MayoClinic.com says that low blood pressure can be caused by a number of heart problems or defects. A low heart rate, also known as bradycardia, can cause a drop in blood pressure. Diastolic pressure can also be affected by heart valve problems, a heart attack or heart failure. In these situations, the heart cannot circulate the amount of blood that is needed.
http://www.livestrong.com/article/142243-causes-low-diastolic-pressure/
MayoClinic.com also states that problems with the endocrine system can often cause low blood pressure. If the thyroid is over- or under-active, low diastolic pressure can result. Other causes include Addison's disease or adrenal insufficiency, hypoglycemia and even diabetes.
http://www.livestrong.com/article/142243-causes-low-diastolic-pressure/
If you have a diastolic number — the bottom number of a blood pressure measurement — less than 90 millimeters of mercury (mm Hg) and a systolic number — the top number of a blood pressure measurement — greater than 140 mm Hg, you have a common type of high blood pressure called isolated systolic hypertension.
http://www.mayoclinic.org/diseases-conditions/high-blood-pressure/expert-answers/hypertension/FAQ-20058527
Isolated systolic hypertension can be caused by underlying conditions such as artery stiffness, heart valve problems or an overactive thyroid (hyperthyroidism). For years, doctors focused primarily on diastolic blood pressure. The theory was that the body could tolerate occasional increases in systolic blood pressure, but consistently high diastolic pressure could lead to health problems. However, doctors now know that high systolic pressure is as important as high diastolic pressure — and even more important in people older than age 50. The recommended goal for systolic pressure for younger people is less than 140 mm Hg.
http://www.mayoclinic.org/diseases-conditions/high-blood-pressure/expert-answers/hypertension/FAQ-20058527
Hypotension is the medical term for low blood pressure (less than 90/60). A blood pressure reading appears as two numbers. The first and higher of the two is a measure of systolic pressure, or the pressure in the arteries when the heart beats and fills them with blood. The second number measures diastolic pressure, or the pressure in the arteries when the heart rests between beats. Optimal blood pressure is less than 120/80 (systolic/diastolic). In healthy people, low blood pressure without any symptoms is not usually a concern and does not need to be treated.
http://www.webmd.com/heart/understanding-low-blood-pressure-basics
MedlinePlus warns that dehydration can cause a drop in blood pressure. When the body is dehydrated, the blood volume drops, which can lead to a marked drop in diastolic blood pressure. A severe lack of certain nutrients, such as vitamin B-12 or folate, has been known to cause anemia.
http://www.livestrong.com/article/142243-causes-low-diastolic-pressure/
The risk of both low and high blood pressure normally increases with age due in part to normal changes during aging. In addition, blood flow to the heart muscle and the brain declines with age, often as a result of plaque buildup in blood vessels. An estimated 10% to 20% of people over age 65 have postural hypotension.
http://www.webmd.com/heart/understanding-low-blood-pressure-basics
The cause of low blood pressure isn't always clear. It may be associated with the following: 1  Pregnancy.  Hormonal problems such as an underactive thyroid (hypothyroidism), diabetes, or low blood sugar (hypoglycemia)
http://www.webmd.com/heart/understanding-low-blood-pressure-basics
Diastolic blood pressure measures the pressure in your blood vessels between heartbeats (when your heart is resting). Represented by the bottom number in a blood pressure reading, diastolic blood pressure is considered low when the blood pressure reading is below 60; a diastolic blood pressure reading higher than 90 is considered high.
http://blood-pressure.emedtv.com/diastolic-blood-pressure/diastolic-blood-pressure.html
Blood pressure is the amount of force (pressure) that blood exerts on the walls of the blood vessels as it passes through them. 1  Pressure while it is relaxed (known as diastolic blood pressure).  Diastolic blood pressure measures the pressure in your blood vessels between heartbeats, when your heart is resting.
http://blood-pressure.emedtv.com/diastolic-blood-pressure/diastolic-blood-pressure.html
what causes low diastolic pressure
When the body is dehydrated, the blood volume drops, which can lead to a marked drop in diastolic blood pressure.