Biology STAAR Practice Test 2025 - Free State of Texas Assessments of Academic Readiness Practice Questions and Study Guide

Capillaries are the smallest blood vessels in the body and play a crucial role in the circulatory system primarily by enabling nutrient and gas exchange between blood and surrounding tissues. Their thin walls, consisting of a single layer of endothelial cells, allow for the efficient diffusion of oxygen, carbon dioxide, nutrients, and waste products. This close proximity to tissue cells facilitates the transfer of essential substances necessary for cellular metabolism and the removal of metabolic byproducts.

The other options, while related to blood transport in some way, do not accurately represent the primary function of capillaries. Blood carrying to the heart is mainly a function of veins, while the transport of hormones is primarily done by blood vessels without the specific functions of capillaries. Oxygen storage is a function more associated with different types of proteins, such as myoglobin in muscle tissues, rather than a role of capillaries themselves. Therefore, option B clearly identifies the essential role of capillaries in maintaining cellular function through nutrient and gas exchange.