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The environment occupied by an organism.


A mechanism or process of learning in which a subject’s behavioural responses decrease to a repetitive stimulus. A simple kind of learning involving a loss of sensitivity to unimportant stimuli, allowing an animal to conserve time and energy.


Quark composites: mesons and baryons. Protons and neutrons are the most common hadrons.

Haemophilus B (HIB)

A serious bacterial infection which can cause meningitis, ear infections, epiglottis (croup) and skin and soft tissue infections. Immunization with the HIB vaccine can protect infants and children from contracting this infection. Recommended immunizations should occur at 2 months, 4 months, 6 months, and 15 months of age.

Hair bulb  

The base of a hair. It contains cells that divide mitotically to produce columns of hair cells

Hair root 

The portion of a hair that extends from the skin's surface to the hair bulb.

Hair shaft  

The portion of a hair that extends above the skin's surface.

Hairpin loop

A section of highly curving, single-stranded DNA or RNA formed when a long piece (string) of the DNA or RNA bends back on itself and hydrogen-bonds (is able to base pair) in some regions to form double-stranded regions. The structure can be visualized by taking a human hair, bending it back on itself and holding it in such a way as to half its original length. The section where the two ends of hair lie next to each other represents the section of double-stranded DNA or RNA. At one end the hair will have to make a sharp turn and will form a loop. This loop represents the single-stranded hairpin loop. Hairpin loops can also form in peptide (molecules).

Half life

The halflife of a specific radioactive isotope is the length of time necessary for half of the radioactive nuclei in a sample to disintegrate; The average time required for the disappearance or decay of one-half of any amount of a given substance. The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element. The halflife of a radioisotope is unique for each isotope

Half plane

The set of all points in a plane that lie on one side of a line in the plane.


A sensory perception without external stimulation of the relevant sensory organ. A hallucination has the immediate sense of reality of a true perception, although in some instances the source of the hallucination may be perceived as within the body, such as an auditory hallucination experienced as coming from within the head rather than through the ears. Hallucinations should be distinguished from illusions, in which an external stimulus is misperceived or misinterpreted and from normal thought processes that are exceptionally vivid. Transient hallucinatory experiences are common in people without mental disorder. Types of hallucinations are: auditory, gustatory, mood-incongruent, olfactory, somatic, tactile, and visual.


A cell or individual with a genetic complement containing one copy of each nuclear chromosome.A single set of chromosomes (half the full set of genetic material) is present in the egg and sperm cells of animals and in the egg and pollen cells of plants; mosses, and many protists and fungi, are haploid, as are some insects, bryophytes, and the gametes of all organisms. Human beings have 23 chromosomes in their reproductive cells. Contrast with Diploid which refers to the condition when a eukaryotic cell possesses two sets of chromosomes.

Haploid life cycle

Occurs when the only multicellular stage in an organism's life cycle is haploid.

Haploid-diploid life cycle

Occurs when a multicellular diploid phase, or sporophyte, alternates with a multicellular haploid phase, or gametophyte. Only plants and certain algae possess this kind of life cycle, which is also called "alternation of generations".

Haptic sensations

Refers to touch sensation and information transmitted through body movement and/or position.

Hardness (of water)

A water-quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium. If the water you use is "hard" then more soap, detergent or shampoo is necessary to raise a lather.

Hard signs

A term used by neurologists to indicate that a child performs in an observably different way than the average child in certain central nervous system functions.
These differences may be quantitative or qualitative. They may include the presence of inappropriate responses and/or the absence of appropriate responses.


A generic term that encompasses all metallic implants. By extension, it also includes nonmetallic materials associated with metallic components such as polyethylene components of joint prostheses. See implants.

Hardy-Weinberg equilibrium

The steady-state relationship between relative frequencies of two or more alleles in an idealized population. At equilibrium both the allele frequencies and the genotype frequencies will remain constant from generation to generation in a population breeding at random in the absence of evolutionary forces.

Hardy-Weinberg theorem

An axiom maintaining that the sexual shuffling of genes alone cannot alter the overall genetic makeup of a population.

Harmonic sequence

A sequence is a harmonic sequence if the reciprocals of the numbers in the sequence form an arithmetic sequence.

Haustorium (plural Haustoria)

In parasitic fungi, a nutrient-absorbing hyphal tip that penetrates the tissues of the host but remains outside the host cell membranes.

