Kids Research Express

The smallest cells in the blood are the platelets, which are designed for a single purpose—to begin the process of coagulation, or forming a clot, whenever a blood vessel is broken. As soon as an artery or vein is injured, the platelets in the area of the injury begin to clump together and stick to the edges of the cut. They also release messengers into the blood that perform a variety of functions: constricting the blood vessels to reduce bleeding, attracting more platelets to the area to enlarge the platelet plug, and initiating the work of plasma-based clotting factors, such as fibrinogen. Through a complex mechanism involving many steps and many clotting factors, the plasma protein fibrinogen is transformed into long, sticky threads of fibrin. Together, the platelets and the fibrin create an intertwined meshwork that forms a stable clot. This self-sealing aspect of the blood is crucial to survival. See also Blood Diseases: Anemia ; leukemia , hemophilia

Blood Type, in medicine, classification of red blood cells by the presence of specific substances on their surface. Typing of red blood cells is a prerequisite for blood transfusion . In the early part of the 20th century, physicians discovered that blood transfusions often failed because the blood type of the recipient was not compatible with that of the donor. In 1901 the Austrian pathologist Karl Landsteiner classified blood types and discovered that they were transmitted by Mendelian heredity (see Mendel's Laws ). The four blood types are known as A, B, AB, and O. Blood type A contains red blood cells that have a substance A on their surface. This type of blood also contains an antibody directed against substance B, found on the red cells of persons with blood type B. Type B blood contains the reverse combination. Serum of blood type AB contains neither antibody, but red cells in this type of blood contain both A and B substances. In type O blood, neither substance is present

Serum, clear, yellowish fluid that remains as the liquid portion of blood after clotting has taken place. Before clotting occurs, the liquid of the blood is called plasma . Both plasma and serum contain proteins, salts, sugars, waste products, vitamins, minerals, fats, and hormones. But plasma also contains the protein fibrinogen and certain other elements necessary for clotting, and it therefore clots as easily as whole blood. Serum, which lacks these elements, does not clot. In order to isolate serum for chemical tests and other medical uses, whole blood is placed in a glass tube and allowed to clot. The clotted blood is then placed in a centrifuge, a machine that spins the blood at high speeds, causing the clot and the cellular parts of the blood to separate from the serum and sink to the bottom of the tube. The serum remains at the top of the tube. Serum from animals like horses and sheep can be used to provide human beings with protection against infections or poisons. Animals ar

Rh Factor, term applied to any of the 30 or more substances, called agglutinogens, found on the surfaces of red blood cells. They are distinct from the main blood types, but their composition is unknown. The Rh factors were named by the American pathologists Karl Landsteiner and Alexander Solomon Wiener, who discovered the first of them in the blood of the rhesus monkey in 1937. This first Rh agglutinogen, corresponding to the one now designated Rh0, is present in the blood of about 85 percent of all humans. Blood reactions involving Rh factors are now chiefly of interest in obstetrics. The presence of Rh factors in the blood is controlled by the laws of heredity. An individual who possesses one gene for the Rh factor will express the factor on the red blood cells. If a woman is Rh-negative, that is, if she has two recessive genes for the Rh0 factor, and a man has two genes that express the Rh-positive factor, then all of their children will be Rh-positive. But if the Rh-positive man h

Cerebrum (the Latin word for brain)—two large, almost symmetrical hemispheres which make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex to fit within the cranium. The cerebrum receives information from all the sense organs and sends motor commands (signals that result in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side a

Cerebellum (“little brain”)—two smaller hemispheres located at the lower back of the brain beneath the occipital lobes, which coordinates body movements. It is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum , the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem —the midbrain, the pons, and the medulla oblongata. The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.

Brain stem, the lowest part of the brain , a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rate, as well as reflexes like eye movement and vomiting. The brain stem has three main parts: the medulla , pons , and midbrain . A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.