Hold your breath-part3 | Freedive Earth

# How Long Can You Really Hold Your Breath-Part3?

## Calculating Blood Volume

The first challenge when trying to work out how much oxygen a person can store in their blood, is to work out how much blood they have, and we have to begin with total body water. The famous statistic is that the human body is 60% water which is a reasonable start, but a little bit too simple for us. The content of water in the body actually depends mainly on the amount of fat that we're carrying, and can vary between 45% for a very obese person to 80% for a new-born baby. Most people fall somewhere in between, which is where the 60% figure comes from. Short of wringing yourself out, or perhaps taking a long-haul flight for 12 hours or so, it's quite difficult (impossible) to extract all of the water out of your body without dying. In fact, as little as 10% dehydration can be fatal. So, basically, although sophisticated methods for calculating total body fluid do exists, we have to take a guess. Here are a few rules of thumb that probably aren't too far off the mark:

## breathhold-howmuchbloodinyourbody.png These body types are based on Body Mass Index (BMI), which is calculated using height and weight: BMI = weight(kg)/height(m)/height(m) Women typically have a lower water content than men because of breast tissue. Of this total water content, only about 7% is contained in the blood. The rest is distributed in the intracellular (66%) and interstitial (27%) fluid. So, for a 60Kg Woman of normal build, total body water is 55 x 0.5 = 27.5l. Of this, 7% is blood, so the total blood volume is 27.5 x 0.07 = 1.925 litres. So how much oxygen is that? Around 98.5% of oxygen in the blood is carried bound to haemoglobin. The amount of haemoglobin per litre of blood can only be determined accurately by a laboratory test on a blood sample, but for a fit healthy freediver we can assume that they fall somewhere in the normal range which is 120-150 grams/litre for Women and 130-170g/l for men. With altitude training this can rise as high as 200g/l for both sexes. Let's take average values of 140g/l for women and 160g/l for men assuming no altitude training. We can work out the total mass of haemoglobin (Hb), then, by multiplying that value by the blood volume. For the woman in our example, that's going to be 1.925 x 140 = 269.5g Hb in total. Add to that the little factoid that in laboratory tests, haemoglobin carries 1.36ml of Oxygen per gram, that works out as 269.5 x 1.36 = 366.52ml Oxygen bound to haemoglobin in the blood. So what about the oxygen not bound to Hb? At a Partial Pressure of 11KPa, oxygen has a solubility of around 2.5ml/l of blood. This means that dissolved oxygen typically contributes around 1.5% of the total blood oxygen (so not a lot.) Nevertheless, we can (and should) include it in our calculations: Dissolved O2 in ml = 2.5 x blood volume in l = 2.5 x 1.925 = 4.81ml Well that's going to make all the difference! It actually will, too, as we'll see in the penultimate section. So, the total oxygen carrying capacity of the blood is the maximal amount of oxygen bound to haemoglobin + the amount dissolved in the blood plasma. For our example that amounts to 366.52ml + 4.81ml = 371.33ml In the next section we're going to look at the final, and much neglected compartment for oxygen storage, muscle.

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