Describe how the circulatory and respiratory systems transport oxygen, carbon dioxide, and nutrients, and explain how their structures (such as alveoli and capillaries) suit gas exchange and delivery (MA STE HS-LS1-2, HS-LS1-3, structure and function).

What this topic is asking

The Massachusetts STE framework (HS-LS1-2) asks you to model how the body's systems interact to carry out the functions of life. For transport, that means the circulatory and respiratory systems working together to supply every cell with oxygen and nutrients and to carry away wastes. On the High School Biology MCAS, this is usually tested by asking how a structure (alveoli, capillaries) suits its function or how the two systems link. The crosscutting concepts are structure and function and systems and system models.

The respiratory system and gas exchange

When you breathe in, air travels down the airways to the alveoli. Here oxygen is at a high concentration in the air and low in the blood, so oxygen diffuses into the blood; carbon dioxide is high in the blood and low in the air, so it diffuses out to be breathed away. The exchange is purely passive, driven by the diffusion you met in the cell membrane and transport.

The alveoli are beautifully suited to this job, and the MCAS rewards explaining why:

• Large surface area. Millions of alveoli give a huge total area for diffusion.
• Thin walls. Each alveolus and capillary wall is only one cell thick, so gases cross a very short distance.
• Rich blood supply. Capillaries constantly carry away oxygenated blood and bring carbon-dioxide-rich blood, keeping the concentration gradients steep.

The circulatory system and transport

The circulatory system links the lungs to the rest of the body. Blood picks up oxygen at the alveoli and nutrients from the digestive system, and the heart pumps it through vessels to every cell. At the cells, oxygen and nutrients are delivered and carbon dioxide and other wastes are collected, then carried back to the lungs (for carbon dioxide) and other organs (for other wastes). The capillaries, the smallest vessels, again have thin walls so substances can pass easily between blood and cells.

Why the two systems must work together

The point the MCAS drives at is that no single system can supply a cell on its own. The respiratory system gets oxygen into the blood, but only the circulatory system can deliver it to a cell deep in a muscle. The oxygen delivered is used in aerobic respiration (see cellular respiration) to release energy, and the carbon dioxide that respiration produces is carried back to the lungs by the same blood. This cooperation is the theme of interacting body systems.

Try this

Q1. Name the process by which oxygen moves from the alveoli into the blood. [1]

• Cue. Diffusion (down its concentration gradient).

Q2. Explain how the circulatory system links the lungs to the body's cells. [2]

• Cue. Blood picks up oxygen at the lungs and the heart pumps it to every cell, while carbon dioxide is carried from the cells back to the lungs.