Life Processes in Animals

Every living being, no matter how simple or complex, has to keep carrying out a set of basic activities just to stay alive. Together, these activities — taking in and using food (nutrition), releasing energy from food (respiration), transporting substances around the body (circulation), getting rid of waste (excretion), and producing new individuals (reproduction) — are called life processes.

Different animals get their food in very different ways. A hummingbird sips nectar from deep inside a flower. A newborn puppy survives entirely on its mother's milk for weeks. A python swallows its prey whole, without chewing at all. A clam sitting at the bottom of a pond filters tiny floating food particles out of the water around it. The methods look nothing alike, yet every one of these animals ends up facing the exact same problem afterward: the food they've taken in is far too complex, in its raw form, for the body to actually use.

Whether it's nectar, milk, prey, or filtered particles, that complex food has to be broken down into much simpler building blocks before cells anywhere in the body can use it for energy or growth. This breaking-down process is called digestion, and in this chapter, we'll trace exactly how it happens inside the human body, then compare it with a couple of very different animal strategies, before moving on to how animals breathe.

Digestion begins the moment food enters your mouth. Your teeth physically crush and grind it into smaller pieces — a purely mechanical first step. At the same time, saliva is released, and it does something more than just moisten the food: it contains a digestive substance that starts breaking down starch (a carbohydrate) into sugar. This is exactly why a plain, starchy food like a piece of roti can start tasting faintly sweet if you chew it for a long, unhurried minute — the starch in it is quite literally being converted into sugar right there in your mouth, before it has even been swallowed. You can confirm that starch has genuinely disappeared using a simple chemistry trick: iodine solution turns a deep blue-black colour wherever starch is still present, so unchewed starchy food reacts strongly with iodine, while thoroughly chewed starchy food barely reacts at all, since most of its starch has already been converted.

Once swallowed, food travels down a muscular tube called the oesophagus (or food pipe). It isn't simply falling down due to gravity — the walls of this tube squeeze and release in a travelling wave, physically pushing the food along, a motion that continues the entire length of the digestive tube from here onward.

In the stomach, muscular walls churn the food vigorously while a mix of digestive juice, strong acid, and protective mucus is added. The acid does double duty: it helps break proteins apart, and it kills off a large share of harmful microbes that might have travelled in with the food. The mucus coating is essential too — without it, that same acid would start damaging the stomach's own lining. By the time food leaves the stomach, it has been turned into a thick, semi-liquid mixture.

This semi-liquid mixture then enters the small intestine — at roughly six metres, it is actually the longest section of the entire digestive tube, despite its name. Here, two other organs join in: the liver sends in bile, a mildly basic fluid that neutralises the acid arriving from the stomach and physically breaks large fat droplets into much tinier ones, making them far easier to digest. The pancreas sends in pancreatic juice, which is also basic and further breaks down carbohydrates, proteins, and fats together. The small intestine's own inner lining adds a final digestive juice to finish the job. Once nutrients are reduced to their simplest, absorbable form, they pass into the bloodstream through thousands of tiny, finger-like projections (called villi) lining the intestine's inner wall — these dramatically increase the surface area available for absorption, letting nutrients enter the blood efficiently and travel onward to every part of the body.

Whatever remains undigested moves into the large intestine. It's shorter than the small intestine (about 1.5 metres) but noticeably wider, which is actually why it earns the name "large." Its main job is absorbing leftover water and some salts from this waste, turning it into a semi-solid material called stool, which sits in the final section (the rectum) until it is eventually pushed out through the anus — a step called egestion. Along the way, helpful bacteria living in the large intestine assist by breaking down fibre, which is one reason fibre-rich foods support a healthy gut.

Cows and buffaloes feed on grass, which is tough and fibrous — difficult to break down properly with just one round of chewing. So these animals swallow partly-chewed grass into a specialised stomach chamber where it sits and partially digests. Later, often while resting rather than actively grazing, that same partly-digested mouthful travels back up into the mouth for a second, much more thorough round of chewing, before finally being swallowed for good. This two-stage process is called rumination, which is why animals that do this are called ruminants — and it also explains why a resting cow can often be seen steadily chewing even though there's no fresh grass anywhere near its mouth. Altogether, a cow may spend several hours a day on this re-chewing alone.

