There are trillions of cells in our body. Different cells have different functions. Each cell has a life cycle. They perform their functions and at the end of their life cycle, they divide (except for neuronal cells). These functions are mostly chemical in nature. What happens when you perform a chemical reaction with an apparatus? With time, the apparatus loses efficiency. Something similar happens with the cells. These reactions, the genetic code and environmental pressures (local pressures in surroundings of cells, hormonal cues, etc.) bring them to the end of their life cycle with time. Then cells divide into two daughter cells (mitosis). The thing with the cellular division is, it isn’t perfect. One crucial aspect of cellular division is the division of chromosomes. Chromosomes are your genetic code (tightly packed DNA). The imperfect cellular division leads to trimming of chromosomes with each division. The trimming takes place at the end of chromosomes. But the end of chromosomes has extra-DNA repeats known as telomeres. I used the word extra because these telomeres don’t have any functional role in cells i.e. they are not involved in any cellular functions as per say. But they act as a protective buffer for the errors caused by cellular divisions. So, the trimming of chromosomes leads to loss of telomeres and not the genetic codes that are involved in actual cellular functions.
The new cells born of cell division are slightly “older” than the parent cells as they have shorter telomere lengths. And when the cells are eventually stripped out of telomere buffers, they stop dividing. Old age people have wrinkles, loose muscles because many of their cells have stopped dividing as their cells have run out of telomere buffers!

That was a short, basic explanation of why our body age. But what has it to do with the second law of thermodynamics? After all, both are taught in different subjects. Well, nature doesn’t work in subjects.

What does the second law tell? According to the second law, the entropy of the universe is always increasing. Entropy is the key word here. For years, I struggled with this term. I will make an attempt to explain entropy in a very basic way.

Look at this building.
Once upon a time, it was a glorious piece of architecture. All doors, windows, walls fully polished, furnished and well structured.
Let’s understand entropy with a question-answer game in relation to this building.

Q.What is this building made of in the beginning?
A. Bricks, cement, wooden doors, other building materials (paint, rod, etc.).

Q. How does it come into existence from those fundamental building materials?
A. The civil engineer made a plan. Many labourers contributed and worked together. These building materials were put together according to the plan in a proper structure.

Q. What are the building materials made of?
A. Particles, molecules, atoms.

Q. What happened to the building with time?
A. Environmental factors, forces such as wind, gravity acted on the building and its constituent particles started disassembling. Building started decaying with time.

Now, let us come back to entropy and understand it in the context of this building. Entropy is the measure of “how probable a state of particles is likely to be in that state”. It is meaningful when we talk about multiple particles. An isolated single particle has only one state. So, talking about its entropy is meaningless. In our building example, the “particles” are the constituent particles of building materials such as sand of cement, iron strips of steel rods. When the building is intact, it is in an organised structure. The particles occupy a specific position in relation to other particles. They have to occupy those specific positions for building to remain structurally sound and intact. Now, let’s assume that we have an isolated location. We throw all the constituent particles of the building at random (rolling a dice). How probable is it that particles would fall in such an exact way that the building would form on itself? It is almost impossible. Almost because there is no law in physics that denies those building particles from not forming the building structure on their own. But for that structure to form, individual particles have to occupy very specific positions. The number of these positions are very less and very specific. So, it’s probability is very low compared to other combinations where particles just pile up at random. The building structure is the state of low entropy.
Now, what the second law says is “universe always moves from a state of low entropy to a state of higher entropy”. It may appear that some systems are moving to a state of lower entropy (flowering of a plant, growth of a child). But, the overall entropy of the universe always increases. The way systems achieve a state of low entropy (organised state) is through the directional use of energy. But in the process, they directly or indirectly increases the entropy of the universe. During building construction, a lot of labourers, types of equipment were involved. Those together spent a lot of energy. During the process, they increased the entropy of the surroundings (sweating of labourer).

The universe is always moving from its lower entropy state to a higher entropy state. This movement is unidirectional. Hence, the movement of time is unidirectional. Past of the universe is a low entropy state than the present. Present will be a low entropy state compared to future. So, the time has to “move” forward.

Coming back to ageing. We age with time. The flow of time is explained by the second law. Hence, ageing in itself is a consequence of the second law. In fact, every change in this universe happening with time is the consequence of the second law of thermodynamics. Dwelling deeper into the life cycle of cells, they too decay with time. What is happening here? Remember the building decaying example. The same concept applies here. Cells too experience environmental pressures and are undergoing constant chemical reactions. They are structurally made of proteins which in turn are made of highly organised amino acids (molecules). So, the low entropy state of cells is a result of delicate and highly organised forces acting between various atoms and molecules. Along with these interactions, cells are burning constant energy and undergoing division to maintain this organisation. As I mentioned before the errors in the division lead to a stage where cellular division stops. The energy burning mechanism becomes less efficient with time. All this happens because some of their constituent atoms and molecules are constantly moving to a higher entropy state with time even though cells are burning calories to maintain its low entropy state (cellular organisation). All this slowly leads to un-organisation. What happens when you un-organise cells and their constituent molecular structures (proteins) in a body? Cellular and bodily functions start becoming lesser efficient than before. Errors start creeping into those functions that ultimately lead to ageing and death.