This chapter studies two key life processes in animals — nutrition (digestion) and respiration — mainly in humans, and then compares them across animals. Digestion is the breaking down of complex food components such as carbohydrates, proteins, and fats into simpler forms the body can use. It happens along the alimentary canal, which runs from the mouth to the anus. In the mouth, teeth carry out mechanical digestion and saliva begins breaking starch into sugar (shown by the iodine test, where chewed rice loses its blue-black colour). Food is pushed down the oesophagus by wave-like movements; in the stomach, digestive juice, acid, and mucus break down proteins and kill bacteria. The small intestine, about 6 metres long, is the longest part; it receives bile from the liver (which neutralises acid and breaks fats into droplets) and pancreatic juice, and its finger-like projections absorb nutrients into the blood. The large intestine absorbs water and salts, forming semi-solid stool stored in the rectum and removed through the anus (egestion). Different animals digest differently — ruminants like cows chew partly digested food again (rumination), and birds use a gizzard. In respiration, oxygen from inhaled air is used to break down glucose, releasing carbon dioxide, water, and energy. Breathing (a physical process) moves air through nostrils, nasal passages, windpipe, and lungs to balloon-like alveoli, where gases are exchanged with the blood; the diaphragm and ribs change the chest space during inhalation and exhalation. Exhaled air carries more carbon dioxide, shown by lime water turning milky. Other animals breathe through lungs, gills (fish), or moist skin (earthworms), and the circulatory system of heart, blood, and blood vessels carries nutrients and oxygen around the body.
Digestion and the alimentary canalDigestion in humans, ruminants, and birdsRespiration and breathing in humansGas exchange in the alveoliBreathing in other animals and the circulatory system
Key terms
Digestion
The process of breaking complex food components into simpler forms that the body can absorb and use.
Alimentary canal
The long tube from the mouth to the anus along which food is digested and absorbed.
Absorption
The passing of digested nutrients from the small intestine into the blood through finger-like projections.
Rumination
The process in animals like cows of bringing partially digested food back to the mouth for thorough chewing.
Respiration
The process by which oxygen is used to break down glucose, releasing carbon dioxide, water, and energy.
Breathing
The physical process of inhaling and exhaling air, moving it in and out of the lungs.
Alveoli
The tiny balloon-like air sacs in the lungs where oxygen and carbon dioxide are exchanged with the blood.
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