Venules
Small veins
Drain capillary blood and begin the return flow back to the heart Significant sites of exchange of nutrients and wastes and WBC emigration Similar in structure to a capillary, will begin to gain a layer of smooth muscle as the venule becomes a vein Veins
Carry blood toward the heart
Composed of the same 3 layers as arteries Contain very little smooth muscle and elastic fibres compared to arteries, causing veins to appear flattened. Very thin walls relative to their overall diameter Pumping action of the heart and contraction of skeletal muscles in the limbs help boost venous return to the heart Average blood pressure in veins is considerably lower than that of arteries Contain valves – thin folds of tunica interna forming flat-like cusps projecting into the lumen and point toward the heart Low BP allows blood returning to the heart to slow and even back up; the valves aid in venous return by preventing backflow. More numerous than arteries (some are paired), roughly 130 million veins.
CAPILLARY EXCHANGE
The mission of the entire cardiovascular system is to keep blood flowing through the capillaries to allow capillary exchange.
Osmosis Review
Osmosis: the movement of water across a selectively permeable membrane by diffusion from an area of higher water concentration to an area of lower water concentration.
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