H

haploid (hap'loid) Denoting cells that have one of each chromosome type and therefore half the number of chromosomes present in most other body cells. Only the gametes (sperm and ova) are haploid. hapten (hap'ten) A small molecule that is not antigenic by itself, but which—when combined with proteins—becomes antigenic and thus capable of stimulating the production of specific antibodies. haversian (ha-ve/shan) system A haversian canal and its concentrically arranged layers, or lamellae, of bone. It constitutes the basic structural unit of compact bone. hay fever A seasonal type of allergic rhinitis caused by pollen. It is characterized by itching and tearing of the eyes, swelling of the nasal mucosa, attacks of sneezing, and often by asthma.

hCG Human chorionic gonadotropin (kor e-on-ik go-nad"o-tro'pin). A hormone secreted by the embryo that has LH-like actions and that is required for maintenance of the mother's corpus luteum for the first 10 weeks of pregnancy.

heart murmur An abnormal heart sound caused by an abnormal flow of blood in the heart. Murmurs are due to structural defects, usually of the valves or septum. heart sounds The sounds produced by closing of the AV valves of the heart during systole (the first sound) and by closing of the semilunar valves of the aorta and pulmonary trunk during diastole (the second sound). helper T cells A subpopulation of T cells

(lymphocytes) that help to stimulate antibody production of B lymphocytes by antigens. hematocrit (he-mato-krit) The ratio of packed red blood cells to total blood volume in a centrifuged sample of blood, expressed as a percentage.

heme (hem) The iron-containing red pigment that, together with the protein globin, forms hemoglobin. hemoglobin (he'md-glo"bin) The combination of heme pigment and protein within red blood cells that acts to transport oxygen and (to a lesser degree) carbon dioxide. Hemoglobin also serves as a weak buffer within red blood cells.

Henderson-Hasselbalch (hen' der-son-hasel-balk) equation A formula used to determine the blood pH produced by a given ratio of bicarbonate to carbon dioxide concentrations. Henry's law The statement that the concentration of gas dissolved in a fluid is directly proportional to the partial pressure of that gas. heparin (hep'ar-in) A mucopolysaccharide found in many tissues, but in greatest abundance in the lungs and liver. It is used medically as an anticoagulant.

hepatic (he-pat ik) Pertaining to the liver. hepatitis (hep"a-if tis) Inflammation of the liver. Hering-Breuer reflex A reflex in which distension of the lungs stimulates stretch receptors, which in turn act to inhibit further distension of the lungs.

hermaphrodite (her-maf ro-dit) An organism with both testicular and ovarian tissue. hetero- (Gk.) Different, other. heterochromatin (hefe-ro-kro'ma-tin) A condensed, inactive form of chromatin. hiatal hernia (hi-a'tal her' ne-a) A protrusion of an abdominal structure through the esophageal hiatus of the diaphragm into the thoracic cavity.

high-density lipoproteins (lip"o-pro'te-inz) (HDLs)

Combinations of lipids and proteins that migrate rapidly to the bottom of a test tube during centrifugation. HDLs are carrier proteins that are believed to transport cholesterol away from blood vessels to the liver, and thus to offer some protection from atherosclerosis. higher motor neurons Neurons in the brain that, as part of the pyramidal or extrapyramidal system, influence the activity of the lower motor neurons in the spinal cord. histamine (his'ta-men) A compound secreted by tissue mast cells and other connective tissue cells that stimulates vasodilation and increases capillary permeability. It is responsible for many of the symptoms of inflammation and allergy. histocompatibility (his'to-kom-pafi-bil' i-te)

antigens A group of cell-surface antigens found on all cells of the body except mature red blood cells. They are important for the function of T lymphocytes, and the greater their variance, the greater will be the likelihood of transplant rejection. histone (histoon) A basic protein associated with DNA that is believed to repress genetic expression. homeo (Gk.) Same.

homeostasis (ho"me-o-sta'sis) The dynamic constancy of the internal environment, the maintenance of which is the principal function of physiological regulatory mechanisms. The concept of homeostasis provides a framework for understanding most physiological processes.

homologous (ho-mol-o-gus) chromosomes The matching pairs of chromosomes in a diploid cell.

hormone (ho/mon) A regulatory chemical produced in an endocrine gland that is secreted into the blood and carried to target cells that respond to the hormone by an alteration in their metabolism.

hormone-response element A specific region of DNA that binds to a particular nuclear hormone receptor when that receptor is activated by bonding with its hormone. This stimulates genetic transcription (RNA synthesis).

humoral immunity (hyoo'-mor-al i-myoo'ni-te) The form of acquired immunity in which antibody molecules are secreted in response to antigenic stimulation (as opposed to cellmediated immunity). hyaline (hi'a-lin) membrane disease A disease affecting premature infants who lack pulmonary surfactant. It is characterized by collapse of the alveoli (atelectasis) and pulmonary edema; also called respiratory distress syndrome.

