Kidney Functions and Filtration Quiz

Kidney Functions

• The kidneys eliminate waste products, regulate extracellular fluid volume, electrolyte content, and pH.
• They receive approximately 1/4 of the cardiac output, filter 125 ml of fluid per minute, and excrete 1.5 L of urine per day.

Glomerular Filtration

• The glomerular filtrate is identical to plasma, except it lacks proteins.
• Factors affecting glomerular filtration rate (GFR) include intra-glomerular hydrostatic pressure, intra-glomerular osmotic pressure, and intra-capsular hydrostatic pressure.

Tubular Reabsorption & Secretion

• Tubular reabsorption is the transport of substances from the tubular lumen to the blood, while tubular secretion is the transport from blood to the tubular lumen.
• The lumenal surface of the tubular cell is the brush border (apical) membrane, while the other sides are the basolateral membrane.
• Tight junctions separate the intercellular space from the lumen.
• Ions and water can move via the transcellular pathway across the tubular cells or the paracellular pathway through tight junctions.

Sodium Reabsorption

• Approximately 67% of Na+ is reabsorbed in the proximal convoluted tubule (PCT).
• Na+ is reabsorbed in exchange for H+ via the Na+/H+ antiporter.
• Intracellular carbonic anhydrase is essential for H+ production.
• Na+ is transported into the blood via Na+/K+ ATPase in the basolateral membrane.
• Na+/H+ antiport is driven by Na+/K+ ATPase.

Bicarbonate Reabsorption

• HCO3- is completely reabsorbed in the PCT.
• HCO3- combines with H+ forming H2CO3, which dissociates into CO2 and water.
• CO2 is passively reabsorbed.

Water Reabsorption

• Water is passively reabsorbed through the paracellular pathway, driven by Na+ reabsorption.
• Na+ secreted by Na+/K+ ATPase raises the osmolality of the lateral intercellular space, driving water movement and Na+ reabsorption by convection.

Chloride Reabsorption

• Cl- is transported into the blood partly via the paracellular pathway and partly in exchange for anions like formate and oxalate.
• The fluid leaving the PCT remains isosmotic to the filtrate entering Bowman's capsule.

Descending Limb of the Loop of Henle

• Water is passively reabsorbed due to the hypertonic interstitial fluid of the medulla.
• The filtrate becomes more concentrated (hyperosmotic).

Ascending Limb of the Loop of Henle

• Approximately 20% of filtered Na+ is reabsorbed.
• Na+, K+, and Cl- enter the cell across the apical membrane via a Na+/K+/2Cl- symporter, powered by Na+/K+ ATPase.
• Na+ is transported into the blood via Na+/K+ ATPase.
• Cl- is transported into the blood via Cl- channels and a K+/Cl- symporter.
• Most of the K+ taken by the Na+/K+/2Cl- symporter returns to the lumen through apical K+ channels.
• The thick ascending limb is impermeable to water, resulting in further dilution of the filtrate.
• The thick ascending limb is the "diluting segment."
• The concentration and dilution of filtrate in the loop of Henle is called the "counter-current multiplier theory."

Distal Convoluted Tubule (DCT)

• Approximately 7% of filtered Na+ is reabsorbed.
• NaCl is reabsorbed without water, further diluting the filtrate.
• NaCl is reabsorbed via a Na+/Cl- symporter, driven by Na+/K+ ATPase.

Calcium Reabsorption

• Ca2+ is transported through channels in the apical membrane and into the blood via a Na+/Ca2+ antiporter.
• Parathyroid hormone (PTH) and calcitriol increase Ca reabsorption.
• Ca2+ is also reabsorbed via the paracellular pathway in the ascending limb.

Hydrogen Ion Secretion

• When all filtered HCO3- is reabsorbed, Na+/H+ exchange continues in the DCT.
• H+ is added to Na2HPO4 to give NaH2PO4 and to NH3 to give NH4+.
• Intracellular carbonic anhydrase is essential for H+ production.

Collecting Tubules

• The collecting tubules consist of principal cells (Na+ reabsorption and K+ secretion) and intercalated cells (H+ secretion).
• Ion and water movement is regulated by hormones:
  • Aldosterone: controls Na+ reabsorption and K+ secretion.
  • Antidiuretic hormone (ADH, vasopressin): controls water reabsorption.
  • ADH binds to V2 receptors in the collecting duct cells, increasing water channel expression and rendering the segment permeable to water.
  • ADH deficiency (diabetes insipidus) can be neurogenic (due to ADH deficiency) or nephrogenic (due to kidney insensitivity to ADH).

Potassium Secretion

• Potassium secretion is coupled to Na+ reabsorption and is increased by:
  • More Na+ reaching the collecting duct (due to diuretics).
  • Diuretics increasing Na+ reabsorption in the collecting duct, leading to K+ loss and hypokalemia.

Aldosterone

• Aldosterone enhances Na+ reabsorption and K+ excretion by:
  • Stimulating Na+/H+ exchange via membrane aldosterone receptors.
  • Activating Na+ channels in the apical membrane.
  • Increasing the number of basolateral Na+/K+ ATPase.

Antidiuretic Hormone (ADH)

• ADH is secreted by the posterior pituitary in response to:
  • Increased blood osmolality, stimulating osmoreceptors in the hypothalamus.
  • Increased blood volume or pressure, inhibiting ADH secretion.
• ADH binds to V2 receptors, increasing water channel expression and allowing passive water reabsorption, resulting in concentrated urine.
• ADH deficiency leads to dilute urine production, polyuria, nocturia, and compensatory polydipsia.
• High ADH concentrations can stimulate V1 receptors in blood vessels, causing vasoconstriction (vasopressin) and increasing blood pressure.

Organic Acid and Base Secretion

• Organic molecules enter the renal tubules by glomerular filtration or are actively secreted directly into the tubules in the PCT.
• Organic anions (A-) are exchanged with α-ketoglutarate via organic anion transporters (OATs) and diffuse into the lumen.
• Organic cations (C+) diffuse into the cell and are actively transported into the lumen in exchange for H+.
• Both systems are powered by Na+/K+ ATPase.

Ammonia Secretion

• Ammonia is formed in the DCT by deamination of glutamine.
• Ammonia diffuses into the lumen and combines with H+ forming NH4+, preventing H+ accumulation in the filtrate.