The symbiotic relationship between the host and the rumen microbiome plays a crucial role in ruminant physiology. One of the most important processes enabling this relationship is urea nitrogen salvaging (UNS). This process is important for both maintaining ruminant nitrogen balance and supporting production of their major energy supply, bacterially-derived short chain fatty acids (SCFA). The key step in UNS is the trans-epithelial movement of urea across the ruminal wall and this is a highly regulated process. At the molecular level, the key transport route is via the facilitative urea transporter-B2, localized to ruminal papillae epithelial layers. Additional urea transport through aquaporins (AQP), such as AQP3, is now also viewed as important. Long-term regulation of these ruminal urea transport proteins appears to mainly involve dietary fermentable carbohydrates; whereas, transepithelial urea transport is finely regulated by local conditions, such as CO₂ levels, pH and SCFA concentration. Although the key principles of ruminal urea transport physiology are now understood, there remains much that is unknown regarding the regulatory pathways. One reason for this is the limited number of techniques currently used in many studies in the field. Therefore, future research in this area that combines a greater range of techniques could facilitate improvements to livestock efficiency, and potentially, reductions in the levels of waste nitrogen entering the environment.

宿主与瘤胃微生物组之间的共生关系在反刍动物生理学中起着至关重要的作用。实现这种关系的最重要的过程之一是尿素氮回收 (UNS)。这一过程对于维持反刍动物的氮平衡和支持其主要能量供应、细菌衍生的短链脂肪酸 (SCFA) 的生产都很重要。UNS 的关键步骤是尿素穿过瘤胃壁的跨上皮运动，这是一个高度调节的过程。在分子水平上，关键的转运途径是通过促进性尿素转运蛋白-B2，定位于瘤胃乳头上皮层。通过水通道蛋白 (AQP)，如 AQP3，额外的尿素运输现在也被认为是重要的。这些瘤胃尿素转运蛋白的长期调控似乎主要涉及日粮可发酵碳水化合物；然而，跨上皮尿素转运受局部条件的精细调节，例如 CO_{2 个}水平，pH 值和 SCFA 浓度。尽管现在了解瘤胃尿素转运生理学的关键原理，但关于调节途径仍有许多未知之处。其原因之一是目前在该领域的许多研究中使用的技术数量有限。因此，该领域的未来研究结合更广泛的技术可以促进牲畜效率的提高，并有可能降低进入环境的废氮水平。