696 Glossary hydrocortisone (hi"drd-koZtt-son) The principal corticosteroid hormone secreted by the adrenal cortex, with glucocorticoid action; also called

Cortisol.

hydrophilic (hi"drd-fil'ik) Denoting a substance that readily absorbs water; literally, "water loving."

hydrophobic (hi"dro-fo' bik) Denoting a substance that repels, and that is repelled by, water; literally, "water fearing." hyper- (Gk.) Over, above, excessive. hyperbaric (hi"per-bar'ik) oxygen Oxygen gas present at greater than atmospheric pressure. hypercapnia (hi"per-kap'ne-a) Excessive concentration of carbon dioxide in the blood. hyperemia (hi"per-e'-me-a) Excessive blood flow to a part of the body. hyperglycemia (hi"per-gli-se'me-a) An abnormally increased concentration of glucose in the blood.

hyperkalemia (hi"per-kai-le me-ai) An abnormally high concentration of potassium in the blood. hyperopia (hi"per- pe-ai) A refractive disorder in which rays of light are brought to a focus behind the retina as a result of the eyeball being too short; also called farsightedness. hyperplasia (hi"per-pla' ze-a) An increase in organ size because of an increase in the number of cells as a result of mitotic cell division. hyperpnea (hi"perp ne-ai) Increased total minute volume during exercise. Unlike hyperventilation, the arterial blood carbon dioxide values are not changed during hyperpnea because the increased ventilation is matched to an increased metabolic rate. hyperpolarization (hi'per-po"lar-X-za'shun) An increase in the negativity of the inside of a cell membrane with respect to the resting membrane potential. hypersensitivity (hi"per-sen"si-tiv'i-te) Another name for allergy; an abnormal immune response that may be immediate (due to antibodies of the IgE class) or delayed (due to cell-mediated immunity). hypertension (hi"per-ten'shun) High blood pressure. Classified as either primary, or essential, hypertension of unknown cause or secondary hypertension that develops as a result of other, known disease processes. hypertonic (hi"per-ton ik) Denoting a solution with a greater solute concentration, and thus a greater osmotic pressure, than plasma. hypertrophy (hi-per'tro-fe) Growth of an organ because of an increase in the size of its cells. hyperventilation (hi-per-ven"ti-la'shun) A high rate and depth of breathing that results in a decrease in the blood carbon dioxide concentration to below normal. hypo- (Gk.) Under, below, less. hypodermis (hi"pot-der mis) A layer of fat beneath the dermis of the skin. hypotension (hi"po-ten shun) Abnormally low blood pressure.

hypothalamic (hi"po-that-lam ik) hormones

Hormones produced by the hypothalamus. These include antidiuretic hormone and oxytocin, which are released from the posterior pituitary, and both releasing and inhibiting hormones that regulate the secretions of the anterior pituitary. hypothalamo-hypophyseal (hi"po-that-lam o-hi"po-fX-se'al) portal system A vascular system that transports releasing and inhibiting hormones from the hypothalamus to the anterior pituitary.

hypothalamo-hypophyseal tract The tract of nerve fibers (axons) that transports antidiuretic hormone and oxytocin from the hypothalamus to the posterior pituitary.

hypothalamus (hi"po-thal at-mus) An area of the brain lying below the thalamus and above the pituitary gland. The hypothalamus regulates the pituitary gland and contributes to the regulation of the autonomic nervous system, among its many functions.

hypothermia (hi"pd-therme-a) A low body temperature. This is a dangerous condition that is defended against by shivering and other physiological mechanisms that generate body heat.

hypovolemic (hi"po-vo-le mik) shock A rapid fall in blood pressure as a result of diminished blood volume.

hypoxemia (hi"pok-se'me-a) A low oxygen concentration of the arterial blood.

ileogastric (il"e-o-gas trik) reflex The reflex in which distension of the ileum causes decreased gastric motility. immediate hypersensitivity Hypersensitivity

(allergy) that is mediated by antibodies of the IgE class and that results in the release of histamine and related compounds from tissue cells.

immunization (im"yu-ni-za'shun) The process of increasing one's resistance to pathogens. In active immunity, a person is injected with antigens that stimulate the development of clones of specific B or T lymphocytes; in passive immunity, a person is injected with antibodies made by another organism. immunoassay (im"yut-no-as a) Any of a number of laboratory or clinical techniques that employ specific bonding between an antigen and its homologous antibody in order to identify and quantify a substance in a sample. immunoglobulins (im"yut-no-glob yut-linz) Subclasses of the gamma globulin fraction of plasma proteins that have antibody functions, providing humoral immunity. immunosurveillance (im"yut-no-ser-va lens) The function of the immune system to recognize and attack malignant cells that produce antigens not recognized as "self." This function is believed to be cell mediated rather than humoral. implantation (im"plan-ta shun) The process by which a blastocyst attaches itself to and penetrates the endometrium of the uterus. infarct (in farkt) An area of necrotic (dead) tissue produced by inadequate blood flow (ischemia).

