1. The rate of blood flow to each tissue of the bodyis almost always precisely controlled in relation tothe tissue need. When tissues are active, they need a greatly increased supply of nutrients
and therefore much more blood flow than when at rest—ccasionallyas much as 20 to 30 times the
resting level. Yetthe heart normally cannot increase its cardiac outputmore than four to seven times
greater than restinglevels. Therefore, it is not possible simply to increaseblood flow everywhere in the
body when a particular tissue demands increased flow. Instead, the microvessels of each tissue
continuously monitor tissue needs, such as the availability of oxygen and other nutrients
Systemic
vessels Arteries
–13% Arterioles and capillaries–7%
