- Define and explain The Digestive System in your own words
- Use key terms such as double circulation accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on The Digestive System: trace food through the gut and learn how nutrients reach the blood.
Trace food through the gut and learn how nutrients reach the blood.
Key ideas
Digestion and absorption
Food is broken down physically by chewing and chemically by enzymes as it travels through the gut. In the small intestine, finger-like villi provide a huge surface area for absorbing digested nutrients into the blood. Each villus has a rich blood supply to carry nutrients away quickly and a wall only one cell thick, so diffusion is rapid.
The hierarchy of organisation
Cells with a similar structure and function form a tissue, such as muscle tissue. Different tissues combine into an organ, such as the stomach, which contains muscle, glandular and epithelial tissue. Organs work together in organ systems, such as the digestive system. Plants are organised the same way: mesophyll tissue forms part of a leaf, which belongs to the shoot system.
Key term — double circulation: The two-loop circulatory system in mammals: one loop carries blood to the lungs, the other to the rest of the body.
Put these in order from smallest to largest: organ, cell, organ system, tissue.
Cell → tissue → organ → organ system.
Answer: Cell → tissue → organ → organ system.
- Mixing up arteries and veins Arteries carry blood away from the heart (usually oxygenated); veins carry blood towards the heart (usually deoxygenated). The direction relative to the heart defines them, not the oxygen content.
- Thinking digestion finishes in the stomach The stomach churns food and starts protein digestion, but most chemical digestion and almost all absorption happen in the small intestine.
Practice
Amylase breaks down starch (carbohydrate); protease breaks down protein; lipase breaks down lipids (fats).
Any two of: large surface area; wall one cell thick for a short diffusion distance; rich blood supply to maintain a concentration gradient; many mitochondria for active transport.
The left ventricle pumps blood all around the body, which needs much higher pressure than pumping blood the short distance to the lungs, so its muscle wall is thicker.
Valves prevent the backflow of blood, keeping it moving in one direction through the heart and veins.
Quick check
Which of these best defines "double circulation"?
A student counts 21 pulse beats in 15 seconds after exercise. Calculate the heart rate.
- The Digestive System: trace food through the gut and learn how nutrients reach the blood.
- Digestion and absorption: Food is broken down physically by chewing and chemically by enzymes as it travels through the gut.
- tissue: A group of similar cells working together to carry out a particular function, such as muscle tissue or xylem tissue.
- Watch out for: mixing up arteries and veins