# Why can our bodies bleed? [duplicate]

Human systolic pressure is 16 kPa and diastolic 11 kPa but Earth atmosphere pressure is 101.325 kPa. I don't understand why when we are hurt we bleed instead of air enter into our body. How can blood be "stronger" than atmosphere pressure?

## marked as duplicate by John Rennie, Community♦Jun 9 '17 at 12:48

• – John Rennie Jun 9 '17 at 12:31

# The Systolic and Diastolic blood pressure are measured relative to the atmospheric pressure, hence they are "stronger" than atmospheric pressure.

The blood pressure measured (Systolic and Diastolic) respectively measure the maximum and minimum pressure between heart beats relative to the atmospheric pressure.

### Gauge Pressure

As John Custer noted in the comments, this type of measurement (relative to an external pressure) is referred to as gauge pressure. An important distinction to make is that gauge pressure measures the pressure of the system with respect to the pressure outside of the system; therefore, the external pressure is not always atmospheric pressure. Additionally, in some cases it may also be referred to as 'relative pressure' though this would be in a more colloquial setting.

# Why

Blood pressure, (and gauge pressure) is measured this way to formulate a numeric representation of the blood pressure independent of external factors. As an example, atmospheric pressure fluctuates with weather and height. If it were measured absolutely, then the 'average' blood pressure would vary over the year and with height. By measuring the gauge pressure, such distinctions are limited considerably, but not removed entirely. For example, your blood pressure would still increase when you go up a mountain; but, now when your blood pressure is measured, the difference is made much more apparent.

Sources:

1. Wikipedia
2. Magder, S. A., MD. (2014). He Highs and Lows of Blood Pressure, Toward Meaningful Clinical Targets in Patients With Shock. Crit Care Med., 2014(42), 5th ser., 1241-1251. Retrieved June 08, 2017, from http://www.medscape.com/viewarticle/823925_2
• I found in Physics.SE this question. So, this means that a 16 kPa of systolic and 11 kPa of diastolic is really 117,325 kPa of systolic and 112,325 kPa of diastolic (pressure + atmospherical pressure) and this is why we can bleed (inner pressure - outside pressure = +16 kPa and +11 kPa = bleeding)?. – Ender Look Jun 9 '17 at 3:09
• The big bold text at the top of your post is misleading : as you said in your answer, they are stronger and the usual measurement is not what OP thinks – Evpok Jun 9 '17 at 6:51