 Just like the digestive system, the respiratory system is lined with a thin epithelial tissue layer. The cells of the epithelium in the respiratory tract release a thin, watery mucus and have cilia pointing outward. Cilia are like little hairs. They are an extra part of the cell's membrane, supported by structures within the cell. Dust, pathogens and small particles entering the lungs are caught in the mucus and movement of the cilia push the dust up and away from the alveoli and towards the throat. That way it can be stopped, swallowed and removed by the digestive system. There are also cilia in the nasal cavity that move the mucus and tract particles down, also toward the throat to be swallowed. When oxygen reaches the alveoli, it dissolves into the fluid there. The alveoli are laced with thin capillaries with low oxygen, high carbon dioxide blood, pumped straight from the heart. The concentration of oxygen is higher in the fluid than it is in the capillaries, so it diffuses into the capillaries. Similarly, the carbon dioxide is at a higher concentration in the capillary than in the fluid in the alveoli, so it diffuses down its gradient, out away from the capillary. Again, the nature and structure of capillaries, very tiny, but so very many of them, facilitates gas exchange. Generally, oxygen and carbon dioxide don't just diffuse into the liquid of the blood. At standard pressure and temperature, the blood can't hold much dissolved gas. We've already mentioned the unique nature of red blood cells and their huge amount of oxygen-carrying molecules, hemoglobins. Hemoglobin is a large protein molecule with four iron atoms. Each iron atom allows the protein to hold onto an oxygen molecule reversibly. It can let go if it needs to, without damaging the protein structure. Most of the oxygen in blood is carried by hemoglobin molecules. Hemoglobin can also carry carbon dioxide, but most of carbon dioxide travels in the blood as a bicarbonate iron. Carbon dioxide reacts with water in blood cells to form carbonic acid and then releases a proton to become bicarbonate. All proteins are sensitive to changes in their environment, such as a change in temperature or acidity. When the pH around hemoglobin lowers slightly, or the environment becomes more acidic, the molecule changes shape a little, which makes it more readily release oxygen molecules. When cells are working hard doing cell respiration, they release carbon dioxide, which makes the blood slightly acidic. With an environment more acidic, hemoglobin then releases oxygen more readily, which the cell can rapidly use because it's working hard. Structural elements of molecules and cells help tissues and organs and organ systems function to keep us alive.