Sun21November0735PM 9

A 23-year-old man suddenly loses 1000 ml blood from a stab wound in the thigh.

Which one of the following is the most effective initial physiological response to a fall in the intravascular blood volume in this man?

(Please select 1 option)

Venoconstriction This is the correct answer

Arteriolar constriction Incorrect answer selected

Sodium and water retention

Transcapillary absorption of interstitial fluid

Increased albumin synthesis

Explanation

The veins of the body contain 70% of the circulating blood volume, in contrast to the 15% in the arterial system.

Veins act as a reservoir, and venous tone is important in maintaining the return of blood to the heart, for example in severe haemorrhage, when sympathetic stimulation causes venoconstriction.

The liver receives approximately 30% of resting cardiac output and is therefore a very vascular organ. The hepatic vascular system is dynamic, meaning that it has considerable ability both to store and release blood - it functions as a reservoir within the general circulation.

In the normal situation, 10-15% of the total blood volume is in the liver, with roughly 60% of that in the sinusoids. When blood is lost, the liver dynamically adjusts its blood volume and can eject enough blood to compensate for a moderate amount of haemorrhage.

The sympathetic nerves constrict the presinusoidal resistance vessels in the portal venous and hepatic arterial systems. More importantly, sympathetic stimulation causes a marked reduction in the capacitance of the portal system and helps to divert blood towards the heart.

During haemorrhage net transcapillary absorption of interstitial fluid from skeletal muscle into the intravascular space compensates effectively for the blood loss. This absorption of fluid is mainly linked to decrease of the capillary hydrostatic pressure (Pc), as caused by reflex adrenergic readjustment of the ratio of pre- to postcapillary resistance. These fluid shifts become significant within a few hours after blood loss further diluting haemoglobin and plasma proteins.

Increased albumin synthesis begins at approximately 48 hours.

Renin is released by the juxtamedullary complex in response to decreased mean arterial pressure, leading to increased aldosterone levels and eventually to sodium and water resorption. Increased levels of antidiuretic hormone (ADH) further contribute to the retention of water.

Answer Statistics

1

49%

2

45%

3

6%

4

2%

Times answered: 268