Haversian system

One of many structural units of vertebrate bone, consisting of concentric layers of mineralized bone matrix surrounding lacunae, which contain osteocytes, and a central canal, which contains blood vessels and nerves.

Head waters

1) the source and upper reaches of a stream; also the upper reaches of a reservoir. (2) the water upstream from a structure or point on a stream. (3) the small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.

Hearing aid

An electronic instrument that amplifies sound waves for a person who has a hearing impairment. Digital hearing aids allow a high degree of control over what frequency (or frequency range) is amplified and to what extent depending on the setting (which can be adjusted for the environment).

Heart block

A block in the conduction of the normal electrical impulses in the heart.

Heart failure

Heart failure (HF) is the inability of the heart to pump a sufficient amount of blood into the blood vessels. Cardiac damage that initiates heart failure usually occurs as a result of other cardiac diseases, such coronary artery disease, coronary heart disease, or a disease of one of the valves of the heart. Heart failure is a disease with devastating effects.

Heart murmur

A sound made by the action of the heart when the valves between the heart chambers do not close completely. Functional heart murmurs are not harmful in any manner. Organic heart murmurs indicate serious problems.

Heart rate

The number of times the heart beats each minute


The total amount of kinetic energy due to molecular motion in a body of matter.
Heat is energy in its most random form.

Heat and moisture exchanger (HME)

A hygroscopic device placed proximal to the patient’s artificial airway that captures the exhaled moisture and evaporates it during inspiration, humidifying the airway

Heat capacity (also known as Thermal capacity)

The energy needed to be delivered to an object in order to increase its temperature by 1 degree.

Heat of vaporisation

The amount of heat required to change a given amount of a liquid into a gas.
540 calories are required to change 1 gram of liquid water into vapor.

Heat shock protein

A protein that helps protect other proteins during heat stress, found in plants, animals, and microorganisms.

Hedonic testing

Assessment of sensory perceptions according to like/dislike. Samples are presented in succession and the subject is told to decide how much they likes or dislike the product and to mark the scales accordingly. The nature of this test is its relative simplicity. The instructions to the panelist are restricted to procedures, and no attempt is made at a direct response. The subject is allowed, however to make their own inferences about the meaning of the scale categories and determine for themselves how they will apply them to the samples. A separate scale is provided for each sample in a test session.


An enzyme that unzips a DNA double helix into single strands.

Helium dilution method for measurement of thoracic gas volume

Air in the thorax is allowed to equilibrate with a known volume of air containing a small concentration of the inert gas, helium. The final dilution of helium is used to calculate the volume of air in the thorax.


A protein structural motif characteristic of certain DNA-binding proteins.

Helper T cells

A type of lymphocyte (white blood cell) that stimulates the production of antibodies by activating B cells when an antigen is present. A type of T cell that is required by some B cells to help them make antibodies or that helps other T cells respond to antigens or secrete lymphokines or interleukins.


A test that measures the percentage of red blood cells in whole blood (normally 35%-40%). A sample of blood is spun around in a centrifuge so that the red cells fall to the bottom of the sample. It is done to determine iron status in the blood.


The collection of blood in tissues or a space following rupture of a blood vessel.
With respect to hematomas in the brain:
Epidural = Outside the brain and its fibrous covering, the dura, but under the skull.
Subdural = Between the brain and its fibrous covering (dura).
Intracerebral =In the brain tissue.
Subarachnoid = Around the surfaces of the brain, between the dura and arachnoid membranes.

Hematopoietic stem cell

A stem cell that gives rise to all red and white blood cells and platelets


The iron-containing group of heme proteins such as hemoglobin and the cytochromes.

Hemianopsia / Hemianopia

Visual field cut. Blindness for one half of the field of vision.This is not the right or left eye, but the right or left half of vision in each eye.


Replacement of one side of a joint with a prosthesis. The prosthesis in a hemiarthoplasty can have a unipolar or bipolar design. In a unipolar design, the prosthetic component is on only on side of the joint. In a bipolar design, there is a prosthetic component on the other side of the joint that is not fixed in place. In a total hip arthroplasty, the across the joint component is rigidly fixed in place. See bipolar hemiarthroplasty.


A type of dietary fiber made up glucose and many other types of sugars and differs from cellulose, which only contains glucose.


Weakness of one side of the body.


Paralysis that involves one side of the body in a lateral fashion.


Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans).


