During absorption, the end products of digestion—amino acids, monosaccharides, glycerol, and fatty acids—are transferred into the circulatory system through blood and lymph vessels in the lining of the small intestine. The structure of this lining provides a huge surface area for absorption to take place. The highly folded lining of the small intestine is covered with millions of fingerlike projections called villi (singular, villus), which are shown in Figure 48-13. The cells covering the villi, in turn, have extensions on their cell membranes called microvilli. The folds, villi, and microvilli give the small intestine a surface area of about 250 m2 (about 2,685 ft2), or roughly the area of a tennis court. Nutrients are absorbed through this surface by means of diffusion and active transport.
Inside each of the villi are capillaries and tiny lymph vessels called lacteals (LAK-tee-uhlz). The lacteals can be seen in Figure 48-13. Glycerol and fatty acids enter the lacteals, which carry them through the lymph vessels and eventually to the bloodstream through lymphatic vessels near the heart. Amino acids and monosaccharides enter the capillaries and are carried to the liver. The liver neutralizes many toxic substances in the blood and removes excess glucose, converting it to glycogen for storage. The filtered blood then carries the nutrients to all parts of the body.
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