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Glossary

This is the elevation above sea level. In the calculators on altitude.org. it can be entered in metres or feet – simply change the units in the drop down menu.

This is the number of breaths you take in a minute. It increases at high altitude, because your body needs to breathe harder in order to take in more oxygen.

A normal respiratory rate would be between 10-14 breaths per minute.

This is a measure of how deeply you breathe. It is defined as the volume of gas inhaled (or exhaled) in a normal breath. At rest, it is normally about 8-10ml/kg body weight. Tidal volume increases at high altitude because your body needs to breathe harder in order to take in more oxygen.

This is the partial pressure of oxygen in your blood. It is normally about 13kPa (kilopascals) or 100mmHg (millimetres of mercury). It can be thought of as the driving force to move oxygen from your blood into the tissues, where it is needed.

Click to read more about partial pressure.

This is a measure of the acidity or alkalinity of the blood. Normal pH is 7.35 to 7.45. The body has may mechanisms to tightly regulate the pH of the blood within this narrow range.

pH is another way of expressing hydrogen ion concentration [H⁺]. pH is calculated using the formula pH = –log10[H⁺]

Use the pH to H⁺ converter.

This is a measure of the acidity or alkalinity of the blood. Normal [H⁺] is 35 – 45 nmol/l. The body has may mechanisms to tightly regulate the pH of the blood within this narrow range.

Hydrogen ion concentration [H⁺] is another way of expressing pH.

Use the pH to H⁺ converter.

This is the partial pressure of CO₂ (carbon dioxide) in your blood. Unlike oxygen, the partial pressure of CO₂ is closely related to the actual number of molecules of CO₂ present. (Oxygen is different because of haemoglobin).

The normal range for PaCO₂ is 4.5-6kPa, or 30-45mmHg.

Carbon dioxide is an acid gas, so when you lose carbon dioxide because of hyperventilation at high altitude, your blood becomes alkaline.

Click to read more about partial pressure.

HCO₃^- is the bicarbonate ion, and it is the most important acid-base buffer in all mammals. This means that it tightly regulates the acidity of the blood. When the blood becomes alkaline, for example because hyperventilation has caused the blood to lose the acid gas carbon dioxide, the kidneys can compensate by removing bicarbonate to return the blood to a normal pH.

The normal range for HCO₃^- is 22 – 28 mmol/l.

We have included HCO₃^- in this calculator because a change in HCO₃^- in the first few days at altitude is an important part of acclimatisation.

A red blood cell (RBC)

Mean corpuscular haemoglobin concentration is the concentration of haemoglobin within the average red cell, and it is usually about 3 times higher than the whole blood haemoglobin concentration. It is calculated as the haemoglobin concentration divided by the haemotocrit. A low MCHC indicates a hypochromic red cell.

2,3 DPG is a substance produced by red blood cells that regulates the binding of haemoglobin to oxygen. A normal value would be around 4.65mmol/l. The full name of 2,3 DPG is 2,3 diphosphoglycerate.

p50 is the partial pressure at which haemoglobin is 50% saturated. A normal value at sea level would be about 3.5kPa (26.8mmHg). p50 is increased by factors that reduce the affinity of haemoglobin for oxygen: H⁺, temperature, CO₂, and a substance called 2,3 DPG.

This is the percentage by volume of blood that is made up of red blood cells. It is closely related to the haemoglobin concentration in blood.

Normal values are 37% to 47% for women and 40% to 54% in men.

Haematocrit increases at altitude as part of the process of acclimatisation.

Haemoglobin is the oxygen-carrying molecule in blood. Click to read more about how haemoglobin binds oxygen.

Normal haemoglobin values at sea level are 11.5 to 16.5g/l for women, and 13 to 18 g/l for men.

Haemoglobin concentration increases at altitude as part of the process of acclimatisaion.

This is the body temperature in degrees Celcius. Normal body temperature is 37ºC. When body temperature changes it alters the binding of haemoglobin to oxygen.

Last updated June 2007

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