Pumping

The dorsal vessel, closed at its posterior end, divides into a posterior heart at the back end of the tube and the aorta in front. In this cardiac or heart area, incurrent openings, and sometimes, external openings, the ostia, permit hemolymph in the heart to exchange with hemolymph in the hemocoel. The aorta contains no ostia. Ostia in honeybees occur in five pairs in abdominal segments two to six inclusive. Blood enters the heart through these ostia. Lips of each ostium project forward into the heart cavity. These lips act as valves preventing backward flow of hemolymph each time the heart contracts.

The walls of the dorsal vessel in the heart area contract, as the walls of the vessel consist of sheets of muscle cells wrapped in spiraling layers around the bore of the tube. Systole is the contraction phase. Heart muscles contract synchronously beginning at the rear of the abdomen, and a wave of contraction spreads forward towards the head along the dorsal vessel. The muscles of the heart then relax. This relaxation begins diastole or the filling phase of the heart. The heart fills as it expands and as its muscles relax. Sometimes elastic fibers that pull the walls open when muscular tension subsides assist filling. During diastole, the resting phase of the heart cycle, the heart, now full of hemolymph, pauses briefly. As the heart rate or frequency of contraction increases, the time the heart spends resting in diastole shortens.

The frequency of the heartbeat varies considerably. Frequency of pumping often is higher in early larvae, and the rate of beating may rise just before molting. Contraction of the heart muscles slows below one to five degrees C and stops when above fifty degrees C. Beating may stop for a few seconds and can reverse periodically when waves of contraction begin at the front and move backward. When reversal occurs, hemolymph passes backwards through the incurrent ostia.

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