It is true that a person suffering from very low blood pressure does not pass sufficient urine. The reason for this is, the blood to pass through the glomerulus of the nephron must have required amount of pressure in it. If the pressure is not sufficient it will not flow through the glomerulus and filtration would not take place, hence no urine would be formed. This is quite harmful to the person as waste products go on accumulating in the body.
To avoid this, a person should be advised to take sufficient amount of water and medicines to keep the blood pressure at an optimum level