Yes. Upper-layer skin cells and the cells in the front surface of the eyes get a significant amount of oxygen directly from the air rather than from the blood. Human bodies have a huge demand for oxygen. As a result, the oxygen that is able to passively diffuse into the body directly from the air is not nearly enough to run the whole body. Fortunately, we have lungs that can actively pull in oxygen and transfer it to the blood, allowing the body to transport oxygen to the cells by using the blood like a fleet of delivery trucks. Most of our cells rely on the blood delivery service. However, the cells in the outer layers or our skin and eyes are in direct contact with the atmosphere and can efficiently get their oxygen right from the air.
Yes, there are nuclear reactions constantly occurring in our bodies, but there are very few of them compared to the chemical reactions, and they do not affect our bodies much. All physical objects are made of molecules. A molecule is a series of atoms linked together by chemical (electromagnetic) bonds.
Photon wave phase can also not be directly detected by humans but can be detected by machines called interferometers. Phase information is often used to determine the flatness of a reflecting surface. In summary, humans can indeed see photons. Humans can see all of the properties of photons except for spin and wave phase.
The nervous system maintains homeostasis by controlling and regulating the other parts of the body. A deviation from a normal set point acts as a stimulus to a receptor, which sends nerve impulses to a regulating center in the brain.
