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What characteristic defines capillaries?

  1. Thick-walled vessels with large diameters

  2. Thin-walled vessels that allow single-file blood flow

  3. Vessels that are only present in the brain

  4. High-pressure vessels supplying oxygen directly

The correct answer is: Thin-walled vessels that allow single-file blood flow

Capillaries are defined by their thin walls, which are only one cell layer thick, allowing for efficient exchange of gases, nutrients, and waste products between the blood and surrounding tissues. This thin structure facilitates the diffusion process, making it easier for oxygen to move from the blood into the tissues and for carbon dioxide and other metabolic waste products to move from tissues into the bloodstream. Additionally, capillaries accommodate single-file blood flow due to their small diameter, which is typically around 5 to 10 micrometers. This arrangement ensures that red blood cells pass through capillaries in single file, maximizing the surface area for interaction with tissue cells. The narrow lumen of capillaries increases the speed at which exchange occurs, effectively delivering oxygen to the cells while removing waste products. In contrast, vessels that are thick-walled with large diameters are characteristic of arteries, which are designed to withstand high pressure as they carry blood away from the heart. The statement that vessels are only present in the brain is incorrect; capillaries are present throughout the entire body, in almost all tissues. High-pressure vessels are associated with arteries as well, not capillaries, since capillary pressure is relatively low to allow for proper exchange without damaging the delicate tissue