Blood consists of plasma and blood cells. One liter of plasma contains 900-9 10g of water, 65-85g of protein and 20g of low molecular weight substances. Low molecular weight substances contain many electrolytes and organic compounds. Blood cells include red blood cells, white blood cells and platelets. The average life span of red blood cells is 120 days, that of white blood cells is 9- 13 days, and that of platelets is 8-9 days.
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The role of blood
1, transportation materials
Various nutrients absorbed from the digestive tract and oxygen inhaled from the lungs should be transported to various organs and tissues of the whole body through blood for their utilization; Metabolites produced by various tissues in the whole body during metabolism, such as urea, creatinine, carbon dioxide, etc., also need to be transported to excretory organs-kidney, lung, skin and intestine through blood.
2. Keep the organization excited
Maintaining the normal excitability of various tissues in the body requires a suitable and constant internal environment, such as temperature, pH, osmotic pressure and various ion concentrations. Blood plays a very important role in maintaining a constant internal environment.
For example, the buffer system in blood (such as sodium bicarbonate and carbonic acid, plasma protein, hemoglobin in red blood cells, etc.). ) has the function of buffering acidic or alkaline substances, and keeps the pH value of blood at 7.35 ~ 7.45. The content and proportion of various inorganic ions in plasma are very important to maintain the normal excitability of nerves and muscles, especially the excitability of myocardium.
3. Adjustment function
Hormones secreted by endocrine glands, special products of tissues and general metabolites, such as thyroxine, epinephrine, renin, histamine, carbon dioxide and lactic acid, are distributed to various organs and tissues of the body through blood circulation, thus regulating the functional activities of various organ systems.
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