Wednesday, January 22, 2014

A Physiological Look at Hypoxia and the Role of Red Blood Cells

Hypoxia is directly related to blood saturation levels and factors that affect that saturation level. 

The chemical composition of oxygen in the air, whether at sea level or in the stratosphere, is approximately 21%. What changes is the partial pressure that oxygen exerts at different altitudes above sea level. This pressure is measured in different ways but for us we will use millimeters of mercury as our standard. The symbol for mercury is Hg. Since oxygen is a molecule and has mass that is affected by gravity it exerts a pressure on whatever it strikes against. Thats right, these little oxygen molecules are constantly moving and they exert a force per unit area on what they strike. That force is pressure.

At sea level the atmosphere consists of nitrogen, oxygen, carbon dioxide and trace gases plus water vapor. Each of these gases exerts a partial pressure unique to their chemical composition. The total pressure of one atmosphere at the surface of the earth is 760 mm of Hg pressure.

If you have a calculator take 21% (Oxygen’s composition in the atmosphere ) of the total pressure exerted by one atmosphere which is 760 mm Hg. Your answer should be close to 159.6 mm Hg partial pressure of oxygen. 

This partial pressure force of159.60 mm of oxygen is what pushes the oxygen into our lungs when we inhale a breath. 

The partial pressure of oxygen will decrease as you climb. This, in turn, affects the blood saturation level of the oxygen in the blood. When the blood saturation level reaches a value where the cells of the brain can’t function properly that is called Hypoxia.

The oxygen carrier cell in humans is the red blood cell found in our circulatory system. It is called an erythrocyte. Now the fun begins. The oxygen molecule passes through tissues to reach the erythrocyte. 

In the lungs (pulmonary lungs) it passes through a very thin membrane shaped like a little balloon called an alveolus. The alveolus, in turn, is surrounded by very small blood vessels called capillaries. The oxygen molecule passes into the capillary through its thin covering capillary membrane. 

The capillary carries a combination of fluid called plasma that contains a lot of things besides the erythrocytes. It is the erythrocyte that we are interested in. Yes, the erythrocyte is surrounded by a plasma membrane. 

The erythrocyte contains a very large complex molecule called hemoglobin. there is a chemical site on the hemoglobin that is specific for the oxygen molecule. The oxygen chemically attaches and stays there until it is delivered to the brain, as one of the very important high energy tissues that need lots of oxygen to function well.

Once there the process described above is reversed. The oxygen molecule is released and travels through several membranes and fluid compartments to reach the brain cell called a neuron. There, the oxygen is used in a chemical process called respiration that ultimately provides the energy to make a neuron work.

You can see, after looking at the pathway just described, an oxygen molecules path can be slowed or prevented by obstacles along the way. Any of these obstacles could increase your chances of developing hypoxia.

Obviously, the partial pressure of oxygen decreases as it passes through the membranes on its journey. The normal result of this journey is saturating the blood erythrocytes so the overall saturation percentage is 96% or above.

What are the symptoms of Hypoxia?

Neural control of night and peripheral vision.

Forgetfulness of common procedures you ordinarily do.

You become euphoric when real danger exists.

Finger nails turn bluish from hypoxia.

Ability to perform common feats like writing begin to deteriorate.

Judgement declines.

Your instructor will have other clues to pass on to you to discover you are hypoxic.

What can go wrong that results in Hypoxia?

First, and most obvious, is flying at an altitude where the partial pressure of oxygen is significantly less than 159.60 mm of Hg.

Second, your lung tissues may be thickened by disease (heat from inhaling smoke)

Third, your lungs may partially fill with excess fluid that prevents gas exchange (pneumonia)

Fourth, the incoming air into your lungs may not contain its normal composition of 21% because it has been replaced or decreased by gaseous pollution.

Fifth, your body may not be producing enough erythrocytes to supply adequate amounts of oxygen. (anemias)

Sixth, your circulatory system may suffer from cardiac disease affecting the heart pump and the delivery system of blood vessels.

Seventh, the hemoglobin molecule itself may be unable to attach oxygen for transportation to the body cells.

Eighth, Your own breathing rate may be insufficient to allow proper ventilation of the lungs with adequate oxygen.

Ninth, Smoking, will increase your exposure to develop hypoxia by causing Carbon Monoxide poisoning of the hemoglobin in your blood cells.

This is just a partial list of problems that may result in hypoxia.

What can you do to prevent Hypoxia?

Make sure your aircraft is supplied with properly working and tested oxygen equipment for the maximum altitude you will fly. 

Don’t smoke while flying.

Have an annual physical performed by an FAA Certified physician.

Recognize your symptoms that indicate you are hypoxic.

In conclusion, safety in flying is paramount and hypoxia will defeat your efforts at safe flying.