what is the function of {紅血球}?

Red blood cells are the most common type of blood cell and the vertebrate body's principal means of delivering oxygen from the lungs or gills to body tissues via the blood.


圖片參考：http://upload.wikimedia.org/wikipedia/commons/1/13/Redbloodcells.jpg

Human red blood cells



Erythrocytes consist mainly of hemoglobin, a complex molecule containing heme groups whose iron atoms temporarily link to oxygen molecules in the lungs or gills and release them throughout the body. Oxygen can easily diffuse through the red blood cell's cell membrane. Hemoglobin also carries some of the waste product carbon dioxide back from the tissues. (In humans, less than 2% of the total oxygen, and most of the carbon dioxide, is held in solution in the blood plasma). A related compound, myoglobin, acts to store oxygen in muscle cells.
The color of erythrocytes is due to the heme group of hemoglobin. The blood plasma alone is straw-colored, but the red blood cells change color depending on the state of the hemoglobin: when combined with oxygen the resulting oxyhemoglobin is scarlet, and when oxygen has been released the resulting deoxyhemoglobin is darker, appearing bluish through the vessel wall and skin. Pulse oximetry takes advantage of this color change to directly measure the arterial blood oxygen saturation using colorimetric techniques.
The sequestration of oxygen-carrying proteins inside cells (rather than having them dissolved in body fluid) was an important step in the evolution of vertebrates; it allows for less viscous blood and higher concentrations of oxygen.
In 2007 it was reported that erythrocytes also play a part in the body's immune response: when lysed by pathogens such as bacteria, their hemoglobin releases free radicals that break down the pathogen's cell wall and membrane, killing it.

Human erythrocytes
The diameter of a typical human erythrocyte disk is 6–8 µm, much smaller than most other human cells. A typical erythrocyte contains about 270 million hemoglobin molecules, with each carrying four heme groups.
Adult humans have roughly 2–3 × 1013 red blood cells at any given time (women have about 4 to 5 million erythrocytes per microliter (cubic millimeter) of blood and men about 5 to 6 million; people living at high altitudes with low oxygen tension will have more). Red blood cells are thus much more common than the other blood particles: There are about 4,000–11,000 white blood cells and about 150,000–400,000 platelets in each microliter of human blood. The red blood cells store collectively about 3.5 grams of iron, more than five times the iron stored by all the other tissues combined.


有興趣深入研究嘅，可以去呢度睇（幾得意）：http://www.dbs.nus.edu.sg/eventlist/happenings/details/2007/dingSTsep07.pdf