Human physiological activities need energy, which comes from the respiration of cells, and the raw materials needed for respiration are transported through the digestive system, respiratory system and circulatory system. (1) Gas exchange in alveoli means that the oxygen concentration of gas entering alveoli from outside is higher than that in alveoli capillaries, and the carbon dioxide concentration in capillaries is higher than that in alveoli. According to the principle of gas diffusion, gas always diffuses from high concentration to low concentration, so oxygen diffuses from alveoli to capillaries. Carbon dioxide diffuses from capillaries to alveoli. In this way, venous blood with less oxygen and more carbon dioxide transported by pulmonary artery becomes arterial blood with more oxygen and less carbon dioxide transported by pulmonary vein, and lung is an important organ for gas exchange in respiratory system. After gas exchange in alveoli, oxygen enters the circulatory system and is carried and transported by red blood cells in blood, which is called arterial blood. Oxygen in blood is transported by red blood cells, reaches the periphery of tissue cells, and finally enters cells through gas exchange in tissue.

(2) After the food is thoroughly digested, the glucose in nutrients is absorbed into the capillaries of the small intestine wall, transported by the plasma in the blood, and enters the cells through substance exchange in the tissue.

(3) According to (1)(2) Blood constantly brings oxygen and glucose to cells during circulation, and the heart is an organ that provides power for blood circulation.

(4) In mitochondria (a structure in cells), glucose is oxidized and decomposed to release energy, which is called respiration. When energy is released, it will produce another gas, namely carbon dioxide.

(5) The energy released by respiration through the oxidation and decomposition of organic matter can not only provide power for life activities, but also maintain normal body temperature.

So the answer is:

( 1)4; Red blood cells (or hemoglobin); artery

(2)7; blood plasma

(3) Heart

(4) Mitochondria; carbon dioxide

(5) Body temperature