The Earth's atmosphere contained almost no oxygen during the Hadesian period, and in the Archean period, with the production and reproduction of seaweed, the concentration of oxygen in the atmosphere slowly increased, creating conditions for the birth of various organisms. Today, oxygen, a special gas, accounts for about 21% of the air, and the vast majority of eukaryotes rely on mitochondria for biological oxidation, "burning" oxygen and thermogenic nutrients for energy.
How does oxygen reach the mitochondria inside cells? In humans, for example, first of all, it enters the trachea through inhalation motion, as if entering a white tunnel. The tunnel continues to bifurcate, getting narrower and narrower, and after 23 bifurcations, it finally reaches the alveolar sac at the end. Each alveolar sac consists of 17 or so alveoli. These alveoli are large and small, communicating with each other. According to the principle of physics, the air of the small alveoli will transfer to the large alveoli, resulting in the collapse of the small alveoli and the expansion of the large alveoli, fortunately with the regulation of alveolar surfactant, the pressure in the alveoli and the surface tension of the alveoli are balanced by each other, and the size of the alveoli fluctuates within the normal range.
Tissues such as alveolar epithelial cells and the capillary basement membrane of the lungs make up the respiratory membrane, which is a barrier between air and blood. The thickness of the respiratory membrane is less than one micron, and the thinnest point is only 0.2 microns, and the small, fat-soluble oxygen molecules easily pass through this "wall" like a Laoshan Taoist priest and enter the blood in the capillaries.
Blood is like an ocean for oxygen molecules, how do they cross the expedition? I saw that oxygen molecules quickly found a means of transportation - hemoglobin on hundreds of millions of red blood cells, which has the ability to bind oxygen. Each hemoglobin molecule can bind 4 oxygen molecules, if it binds more, hemoglobin appears bright red, if it binds less, hemoglobin appears dark red. This makes arterial blood redder than venous blood, and if the body lacks oxygen, the skin and mucous membranes appear bluish purple, which clinicians call cyanosis.
Blood circulation with oxygen molecules transported in the body, reaching the tissue organs, because the tissue organs consume oxygen to reduce the oxygen content in the interstitial fluid and capillary blood, oxygen molecules are dissociated from hemoglobin, through the blood vessel wall into the interstitial fluid, and then through the cell membrane into the cell, and finally into the mitochondria, becoming the fuel for biological oxidation. The "slag" produced after biological oxidation, carbon dioxide, returns to the bloodstream in the same way, enters the pulmonary circulation and is excreted from the alveoli.
Oxygen molecules don't have a compass, how do they drift in the human body without losing their way? What determines its direction is the invisible partial pressure of oxygen. Gas molecules are constantly moving to produce pressure, and the pressure generated by one of the gas molecules is called partial pressure. The partial pressure of a gas is equal to the total pressure of the gas mixture multiplied by the volume percentage of the gas. For example, the proportion of oxygen in the air is about 21%, and the partial pressure of oxygen is 760 mm Hg multiplied by 21%, which is about 159 mm Hg. In the human body, oxygen must be dissolved in body fluids to be transported, and its partial pressure is related to factors such as solubility and oxygen consumption in various parts. The partial pressure of oxygen in arterial blood is about 100 mm Hg, in venous blood it is about 40 mm Hg, and to the tissue is about 30 mm Hg. The oxygen molecule always shifts in the direction of low partial pressure of oxygen, which allows it to eventually find and contribute to the tissue.
Hemoglobin oxygen saturation (hereinafter referred to as oxygen saturation) is an important indicator to evaluate oxygen transport. The ratio of the actual oxygen content in the blood to the maximum oxygen content is called oxygen saturation. The oxygen saturation of arterial blood is about 97.4%. As long as the hemoglobin oxygen saturation in arterial blood is maintained at more than 90%, the blood can carry enough oxygen, even if humans are in a plateau and the air is thin, as long as the partial pressure of oxygen in arterial blood is not less than 60 mm Hg, more than 90% oxygen saturation can be maintained, and the body's oxygen supply is normal. However, if the outside air is too thin or oxygen cannot enter the blood smoothly due to dyspnea, alveolar fibrosis, alveolar congestion and edema, etc., and the partial pressure of oxygen is lower than 60 mm Hg, hemoglobin oxygen saturation will drop sharply. Depending on the cause, clinicians increase the patient's oxygen saturation with oxygen, ventilators, or extracorporeal membrane oxygenation.
Oxygen is the fuel of the human body and the source of energy for cells, although it is very important, but the more oxygen intake, the better. If there is too much oxygen in the human body, by-products such as oxygen free radicals will be produced, which will accelerate aging and cause various diseases.
As long as we maintain a normal routine, do not abuse drugs and health products, and prevent and treat respiratory diseases, the oxygen in the body can be properly transported normally and provide an endless stream of power.