Chapter 9 — Life Processes in Animals

It covers two key life processes in animals: nutrition (how food is digested and nutrients are absorbed) and respiration (how oxygen is used to release energy from food). The chapter focuses on humans and then compares these processes across other animals such as ruminants, birds, fish, and frogs.

Digestion is the process of breaking complex food components into simpler forms. In humans it occurs along the alimentary canal — a long tube that starts at the mouth and ends at the anus, passing through the oesophagus, stomach, small intestine, and large intestine.

Saliva contains a digestive juice that breaks down starch (a carbohydrate) into sugar. It also moistens food, making it soft and easy to swallow. This is why starchy foods like chapati taste sweet after prolonged chewing — the starch has been converted to sugar by saliva.

The stomach walls contract and relax to churn the food, which is then mixed with a secretion containing digestive juice, acid, and mucus. The digestive juice breaks down proteins, the acid helps break down proteins and kills many harmful bacteria, and the mucus protects the stomach lining. Food leaves the stomach as a semi-liquid mass.

The small intestine (about 6 metres long — the longest part of the alimentary canal) receives bile from the liver (which neutralises acid and breaks down fats) and pancreatic juice from the pancreas (which breaks down carbohydrates, proteins, and fats). Its own lining also secretes digestive juice. Thousands of finger-like projections on its inner lining increase surface area so that digested nutrients pass efficiently into the blood, which carries them to all parts of the body.

The small intestine is about 6 metres long and is where most digestion and nutrient absorption takes place. The large intestine is only about 1.5 metres long but is wider — hence the name 'large'. It absorbs water and some salts from undigested food, turning it into semi-solid stool, which is stored in the rectum until expelled through the anus (egestion).

Breathing is a physical process — the inhalation and exhalation of air. Respiration is a chemical process that occurs inside the body: oxygen is used to break down glucose into carbon dioxide, water, and energy (Glucose + Oxygen → Carbon dioxide + Water + Energy). Breathing brings in the oxygen needed for respiration and removes the carbon dioxide produced.

The diaphragm is a dome-shaped muscle below the lungs. During inhalation, it moves downward while the ribs move up and outward, increasing the space inside the chest so air enters the lungs. During exhalation, the diaphragm moves upward and the ribs move down and inward, reducing chest space and pushing air out of the lungs.

Gas exchange takes place in the alveoli — small balloon-like sacs at the end of the finest branches of the windpipe inside the lungs. Alveoli have thin walls surrounded by fine tubes containing blood. Oxygen from the alveoli passes into the blood and is carried to all parts of the body; carbon dioxide from the blood is released into the alveoli and breathed out.

Rumination is the process in which grass-eating animals (such as cows and buffaloes) partially chew and swallow food into the stomach, then bring the partially digested food back to the mouth for thorough chewing. These animals are called ruminants. A cow spends about 8 hours a day chewing in this way.

Birds do not have teeth. Instead, they have a chamber called a gizzard. The gizzard's walls contract and relax to break down food, often with the help of grit (small stones) that the birds swallow, which assist in grinding the food mechanically.

Most aquatic animals like fish breathe through gills — specialised structures richly supplied with blood vessels through which oxygen and carbon dioxide are exchanged with the gases dissolved in water. Amphibians like frogs use different structures at different life stages: tadpoles breathe through gills, adult frogs use lungs on land and skin for gas exchange when in water. Earthworms use their moist skin for gas exchange.

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