Shonika has just donated a pint of blood. (Hint: Remember Starling’s Law) A. Bef

Question

Shonika has just donated a pint of blood. (Hint: Remember Starling’s Law)

A.        Before her body has time to begin any intentional homeostatic attempts, will this cause her heart rate to increase, not change, or decrease? (½)

B.        Before her body has time to begin any intentional homeostatic attempts, will this cause her stroke volume to increase, not change, or decrease? (½)

C.        Before her body has time to begin any intentional homeostatic attempts, will this cause her cardiac output to increase, not change, or decrease? (½)

D.        After her body has time to initiate the appropriate homeostatic attempts, will this cause her heart rate to increase, not change, or decrease? (½)

Explanation / Answer

1. Donation of 1 point of blood means that she has donated 350 ml blood out of 5 L present in the circulatory system which roughly equals to 13% of blood present in body. In this case heart rate DOES NOT CHANGE as homeostatic mechanisms have not begun yet.

2. Stroke volume = cardiac output/ heart rate

In this case stroke volume DECREASES because of decrease in VENOUS RETURN as a result of which PRELOAD DECREASES and according to Frank starling law, the force of contraction decreases

3. Cardiac output= stroke volume * heart rate.

In this case since blood quantity is reduced, it causes a decrease in VENOUS RETURN which decreases the PRELOAD hence as per the Frank starling law, cardiac output DECREASES.

4.After initiation of homeostatic mechanism, SYMPATHETIC SYSTEM becomes dominant. Hence heart rate INCREASES.

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