Solution: (1) The inner surface of respiratory tract is covered with mucous membrane. Glandular cells on mucous membrane can secrete mucus, adhere to and block dust, and moisten the air. Mucosa is also rich in capillaries, which can warm the air and play a role in cleaning, warming and moistening the inhaled air. Therefore, the respiratory tract can treat the inhaled gas to make the gas reaching the lungs clean, warm and moist.

As the first line of defense, respiratory mucosa is immune to many pathogens, so it belongs to nonspecific immunity.

(2) In the ecosystem, only green plants can fix solar energy through photosynthesis. Green plants use light energy to synthesize organic matter from carbon dioxide and water through chloroplasts, and at the same time release oxygen and store energy from sunlight. Therefore, the energy in the ecosystem ultimately comes from solar energy.

(3) The blood circulation route is as follows:

According to the blood circulation chart, in the process of exercise, tissue cells will produce a lot of carbon dioxide, which will enter the blood circulation through gas diffusion, pass through the superior and inferior vena cava, the right atrium, the right ventricle and the pulmonary artery, and then be transported to the pulmonary capillaries, and then enter the lungs from the blood through gas diffusion and be excreted through exhalation.

(4) Nephron consists of glomerulus, renal capsule and renal tubule. The formation of urine mainly goes through two continuous processes: filtration and reabsorption. When blood flows through the walls of glomerulus and renal capsule, in addition to protein and macromolecules of blood cells, some substances in plasma, such as water, inorganic salts, glucose and urea, can enter the renal capsule through glomerular filtration. The fluid in the renal capsule is called protourine. The raw urine formed by human body every day is about 65438 0.50 liters. When raw urine flows through the renal tubules, all glucose, most water and some inorganic salts are reabsorbed by the renal tubules, enter the capillaries around the renal tubules and are sent back to the blood, while the remaining water, inorganic salts and urea are formed.

So the answer is:

(1) Clean, warm and moist (indispensable); Nonspecific;

(2) solar energy;

(3)④、③、①、②;

(4) nephron.