Blood Sucker: Like the adult heart, the developing heart takes advantage of suction

The embryonic heart, though only a simple tube, uses the same basic mechanism to move blood as an adult heart does, new observations in zebrafish suggest.

Adult hearts in vertebrates, such as zebrafish and people, pump blood using valves and muscle contractions to create suction, but the early heart in these animals is a valveless tube.

Nevertheless, this simple organ begins pumping blood when an embryo is just a few days old. Because of its austere anatomy, researchers long assumed that the developing heart uses a mechanism called peristalsis, in which a series of muscle contractions move material from one end of a tube to the other.

“Peristalsis is like squeezing toothpaste out of a tube—if you hold one end and run your fingers down it, the contents will come out,” says bioengineer Morteza Gharib of the California Institute of Technology in Pasadena. Peristalsis commonly moves fluids in other parts of the body. For example, in the esophagus, it moves food from the throat to the stomach.

Peristalsis has certain defining characteristics, Gharib says. For one, increasing the frequency of muscle contractions increases the flow rate of contents leaving the tube. Moreover, all contents in the tube head in one direction with no material backing up.

While studying embryonic zebrafish hearts, Gharib and his colleagues noticed that neither of these conditions was satisfied all the time. “That put the whole claim of peristalsis in doubt,” he says.

To investigate what pumping mechanism the early heart might be using, Gharib’s team used a powerful microscope to view the beating hearts of zebrafish embryos. The researchers got a good look at their target by using fish genetically modified so that their hearts glowed red and their blood glowed green.

The studies revealed that the hearts move fluid much as does a device called an impedance pump, which creates a low-pressure area within a tube by squeezing and quickly releasing one end. Gharib explains that in response to the pressure drop, new fluid flows in. This process continues in a wave down the length of the tube.

The researchers report in the May 5 Science that they noticed a similar phenomenon when a small group of cells contracted and relaxed at one end of each fish’s tubular heart. With each cycle, blood rushed into the tube and continued out the other end.

The finding that embryos have impedance pumps indicates that both embryonic and adult hearts use the basic mechanism—suction—for pumping blood, Gharib notes. Previously, he adds, researchers were at a loss to explain how the embryonic heart transitions away from peristalsis as valves develop. “It was as if two different designers made the embryonic and adult hearts,” he says. “Our paper suggests that suction is really the way the heart does pumping.”

Steven Vogel of Duke University in Durham, N.C., who has studied the mechanics of a variety of other natural pumps in animals and plants, notes that a similar mechanism might be lying undiscovered in the hearts of other organisms, such as insects. “I have a strong feeling this [mechanism] will turn up elsewhere,” he says.