L7 Control Of ECF II Flashcards
When a disturbance is called isometric, hyperosmotic, or hyposmotic, the terms refer to the…
Osmolarity of the ECF
Dehydration is a classic example of ________________.
Hyperosmotic volume contraction
Clinical presentation of Diabetes Insipidus
High plasma osmolality (hyperosmotic volume contraction)
Low urine osmolality
Polyuria (lare volume of dilute urine)
Polydipsia (excessive thirst due to hyperosmotic plasma)
In neurogenic (central) diabetes insipidus, plasma ADH is low due to…
Hypothalamic-pituitary injury
Patient cannot secrete sufficient amounts of ADH
Will respond to exogenous ADH agonists (ie desmopressin)
In nephrogenic diabetes insipidus, the plasma ADH is …
High (since pituitary is functioning normally) but kidneys are unable to respond to it.
Several causes:
• Defect in V2 receptor or elsewhere
• Lithium toxicity
• Hypercalcemia
Acute water load is an example of …
Hyposmotic volume expansion
Excess free water —> decreased plasma osmolality —> decreased plasma ADH —> decreased collecting duct water permeability —> diuresis
Classic medical condition that demonstrates the concept of hyposmotic volume expansion
SIADH: Syndrome of Inappropriate ADH secretion
Head injury and some lung tumors can cause excessive amounts of ADH to be secreted
The results of chronic ECF dilution in SIADH?
HYPONATREMIA
ECF volume may transiently expand, but euvolemia common
Excess renal sodium loss (decreased aldosterone, increased ANP) —> dilution of the ISF but no hypertension
Pathophysiology of SIADH
Super increased ADH —> excess free water retained —> decreased P(osm), hyponatremia, low aldosterone and increased ANP —> increased renal sodium loss —> more hyponatremia
Classic example of hyposmotic volume expansion
Why must plasma Na+ be carefully controlled?
Na+ is the major cation of the ECF and determines the volume of the ECF compartment
Na+ loss is often ______.
Isotonic (diarrhea, vomiting)
Changes in Na+ concentration of ECF generally caused by changes in body water content rather than changes in Na+ content
Hyponatremia is defined as plasma [Na+] of less than ______.
135 mEq/L
Common causes of hyponatremia
Hyponatremia secondary to blood volume depletion
• High ADH: maximal reabsorption of water
• Thirst - ingested water and dilution of ECF [Na+]
Hyponatremia secondary to excessive water conservation (ie SIADH)
Hyponatremia secondary to excessive water intake (water intoxication, exercise associated hyponatremia)
Hypernatremia is defined as plasma [Na+] greater than ______.
145 mEq/L
Causes of hypernatremia
Loss of water (dehydration, diabetes insipidus)
Gain of sodium
These rarely produce PERSISTENT hypernatremia in normal subjects. Excess [Na+] causes hyper osmolarity and triggers thirst. Drinking water will quickly dilute the plasma sodium to normal.