Birds take an entirely different approach, mainly because they have no teeth at all. Instead, swallowed food travels into a muscular chamber called the gizzard, where powerful contractions grind it down — often with help from small stones or grit that the bird deliberately swallows beforehand, which act almost like built-in grinding tools inside the gizzard.

Both strategies solve the exact same underlying problem — breaking tough, complex food down into something the body can absorb — using completely different structures suited to what each animal actually eats.

These two words get mixed up constantly, but they describe two genuinely different things. Breathing is the physical, mechanical act of drawing air into the body and pushing it back out. Respiration is the chemical process happening inside your cells that actually uses the oxygen from that air to release energy from digested food. Breathing is what delivers the raw material; respiration is what actually puts it to use.

Air enters through the nostrils, where tiny hairs and mucus trap dust before it can travel further in — which is exactly why breathing through the nose, rather than the mouth, filters incoming air far more effectively. From there, air passes through the nasal passages, down the windpipe, and into branching tubes inside the lungs that split into finer and finer pathways, finally ending in millions of tiny, balloon-like air sacs called alveoli. Each alveolus is wrapped in a dense mesh of extremely fine blood vessels, and it's here — and only here — that oxygen actually crosses into the blood while carbon dioxide crosses out of it.

The physical act of breathing works through a neat mechanical trick. On inhalation, the ribs move upward and outward while a dome-shaped muscle beneath the lungs, called the diaphragm, moves downward — acting together, these two movements make the chest cavity bigger on the inside, pulling air inward to fill that extra room. On exhalation, both movements reverse: the ribs move down and inward while the diaphragm moves back upward, shrinking that space and pushing air back out. You can demonstrate this exact principle with a simple model built from balloons tied inside a bottle with a stretched rubber sheet across its open base — pulling the sheet downward enlarges the space inside and inflates the balloons, mimicking inhalation, while releasing it does the reverse.

Because of respiration happening continuously inside the body, air you breathe out is chemically different from air you breathe in — it carries noticeably more carbon dioxide (roughly 4-5%, compared to around 0.04% in the air you inhale) and somewhat less oxygen (roughly 16-17%, down from about 21% in inhaled air). Notice that exhaled air still contains a fair amount of oxygen — not all of the inhaled oxygen gets used up in a single breath. This whole chemical process, happening inside cells throughout the body, is summarised by one equation:

Glucose + Oxygen → Carbon dioxide + Water + Energy

Finally, none of this oxygen would be useful if it couldn't actually reach the rest of the body. That job belongs to the circulatory system — the heart, blood, and blood vessels working together. The heart pumps blood continuously through the vessels, carrying oxygen (picked up at the alveoli) and digested nutrients (absorbed in the small intestine) to every part of the body, while also carrying waste products away.

Fish live permanently underwater, so lungs — built for air — wouldn't work for them at all. Instead, fish rely on gills: thin, feathery structures richly packed with fine blood vessels. As water flows continuously over the gills, dissolved oxygen in that water crosses directly into the blood, while carbon dioxide crosses the other way, out into the water.

Amphibians like frogs tell an even more interesting story, because they actually switch breathing methods as they grow. A tadpole, which lives entirely underwater, breathes using gills, just like a fish. As it develops into an adult frog capable of living on land, it develops lungs for breathing air — but it doesn't abandon gas exchange through its skin either, which stays useful especially while the frog is back in water. This flexible, two-stage system is exactly what lets the same animal survive comfortably in both environments at different points in its life.

Earthworms take a completely different route: they have neither lungs nor gills. Instead, their constantly moist skin itself acts as the exchange surface, letting oxygen and carbon dioxide pass directly through it.

Looked at together, lungs, gills, and moist skin are really three different engineering solutions to the exact same underlying need — getting oxygen into the body and carbon dioxide out of it — each one shaped by the specific habitat the animal actually lives in.