Nutrition in Animals

Animals, including human beings, are heterotrophs — they cannot make their own food and must take it in ready-made from plants or other animals. But food cannot be used by the body in the form in which it is eaten; a piece of bread or a fruit must be broken down into simple, soluble substances before the body can use it. The whole sequence of taking in food and processing it is called nutrition in animals, and it takes place in a series of clear steps.

There are five steps of nutrition. The first is ingestion, which is the taking in of food into the body, usually through the mouth. The second is digestion, the breaking down of complex food into simple, soluble substances that the body can absorb; this is done with the help of digestive juices containing chemicals called enzymes. The third step is absorption, in which the digested, soluble food passes through the wall of the digestive tract into the blood.

The last two steps complete the process. The fourth step is assimilation, in which the absorbed food is carried by the blood to all the cells of the body, where it is used for energy, growth, and repair. The fifth and final step is egestion, the removal of the undigested part of the food from the body as waste. Together, these five steps — ingestion, digestion, absorption, assimilation, and egestion — make up the complete process of nutrition in animals.

Different animals carry out these steps in different ways, depending on their food and body design. Some, like the tiny Amoeba, have very simple methods, while others, like humans and cattle, have a long and complex digestive system with many specialised parts. In the rest of this chapter we look mainly at how nutrition takes place in human beings, and then at a few other animals, but in every case the same five basic steps are involved.

In human beings, nutrition is carried out by the digestive system, which is a long, coiled tube called the alimentary canal along with several digestive glands. The alimentary canal runs from the mouth to the anus and includes the mouth (buccal cavity), the food pipe (oesophagus), the stomach, the small intestine, and the large intestine. Food travels through these parts in order, being digested step by step along the way. In this topic we follow the food from the mouth to the stomach.

Digestion begins in the mouth (buccal cavity). Here the teeth cut, tear, and grind the food into smaller pieces (a kind of physical digestion called mastication or chewing), while the tongue mixes the food and helps in tasting and swallowing. The salivary glands pour out saliva, which moistens the food and contains an enzyme (salivary amylase) that begins the digestion of starch, turning it into sugar. This is why a piece of bread or rice begins to taste slightly sweet if chewed for a while.

The chewed, moistened food, now a soft ball called a bolus, is swallowed and passes into the food pipe (oesophagus). The oesophagus does not simply let the food drop; its muscular walls push the food downward by rhythmic waves of contraction called peristalsis. This same wave-like movement, peristalsis, moves food along the entire alimentary canal, which is why we can even swallow while lying down or upside down.

The food then reaches the stomach, a stretchable, bag-like organ. The muscular walls of the stomach churn the food and mix it with gastric juice, which contains hydrochloric acid (that kills germs and provides an acidic medium), mucus (that protects the stomach lining), and enzymes that begin the digestion of proteins. After a few hours of churning and chemical action, the food becomes a semi-liquid paste, ready to move on to the small intestine for the next stage of digestion.

After leaving the stomach, the semi-digested food enters the small intestine, which is the longest part of the alimentary canal — coiled up to fit inside the body, it would measure several metres if stretched out. The small intestine is where digestion is completed and where most of the absorption of digested food takes place. To do its work, it receives the help of two important digestive glands: the liver and the pancreas.

The liver is the largest gland in the body. It produces a greenish-yellow juice called bile, which is stored in the gall bladder and released into the small intestine. Bile does not contain enzymes, but it helps in the digestion of fats by breaking large fat droplets into tiny ones (a process like the way soap breaks up grease), making them easier to digest. The pancreas produces pancreatic juice, which contains enzymes that digest carbohydrates, proteins, and fats. The walls of the small intestine also release intestinal juice, completing the digestion of all the food types into their simplest forms — carbohydrates into glucose (simple sugars), proteins into amino acids, and fats into fatty acids and glycerol.

