Blood Volume Status & Hyponatremia

Blood Volume Status & Hyponatremia

• Sodium excretion is regulated by aldosterone, angiotensin II, and ANP in relation to blood volume.
• Hyponatremia is a common electrolyte disorder defined as a serum/plasma sodium level below 135 mmol/L.
• Clinically significant hyponatremia occurs when levels are below 130 mmol/L.
• Causes of hyponatremia are categorized as dilutional (increased water in the body), depletional (loss of sodium in the body), and artifactual (analytical error):
  • Dilutional Hyponatremia
    • SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) leads to increased water retention due to increased ADH production
    • General edema (swelling) can be caused by conditions like congestive heart failure, cirrhosis, and nephrotic syndrome.
    • Hyperglycemia (high blood sugar) can lead to hyponatremia by drawing water out of the cells and into the vascular space.
  • Depletional Hyponatremia
    • Renal Losses:
      • Diuretics (medications that increase urine production)
      • Hypoaldosteronism (low levels of aldosterone, a hormone that regulates sodium reabsorption)
      • Addison's Disease (a disorder of the adrenal glands)
    • Nonrenal Losses:
      • Gastrointestinal losses (vomiting, diarrhea)
      • Skin losses (burns, trauma, excessive sweating) -Artifactual Hyponatremia:
    • Analytical errors in blood tests can falsely indicate hyponatremia.
    • This is also known as pseudohyponatremia.
• Hyponatremia symptoms can include altered mental status, lethargy, irritability, restlessness, seizures, muscle twitching, hyperreflexes, fever, nausea/vomiting, difficult respiration, and increased thirst.

Sodium

• Sodium is the most abundant extracellular cation, representing 90% of all extracellular cations.
• It plays a major role in determining plasma osmolality.
• Sodium regulation involves three primary processes:
  • Water intake in response to thirst, stimulated or suppressed by plasma osmolality.
  • Water excretion, primarily regulated by ADH release in response to blood volume and osmolality changes.
    • Decreased water intake increases plasma osmolality.
    • ADH minimizes renal water loss.
  • Sodium excretion, regulated by aldosterone, angiotensin II, and ANP in relation to blood volume.

Hypernatremia

• Hypernatremia occurs with a plasma sodium concentration >145 mmol/L.
• It is caused by water loss or sodium gain.
• Symptoms can include altered mental status, lethargy, irritability, restlessness, seizures, muscle twitching, hyperreflexes, fever, nausea/vomiting, difficult respiration, and increased thirst.

Chloride (Cl-)

• Chloride is the major extracellular anion, serving as an enzyme activator and the primary counterion for sodium in the extracellular fluid.
• Maintaining electrical neutrality is crucial:
  • Sodium is reabsorbed with chloride in the proximal tubules.
  • Chloride helps maintain electroneutrality through the chloride shift.
    • When carbon dioxide diffuses from tissues into both plasma and red blood cells, bicarbonate (HCO3-) diffuses out into plasma, and chloride diffuses into the red blood cell to maintain electrical balance.

Urine Formation

• Urine formation occurs through a series of processes:
  • Glomerular filtration - The glomerulus filters blood, producing filtrate.
  • Tubular reabsorption - Substances move from the tubular lumen back into the peritubular capillary plasma.
    • This is an active process for most substances, requiring energy.
  • Tubular secretion - Substances move from the peritubular capillary plasma into the tubular lumen.
    • This can be either active or passive.
  • Excretion - The final urine product is excreted.
• The nephron loop, made up of the thin descending and thick ascending loops of Henle, plays a crucial role in reabsorbing water and solutes.
• The proximal convoluted tubule reabsorbs about 65% of the filtrate.
• The loop of Henle reabsorbs 15%.
• The distal convoluted tubule reabsorbs 15%.
• The collecting duct reabsorbs 4%.
• Urine formation is essentially Glomerular Filtration - Tubular Reabsorption.

Glomerular Filtration Rate (GFR)

• GFR is the volume of blood filtered per minute by the glomerulus.
• It is essential for evaluating renal function.
• Glomerular filtration starts with blood entering the afferent arteriole, passing through the glomerulus (which filters the blood), and exiting through the efferent arteriole.
• Peritubular capillaries extend from the efferent arteriole.
• Only about 20% of the blood entering the afferent arteriole goes through the glomerulus.
• Only about 1 L of blood is filtered per minute, with only the plasma being filtered.
• About 600 mL of plasma are filtered, and roughly 120 mL per minute of filtrate is produced by both kidneys combined.
• 99% of the filtrate is reabsorbed back into the blood.
• Only 1% of the 175 L of filtrate/day is excreted as urine (1.73 L/day).