(1) After getting up in the morning, Liu Ming's mother has prepared breakfast for him, including salted fish, bacon, bread and egg soup. Breakfast is unreasonable because of excessive salt intake and lack of vegetables and fruits.

(2) The main ingredients in bread can only be absorbed if they are completely digested and decomposed into glucose. The digestive juices involved are saliva, intestinal juice and pancreatic juice. The main ingredient in fish is protein, and the stomach is where digestion begins.

(3) Lung is suitable for human respiratory system. According to "particulate matter with a diameter less than or equal to 2.5 microns in the atmosphere is called PM2.5, which is a fine particulate matter that can directly enter the lungs. PM2.5 is the main reason for the formation of "smog", which is rich in toxic and harmful substances, stays in the atmosphere for a long time and has a long transportation distance. " It can be seen that PM2.5 mainly harms the human respiratory system. Air enters the lungs and passes through the nasal cavity, pharynx, larynx, trachea and bronchus. However, PM2.5 particles are small and are not easily blocked by organs such as the nose. It can directly enter the bronchus and alveoli, affecting the gas exchange in the lungs and causing diseases such as asthma. After entering the lungs, it can be quickly absorbed by capillaries outside alveoli and distributed throughout the body through blood circulation. Therefore, PM2.5 is the most harmful to the human respiratory system;

(4) Because PM2.5 is the main cause of "smog", it is rich in toxic and harmful substances and has the characteristics of long stay in the atmosphere and long transportation distance, so the more PM2.5 content, the worse the air quality, and vice versa.

So the answer is: (1) too much salt and not enough vegetables and fruits.

(2) Glucose? Saliva, intestinal juice, pancreatic juice protein? Stomach?

(3) the respiratory system?

⑷D