Kids Research Express

Gene, basic unit of heredity found in the cells of all living organisms, from bacteria to humans. Genes determine the physical characteristics that an organism inherits, such as the shape of a tree’s leaf, the markings on a cat’s fur, and the color of a human hair (see Heredity ). Genes are composed of segments of deoxyribonucleic acid (DNA), a molecule that forms the long, threadlike structures called chromosomes. The information encoded within the DNA structure of a gene directs the manufacture of proteins, molecular workhorses that carry out all life-supporting activities within a cell (see Genetics ). Chromosomes within a cell occur in matched pairs. Each chromosome contains many genes, and each gene is located at a particular site on the chromosome, known as the locus. Like chromosomes, genes typically occur in pairs. A gene found on one chromosome in a pair usually has the same locus as another gene in the other chromosome of the pair, and these two genes are called alleles. All

Hemoglobin, iron-protein compound in red blood cells that gives blood its red color and transports oxygen, carbon dioxide, and nitric oxide. Hemoglobin is present in all but the least complex of animals. It carries oxygen from the lungs or gills, where blood is oxygenated, to body cells. When saturated with oxygen, hemoglobin is called oxyhemoglobin. After releasing oxygen to the body tissues, hemoglobin reverses its function and picks up carbon dioxide, the waste product of cellular respiration, for transport to the lungs, where it is expired. When saturated with carbon dioxide, hemoglobin is known as carboxyhemoglobin. In 1996 scientists discovered that, in addition to oxygen and carbon dioxide, hemoglobin takes up and releases a third gas, nitric oxide. Nitric oxide plays an important role in regulating blood pressure by relaxing the blood vessel walls, thus increasing blood flow. Hemoglobin controls the expansion and contraction of blood vessels, and thus blood pressure, by regul

Urine, pale yellow fluid produced by the kidneys , composed of dissolved wastes and excess water or chemical substances from the body. It is produced when blood filters through the kidneys, which remove about 110 liters (230 pints) of watery fluid from the blood every day. Most of this fluid is reabsorbed into the blood, but the remainder is passed from the body as urine. Urine leaves the kidneys, passes to the bladder through two slender tubes, the ureters, and exits the body through the urethra. A healthy adult can produce between 0.5 to 2 liters (1 to 4 pints) of urine a day, but the quantity varies considerably, depending on fluid intake and loss of fluid from sweating, vomiting, or diarrhea. Water accounts for about 96 percent, by volume, of the urine excreted by a healthy person. Urine also contains small amounts of urea, chloride, sodium, potassium, ammonia, and calcium. Other substances, such as sugar, are sometimes excreted in the urine if their concentration in the body beco

Tendon, in anatomy, extremely strong cord that is flexible but does not stretch, made of large bundles of white, fibrous protein known as collagen. A tendon joins a muscle with a bone or another muscle. A tendon delivers the pulling force of a muscle to a bone, which makes body motion possible. A tendon attached to a bone is cylindrical while one connected to a flat muscle on the wall of the abdomen is a wide sheet of fibers. The tendons in the hands, wrists, and feet are enclosed in protective membranes and lubricated by a fluid to prevent excessive friction. A larger tendon includes a system of nerves that registers pain when the tendon is squeezed, and triggers a reflex contraction in the adjoining muscle when the tendon is stretched. The Achilles tendon , which extends from midcalf to the heel, is the thickest and strongest tendon in the human body. Rupture of this tendon is a serious injury that most commonly affects athletes participating in events involving sprinting and jumpin

Tears, fluid produced in the lacrimal glands above the outside corner of each eye . The fluid lubricates the eyes and protects them from foreign matter and infection—the latter because it contains salt and lysozyme, an enzyme that kills microorganisms. During normal flow, tears constantly wash over the exterior of the eye and collect at its inside corner, where they drain through two small lacrimal ducts into the nasal cavity. During heavy tear flow, such as that caused by irritation, the excess fluid that is not taken up by the lacrimal ducts spills over the eyelids, carrying off foreign bodies too large to be flushed through the lacrimal ducts.

The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hingelike action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles , to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system.

Menopause, permanent ending of menstruation in women. Menopause marks the end of a woman’s natural ability to bear children. Menopause is usually preceded by 10 to 15 years during which the ovaries gradually stop producing eggs and sex hormones, a period called the climacteric. Perimenopause encompasses this period of changing ovarian activity and also the first few years without menstrual cycling, typically characterized by hormonal and physical changes and sometimes emotional and psychological changes as well.