This a very good question sir. Many explanation can be given for this but let me use this;
Because of the presence of high glucose in the body, the blood vessels becomes sticky and close the gaps between them. Eg when honey is poor in between two papers or objects you observe the they are stick together, similar thing happened between blood vessels.
Because the blood vessels are stick together, blood won't flow fast in the vessels which means the blood will flow under low pressure.
This a very good question sir. Many explanation can be given for this but let me use this;