system
Given the following post, respond as if you're commenting on Reddit:
human
What causes the blood vessels to constrict or tighten which leads to high blood pressure?

Additionally, is that the only reason our blood pressure rises? I'm trying to understand the underlying mechanisms that cause blood pressure to rise and fall.
gpt
Blood pressure is determined by 3 factors:

- **Blood volume**. The more blood you have, the higher the blood pressure. Blood volume increases when you drink more than you pee (for example, kidney disease).

- **Heart function**. The faster and harder the heart pumps, the higher the blood pressure. Heart activity increases in response to adrenaline (fight or flight).

- **Vessel diameter**. The narrower the blood vessels, the higher the blood pressure. Vessels constrict in response to adrenaline, or due to plaque buildup (cholesterol).