2.7.1Unit 2 — Multicellular Organisms
Exchange Surfaces
5 min read 5 parts Lesson 35 of 39
Section 1 of 5The Need for Exchange
The Need for Exchange
Respiration
- Cells need a constant supply of oxygen and nutrients for respiration, which releases energy in the form of ATP.
Oxygen uptake
- Oxygen is absorbed in the lungs, where it diffuses from the air sacs called alveoli into the blood.
Nutrient uptake
- Glucose and amino acids are absorbed from digested food in the small intestine and pass into the blood through the villi.
Delivery
- The heart pumps the oxygenated, nutrient-rich blood in arteries, arterioles and capillaries to the cells that need them.
Section 2 of 5Carrying and Delivering Materials
1
Oxygen is carried around the body by the red blood cells, whereas dissolved nutrients are carried in the blood plasma.
2
At the capillaries within tissues, substances move between the blood and the cells, mostly by diffusion.
3
Respiring cells take in oxygen, glucose, amino acids, water and minerals from the blood.
4
Cells release carbon dioxide, urea and some water into the blood, which carries these wastes away.
5
Carbon dioxide is mostly carried dissolved in plasma to the lungs, where it diffuses into the alveoli and is breathed out.
Section 3 of 5Capillary Networks
Close to cells
- Capillary networks surround the tissues, so substances only have a short way to move between blood and cells.
Very thin walls
- The capillary wall is one cell thick, so the diffusion distance is short.
Narrow lumen
- The lumen is narrower than a red blood cell, which slows blood flow and gives more time for exchange.
Large total area
- There are so many capillaries that their combined surface area is very large, giving rapid exchange of materials.
| Moves into cells | Moves out of cells |
|---|---|
| Oxygen, glucose, amino acids | Carbon dioxide |
| Water and minerals | Urea and some water |
Section 4 of 5Maintaining Diffusion
Maintaining Diffusion
Ventilation
- Breathing keeps the carbon dioxide concentration in the alveoli low so that diffusion out of the blood continues.
Blood flow
- Moving blood carries absorbed substances away, so the concentration gradient at each exchange surface stays steep.
Waste substances
- Other waste substances carried in the blood include urea, excess ions and excess water.
Overall idea
- Exchange surfaces, the heart and blood vessels work together as one system to supply cells and remove wastes.
Section 5 of 5Exam Checklist & Tips
2.7.1 – Key Points for the Exam
- Exchange surface A surface across which substances are absorbed or released.
- Diffusion The net movement of particles from a region of higher concentration to lower concentration.
- Alveolus A tiny air sac in the lungs where gas exchange occurs.
- Villus A finger-like projection of the small intestine that absorbs nutrients.
- ATP The molecule that provides the energy released by respiration for cell activities.
Common Mistakes
- Saying that oxygen is carried in the plasma, when it is carried by red blood cells.
- Writing that exchange takes place in arteries and veins; it takes place in the capillaries.
- Forgetting that waste substances such as urea must be removed as well as carbon dioxide.
- Stating that cells make oxygen for respiration, instead of taking it up from the blood.