(1) The process that nutrients enter the circulatory system through the digestive tract wall is called absorption. C in the figure represents the process of glucose, the final product of starch digestion, entering the blood circulation through the digestive tract wall, which is called absorption. Small intestine is the main organ to absorb nutrients, because it is very long, about 5 ~ 6 m, with annular folds on the inner wall of small intestine and small intestinal villi on the folds, which increases the absorbed internal surface area; There are capillaries and lymphatic capillaries in the villi of small intestine, and the villi wall, capillary wall and lymphatic capillary wall are all composed of a layer of epithelial cells, which is beneficial to the absorption of nutrients into the blood. Protein in milk is a macronutrient, which needs to be digested into small amino acids in the digestive tract before it can be absorbed into the blood circulation.

(2) The gas exchange between the lung and the outside world is called lung ventilation, which is realized by breathing movement; The gas exchange between alveoli and blood is realized by gas diffusion; When blood flows through pulmonary capillaries, carbon dioxide in blood enters alveoli, and oxygen in alveoli enters blood, so blood changes from venous blood to arterial blood.

(3) There is a red iron protein in red blood cells called hemoglobin, which is easy to combine with oxygen in places with high oxygen content; Where the oxygen content is low, it is easy to separate from oxygen; Oxygen in the outside air enters the blood, combines with hemoglobin, is transported to tissues and cells with the blood, participates in the oxidative decomposition of organic matter, and releases energy for the needs of human life activities.

So the answer is: (1) absorption; Small intestine; amino acid

(2) respiratory movement;

(3) hemoglobin; histocyte