There are plenty of reasons why humans faint—known as syncope in medicine—but they're all indirect causes, since fainting occurs when there's a temporary shortage of blood supply to the brain. And sure enough, some diabetes complications can make us faint.
Two major body systems are involved when we faint: the autonomic nervous system (brain, nerves, spinal cord), and the cardiovascular system (heart, blood vessels).
Nervous System Fainting
The autonomic nervous system automatically regulates our heart rate, our breathing, and our blood pressure. Systemic failure to maintain that blood pressure at the appropriate level in the body can often result in a fainting spell, and in fact this is the most common reason for fainting.
While it can be brought on by coughing or laughing, it can also occur as a consequence of poorly controlled blood glucose. When poorly controlled, nerve fibers in the body are damaged; if damage happens to t1he nerves that control blood pressure, fainting can become increasingly common (and increasingly dangerous).
More common in older folks is what's known as orthostatic hypotension: blood pressure suddenly drops when a person stands up. Having been in a sitting position, when they stand, gravity floods the legs with blood. Our autonomic nervous system is supposed to meet this burden with a boost in heart rate and a tightening of the blood vessels, but it doesn't always happen like that, especially as we age.
Diabetes increases the risk of orthostatic hypotension if the patient is not well-hydrated and does not have control of their disease. The consequence is a lower fluid volume in the blood, which undercuts blood pressure and makes fainting more likely.
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Poorly controlled blood glucose can lead to fainting.