Chapter 9: Life Processes in Animals

Nutrition, circulation, respiration, excretion and reproduction are called life processes because every living being needs them to survive. The food we eat contains complex substances such as carbohydrates, proteins and fats. They must be broken into simpler forms before the body can use them. This breaking down is called digestion. It takes place in a long tube called the alimentary canal, which starts at the mouth and ends at the anus. The parts of this canal, in order, are the mouth, food pipe (oesophagus), stomach, small intestine, large intestine, rectum and anus. The liver and the pancreas are helper organs that pour their juices into the canal. Teeth crush and chew food into small pieces; this is mechanical digestion. Digestive juices then break food into simple substances, which is chemical digestion.

In the mouth, teeth chew the food and saliva softens it. Saliva also contains a digestive juice that begins to break starch into sugar, which is why starchy food tastes sweet when chewed for long. The tongue mixes the food with saliva and pushes it into the food pipe (oesophagus). The walls of the food pipe contract and relax in waves and push the food down to the stomach. In the stomach, the walls contract and relax to churn the food, which is mixed with a secretion of the stomach lining. This secretion has a digestive juice that breaks down proteins, an acid that helps in this work and kills many harmful bacteria, and mucus that protects the stomach lining from the acid. The food becomes a semi-liquid mass.

The small intestine is the longest part of the alimentary canal, about 6 metres long, though it is called small because it is narrow. It receives juices from three sources: its own lining, the liver and the pancreas. The liver makes bile, which is mildly basic; it neutralises the acid coming from the stomach and breaks fats into tiny droplets. The pancreas makes pancreatic juice, also basic, which neutralises acid and digests carbohydrates, proteins and fats. The juice of the small intestine finishes the digestion. The digested nutrients pass into the blood through the walls of the small intestine; this is absorption. The inner lining has thousands of tiny finger-like projections, called villi, which greatly increase the surface area for absorption. The blood carries the nutrients to all parts of the body, where they give energy and help in growth and repair.

The part of food that the small intestine cannot digest moves into the large intestine. It is about 1.5 metres long, shorter than the small intestine, but called large because it is wider. It absorbs most of the remaining water and some salts from the undigested food, making the waste semi-solid; this is the stool. The stool is stored in the lowest part, the rectum, until the body is ready to remove it through the anus. This removal of undigested waste is called egestion. Fibre in fruits, vegetables and whole grains makes the stool easy to pass. Useful bacteria in the large intestine help digest fibre and make some nutrients, and fermented foods such as curd and buttermilk support them. Eating at regular times, chewing food well and avoiding overeating keep the gut healthy.

Different animals have different kinds of alimentary canals, suited to the food they eat. Grass-eating animals such as cows, buffaloes and goats first chew the grass only a little and swallow it into a large stomach part called the rumen, where partial digestion takes place. The partly digested food is brought back to the mouth and chewed again slowly; this is rumination, and such animals are called ruminants. A cow spends about eight hours a day chewing the cud. The well-chewed food then passes down the canal for further digestion. Birds have no teeth, but they have a muscular chamber called the gizzard. Its walls contract and relax and crush the food, often with the help of grit (tiny stones) that the bird has swallowed.

Taking air in is inhalation and sending air out is exhalation; together they are called breathing. The organs that help us breathe form the respiratory system. Air enters through the nostrils into the nasal passages, where tiny hairs and mucus trap dust and dirt, so it is better to breathe through the nose. The air then goes down the windpipe, which divides into two branches entering the two lungs. The branches divide into finer tubes ending in tiny balloon-like sacs called alveoli. The lungs are protected by the rib cage, and below them lies a dome-shaped muscle, the diaphragm. During inhalation, the ribs move up and outwards and the diaphragm moves down, so the chest cavity becomes larger and air rushes in. During exhalation, the ribs move down and inwards and the diaphragm moves up, so the chest becomes smaller and air is pushed out.

Fresh air fills the alveoli, which have thin walls surrounded by fine blood vessels. Here oxygen passes from the air into the blood, and carbon dioxide passes from the blood into the air, which is breathed out. This is the exchange of gases. Inhaled air has about 21% oxygen and about 0.04% carbon dioxide, while exhaled air has about 16 to 17% oxygen and 4 to 5% carbon dioxide; so not all the oxygen is used. When we pass exhaled air into lime water, it turns milky because of carbon dioxide. Inside the body's cells, oxygen breaks down glucose to release energy: glucose + oxygen gives carbon dioxide + water + energy. This is respiration. Breathing is a physical process and respiration is a chemical process inside the body. The circulatory system, with the heart, blood and blood vessels, carries oxygen and nutrients to all parts of the body and brings back waste.

Different animals have different ways of breathing to suit their habitats. Animals such as birds, elephants, lions, cows, goats, lizards and snakes breathe through lungs, though the structure of the lungs differs. Most water animals such as fish breathe through gills, which are rich in blood vessels; oxygen dissolved in water passes into the blood and carbon dioxide passes out across the gills. Amphibians such as frogs live on land and in water and use different parts at different stages: tadpoles breathe through gills, while adult frogs use lungs on land and their skin when in water. An earthworm breathes through its moist skin, which takes in oxygen and gives out carbon dioxide. This shows how animals have adapted over time to different environments.