Sun10October0740PM 7

1 litre of 5% dextrose (d-glucose) is intravenously infused into a normally fit and well 54-year-old female over a period of 10 minutes.

Which one of the following physiological responses is most likely to influence a change in the urine output in this patient?

(Please select 1 option)

Osmotic diuresis

Stimulation of carotid sinus baroreceptors Incorrect answer selected

Inhibition of arginine vasopressin (AVP) secretion This is the correct answer

Inhibition of renin secretion

Stimulation of atrial stretch receptors

Explanation

Basic assumptions are:

Total body water (TBW) is one-third extracellular fluid (ECF) and two-thirds intracellular fluid (ICF).

ECF is one-quarter plasma and three-quarters interstitial fluid (ISF).

The threshold of the volume receptors is 7-10% blood volume change. The osmoreceptors are sensitive to a 1-2% change in osmolality.

Plasma osmolality is normal prior to the transfusion (that is, 287-290 mOsm/kg).

5% dextrose (d-glucose) is isosmotic and an acute infusion of 1000 ml is the same as administering 1000 ml of water because the glucose is taken up into the cells. The water is distributed evenly throughout the body in the proportions of the body compartments (2/3 ICF and 1/3ECF; 25% plasma and 75% interstitial fluid).

Volume increase of plasma will be approximately 2%, below the threshold for activation of the atrial stretch receptors. The osmolality of the plasma will fall by approximately 2.5%, above the threshold. The primary function of arginine vasopressin (AVP) or antidiuretic hormone (ADH) in the body is to regulate extracellular fluid volume by affecting renal handling of water, although it is also a vasoconstrictor and pressor agent (hence, the name "vasopressin"). AVP acts on renal collecting ducts via V2 receptors to increase water permeability, which leads to decreased urine formation (hence, the antidiuretic action of "antidiuretic hormone"). This increases blood volume, cardiac output and arterial pressure.

Five per cent dextrose solution is isotonic and is not sufficiently concentrated to produce an osmotic diuresis.

Specialised cells (macula densa) of distal tubules lie adjacent to the juxta-glomerular cells of the afferent arteriole. The macula densa senses the amount of sodium and chloride ion in the tubular fluid. When NaCl is elevated in the tubular fluid, renin release is inhibited.

The hormonal changes are slower in onset than the physical changes governing glomerulotubular balance. Any potential fall in serum sodium would be accompanied by a stimulus of renin secretion.

Answer Statistics

1

21%

2

10%

3

25%

4

4%

5

42%

Times answered: 254