Solution: (1) The heart has four chambers: left atrium, right atrium, left ventricle and right ventricle; The left atrium connects with pulmonary vein, the right atrium connects with superior and inferior vena cava, the left ventricle connects with aorta, and the right ventricle connects with pulmonary artery. That is, A connected with superior and inferior vena cava is the right atrium, and D connected with aorta is the left ventricle.

(2) Blood exchanges substances with tissue cells through capillaries at the tissue, oxygen and nutrients in the blood enter the tissue cells, and wastes such as carbon dioxide generated by the tissue cells enter the blood; Judging from the arrows in the diagram, ① is carbon dioxide and ② is oxygen.

(3) The way of blood circulation and the changes of blood components in the process of circulation are shown in the figure: Therefore, the four blood vessels in the figure contain arterial blood, namely aorta and pulmonary vein.

(4) After intravenous injection of drugs, drugs first enter the right atrium through the superior and inferior vena cava, and any drug that passes through the right ventricle → pulmonary artery → pulmonary capillary → pulmonary vein → left atrium → left ventricle → aorta → systemic capillary → superior and inferior vena cava → right atrium. That is, the sequence of passing through the heart is: A → C → B → D. 。

(5) The small intestine is the main place to absorb nutrients, so the nutrients will increase obviously after the blood flows through the capillaries in the small intestine; At the same time, the oxygen in the blood is utilized by the cells that make up the small intestine, so the blood flowing out of the small intestine is venous blood rich in nutrients.

Therefore, the answer is: (1) right atrium; left ventricle

(2) Carbon dioxide and oxygen

(3) Aorta and pulmonary vein

（4）A→C→B→D

(5) nutrients; vein