Use a model to explain how the circulatory and respiratory systems transport materials and exchange gases to supply cells and remove wastes (Tennessee Academic Standards for Science, Biology I, BIO1.LS1).

What this topic is asking

The Tennessee LS1 standards ask you to model how the circulatory and respiratory systems transport materials and exchange gases so that cells get what they need and wastes are removed. For the Biology I EOC that means knowing the basic path of blood through the heart and body, how the lungs exchange gases, that gas exchange happens by diffusion, and how the two systems work together. Items often pair the two systems or ask how a gas crosses a membrane.

The circulatory system: transport

The basic path of blood is a double loop driven by the heart:

• Oxygen-poor blood from the body is pumped to the lungs, where it picks up oxygen and releases carbon dioxide.
• The now oxygen-rich blood returns to the heart and is pumped out to the body, where cells take up oxygen (and nutrients) and release carbon dioxide.
• The oxygen-poor blood then returns to the heart, and the cycle repeats.

Capillaries are the tiny vessels where exchange with cells happens; their thin walls let materials pass between blood and tissue. This transport role links to membrane transport, because materials cross cell membranes at the destination.

The respiratory system: gas exchange

The respiratory system (the lungs and the airways leading to them) brings air in and out and is the site of gas exchange. Deep in the lungs are millions of tiny air sacs called alveoli, surrounded by capillaries. Here:

• Oxygen in the inhaled air is at a higher concentration in the alveoli than in the blood, so it diffuses into the blood.
• Carbon dioxide is at a higher concentration in the blood than in the alveoli, so it diffuses out into the air sacs to be exhaled.

The alveoli are well suited to this: there are a huge number of them (a large surface area), and their walls are very thin (a short diffusion distance), so exchange is fast.

Gas exchange is diffusion

The EOC tests that gas exchange is diffusion, not active transport or osmosis. The heart pumps the blood, but the gases themselves cross by diffusion.

The two systems work together

The whole point of the standard is that the circulatory and respiratory systems are interdependent. The respiratory system exchanges gases with the air at the lungs; the circulatory system transports those gases between the lungs and the body cells. Oxygen picked up at the lungs is carried to cells, where it is used in cellular respiration to release energy; the carbon dioxide produced is carried back to the lungs to be exhaled. Neither system can supply the cells alone, which is why the body is described as a system of cooperating systems.

Try this

Q1. State how oxygen and carbon dioxide are exchanged in the lungs and by what process. [2]

• Cue. Oxygen diffuses from the air sacs (alveoli) into the blood, and carbon dioxide diffuses from the blood into the air sacs; the process is diffusion (down concentration gradients).

Q2. Explain how the circulatory and respiratory systems work together to supply a body cell with oxygen. [2]

• Cue. The respiratory system takes oxygen into the lungs and exchanges it into the blood; the circulatory system transports that oxygen in the blood to the body cells (and carries carbon dioxide back to the lungs).