Distribution of cardiac output

At rest and during light, moderate or heavy exercise the distribution of cardiac output

through specific vascular beds varies. For a typical 70 kg male, the following statements

are true EXCEPT which one?

a) During heavy exercise, the cerebral blood flow is 750 mL/min

b) At rest, renal blood flow is 450 mL/min per 100 g of tissue

c) At rest, skeletal muscle receives 20% of total cardiac output

d) During heavy exercise, coronary blood flow increases eight-fold

e) During heavy exercise, splanchnic blood flow falls to around 1%of total cardiac output

Answer: d

Explanation

The distribution of cardiac output at rest and during exercise is to some extent intuitive,

with blood flow being diverted to rapidly respiring tissues, sacrificed to those tissues

less important during exercise and preserved unchanged to those tissues whose

tolerance of relative ischaemia is poor (i.e. the central nervous system). Beware of

confusing the factors by which cardiovascular and respiratory variables can be elevated.

Minute ventilation can increase 30-fold; however, increase in cardiac output is a

more modest five-fold, from around 5 L/min at rest to a maximum in the region of

25 L/min. The greatest proportional increase in blood flow during exercise is unsurprisingly

through skeletal muscle. At rest, skeletal muscle receives around 1000mL/

min, which although representing 20% of cardiac output when considered relative to

total muscle mass supplied (34 kg) is only 2.9 mL/min per 100 g of tissue. During heavy

exercise, skeletal muscle blood flow can increase 20-fold to 20 L/min (80% of cardiac

output and 58 mL/min per 100 g). Compare this to the kidneys, which at rest also

receive 1000 mL/min (20% of cardiac output), but given their much smaller mass, this

represents 450 mL/min per 100 g. This is second only to the carotid bodies in terms of

blood flow per mass of tissue. During exercise, renal blood flow can fall to 250 mL/min,

at that stage just 1% of cardiac output. Splanchnic blood flow falls similarly. Coronary

blood flow is 250 mL/min at rest and increases to a maximal 1000 mL/min during

heavy exercise (a four-fold increase), thus as a proportion of total cardiac output it

remains fairly consistent, which is also intuitive, given the determinants of coronary

blood flow. As mentioned above, cerebral blood flow remains at 750 mL/min at rest or

during exercise (54 mL/min per 100 g), but obviously as a proportion of total cardiac

output, it falls from 15% at rest to 3% during heavy exercise.