In addition to their blood collection and exchange functions, the venules are an important component of the blood reservoir system in the venous circulation. At rest, approximately two thirds of the total blood volume is within the venous system, and perhaps more than half of this volume is within venules. Although the blood moves within the venous reservoir, it moves slowly, much like water in a reservoir behind a river dam. If venule radius is increased or decreased, the volume of blood in tissue can change up to 20 mL/kg of tissue,- therefore, the volume of blood readily available for circulation would increase by more than 1 L in a 70-kg (154-pound) person. Such a large change in available blood volume can substantially improve the venous return of blood to the heart following depletion of blood volume caused by hemorrhage or dehydration. For example, the volume of blood typically removed from blood donors is about 500 mL, or about 10% of the total blood volume, usually no ill effects are experienced, in part because the venules and veins decrease their reservoir volume to restore the circulating blood volume.
Lymphatic vessels: basic structure and
"functions. The contraction-relaxation cycle of lymphatic bulbs (bottom) is the fundamental process that removes excess water and plasma proteins from the interstitial spaces. Pressures along the lymphatics are generated by lymphatic vessel contractions and by organ movements.
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