Develop and use a model to explain how the circulatory and respiratory systems transport substances and exchange gases to support cells (Louisiana Student Standards for Science, High School Biology, HS-LS1-2).

What this topic is asking

Louisiana's LS1 standards (HS-LS1-2) ask you to model how organ systems supply the body's cells, and the circulatory and respiratory systems are the central example. For LEAP 2025 Biology you should know what the circulatory system transports, how the respiratory system carries out gas exchange, that gases move by diffusion, and how the structure of exchange surfaces suits their function. The test often gives a model of the lungs or blood vessels and asks you to explain a feature or trace a substance.

The circulatory system: transport

The heart pumps blood through the vessels: arteries carry blood away from the heart, veins return it, and tiny capillaries allow exchange with the cells. Blood is the transport fluid, with red blood cells carrying oxygen. The circulatory system is what physically delivers the materials every cell needs and removes what it must get rid of.

The respiratory system: gas exchange

The respiratory system brings air into the lungs, where gas exchange takes place. The lungs contain millions of tiny air sacs called alveoli, surrounded by capillaries. Here, oxygen from the breathed-in air passes into the blood, and carbon dioxide from the blood passes into the air to be breathed out. The respiratory system supplies the oxygen and removes the carbon dioxide; the circulatory system carries them between the lungs and the cells.

Gases move by diffusion

This links directly to membrane transport: gas exchange is passive diffusion, not active transport.

Structure suits function at exchange surfaces

Exchange surfaces like the alveoli are adapted to make diffusion fast, an example of structure suiting function:

• A large surface area (millions of alveoli) gives more room for gases to cross.
• A thin wall (often one cell thick) means a short distance for gases to diffuse.
• A rich blood supply (many capillaries) keeps the concentration gradient steep by constantly bringing and removing blood.

The same principles apply to other exchange surfaces in the body, such as the lining of the small intestine, which absorbs nutrients.

Try this

Q1. State the main function of the circulatory system and name two things it transports. [2]

• Cue. Transport of materials around the body; it carries any two of oxygen, nutrients, carbon dioxide, wastes, or hormones.

Q2. State two features of the alveoli that make them well suited for gas exchange. [2]

• Cue. Any two of: large surface area, thin (one-cell-thick) wall, rich blood supply (to keep the concentration gradient steep).