Today I am traveling. In fact, I will be away for the next 8 days, and so have prepared a number of posts in advance to be able to continue the Guide while on the road. In addition, we should have a guest post by K. Mata, Goddess of Lab Rats, coming up soon.
In today's society, we take long distance travel for granted. One steps onto a jet in cool, dry Dallas, and deplanes 8 hours later in warm, humid Hawaii. In the meantime, we have traveled nearly 7000 kilometers at an altitude of over 10,000 meters. Of course that beats the Google Maps directions to "kayak across the Pacific Ocean" for 3500 kilometers, but it is still a trip through inhospitable environment! We are sheltered by virtue of the heated, pressurized cabin of the airliner, providing us with a key indispensible component of survival – oxygen.
The neurons that comprise the brain are unique compared to the vast majority of cells in the body – they cannot store essential energy-producing compounds (glucose, glycogen, fats) nor can they utilize the "anaerobic" (non-oxygen-requiring) enzymes to provide energy during short periods of activity. Muscle cells store glucose, they can synthesize a polymer-like derivative of glucose – glycogen – that provides a reserve supply for exercise and exertion. Excess glucose is converted to fatty acids and eventually stored as fat in adipose tissue. For slow, steady exertion, glucose is "burned" – in fact, broken down enzymatically – via a process called the "Kreb's cycle" or "citric acid cycle" which converts glucose to carbon dioxide, water, and energy (in the form of the energy transport molecule ATP). This cycle is "aerobic," it requires oxygen for completion. For brief bursts of intense activity, glucose can also be broken down to lactic acid in the absence or low availability of derivative via anerobic catalysis. The lactic acid builds up and is responsible for muscle fatigue, oxygen is required to convert the lactic acid back to intermediate molecules, and then to CO2 and water.
The brain, however, does none of this. It does not store glucose, nor does it manufacture glycogen or fatty acids. Neurons are very limited in anaerobic capabilities, sufficient oxygen for aerobic processing of glucose is necessary at all times – build-up of lactic acid is damaging at best, and fatal at worst. For this reason, the brain is most sensitive to low oxygen, low blood glucose and dehydration/low blood flow. Often the first symptom of any of these conditions is altered state of consciousness, delirium, then coma. It is the primary reason why hospitals monitor the O2 saturation of a patient's blood, it is why diabetics must watch out for low blood sugar as well as high. Low oxygen – "ischemia" – is the main culprit in stroke and brain injury.
Fortunately, restoring blood flow and oxygen to the neurons of the brain usually results in a rapid restoration of consciousness and normal function. One of the first things taught in first aid is to treat the body for shock, which is primarily due to low blood flow to the brain. Keep the patient warm, lower the head and raise the feet to keep the brain well fed. Normal body reactions to extreme conditions is to preserve blood flow to the brain at the expense of the body, but shock represents the opposite, and it is necessary to *help* the body and brain along.
Fortunately the body and brain are pretty adaptable. If a person moves to a high altitude, with lower oxygen content in the air, the body starts making more hemoglobin – the oxygen carrying molecule in the blood – and more red blood cells to carry the oxygen. However, the process takes several days, so the immediate adaptation is to increase the density of red blood cells by concentrating (dehydrating) the blood. Dehydration is a definite risk for high altitude athletes and hikers, so acclimating for about a week is often necessary to ensure peak capabilities.
The necessity for oxygen and glucose to be constantly supplied to the brain is also the basis for two of the most important means of viewing brain activity from the outside. Functional magnetic resonance imaging relies on movement of oxygen into neurons, while some versions of positron emission tomography use an isotope labeled version of glucose to track the most active neurons. Both methods give us a picture of active regions of brain on the basis of the uptake of their most essential nutrients.
So, next time you take a breath – appreciate it. Your brain is depending on it!