A study guide to two body systems. It follows food through the digestive system, explains mechanical and chemical digestion, what the enzymes amylase, protease and lipase do and how villi absorb nutrients. It then follows air through the respiratory system, explains how the diaphragm and rib muscles move air in and out, how gas exchange happens in the alveoli, and how both systems supply cells for cellular respiration, with check-yourself questions and answers.
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Curriculum: Texas grade levels (TEKS). We show specific outcome codes only where they have been verified against the official curriculum document.
Completed: The Digestive and Respiratory Systems
8th Grade students (typically ages 13–14) working on the human body. It is pitched at level 2 · standard.
This is a study guide, so there is no separate answer sheet; the 'Check yourself' questions include answers.
About 30 minutes. Short, regular sessions work best: two or three a week beats one long one.
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Try The Circulatory System: The Heart, Blood Vessels and Blood.
Success TutoringWorksheets8th Grade · Science · The Human BodyEvery cell in your body needs energy. Cells release energy from glucose in a process called cellular respiration: glucose + oxygen → carbon dioxide + water (energy is released). The digestive system supplies the glucose and other nutrients, the respiratory system supplies the oxygen and removes the carbon dioxide, and the circulatory system carries all of these to and from the cells.
Digestion breaks large, insoluble food molecules into small, soluble molecules that can pass into the blood. Mechanical digestion physically breaks food into smaller pieces (chewing, and the churning of the stomach). Chemical digestion uses enzymes to break the molecules themselves into smaller molecules.
| Organ | What happens there |
|---|---|
| Mouth | Teeth chew food (mechanical digestion). Saliva moistens food and contains amylase, which starts breaking down starch |
| Esophagus | A muscular tube that pushes food to the stomach by waves of muscle contraction called peristalsis |
| Stomach | Muscular walls churn food. Gastric juice contains hydrochloric acid, which kills many microbes, and protease enzymes, which start digesting protein |
| Small intestine | Enzymes from the pancreas and the intestine wall finish digestion; bile from the liver breaks fat into small droplets. Digested nutrients are absorbed into the blood through the villi |
| Liver | Makes bile, which is stored in the gall bladder and released into the small intestine |
| Pancreas | Makes enzymes (amylase, protease and lipase) and releases them into the small intestine |
| Large intestine | Absorbs most of the remaining water from undigested food; helpful bacteria live here |
| Rectum and anus | The rectum stores solid waste until it leaves the body through the anus |
| Enzyme | Breaks down | Into |
|---|---|---|
| Amylase | Starch | Sugars |
| Protease | Proteins | Amino acids |
| Lipase | Fats and oils (lipids) | Fatty acids and glycerol |
The lining of the small intestine is covered in millions of tiny finger-like folds called villi. They give a very large surface area, have thin walls and a rich blood supply, so digested nutrients pass quickly into the blood.
Air enters through the nose or mouth, where it is warmed and moistened. It travels down the trachea (windpipe), which is held open by rings of cartilage, then into two bronchi (one to each lung), which branch into thousands of smaller tubes called bronchioles. These end in tiny air sacs called alveoli.
| Breathing in (inhaling) | Breathing out (exhaling) | |
|---|---|---|
| Diaphragm | Contracts and flattens (moves down) | Relaxes and domes upwards |
| Rib (intercostal) muscles | Pull the ribs up and out | Ribs move down and in |
| Volume of chest | Increases | Decreases |
| Air pressure in lungs | Decreases, so air flows in | Increases, so air flows out |
Oxygen diffuses from the air in the alveoli into the blood in the surrounding capillaries, because there is more oxygen in the alveoli than in the blood. Carbon dioxide diffuses the other way, from the blood into the alveoli, and is breathed out. Alveoli are well suited to this: there are millions of them, giving a huge surface area; their walls are only one cell thick; they are moist; and they are surrounded by many capillaries.
| Gas (approximate) | Inhaled air | Exhaled air |
|---|---|---|
| Oxygen | 21% | 16% |
| Carbon dioxide | 0.04% | 4% |
Most of the rest of the air is nitrogen, which the body does not use. Exhaled air also contains more water vapor than inhaled air.
Calculating air breathed per minute
Answers: 1. glucose + oxygen → carbon dioxide + water (energy is released). 2. Mechanical digestion breaks food into smaller pieces without changing the molecules (for example, chewing or stomach churning); chemical digestion uses enzymes to break large molecules into smaller ones (for example, amylase breaking starch into sugars). 3. Protease; amino acids. 4. Any two of: very large surface area; thin walls; rich blood supply. 5. Nose, trachea, bronchi, bronchioles, alveoli. 6. It contracts and flattens. This increases the volume of the chest, lowering the air pressure inside the lungs, so air flows in from outside where the pressure is higher. 7. It is produced by cellular respiration in the body's cells, carried by the blood to the lungs and diffuses into the alveoli. 8. 12 × 2 = 24 breaths per minute.
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