hair: A keratinous strand produced by epithelial cells of the hair follicle.
hair cells: Sensory cells of the inner ear.
hair follicle: An accessory structure of the integument; a tube lined by a stratified squamous epithelium that begins at the surface of the skin and ends at the hair papilla.
hair root: A thickened, conical structure consisting of a connective tissue papilla and the overlying matrix, a layer of epithelial cells that produces the hair shaft.
hallux: The big toe.
haploid: Possessing half the normal number of chromosomes; a characteristic of gametes.
hapten: A partial antigen that can bind to an antibody but cannot stimulate antibody production; a foreign compound that has only one antigenic determinant site.
hard palate: The bony roof of the oral cavity, formed by the maxillary and palatine bones.
Hassall's corpuscles: Aggregations of epithelial cells in the thymus whose functions are unknown.
haustra: Saclike pouches along the length of the large intestine that result from tension in the taenia coli.
heat exhaustion: A condition that is characterized by excessive perspiration and results in dangerous losses of fluids and salts.
heat stroke: A condition resulting from the failure of the body’s normal temperature control mechanisms; characterized by a cessation of sweating and a potentially fatal elevation of body temperature.
Heimlich maneuver: A technique for removing an airway blockage by external compression of the abdomen and forceful elevation of the diaphragm.
helper T cells: Lymphocytes whose secretions and other activities coordinate cell-mediated and antibody-mediated immunities; also called TH cells.
hematocrit: The percentage of the volume of whole blood contributed by cells; also called volume of packed red cells (VPRC) or packed cell volume (PCV).
hematoma: A tumor or swelling filled with blood.
hematuria: The presence of abnormal numbers of red blood cells in urine.
heme: A porphyrin ring containing a central iron atom that can reversibly bind oxygen molecules; a component of the hemoglobin molecule.
hemocytoblasts: Stem cells whose divisions produce each of the various populations of blood cells.
hemodialysis: Dialysis of the blood.
hemoglobin: A protein composed of four globular subunits, each bound to a heme molecule; gives red blood cells the ability to transport oxygen in the blood.
hemolysis: The breakdown (lysis) of red blood cells.
hemophilia: A congenital condition due to the inadequate synthesis of one of the clotting factors.
hemopoiesis: Blood cell formation and differentiation.
hemorrhage: Blood loss.
hemorrhoids: Swollen, varicose veins that protrude from the walls of the rectum and/or the anorectal canal.
hemostasis: The cessation of bleeding.
hemothorax: The entry of blood into one of the pleural cavities.
heparin: An anticoagulant released by activated basophils and mast cells.
hepatic duct: The duct that carries bile away from the liver lobes and toward the union with the cystic duct.
hepatic portal vein: The vessel that carries blood between the intestinal capillaries and the sinusoids of the liver.
hepatitis: An inflammation of the liver, resulting from exposure to toxic chemicals, drugs, or viruses.
hepatocyte: A liver cell.
hernia: The protrusion of a loop or portion of a visceral organ through the abdominopelvic wall or into the thoracic cavity.
herniated disc: The rupture of the connective tissue sheath of the nucleus pulposus of an intervertebral disc.
heterotopic: Ectopic; outside the normal location.
heterozygous: Possessing two different alleles at corresponding sites on a chromosome pair; the individual’s phenotype is determined by one or both of the alleles.
hexose: A six-carbon simple sugar.
hiatus: A gap, cleft, or opening.
high-density lipoprotein (HDL): A lipoprotein with a relatively small lipid content; thought to be responsible for the movement of cholesterol from peripheral tissues to the liver.
hilum/hilus: A localized region where blood vessels, lymphatic vessels, nerves, and/or other anatomical structures are attached to an organ.
hippocampus: A region, beneath the floor of a lateral ventricle, involved with emotional states and the conversion of short-term to long-term memories.
histamine: The chemical released by stimulated mast cells or basophils to initiate or enhance an inflammatory response.
histology: The study of tissues.
histones: Proteins associated with the DNA of the nucleus; the DNA strands are wound around them.
holocrine: A form of exocrine secretion in which the secretory cell becomes swollen with vesicles and then ruptures.
homeostasis: The maintenance of a relatively constant internal environment.
homologous chromosomes: The members of a chromosome pair.
