Forebrain
Your ability to run depends on far more than mechanics. You also need mental faculties like motivation, judgment and wisdom. These faculties are processed in the front of the brain, and researchers think they may be among the first to falter when the fuel runs low.
In back of the brain MRI studies show that the occipital lobe, a network center for visual processing, can fire at a diminished rate when the blood sugar runs dangerously low-a condition called central fatigue. This may explain the hallucinations that bonking runners sometimes experience.
Heart
Your heart rate accelerates when you do, of course. It also speeds up as your muscle cells release lactic acid, and as your thick, dehydrated blood makes its sluggish way through collapsing, dehydrated vessels. Drinking too much water can also make your heart beat faster by flushing the electrolytes that maintain osmotic pressure in the cells, a condition called hyponotremia.
Upper body muscles
Life's unfair: Your upper body may have plenty of fuel when your legs are out of gas. But glycogen stored here can't feed your legs.
Leg Muscles
Muscles store carbohydrate fuel in the form of glycogen-chains of glucose molecules-and use this supply as a sort of biological debit card to draw down glycogen and help fire contractions. Glycogen in the bloodstream is a liquid asset; your muscles will use it before tapping its energy savings stored muscle membranes. The fuel byproducts are carbon dioxide, water, and our old friend lactic acid.
Portal vein
This is the HOV lane of blood vessels, zooming glucose from the gastric tube directly to the liver-which, in turn, supplies the fuel for your brain.
Gastric tube
Overexertion can redirect most of the blood to the legs, leaving too little for your stomach and small intestine. If you drink too little water you can slow "gastric emptying" and cause stomach cramps-the number one reason runners fail to finish races.
Pancreas
Your insulin factory. This hormone regulates energy storage and helps convert fat to glucose. Nutrition renegades believe that protein stimulates insulin production and speeds the supply of glucose to your muscles. At any rate, muscles usually recharge on their own within 24 hours.
