na+ reabsorption from PCT.
• Loop diuretics (Lasix) decreases in Na+ reabsorption from Henle's loop.
• Aldosterone antagonists e.g. Aldactone opposes aldosterone leading to decreases in Na+ reabsorption from DCT.
• Aldosterone is the most important factor for controlling Na+ reabsorption not only in kidney tubules, but also in the intestine and sweat glands.

Events inside PCT

• 70% of Na+ load is reabsorbed.
• 70% of the water load is reabsorbed, known as obligatory water reabsorption.
• Co-transport of K+, glucose, amino acids, & other organic acids occur at the first half.
• Absorption of CL- & secretion of H+ ions occur in the 2nd half.
• Reabsorption & synthesis of NaHCO3 occur.
• The remaining tubular fluid is isotonic (300mosmol) but slightly acidic (pH<7.35).

Renal Handling of Chloride

• Chloride ions are the primary extracellular anion amounting to approximately 104 mEq/L.
• More than 99% of the filtered chloride is reabsorbed.
• Passive reabsorption occurs in the PCT and collecting tubules and is secondary to Na+ reabsorption.
• Active reabsorption occurs in the thick ascending limb of Henle's loop (Na+, K+ & 2 Cl-).

Renal Handling of Potassium

• Potassium (K+) is the primary intracellular cation, with a concentration of 150mEq/L, while its ECF concentration is 4.2mEq/L.
• Accurate regulation of K+ balance is done mainly by the effect of Aldosterone on the kidney.

Potassium Functions

• Controls osmotic pressure inside the cells & regulate intracellular fluid content.
• Maintains electrical activities of the cell as the resting membrane potential & the depolarization & repolarization processes.
• Maintains intracellular pH (acidosis moves K+ outside cell).
• Tissue repair & growth are K+ dependent.
• Helps glucose transport inside cells from extracellular compartments.
• Cardiac contraction, gland secretion and other body functions, which are all preceded by action potential, are K+ dependent.

Potassium Reabsorption

• A K+ load of 760/mEq per day is completely reabsorbed.
• 65% is reabsorbed by the Proximal Convoluted Tubule (PCT).
• 25% is reabsorbed through secondary active co-transport with Na+ & Cl- in the thick ascending limb of the loop of Henle through 1 Na+, 1 K+, and 2Cl-.
• 10% is absorbed from the cortical collecting duct only when aldosterone is absent.

Potassium Secretion and Excretion

• Excreted K+ is derived from secretion rather than from filtration.
• This process is mainly occurring in the Distal Convoluted Tubule (DCT) and cortical collecting tubules.
• K+ is secreted in these sites by the principal cells in exchange with Na+ by counter transport when under the effect of aldosterone.
• At principal cells, Na+, K+ ATPase pumps Na+ outside the basilar membrane & K+ to the inside of the cell, then it diffuses out to the lumen.
• Intracellular migration of Na+ leads to decreased potential difference across the tubular cell which leads to increased movement of K+ into the tubular lumen.
• ↑ H+ secretion and ↓ K+ secretion.

Factors That Increase K+ Excretion

• Increase K+ intake.
• Increased Aldosterone hormone.
• Increase tubular flow rate the distal nephron.
• Increase excretion of anions such as CI-, HCO3-, and HPO4
• Certain diuretics.
• Chronic acidosis decreases in Na+ & water reabsorption by PCT, and increases in tubular flow rate to distal tubules.

Factors That Decrease K+ Excretion

• Decrease K+ intake.
• Decrease Aldosterone hormone.
• Decrease tubular flow rate through the distal portions.
• Endothelins, IL-1, PGE2, ANP.
• Amiloride drug inhibits in Na+-channels.
• Acute Increase in H+ concentration inside the distal convoluted tubules suppresses the activity of Na+ K+ ATPase pump alongside competition for Na+ in the tubular fluid.

Explore kidney function, sodium reabsorption, and factors affecting sodium balance in the body. Learn about glomerulotubular balance, aldosterone antagonists, pressure diuresis, and osmotic diuretics. Understand non-exchangeable sodium and transport mechanisms in renal cells.