Blood Chemistry
Blood in the human body is a complex fluid. It is responsible for transport of dissolved gases, nutrients and waste products. It regulates the pH of not only itself, but of all the intercellular fluid in the body. In addition, blood carries hormones and critical parts of the immune system throughout the body, and aids in thermoregulation by redistributing heat. Blood also contains the factors needed for clotting, thereby preventing its own loss in the event of injury.
An adult human has 4-6 liters of blood. Approximately half of this is water, and slightly less than half is red blood cells. The rest is made up of proteins, sugars, salts, and other small molecules, plus white blood cells and platelets. The noncellular portion is termed plasma, while the cellular parts are collectively referred to as the formed elements.
While a small amount of oxygen can be dissolved directly in plasma, the quantity is far too low to support human life. Instead, transport of oxygen and carbon dioxide is accomplished by the red blood cells, of which there are approximately 25 trillion in circulation at any one time. Red blood cells are packed full of the protein hemoglobin, whose iron-containing heme group binds oxygen at high concentrations, and releases it at low concentrations. This allows hemoglobin to pick up oxygen in the lungs where the concentration is high, and deposit it in the tissues where it is low. Similarly, hemoglobin binds carbon dioxide (not with the heme group), and carries it to the lungs.Carbon monoxide, a product of incomplete combustion, binds to heme directly, and much more tightly than oxygen. For this reason, carbon monoxide is a poison. Genetic changes in the hemoglobin molecule that affect the shape of the red blood cell and the oxygen carrying capacity of the cell result in the inheritable diseases of sickle-cell anemia and beta-thalassemia.
Nutrients from the gut are dissolved directly in the plasma for transport. Several mechanisms prevent wide oscillations in plasma nutrient levels, despite irregular supply from the intestine. All materials absorbed into the bloodstream first pass directly to the liver, where excess nutrients are stored and then released as needed. (In contrast to all other nutrients, fats are not absorbed into the bloodstream, but instead are picked up by the lymphatic system.) The concentration of sugar in the blood is regulated by the hormones insulin and glucagon, which, respectively, lower or raise the blood sugar level in response to changes in supply and demand. Excesses that cannot be absorbed by the liver are excreted in the kidney, whose active transport and reclamation mechanisms make it one of the most finely tuned regulators of blood composition. The kidney is principally responsible for regulating the levels of ions, water, and wasted products in the blood.
The pH of the blood and fluid surrounding cells is normally 7.4. The most common threat to the maintenance of this pH is the production of organic acids by metabolic processes in the cell. Of most significance is lactic acid, produced by fermentation in the absence of oxygen, especially in muscle cells during strenuous exercise. To prevent lactic acid and other acids from changing the blood's pH, the blood uses a buffering system. While there are several such systems, the most important is the bicarbonate buffer system. In this system, the carbon dioxide released during cell metabolism is acted upon by the enzyme carbonic anhydrase to create carbonic acid (H2CO3), which immediately dissociates to form H+ and HCO3-, the bicarbonate ion. H+ ions released by lactic acid react with the bicarbonate ion to form carbonic acid again, preventing a change in pH. The kidney also participates in pH regulation by secreting excess H+ as necessary.
While carbon dioxide is central to pH control, it is itself an acid, and too much CO2 accumulation can cause acidosis, or a blood pH below normal. Respiratory acidosis most commonly occurs from hypoventilation, or too little gas exchange in the lungs. As blood pH falls, the respiratory centers of the brain increase the breathing rate, to try to remove the excess carbon dioxide. Conversely, hyperventilation, or too-rapid breathing, can deplete the blood of carbon dioxide, leading to respiratory alkalosis.
The process of preventing blood loss from a damaged blood vessel is called hemostasis. While immediate hemostasis involves contraction of the muscles surrounding the blood vessel, long-term hemostasis requires the formation of a blood clot. This process involves a complex cascade of more than a dozen different plasma proteins. Clot formation begins when cell fragments known as platelets adhere to the wound, due to chemical changes undergone by wounded tissue. Once the platelets stick, they release "platelet thromboplastic factor", which, in conjunction with calcium ions in the plasma, triggers the activation of a series of other factors. This cascade acts to amplify the original signal from the platelets, rapidly causing more and more factors to become activated at each level. The next to last step in this chain is the activation of the circulating proenzyme prothrombin into its active form, thrombin. This enzyme then activates the circulating fibrous protein fibrinogen, into its active form, fibrin. Fibrin binds to the platelets at the wound site, forming a meshwork in which red blood cells and other elements become trapped, forming a clot. Clotting usually begins within 15 seconds after the wound occurs.
Hemophilia is an inherited disorder of clotting, due to a defect in the gene for one of the clotting factors. Hemophilia can be treated by regular administration of the factor, isolated from donated blood. More recently, this factor has been made by genetic engineering in a bacterium.
Blood is routinely analyzed during the diagnosis or treatment of disease, because it provides a window on the metabolic processes occurring in the body. For instance, immediately after a heart attack, the level of a certain muscle enzyme is elevated in the blood, due to the damaged heart muscle. Within several days, the level of this enzyme falls, but another rises, and then it falls in turn. By measuring the concentrations of each after a heart attack, the exact time of the attack can be estimated, providing important information for treatment planning.
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