baroreceptors (ba/'o-re-sep'torz) Receptors for arterial blood pressure located in the aortic arch and the carotid sinuses. Barr body A microscopic structure in the cell nucleus produced from an inactive X chromosome in females. basal ganglia (ba'sal gang'gle-a) Gray matter, or nuclei, within the cerebral hemispheres, forming the corpus striatum, amygdaloid nucleus, and claustrum. basal metabolic (ba'sal mefa-bol'ik) rate (BMR) The rate of metabolism (expressed as oxygen consumption or heat production) under resting or basal conditions 8 to 12 hours after eating. basophil (ba'so-fil) The rarest type of leukocyte; a granular leukocyte with an affinity for blue stain in the standard staining procedure. B cell lymphocytes (lim'fd-sTts) Lymphocytes that can be transformed by antigens into plasma cells that secrete antibodies (and are thus responsible for humoral immunity). The B stands for bursa equivalent, which is believed to be the bone marrow. benign (bi-nin') Not malignant or life threatening. bi- (L.) Two, twice.

bile (biil) Fluid produced by the liver and stored in the gallbladder that contains bile salts, bile pigments, cholesterol, and other molecules. The bile is secreted into the small intestine. bile salts Salts of derivatives of cholesterol in bile that are polar on one end and nonpolar on the other end of the molecule. Bile salts have detergent or surfactant effects and act to emulsify fat in the lumen of the small intestine. bilirubin (bil"i-roo'bin) Bile pigment derived from the breakdown of the heme portion of hemoglobin.

blastocyst (Mas'to-sist) The stage of early embryonic development that consists of an inner cell mass, which will become the embryo, and surrounding trophoblast cells, which will form part of the placenta. This is the form of the embryo that implants in the endometrium of the uterus beginning at about the fifth day following fertilization. blood-brain barrier The structures and cells that selectively prevent particular molecules in the plasma from entering the central nervous system.

Bohr effect The effect of blood pH on the dissociation of oxyhemoglobin. Dissociation is promoted by a decrease in the pH.

Boyle's law The statement that the pressure of a given quantity of a gas is inversely proportional to its volume. bradycardia (brad"i-kar'de-a) A slow cardiac rate;

less than sixty beats per minute. bradykinin (brad"i-ki'nin) A short polypeptide that stimulates vasodilation and other cardiovascular changes. bronchiole (brong'ke-ol) the smallest of the air passages in the lungs, containing smooth muscle and cuboidal epithelial cells. brown fat A type of fat most abundant at birth that provides a unique source of heat energy for infants, protecting them against hypothermia. brush border enzymes Digestive enzymes that are located in the cell membrane of the microvilli of intestinal epithelial cells. buffer A molecule that serves to prevent large changes in pH by either combining with H+ or by releasing H+ into solution. bulk transport Transport of materials into a cell by endocytosis or phagocytosis, and out of a cell by exocytosis. bundle of His (hiss) A band of rapidly conducting cardiac fibers originating in the AV node and extending down the atrioventricular septum to the apex of the heart. This tissue conducts action potentials from the atria into the ventricles.

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