Let's first take a look at what drives your blood pressure.
Unlike a river, which flows continuously downstream, blood flow is pulsatile, with ebb and flow driven by pumping heart muscle. This cycle repeats itself 60 or so times each minute, every time your heart beats. With each heartbeat occurring about once per second, the heart squeezes, or contracts, within a split-second. The rapid contraction forces about 90 cc of blood (approximately 1/3 cup) up, pushing the aortic valve open, and blood passes up into the aorta and rapidly distributes upward to the brain (via the carotid arteries on either side of the neck), the arms, and downward to the abdomen, pelvis, and legs. The force of heart contraction and its rapid distribution to the body is measured as systolic pressure, or the top number in blood pressure.
As the 90 cc or so of ejected blood distributes rapidly throughout the body, pressure in all the arteries drops over the ensuing half second, and the aortic valve closes. This resting phase of blood pressure is measured as diastolic pressure.
Thus, blood pressure is the driving force within the arteries of the body, literally the hydraulic pressure that drives blood flow to all organs.
Just understanding the basic phenomena behind blood pressure allows us to immediately identify some ways blood pressure can go haywire:
If the volume of blood contained in the entire circulation is increased (e.g., fluid retention resulting from excessive sodium in the diet or poor kidney function), then the volume of blood ejected from the heart increases and the entire network of arteries and veins in the body are filled towards greater distention, raising systolic pressure.
If the volume of blood contained in the entire circulation is decreased (e.g., dehydration, hemorrhage), then systolic pressure will drop. This is hypotension, the opposite of hypertension.
A factor that affects both systolic and diastolic pressure is the flexibility, or "give," of the arteries of the body. Soft, flexible arteries expand with the increased blood volume of systole and easily relax with diastole. Rigid arteries can't accommodate the increased volume of systole, nor relax with diastole; this is common in arteries lined with semi-rigid atherosclerotic plaque (often appropriately called "hardening of the arteries") or with any condition that simply adds rigidity to the artery walls.
- If the aortic valve is leaky, and blood flows backward back into the heart during diastole, then diastolic pressure drops to abnormal levels, e.g., 50 mmHg, indicating abnormally rapid distribution of blood.
We require pressure sufficient to feed organs adequately and meet their varied needs, including under conditions of increased demand (e.g., walking on a treadmill, digesting dinner, sexual activity, etc.).
But what does it mean if blood pressure is low? From the above list, you can see that several factors can result in low blood pressure:
Reduced blood volume--Dehydration is the most common cause. Dehydration is, in my view, common and terribly underappreciated. The treatment is simple: Drink lots of fluids, preferably unsweetened and non-carbonated (e.g., water, unsweetened teas). Reduced blood volume can also result from blood loss and fluid loss (diarrhea, diuretics, fever, etc.); this needs to be assessed by your doctor.
- Poor heart function--A weakened heart from heart attacks, viral infections, or excessive alcohol can lead to reduced volume of output from the heart, thereby reducing systolic pressure.
Low thyroid function--An underactive thyroid (hypothyroidism) results in reduced heart function that can result in low blood pressure.
Leaky aortic valve--If the aortic valve leaks and no longer seals properly on closure, a condition called aortic valve regurgitation or insufficiency results. This leads to unusual reductions in diastolic pressure.Excessive blood pressure medication--or medications that reduce blood pressure, including some anti-depressants, diuretics, heart rhythm medication, and others.
Nervous system dysfunction--Also known as "neuropathies," this simply means that factors controlling the constriction of arteries in the body are not well controlled. This can happen with diabetes (diabetic peripheral neuropathy) or other unusual neurologic diseases.
Beyond these common causes, there are uncommon causes of reduced blood pressure, such as infections, particularly if bacteria enter the blood, and Addison's disease, a rare condition of low adrenal gland function.
What is too "low"?
First of all, normal--truly normal--blood pressure is generally in the range of 90 to 115 mmHg systolic, 60 to 80 mmHg diastolic, a lot lower than most people think. We have all been numbed by talk of 140/90 being normal or borderline, while emerging evidence has shown that blood pressure values in this range are clearly hypertensive and are associated with heightened risk of stroke and heart attack.
Ideal blood pressure is therefore in the 90 to 115 systolic range, 60 to 80 mmHg diastolic range. An occasional person may have an even lower blood pressure yet be perfectly healthy. The real test of whether blood pressure is too low is if it results in lightheadedness. While we all experience lightheadedness on occasion, particularly when arising from sleep or from a seated position, persistent or long-lasting lightheadedness may be a sign that blood pressure is too low. You and your doctor may need to consider one of the items on the list of causes of excessively low blood pressure, such as dehydration. High blood pressure is not good for you, of course, but neither is excessively low blood pressure.
The first thing to consider with low blood pressure is dehydration. However, if purposeful hydration fails to fix the issue, a conversation with your health care provider is in order.