Sodium and Water Travel Together

• In general, sodium movement defines water movement because sodium is the primary osmotically-active particle in the body.

• Clinical evaluation is crucial for interpreting serum sodium and water balance data

  The sodium concentration may be misleading because in some disorders water and sodium may not be lost or gained proportionally. Clinical evaluation (e.g., assessment of skin turgor, dryness of mucous membranes, thirst and water consumption history, and body weight changes) helps to define dehydration or volume overload.

• Renal management of plasma sodium is primarily dependent on distal tubule function (Na/K/H+ exchange)

• Decreased sodium concentration = Hyponatremia
  • Depletion of both sodium and water occurs with:
    1. Replacement of lost fluid (vomiting, burns, etc.) with hypotonic solutions
    2. Chronic renal disease or diuretic therapy
    3. Ketoacidosis (diabetic or starvation)
    4. Low steroid conditions (Addison's disease or steroid withdrawal)

  • Excess water (with dilution of sodium) accumulates in:
    1. Congestive heart failure
    2. Cirrhosis
    3. Nephrotic syndrome
    4. Acute renal failure with oliguria

  • Inappropriate ADH Syndrome will be discussed in the lecture on pituitry diseases

• Increased sodium concentration = Hypernatremia
  1. Poor fluid intake or excess loss from skin (sweating, burns)

  2. Water loss from the kidney (osmotic diabetes)

  3. GI loss (vomiting, diarrhea)

  4. Diabetes insipidus will be discussed in the pituitary lecture

• Evaluate water balance problems by comparing:
  • Serum sodium with urine sodium

  • Serum osmolality with urine osmolality

    If serum sodium or osmolality is low, then urine sodium and osmolality should be very low because the kidney would normally retain sodium in response to the deficit in the body. The opposite should occur with high serum sodium or osmolality. If this is the case, the kidney is responding appropriately to the physiologic problem and the genesis of the problem probably isn't renal. If the kidney does not conserve or excrete sodium appropriately, then consider a primary problem in the kidney.

  • And careful clinical examination