Once digestion is complete, the absorption of this simple, soluble food takes place in the small intestine. Its inner wall is covered with millions of tiny finger-like projections called villi (singular: villus). The villi greatly increase the surface area of the intestine, so that the digested food can be absorbed quickly and efficiently. Each villus has a network of fine blood vessels, into which the absorbed glucose, amino acids, and other nutrients pass to enter the bloodstream.

The blood then carries the absorbed nutrients to all the cells of the body in the step called assimilation, where the cells use them for energy, growth, and repair. Thus the small intestine, with the help of the liver and pancreas and its absorbing villi, is the most important region for completing digestion and absorbing food — making it the true centre of the digestive process.

After the small intestine has absorbed the useful, digested food, the remaining material — which is undigested food and water — passes into the large intestine. The large intestine is shorter but wider than the small intestine. Its main job is not digestion but the absorption of water and some salts from the undigested food. As water is absorbed, the leftover material becomes more solid, forming the waste called faeces.

The faeces are stored temporarily in the last part of the large intestine (the rectum) and are finally removed from the body through the anus. This removal of undigested waste is the step called egestion, the last of the five steps of nutrition. Egestion should happen regularly; if the waste stays too long and too much water is absorbed, it becomes hard and difficult to pass, a condition called constipation. Eating fibre-rich food (such as fruits, vegetables, and whole grains) and drinking enough water help keep egestion smooth and the digestive system healthy.

Because digestion begins in the mouth with the teeth, taking care of our teeth is an important part of healthy nutrition. Humans have different kinds of teeth for different jobs: incisors for cutting and biting, canines for tearing, and premolars and molars for grinding and chewing. We grow two sets of teeth in life — a first set of milk teeth in childhood, which are later replaced by the permanent teeth.

Teeth must be cared for to keep them healthy. When we eat sugary food and do not clean our teeth, bacteria in the mouth act on the food particles and produce acids, which slowly damage the hard enamel of the teeth and cause tooth decay (cavities). To prevent this, we should brush our teeth twice a day, especially after meals, avoid eating too many sweets, rinse the mouth after eating, and visit the dentist regularly. Good dental care protects the very first stage of digestion and keeps our whole nutrition process working well.

Although the basic steps of nutrition are the same in all animals, different animals have special ways of digesting their food to suit what they eat. Two interesting examples are the ruminants (grass-eating animals like cows and buffaloes) and the tiny single-celled Amoeba, which show how digestion can be adapted both in large and in microscopic animals.

Ruminants are grass-eating (herbivorous) animals such as cows, buffaloes, goats, and deer. Grass contains a large amount of cellulose, which is hard to digest. Ruminants have a special four-part stomach, the first part of which is a large chamber called the rumen. When a ruminant grazes, it quickly swallows large amounts of grass, which is stored and partly digested in the rumen, where helpful bacteria break down the cellulose. Later, when the animal is resting, it brings this partly digested food (called cud) back into its mouth in small lumps and chews it again slowly and thoroughly. This chewing of the cud is called rumination, which gives ruminants their name. The well-chewed food is then swallowed again for complete digestion.

The Amoeba is a tiny, single-celled organism that lives in water. It has no mouth or digestive system, yet it still carries out all the steps of nutrition. Amoeba moves and feeds using temporary finger-like extensions of its body called pseudopodia ("false feet"). When it senses food, it surrounds the food particle with its pseudopodia and engulfs it, trapping it inside a food vacuole — a little bag of fluid within its body. This process of engulfing food is its method of ingestion.

Inside the food vacuole, digestive juices break the food down into simple substances, which are then absorbed into the body of the Amoeba and used for energy and growth (assimilation). The undigested remains are removed when the food vacuole reaches the surface and the waste is thrown out (egestion). Thus, even a single tiny cell like Amoeba performs ingestion, digestion, absorption, assimilation, and egestion, showing that the five steps of nutrition occur in animals of every size — from the microscopic Amoeba to the large grazing cow.