Organ Metabolism: Facts and Figures About Metabolic Systems Activity of Our Organs in the Human Body
When discussing weight loss and dieting, it's common to hear the term "metabolism" thrown around. Most people are aware that metabolism has something to do with bodily energy consumption, but few know that each of the human body's organs and organ systems play a vital role in metabolic function. (The definition of metabolism is "the chemical processes that occur within a living organism to maintain life.")
The brain is important in respiratory metabolism, consuming 20% of the oxygen intake of the entire body! This is clearly a lot of oxygen, especially considering that the brain makes up only about 2% of the weight of an average adult. The brain’s oxygen consumption remains relatively constant regardless of levels of mental or physical activity. The brain uses glucose (blood sugar) as a fuel to provide energy for all brain functions. (It uses glucose to synthesize ATP, which is then used to maintain membrane potentials and nerve impulses.)
During periods of starvation, the brain can also consume circulating ketone bodies formed from body fat deposits. The brain needs a constant supply of energy, and loss of energy for even a short period (for example, in a stroke) can lead to catastrophic problems.
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Muscle also accounts for a large portion of the body’s energy use: 30% when sleeping, and up to 90% when engaging in vigorous physical activity. Muscle metabolism, like brain metabolism, produces ATP. Unlike in the brain, however, the ATP is used to power muscle contraction and relaxation. Muscle contraction is powered by motor nerve impulses, which cause calcium ions to be released from inside the membrane. These ions bind to regulatory proteins (troponin C) in the sarcoplasm, causing mechanical movement powered by ATP. The muscle shortens (contracts) as a result of these motions. The muscle relaxes when the calcium is pumped back into the sarcoplasmic reticulum.
Muscle fatigue occurs when the muscle can no longer produce enough power to fuel contractions and relaxations. Depending on your level of fitness, this can occur anywhere between twenty seconds and a minute.
Unlike the brain, the muscle is designed to utilize varying amounts of energy to serve the body’s needs.
The liver is a major player in human metabolism. Almost every nutrient that the body takes in is routed through the liver, which is responsible for processing the nutrients into usable forms and sending them where they need to go. One of the liver’s most important functions is to keep the level of blood glucose steady. It accomplishes this by releasing an optimal mix of hormones like glucagon, insulin, and epinephrine, all of which regulate blood sugar levels.
The liver also plays a role in the degradation of fatty acids. Under starvation conditions, the liver can break down triacylglycerols to form ketone bodies, which, as discussed earlier, can be utilized by the brain as an energy source in the absence of adequate blood sugar. On the other hand, when energy levels are high, the liver can reverse the process to store excess energy in the form of fat molecules.
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Like the brain, the heart acts in a very constant and predictable manner. Unlike the brain, however, the heart has the capacity to vary output to pump more oxygenated blood to active tissues under conditions of duress. The heart is aerobic and uses fatty acids as fuel using the citric acid cycle. The ATP produced stimulates contractions, which pump the blood through the arteries.
Body fat cells, also known as adipose tissue, actually form an organ system in and of itself. Besides storing energy for future use, fat cells manufacture various hormones that control appetite and body energy usage. Brown fat, one of the two types of adipose tissue, can also use a protein called thermogenin to produce heat. Brown fat is especially important in infants, who do not have any other voluntary means of regulating body temperature (such as putting on clothes, moving to warmer areas, etc). Of course, fat cells are also critical repositories of energy between meals and during periods of starvation.
Metabolism is an integrated process involving various different biochemical pathways such as the Krebs cycle, gluconeogenesis, and lipid catabolism. The human body has assigned specific roles to each organ, and all the organs work together to keep our body's metabolism functioning in tip top condition.