B3.2 Transport

Transport systems in living organisms are designed to move essential materials , oxygen, nutrients, water, and waste , efficiently across distances that simple diffusion alone cannot cover. In this subtopic, we explore how animals use a blood circulatory system built around capillaries, arteries, and veins, and how plants use xylem vessels to move water and phloem to transport sugars and other assimilates. We also examine the structure of a dicotyledonous (dicot) stem in cross-section.

Plant transport therefore involves two systems:

• Xylem , transports water and dissolved minerals from roots to leaves.
• Phloem , transports sugars and other assimilates from sites of production (sources) to sites of use or storage (sinks).

The unifying theme throughout is form follows function: every structural feature of a transport vessel can be explained by what it needs to do.

Capillaries are the site of all material exchange between blood and tissues. Their entire structure is optimised for this single purpose.

Capillary: The smallest blood vessels in the body, with diameters of approximately 5–10 μm, forming a dense branching network that reaches almost every cell in the body.

Why branching matters:

• Capillaries divide extensively from arterioles, forming a dense network throughout every tissue.
• This branching creates an enormous combined surface area , estimated at approximately 500–700 m² across the entire human body.
• A larger surface area means more sites for diffusion of oxygen, glucose, carbon dioxide, and metabolic waste to occur simultaneously.

Why diameter matters:

• The narrow lumen (5–10 μm) forces red blood cells to pass in single file, slowing blood flow significantly compared to arteries.
• This slowed velocity increases contact time between blood and surrounding tissues, maximising the opportunity for exchange.