Kids Research Express

. Thyroxine, also tetraiodothyronine, main hormone secreted by the thyroid gland. The function of this and the other thyroid hormone, triiodothyronine, is to increase the cellular rate of carbohydrate metabolism and of protein synthesis and breakdown. The hormones are synthesized in the thyroid by combining iodine with the amino acid tyrosine and are secreted into the blood as a complex with the protein plasma globulin. Both synthesis and secretion are regulated by, and in turn regulate the formation of, a hormone secreted by the pituitary gland . Thyroxine was first isolated in 1919 by the American biochemist Edward Calvin Kendall and was synthesized in 1927 by the British biochemist Charles Harington. Synthetic thyroxine is now used in treating thyroid-deficiency conditions such as cretinism and goiter .

. Adrenocorticotropic Hormone (ACTH), also known as corticotropin, hormone secreted by the anterior part of the pituitary gland . The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layer) of the adrenal gland . One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolism . ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis . ACTH is a complex protein molecule containing 39 amino acids . Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increase in response to stress, disease, and decreased blood pressure.

. Testosterone, principal male hormone, or androgen , produced mainly in the Leydig cells in the male testes. The Leydig cells also produce two other androgens of less potency and in much smaller quantities. Testosterone stimulates the development of the male secondary sex characteristics after puberty , causing growth of the beard and pubic hair, development of the penis, and change of voice. The hormone also aids in the growth, muscular development, and masculine body contour of the adult male. See Hormone . If, before puberty, little or no testosterone secretion occurs, secondary sexual characteristics fail to develop. In addition, the long bones continue to grow abnormally and give the patient a characteristic tall but effeminate build. If testicular failure follows puberty, less obvious changes occur, although gradual recession of beard, weakening of muscles, increased deposition of fat, and change in voice may develop slowly, with infertility usually present and decreased libido ...

Androgen, term embracing any of the male sex hormones, substances that induce and maintain secondary sex characteristics in males. The principal androgen is testosterone, produced in the testes of males and in the adrenal glands of both sexes. Testosterone circulates in the blood and is excreted in the urine. Androgens function principally, beginning at puberty, in the stimulation of such male secondary sex characteristics as development of the genital organs and maturation of sperm, growth of body hair, and changes in the larynx that lower the voice. Androgens have an anabolic effect—that is, they stimulate protein synthesis and inhibit protein breakdown, enhancing the growth of muscle and bone tissue in the developing male. An excess of androgens can lead to a variety of disorders, including the accentuation of male secondary sex characteristics in both males and females. Less frequently, an excess of androgens may cause testicular or ovarian tumors. For a discussion of the related f...

About 55 percent of the blood is composed of a liquid known as plasma . The rest of the blood is made of three major types of cells: red blood cells (also known as erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes). See also Blood Type .

Plasma consists predominantly of water and salts. The kidneys carefully maintain the salt concentration in plasma because small changes in its concentration will cause cells in the body to function improperly. In extreme conditions this can result in seizures, coma, or even death. The pH of plasma, the common measurement of the plasma’s acidity, is also carefully controlled by the kidneys within the neutral range of 6.8 to 7.7. Plasma also contains other small molecules, including vitamins, minerals, nutrients, and waste products. The concentrations of all of these molecules must be carefully regulated. Plasma is usually yellow in color due to proteins dissolved in it. However, after a person eats a fatty meal, that person’s plasma temporarily develops a milky color as the blood carries the ingested fats from the intestines to other organs of the body. Plasma carries a large number of important proteins, including albumin, gamma globulin , and clotting factors. Albumin is the main prot...

Red Blood Cell or Erythrocyte, the oxygen-carrying component of the blood. Also referred to as red corpuscles, they are by far the most numerous type of blood cell. Red blood cells make up almost 45 percent of the blood volume. Their primary function is to carry oxygen from the lungs to every cell in the body. Red blood cells are composed predominantly of a protein and iron compound, called hemoglobin , that captures oxygen molecules as the blood moves through the lungs, giving blood its red color. As blood passes through body tissues, hemoglobin then releases the oxygen to cells throughout the body. Red blood cells are so packed with hemoglobin that they lack many components, including a nucleus, found in other cells. The membrane, or outer layer, of the red blood cell is flexible, like a soap bubble, and is able to bend in many directions without breaking. This is important because the red blood cells must be able to pass through the tiniest blood vessels, the capillaries, to deliver...