With reference to human body systems, their organs, and functions, consider the following statements:

1. 1. In the human digestive system, the liver is the largest gland of the body, which secretes bile; however, this juice contains no digestive enzymes, yet it plays an important role in the emulsification of fats.
2. 2. In the human respiratory system, the exchange of gases takes place through the walls of the tiniest parts of the lungs—alveoli—via the process of diffusion, where hemoglobin present in the blood binds with oxygen to form oxyhemoglobin.
3. 3. The neuron is the basic structural and functional unit of the nervous system, in which the conduction of impulses always starts from the dendrites and proceeds unidirectionally towards the terminal end of the axon.
4. 4. In the human excretory system, the functional unit of the kidney is the 'nephron', in whose Bowman's capsule ultrafiltration of blood takes place at high pressure by the glomerulus, and during this urine formation process, the active reabsorption of glucose and amino acids is only possible in the loop of Henle.

Which of the above statement(s) is/are completely correct?