There are different types of pumping structures in animal circulatory systems. D

All circulatory systems have some type of pumping structure that propels fluids around the system. There are three main types of pumping structures in animal circulatory systems.

a.   External pump: Organs not strictly associated with the circulatory system, such as skeletal muscle, can be used to develop pressure gradients. Contraction and relaxation of skeletal muscles, such as during movements of the legs, can alternately compress and expand a blood vessel, forcing fluid along its pressure gradient. A one-way valve guarantees one-way flow.

b.   Peristaltic contraction: This rhythmic wave of muscle contraction along a blood vessel wall proceeds in a coordinated fashion from one end to the other, similar to squeezing toothpaste from a tube. Because the contractions usually occur in a specific direction, the flow is usually one-way even when no valves are present.

c.   Contractile chambers, such as the vertebrate heart: Muscular contraction of the walls of the chamber increase pressure, and when this pressure exceeds that in the rest of the circulatory system, blood will flow down the pressure gradient. Blood does not flow backwards during the contraction because a one-way valve prevents such flow.