What Is Blood Volume, and Why Does It Matter in POTS?
A plain-language explainer on blood volume, the standing response, the hypovolemic subtype of POTS, and where sodium and fluids fit in.
So much POTS advice circles back to “salt and water” and “blood volume,” but the term rarely gets explained. Understanding what blood volume actually is — and what it does when you stand — makes the rest of POTS management click into place.
How blood shifts when you stand
Blood volume simply means the total amount of blood circulating in your body. It is the fluid your heart pumps and your vessels carry, and it has to keep your brain, muscles, and organs supplied no matter what position you are in.
Gravity makes that harder than it sounds. When you stand up, gravity immediately pulls a meaningful share of your blood downward, toward your legs and abdomen. For a moment, less blood is returning to your heart, which means less is being pumped up to your brain. In a body without POTS, this is handled automatically: blood vessels in the lower body tighten to push blood back up, and the heart adjusts, all within seconds, so you barely notice.
This constant, invisible correction is what lets most people stand without thinking about it. The standing response is one of the autonomic nervous system’s everyday jobs.
The hypovolemic subtype of POTS
POTS is not a single uniform thing — it has different patterns, and one of the most discussed is the hypovolemic subtype. Hypovolemia simply means lower-than-ideal blood volume.
If you are starting with less blood volume than your body would like, the standing challenge becomes much harder. There is less fluid in the system to begin with, so when gravity pulls blood down on standing, the shortfall reaching your brain is greater. Your body compensates by driving your heart rate up sharply — the racing heart that defines POTS — to try to keep enough blood moving. That compensation is why the heart rate climbs, and it is also why it feels so taxing: your system is working overtime to manage a volume deficit.
Not everyone with POTS has low blood volume, and the condition involves other mechanisms too. But for the substantial group in whom volume runs low, this piece explains a great deal about why they feel the way they do — and why volume-focused strategies tend to help them.
Why sodium and fluids enter the picture
Once you see POTS partly as a blood-volume problem, the obsession with fluids and sodium stops seeming arbitrary. If low volume is part of the issue, then supporting volume is a logical lever.
| Step in the logic | What it means |
|---|---|
| Standing pulls blood downward | Less returns to the heart and brain |
| Low blood volume worsens the shortfall | Heart rate climbs to compensate |
| Fluids add volume | More circulating blood to work with |
| Sodium helps hold that fluid | Volume is retained rather than flushed |
This is the core reason “salt and water” is everywhere in POTS care. Fluids increase the volume available; sodium gives that fluid a reason to stay in the bloodstream instead of passing straight through. Together they aim to nudge blood volume up so the standing response is less of a struggle. It is support, not a cure — a way to give your body more to work with, not a fix for the underlying autonomic pattern.
How far volume strategies help, and what targets make sense, varies from person to person and is something to work out with your clinician. There are also medical ways of assessing and addressing volume that go beyond diet, which is another reason this is a conversation for your care team rather than a solo project.
The bottom line
Blood volume is the total blood circulating in your body, and standing always pulls some of it downward. In the hypovolemic pattern of POTS, that volume runs low, so the standing shortfall is bigger and the heart races harder to compensate. Fluids add volume and sodium helps hold it, which is the whole rationale behind “salt and water.” It supports the system rather than curing it, and the right approach is one to dial in with your clinician.