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In the leech, the physiological significance of high-pressure phases (HIP) and lowpressure phases (LOP) of the lateral vessels was studied by intravascular pressure recordings and observation of blood flow in different parts of the circulatory system, and by measurements of the blood flow velocities in the dorsal vessel. Different shapes of the pressure pulses were found in the anterior lateral vessel segments during HIP and LOP phases, according to the different modes of diastolic filling in both phases. Pressure recordings in the lateral abdominal vessels showed the different action of the lateral abdominal sphincters in the HIP and LOP phases of the ipsilateral lateral vessel. The LOP contractions were responsible for the blood supply to the capillary networks of the organs and the body wall, with the possible exception of the intestine. The HIP contractions caused a forward bloodflow within the lateral vessel. In the dorsal vessel, the blood pressure was about 0.9-1.9kPa in different animals. The blood flow was discontinuous with velocities of 0.5-10 mms−1. Average blood flow in the dorsal vessel was 22.9μl min−1. An improved model of the circulation in the leech is presented.
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