homozygous: Having the same allele for a given phenotypic character on two homologous chromosomes.
hormone: A compound that is secreted by one cell and travels through the circulatory system to affect the activities of cells in another portion of the body.
human chorionic gonadotropin (hCG): The placental hormone that maintains the corpus luteum for the first 3 months of pregnancy.
human immunodeficiency virus (HIV): The infectious agent that causes acquired immune deficiency syndrome (AIDS).
human leukocyte antigen (HLA): See MHC protein.
human placental lactogen (hPL): The placental hormone that stimulates the functional development of the mammary glands.
humoral immunity: See antibody-mediated immunity.
hyaluronan: A carbohydrate component of proteoglycans in the matrix of many connective tissues.
hyaluronidase: An enzyme that breaks down the bonds between adjacent follicle cells; produced by some bacteria and found in the acrosomal cap of a spermatozoon.
hydrocephalus: A condition resulting from excessive production or inadequate drainage of cerebrospinal fluid.
hydrogen bond: A weak interaction between the hydrogen atom on one molecule and a negatively charged portion of another molecule.
hydrolysis: The breakage of a chemical bond through the addition of a water molecule; the reverse of dehydration synthesis.
hydrophilic: Freely associating with water; readily entering into solution.
hydrophobic: Incapable of freely associating with water molecules; insoluble.
hydrostatic pressure: Fluid pressure.
hypercapnia: High plasma carbon dioxide concentrations, commonly as a result of hypoventilation or inadequate tissue perfusion.
hyperglycemia: Elevated plasma glucose concentrations.
hyperkalemia: Abnormally high potassium concentrations in extracellular fluid.
hypernatremia: Abnormally high sodium concentrations in extracellular fluid.
hyperopia: Farsightedness, characterized by an inability to focus on nearby objects.
hyperplasia: An abnormal enlargement of an organ due to an increase in the number of cells.
hyperpnea: Abnormal increases in the rate and depth of respiration.
hyperpolarization: The movement of the transmembrane potential away from the normal resting potential and farther from 0 mV.
hyperreflexia: Abnormally exaggerated reflex responses to stimulation.
hypersecretion: The overactivity of glands that produce exocrine or endocrine secretions.
hypersensitivity: An overreaction to an allergen that results in tissue damage and inflammation.
hypertension: Abnormally high blood pressure.
hyperthermia: Excessively high body temperature.
hyperthyroidism: An excessive production of thyroid hormones.
hypertonic: In comparing two solutions, the solution with the higher osmolarity.
hypertrophy: An increase in the size of tissue without cell division.
hyperventilation: A rate of respiration sufficient to reduce plasma PCO2 to levels below normal.
hypervitaminosis: A clinical condition caused by the excessive ingestion and uptake of vitamins.
hypesthesia: An abnormally decreased sensitivity to stimuli.
hypocapnia: An abnormally low plasma PCO2, commonly as a result of hyperventilation.
hypodermic needle: A needle inserted through the skin to introduce drugs into the subcutaneous layer.
hypodermis: See subcutaneous layer.
hypokalemia: Abnormally low plasma potassium concentrations.
hyponatremia: Abnormally low plasma sodium concentrations.
hyponychium: A thickening in the epidermis beneath the free edge of a nail.
hypophyseal portal system: The network of vessels that carry blood from capillaries in the hypothalamus to capillaries in the anterior pituitary gland (hypophysis).
hypophysis: The anterior pituitary gland, which is subdivided into the pars distalis and the pars intermedia.
hyporeflexia: Abnormally depressed reflex responses to stimuli.
hyposecretion: Abnormally low rates of exocrine or endocrine secretion.
hypothalamus: The floor of the diencephalon; the region of the brain containing centers involved with the unconscious regulation of visceral functions, emotions, drives, and the coordination of neural and endocrine functions.
hypothermia: An abnormally low body temperature.
hypothesis: A prediction that can be subjected to scientific analysis and review.
hypotonic: In comparing two solutions, the solution with the lower osmolarity.
hypoventilation: A respiratory rate that is insufficient to keep plasma PCO2 within normal levels.
hypovitaminosis: A clinical condition resulting from inadequate vitamin ingestion and uptake; vitamin deficiency.
hypovolemic: An abnormally low blood volume.
hypoxia: A low tissue oxygen concentration.