Proteins are polymers of amino acids. Amino acids have two
functional groups, the basic amino group, NH2, and the acidic
carboxyl group, COOH:
About 20 different amino acids make up the proteins in the body.
DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are complex molecules whose biological activity is dependent on the composition of organic nitrogen bases present in their structures. DNA is the essential hereditary material of all living cells. RNA controls the production of proteins, the building blocks of living cells.
Physical activity is a series of muscle contractions each requiring energy.
Muscles store energy as glycogen. This glycogen is the storage form of glucose in the blood arising from carbohydrate in the diet (candy bar, fruit, potatoes, pancake syrup, etc.). When muscles contract, the glycogen is converted to pyruvic acid and, if sufficient oxygen is present, to CO2 and H2O. This is aerobic exercise or aerobics. If sufficient oxygen is unavailable, pyruvic acid is converted to lactic acid. This is anaerobic exercise. If the buildup of lactic acid continues, the muscles are less responsive to contraction, and the person experiences some fatigue and even pain. After exercising, more oxygen becomes available to convert the lactic acid 
to pyruvic acid and the latter is converted to CO2, H2O, and energy. Sprinting and weight lifting are typical anaerobic activities; a marathon race is largely aerobic.
To insure necessary biological functions, body fluids are maintained at a narrow pH range. For example, protein molecules within cells can be altered if the pH changes too much in either direction of the optimum pH. This alteration can change the usual properties of the protein such that the protein would not be able to carry out its normal function.
The H2CO3/HCO3- buffer is the major buffer in the blood. This
buffer counteracts either added acid or bases. Acidic by-products
(H+) of metabolic cycles are neutralized by HCO3-(aq)
forming H2CO3(aq). Excess H2CO3(aq) is removed from the body as CO2(g) in the lungs.
Excess base is neutralized by H2CO3(aq) forming HCO3-(aq). To
compensate for the diminished H2CO3(aq), CO2 (g) dissolves in water to form
carbonic acid.
