How Blood Clots (1969)

[Music] Every human cell requires food and oxygen and a way to remove waste products. This need is satisfied in a remarkable way by our circulatory system. The heart at the center of this system pumps a fluid tissue called blood through the arteries, capillaries and veins which make up the blood vessels. Here we see blood vessels lying close to the surface of the skin. Blood circulating in these vessels contains cell foods, waste products, and other substances in the amounts needed to maintain good health. When a vessel is damaged, some of the materials in the blood help the system to repair itself. Escaping blood is made to clot. and the injured vessel is resealed. Here is another example of blood clotting at the site of an injury to a blood vessel. We can learn more about how blood clots by observing a sample of human blood. At room temperature, our blood sample begins to clot in a few minutes. We are watching this clot formation in fast motion. [Music] After several hours, the blood has separated into a red clot and yellow serum. What role does each part of the blood play in the clotting process? Using a centrifuge to spin blood samples at a rapid rate, we can separate blood into solid and liquid parts. We prevent the blood from clotting in the centrifuge by adding a special chemical. The blood separates into solid particles at the bottom and liquid plasma at the top. We can then find out what plasma is composed of by treating it with certain chemicals. These chemicals react with substances in plasma to form solids. The solids are then removed and identified. All of the components of plasma can be isolated by repeating these separations with different chemicals. When plasma is broken down this way, scientists find that 90% is simply water. The remaining 10% consists of cell foods. calcium, the protein fibbrronogen, the enzyme prothroin and many other substances. Calcium, fibbrronogen, and prothroin are among a dozen known clotting agents found in plasma. In addition to plasma, human blood contains solid particles called blood cells. Over 25 trillion blood cells will be found in the blood system of the average adult. Most of the cells are red corpusles. They give the blood its red color. Here we see them being carried along by the flow of plasma in the blood vessel. The red corp pusles transport oxygen from the lungs to the body cells and return carbon dioxide to the lungs. Another type of blood cell is the white corpusle. Although they differ in size and nucleus structure, all white cells are thought to help destroy bacteria and other foreign bodies in the bloodstream. The only other solid particles in the blood are called platelets. These appear as clusters of tiny bodies among the red and white cells. Platelets like red corpusles are not whole cells. They have no nuclei and cannot reproduce. But within these fragments lies the key to the process of blood clotting. Scientists know that red and white corpusles develop from cells in the bone marrow. But where do platelets come from? They looked for the answer in bits of bone marrow grown in cultures like this. Through the microscope, they saw many cells in the marrow. But they focused their attention on one type. It was large and spherical. During a 24-hour period, the cell underwent an unusual transformation, which we see here in fast motion. [Music] Beadlike pieces form. formed from the aging cell. They seem to be exactly like platelets found in the blood except for the threads connecting them. Scientists now believe that something in the body dissolves these threads and releases the beads into the bloodstream where we recognize them as platelets. Platelets play two important roles in the process of blood clotting. At the injured area of a blood vessel, platelets gather in large numbers, forming a white clot that temporarily seals the wound, but some of the platelets escape with the blood. They begin a chain reaction which causes the blood to clot. Aided by other clotting agents, the platelets react with calcium to produce a substance called thromboplastin. Then the thimoplastin reacts with prothroin and calcium forming a new compound thromben. Finally, thromben combines with fibbrronogen to produce a threadlike substance called fibbrin. Although the formation of fibbrin is near the end of the clotting process, it is the only part which is visible under the microscope. Here we see fibram forming in slow motion. The white dots are platelets. [Music] The fibbrin threads form a mesh which can trap red and white cells. The trapped cells give the clot its red color. As the mass dries out, it forms a scab. [Applause] [Music] Now, let's watch the clot form at normal speed. Do you think that blood would clot if it had no red or white cells? This fresh plasma sample is essentially blood without red or white cells. [Music] The formation of a white clot shows that red and white cells are not necessary for clotting. Red and white cells appear in a normal blood clot simply because they are trapped by fibbrin. The blood system of a healthy person forms clots as they are needed to prevent blood loss. But what happens to a clot in a blood vessel after a wound heals? The clot dissolves. A substance in human blood called plasine dissolves blood clots when they are no longer necessary. [Music] Plasmine reverses the clotting process by dissolving fibbrin and releasing trapped blood cells. Healthy blood contains the necessary chemicals for both forming and dissolving clots. Although scientists now know how blood clots, many problems related to the clotting process remain unsolved. How can we prevent the disease in which dangerous clots form in blood vessels for no apparent reason? How can we cure hemophilia or bleeders disease and break the cycle which passes it on to future generations? Today we do not know. But further studies into the complicated process of blood clotting may enable us to find the answers. [Music]