inhibin (in-hib in) Believed to be a water-soluble hormone secreted by the seminiferous tubules of the testes that specifically exerts negative feedback control of FSH secretion from the anterior pituitary. inositol triphosphate (X-no' sX-tol tri-fos'fcit) A second messenger in hormone action that is produced by the cell membrane of a target cell in response to the action of a hormone. This compound is believed to stimulate the release of Ca2+ from the endoplasmic reticulum of the cell.

insulin (in sut-lin) A polypeptide hormone secreted by the beta cells of the islets of Langerhans in the pancreas that promotes the anabolism of carbohydrates, fat, and protein. Insulin acts to promote the cellular uptake of blood glucose and, therefore, to lower the blood glucose concentration; insulin deficiency produces hyperglycemia and diabetes mellitus. integrins (in-te grinz) A family of glycoproteins that extend from the cytoskeleton, through the plasma membrane of cells, and into the extracellular matrix. They serve to integrate different cells of a tissue and the extracellular matrix, and to bind cells to other cells, such as neutrophils to the endothelial cells of capillaries for extravasation. inter- (L.) Between, among.

interferons (in"ter-fer unz) Small proteins that inhibit the multiplication of viruses inside host cells and that also have antitumor properties. interleukin-2 (in"ter-loo' kin-2) A lymphokine secreted by T lymphocytes that stimulates the proliferation of both B and T lymphocytes. interneurons (in"ter-noor onz) Those neurons within the central nervous system that do not extend into the peripheral nervous system. They are interposed between sensory (afferent) and motor (efferent) neurons; also called association neurons. interoceptors (in"ter-o-sep torz) Sensory receptors that respond to changes in the internal environment (as opposed to exteroceptors). interphase The interval between successive cell divisions, during which time the chromosomes are in an extended state and are active in directing RNA synthesis. intestino-intestinal (in"testX-no-in-tes'tX-nal) reflex The reflex in which overdistension to one region of the intestine causes relaxation throughout the rest of the intestine. intra- (L.) Within, inside.

intrafusal (in"tra-fyoo'sal) fibers Modified muscle fibers that are encapsulated to form muscle spindle organs, which are muscle stretch receptors.

intrapleural (in"trat-ploor al) space An actual or potential space between the visceral pleura covering the lungs and the parietal pleura lining the thoracic wall. Normally, this is a potential space; it can become real only in abnormal situations. intrapulmonary (in"trat-pul mot-nar"e) space The space within the air sacs and airways of the lungs. intron (in tron) A noncoding nucleotide sequence in DNA that interrupts the coding regions (exons) for mRNA. inulin (in yut-lin) A polysaccharide of fructose, produced by certain plants, that is filtered by the human kidneys but neither reabsorbed nor secreted. The clearance rate of injected inulin is thus used to measure the glomerular filtration rate.

in vitro (in ve'tro) Occuring outside the body, in a test tube or other artificial environment. in vivo (in ve vo) Occuring within the body. ion (i on) An atom or a group of atoms that has a net positive or a net negative charge because of a loss or gain of electrons. ionization (i"on-i-za'shun) The dissociation of a solute to form ions. ipsilateral (ip"st-lat er-al) On the same side (as opposed to contralateral). IPSP Inhibitory postsynaptic potential. A

hyperpolarization of the postsynaptic membrane in response to a particular neurotransmitter chemical, which makes it more difficult for the postsynaptic cell to attain the threshold level of depolarization required to produce action potentials. IPSPs are responsible for postsynaptic inhibition. ischemia (i-ske'me-ai) A rate of blood flow to an organ that is inadequate to supply sufficient oxygen and maintain aerobic respiration in that organ.

islets of Langerhans (f letz of lang'er-hanz)

Encapsulated groupings of endocrine cells within the exocrine tissue of the pancreas, including alpha cells that secrete glucagon and beta cells that secrete insulin; also called pancreatic islets. iso- (Gk.) Equal, same. isoenzymes (i"so-en' zimz) Enzymes, usually produced by different organs, that catalyze the same reaction but that differ from each other in amino acid composition. isometric (i"so-met rik) contraction Muscle contraction in which there is no appreciable shortening of the muscle. isotonic (i"so-ton'ik) contraction Muscle contraction in which the muscle shortens in length and maintains approximately the same amount of tension throughout the shortening process. isotonic solution A solution having the same total solute concentration, osmolality, and osmotic pressure as the solution with which it is compared; a solution with the same solute concentration and osmotic pressure as plasma.

jaundice (Jawn' dis) A condition characterized by high blood bilirubin levels and staining of the tissues with bilirubin, which imparts a yellow color to the skin and mucous membranes. junctional (Jungk shun-al) complexes Structures that join adjacent epithelial cells together, including the zonula occludens, zonula adherens, and macula adherens (desmosome). juxta- (L.) Near to, next to.

juxtaglomerular (Juksta-glo-mer'yii-lar apparatus A renal structure in which regions of the nephron tubule and afferent arteriole are in contact with each other. Cells in the afferent arteriole of the juxtaglomerular apparatus secrete the enzyme renin into the blood, which activates the renin-angiotensin system.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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