Question

Compare the decrease in muscle contractions with and without blood flow. Why is there a difference?

Answer 1

Contracting muscle consumes large amounts of oxygen to replenish ATP that is hydrolyzed during contraction; therefore, contracting muscle needs to be able to increase its blood flow and oxygen delivery in order to support its metabolic and contractile activities. Second, about 20% of the cardiac output at rest goes to skeletal muscle, which means that about 20% of resting systemic vascular resistance is determined by the vascular resistance of skeletal muscle. For this reason, changes in skeletal muscle resistance and blood flow can greatly influence arterial pressure.

When the contractions first begin, blood flow briefly decreases because of compressive forces exerted by the contracting muscles on the vasculature within the muscle. Each time the muscles contract arterial inflow decreases due to extravascular compression, and then arterial inflow increases as the muscles relax. This is repeated each time the muscles contract and relax. If flow were measured in the outflow vein, the venous outflow would increase during contraction and decrease during relaxation - the opposite of what occurs on the arterial side of the circulation. After just a couple of seconds, mean and peak flows begin to increase (active hyperemia). After 15-20 seconds the increased flow will reach a steady state that is determined by the force and frequency of contraction, and the metabolic demands of the tissue. When contractions cease, blood flow may transiently increase because of the loss of compressive forces, and then over the next minute or so the flow will return to control.