Biochemical Oxygen Demand
Oxygen helps liberate biochemical energy from food by acting as the electron acceptor for the reaction that metabolizes adenosine triphosphate, ATP, one of the body's major chemical energy sources. Metabolic processes that require oxygen are called aerobic. Naturally occurring oxygen is in the form of molecular oxygen, O2. Atmospheric oxygen is obtained by the body in the lungs in tiny air sacs called alveoli. Within the alveoli, red blood cells (rbc's) in narrow blood vessels absorb oxygen and carry it to cells throughout the body. Respiration, inhaling and exhaling of air, is subconsciously controlled by the brain in response to fluctuations in carbon dioxide levels. The average human body of 139 lb (63 kg) consumes 250 ml of O2 each minute. The major single-organ oxygen consumers are the liver, brain, and heart (consuming 20.4%, 18.4%, and 11.6%, respectively), while the sum total of all the body's skeletal muscles consume about 20%. In addition, the kidneys use up about 7.2%, and the skin uses 4.8% The rest of the body consumes the remaining 17.6% of the oxygen. Oxygen use can also be measured per 100 gm of an organ to indicate concentrations of use; as such, heart usage is highest, followed by the kidneys, then the brain, and then the liver. During exercise, the biochemical oxygen demand increases for active tissues including the heart and skeletal muscles.
Oxygen is the molecule used by animals as a final electron acceptor for metabolism. Two electrons (one at a time) from metabolic products can chemically bind each oxygen molecule. While numerous molecules combine with oxygen in the human body, one of the major chemical reactions involving oxygen is the synthesis of the high-energy phosphate bonds in ATP. ATP is the cell's currency for generating muscle contractions and driving certain ions through membrane-bound ion channels. Oxygen facilitates aerobic ATP production in mitochondria of cells throughout the body. Aerobic production of 36 molecules of ATP from one glucose molecules occurs in the citric acid metabolic cycle. About 1 L of oxygen can release the chemical energy stored in 1 g of food.
Oxygen is carried through the body in a number of chemical forms including simple O, water (H2O), carbon dioxide (CO2), and oxyhemoglobin. Unbound oxygen radicals can be highly toxic to cells. Allowing random oxidation reactions to occur throughout the cell, these radicals can be very destructive, and cellular defenses have evolved to combat them. In fact, the oxygen radical H2O2 is highly toxic to cells and can be used as a bactericidal agent. H2O and CO2 are end products for several aerobic reactions. And oxyhemoglobin is the oxygen shuttle complex that carries oxygen to needy cells. One rbc contains around 350 million hemoglobin molecules. Hence, one rbc can carry about 1.5 billion oxygen molecules.
Hemoglobin is a large globular protein made up of four polypeptide chains (two alpha and two beta hemoglobins, in adults) that each contain one heme complex. Heme complexes are sophisticated ring structures that contain a central ferrous iron atom. The iron atoms can each bind one O2 molecule. Hence, one hemoglobin molecule can bind four O2 molecules. This is conventionally represented as Hb4O8. The four Hb components can alter their orientation to favor uptake or release of the oxygen. When the Hb bonds are relaxed, they favor uptake, and when they are tense they favor release. Affinity is the chemical term used to indicate how eager multiple units are to interact with one another. The chemical affinity for the first oxygen to bind is lower than the affinity for the later oxygens to bind. In other words, once one oxygen has bound to the hemoglobin, the binding of the other three oxygen molecules is more favorable. In addition, the amount of oxygen bound or released depends on the concentration of oxygen in two locations (where it is coming from and where it is being absorbed).
Oxygen flow is greatly determined by local partial pressure gradient. Just like it is more difficult to push water up a waterfall, so it is difficult to absorb oxygen into an area that already has more oxygen than the place it is coming from. In both cases, a certain amount of pressure is causing something to flow one way. As rbcs travel through arteries and veins around the human body, they collect oxygen in the alveoli where the partial pressure of oxygen is higher than it is in the rbcs. Usual average atmospheric pressure is measured as 1 atmosphere (1 atm) or 14.7 pounds per square inch (14.7 psi). Since oxygen makes up about 21% of atmospheric air, the partial pressure of oxygen is 0.21 atm or 3.09 psi. Because venous oxygen partial pressure is less than 3.09 psi, oxygen is driven into the blood in the lungs. Although a small amount of O2 gas dissolves into the plasma (the fluid surrounding the blood cells), most is bound by hemoglobin. The reverse process occurs as capillaries supply tissues with oxygen. The partial pressure of oxygen in the tissue is lower than in the blood, so oxygen flows into the tissue. CO2 travels a reverse course where high tissue partial pressures push CO2 out into the veins that carry it to the lungs for release into the atmosphere. The CO2 partial pressure of the atmosphere is significantly lower than that of body tissues. The relationship of gaseous absorption to atmospheric pressure makes it crucial for mountain climbers and scuba divers to calculate their expected partial pressure gaseous exposure before climbing or diving. Miscalculations could lead to death.
Body tissues vary in their oxygen dependency. Hypoxia is the condition of existing with a lowered oxygen supply. The brain and heart are the two most hypoxia sensitive organs. A severe drop in available oxygen can cause brain death in five minutes. Less severe hypoxia can lead to other mental problems such as dizziness, headache, disorientation, drowsiness, or impaired judgment. Although basic brain functions may recover fully from a short hypoxic period, higher neural functions can be severely impaired.
During rigorous exercise, oxygen demand may increase to up to 15 times the normal demand. As muscles deplete their oxygen supplies, the lowered muscular oxygen partial pressure steepens the pressure gradient, so that even more oxygen leaves the blood and enters the muscles. If aerobic metabolism is unable to supply enough ATP to muscle cells, then anaerobic metabolism can provide some ATP. However, anaerobic metabolism temporarily adds lactic acid to the muscle creating an oxygen debt whereby the muscle fatigues and requires a recovery period to get rid of the lactic acid. An initial oxygen debt is also always present for about the first 30 seconds of exercise until circulation can accelerate to provide additional oxygen.
Various life forms are classified on the basis of their tolerance or requirement of oxygen. Different types of bacteria are aerobic, facultatively aerobic, or anaerobic. Aerobes use oxygen to generate energy. Facultative aerobes can use oxygen but survive without it. Oxygen is highly toxic to anaerobes which die rapidly when exposed to it.
Guyton & Hall. Textbook of Medical Physiology. 10th ed. New York: W. B. Saunders Company, 2000.
Rhoads R., and R. Pflanzer, eds. Physiology. 2nd ed. New York: Saunders College Publishing, 1992.