There are 5 methods on how is energy used in the human body like
1. Physical exertion
2. Mental exertion
3. Physiological functions of the body
4. metabolic reactions.
5. Energy lost by dissipation.
Energy is the critical requirement for the body that it doesn’t survive even a minute without it.
Hence, cyanide pill used as for suicide is so effective in causing death. It causes rapid energy failure (mitochondrial arrest) leading to immediate death.
Even during sleep, huge amounts of energy is consumed by the body.
But unlike animals, the human body requires more energy as his needs and activities are more.
Before we go into details on how is energy used in the human body, we need to first see how energy is present and also produced in the human body.
There are three types of energies in the body.
1. Chemical energy
2. Electrical energy (nerves and muscles)
3. Heat energy.
Though we see three types of energies above, the chemical energy in the form of adenosine triphosphate (ATP) is the base.
It is produced by biological oxidation of food in the presence of oxygen.
From this chemical energy, electric energy is formed in nerves to conduct signals with electric speed.
And also heat energy is formed in the muscles, to maintain body temperature.
How is Energy Used in the Human Body
Physical exertion: This is the way most of the energy of the body is spent in living animal and humans beings. This is done due to the intention to move, perform or work. This may not be seen in plants as they do not move. The body energy is also consumed for muscle contractions and relaxation. We see this during walking, running, lifting, jumping, talking, shouting, etc. Further body language also consumes energy. So you can notice that when you are hungry your body language will be low and ineffective.
Mental exertion: This is the next way how energy is used in higher animals like humans. Good amount of energy is consumed for thinking, learning, memorizing. Hence during the heartbeat, at least 20% of blood pumped from the heart goes directly into the brain.
This brain is a high energy demanding organ, so it requires more glucose and oxygen from the blood. Besides, anxiety and other behavior related functions need energy. Hence under stress, you can notice eating more will reduce stress. So Body first tries to fill up this energy need more than other energy needs. Even animals do some sort of brain and behavioral activity but low, unlike humans.
Physiological functions: All the physiological functions in the body need energy. These include heartbeat, blood flow, breath (inspiration & expiration), digestion, intestinal contractions, etc. require a constant supply of energy. These are involuntary in nature without the intention of the animals. But are always active and energy demanding. The body energy is mostly used up due to smooth muscle contractions in the gut, blood vessels, lungs. The cardiac muscle in the heart is even more active and high energy is demanding. Also, blood vessel carries blood to corners of the body and bring back by veins by contractions and relaxations.
Body metabolic and synthetic reactions: Most of the body cell reactions like the formation of proteins (translation), multiplication of genes, the formation of genes demand energy. Also, transport of salts, nutrients across the cell membrane, degradation of toxic waste (metabolism) require energy.
Further enzymes are the key factors involved in the above reactions. These enzymes perform only at body temperature. So the body tries to maintain heat at a constant by heat generation.
Energy wastage: The body energy formed is quite valuable, but still, it is lost. Here the energy is dissipated from the body in the form of heat. The body tries to maintain a constant fixed temperature to keep up the physiology. This temperature is generated by burning calories in muscles. During cold weather or atmospheric temperature, the body loses heat. So you can remember during winter you will feel more hunger and crave for food.
Also, energy is lost when the material goes out of the body. This we prominently see during breathing, hot air goes out and fresh air comes in. Also to some extent heat is lost by due to urination, bowels evacuation.
So the energy formed is thus used up by four ways and lost by one way.