What this chapter is about
All living things carry out certain activities to stay alive. These activities are called life processes. Animals, including humans, must eat food, breathe air, remove waste, move, grow and respond to their surroundings. This chapter looks at how these processes happen inside an animal's body.
You will learn how animals get energy from food through digestion, how oxygen reaches every part of the body through respiration and circulation, and how waste products are removed through excretion. You will also see how the nervous system helps animals sense and respond to changes around them.
By the end of this chapter, you should be able to explain how different organ systems work together to keep an animal alive and healthy. You will understand why eating, breathing and removing waste are all connected.
Key ideas
- Nutrition is the process of taking in food, breaking it down and using it for energy and growth. Animals are heterotrophs, meaning they cannot make their own food and must eat plants or other animals.
- Digestion breaks large food molecules into smaller ones that can be absorbed into the blood. In humans, this happens in the alimentary canal (mouth, food pipe, stomach, small intestine, large intestine).
- Respiration is the process of releasing energy from food. It needs oxygen and produces carbon dioxide and water as waste. Lungs help land animals breathe; gills help fish breathe underwater.
- Circulation is the movement of blood through the body. The heart pumps blood. Blood carries oxygen and digested food to all cells and brings back waste like carbon dioxide.
- Excretion is the removal of waste products made inside the body. The kidneys filter blood and produce urine. The lungs remove carbon dioxide. The skin removes some salts and water as sweat.
- Response to stimuli means reacting to changes in the environment. The nervous system (brain, spinal cord, nerves) and sense organs (eyes, ears, nose, tongue, skin) help animals detect and respond.
- All life processes are connected. Without digestion, the body has no energy. Without respiration, cells cannot use food. Without circulation, nutrients and oxygen cannot reach cells.
Formulas and facts to remember
1. Alimentary canal: mouth → food pipe (oesophagus) → stomach → small intestine → large intestine → anus.
2. Digestion in the mouth: saliva contains enzymes that start breaking down starch into sugar.
3. Stomach acids: the stomach makes hydrochloric acid, which kills germs and helps digest proteins.
4. Small intestine: most digestion and absorption happens here; finger-like villi increase the surface area.
5. Respiration equation (in words): food (glucose) + oxygen → energy + carbon dioxide + water.
6. Heart chambers: the human heart has four chambers — two atria (upper) and two ventricles (lower).
7. Kidneys: two bean-shaped organs that filter blood and remove waste as urine.
8. Sense organs and what they detect: eyes (light), ears (sound), nose (smell), tongue (taste), skin (touch, heat, pain).
Worked examples
Example 1: Tracing the path of a chapati
Rahul eats a small piece of chapati. Describe what happens to it inside his body.
Step 1: In the mouth, teeth chew the chapati into smaller pieces. Saliva mixes with it and starts breaking starch into sugar.
Step 2: The food travels down the food pipe to the stomach. Stomach juices mix with it and break it down further.
Step 3: In the small intestine, digestive juices complete the breakdown. The nutrients pass through the intestine wall into the blood.
Step 4: Blood carries the nutrients to all body cells. Undigested parts move to the large intestine, where water is absorbed. The remaining waste is removed through the anus.
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Example 2: Why do we breathe faster after running?
Priya runs 100 metres and notices she is breathing fast. Explain why.
Step 1: When Priya runs, her muscles work hard and need more energy.
Step 2: Energy comes from respiration, which uses oxygen and releases carbon dioxide.
Step 3: To supply more oxygen and remove extra carbon dioxide, her lungs must take in more air. So she breathes faster.
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Example 3: Role of kidneys
Arjun drinks two litres of water on a hot day. How does his body handle the extra water?
Step 1: Blood absorbs the water from the intestine.
Step 2: As blood passes through the kidneys, they filter out excess water and waste.
Step 3: The extra water leaves the body as urine. Arjun may need to urinate more often.
Common mistakes
Thinking respiration and breathing are the same thing → Breathing is inhaling and exhaling air; respiration is the process inside cells that releases energy from food.
Believing digestion happens only in the stomach → Most digestion and absorption happen in the small intestine, not the stomach.
Confusing arteries and veins → Arteries carry blood away from the heart; veins carry blood towards the heart.
Thinking the heart is on the left side only → The heart is in the middle of the chest, tilted slightly to the left.
Believing plants do not respire → All living things respire; plants respire day and night but photosynthesise only in light.
Quick revision
- Life processes keep an organism alive: nutrition, respiration, circulation, excretion, response to stimuli.
- Digestion turns food into simple, absorbable nutrients; the small intestine does most of this work.
- Respiration releases energy from food using oxygen; it happens in every cell.
- The heart pumps blood, which carries oxygen, nutrients and waste around the body.
- Kidneys filter blood and remove liquid waste as urine.
- The nervous system and sense organs help animals detect and respond to their surroundings.