An iron-containing protein in red blood cells that reversibly binds oxygen. A protein that binds and transports molecular oxygen in animals. It is a tetrameric protein (a protein with four subunits) where each subunit binds one heme, a co-factor responsible for oxygen binding. Oxygen binding on individual subunits shows positive cooperativity, a form of interaction between subunits where binding of oxygen to the heme in one subunit increases the strength of binding of oxygen to hemes in all other subunits.


In invertebrates with an open circulatory system, the body fluid that bathes tissues.


A group of hereditary disorders characterized by failure of the blood to clot and consequent excessive bleeding from even minor wounds. A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting factors, usually one known as Factor VII. Hence hemophiliacs suffer from an inability to clot their blood.


Spitting of blood


Pertaining to the liver.

Hepatic portal vessel

A large circulatory channel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood's nutrient content.

Hepatitis B

A potentially serious viral disease that affects the liver. Can be transmitted through sexual contact or through contact with infected blood


An organism that feeds on plant material.
Hence: Herbivores, herbivorous


The consumption of living plant material.


The transmission of characteristics from parent to offspring.

Hering Breuer reflex

The nervous mechanism that tends to limit inspiratory excursions. Mediated by stretch receptors in the intercostal muscles whose activity leads to inhibition of inspiratory neurons in the brain stem


An individual that functions as both male and female in sexual reproduction by producing both sperm and eggs. An organism which has both female and male reproductive organs and is therefore capable of producing both eggs and sperms. A simultaneous hermaphrodite has both kinds of reproductive organs at the same time. A sequential hermaphrodite has one kind early in life and the other later in life.


Abnormal protrusion of a structure out of its normal anatomical position

Heron’s formula

A formula for the area of a triangle

Herpes zoster

An acute inflammatory disease of the dorsal root ganglion caused by the virus of chickenpox. Characterized by small vesicles occurring on the skin supplied by the affected nerve.


Nontranscribed eukaryotic chromatin that is so highly compacted that it is visible with a light microscope during interphase.

where a, b, and c are the lengths of the sides of the triangle, and s is half the perimeter.


Evolutionary changes in the timing or rate of development.

Heterogeneous Immunoassay

Heterogenous immunoassays are reactions in which a physical separation step is required to separate the bound ligand from the free ligand before either fraction can be measured.

Heterogeneous magnetic fields

Magnetic field that exhibits a gradient depending on the nature of the magnet

Heterogeneous nuclear RNA (hnRNA)

Refers collectively to the variety of RNAs found in the nucleus, including primary transcripts, partially processed RNAs and snRNA. The term hnRNA is often used just for the unprocessed primary transcripts, however.


A term used variously to refer to some or all of those algae with chloroplasts having chlorophylls a and c (i.e. stramenopiles, cryptomonads, haptophytes and dinoflagellates).


A condition in the life cycle of all modern plants in which the sporophyte and gametophyte generations differ in morphology.

Heteroscedastic Variances

Heteroscedastic variances are unequal variances. One of the fundamental characteristics of most immunoassays is that the variances of the sample response change throughout the range of the standard curve.


An organism that which cannot synthesize its own organic food material so obtains them by eating other organisms or their by-products.
Hence:Heterotrophic = Petaining to a hetertroph; A mode of nutrition in which the consumer relies upon molecules created by other organisms for energy and nutrients. Osmotrophic = absorbing soluble organic matter.Phagotrophic = ingesting particles of food.


A diploid organism that carries two different alleles at one or more genetic loci.

Heterozygote advantage

A mechanism that preserves variation in eukaryotic gene pools by conferring greater reproductive success on heterozygotes over individuals homozygous for any one of the associated alleles.


The presence of different alleles at one or more loci on homologous chromosomes.Genetic variability among individuals within populations and variabilty among populations. See homozygosity.


Identifies the two alleles of a diploid organism as being different from each other. If both alleles are genetically identical, the cell or organism is homozygous.Often, one of the two alleles is the dominant, the other the recessive allele. If a dominant allele is present, one copy is enough to establish the corresponding phenotype (e.g. dark eye color). The recessive allele for light eye color cannot be expressed, until two copies of the allele (homozygous) are present.

Hexadecimal number

A number written in base sixteen.


A physiological state that allows survival during long periods of cold temperatures and reduced food supplies, in which metabolism decreases, the heart and respiratory system slow down, and body temperature is maintained at a lower level than normal.


A series in which each element is categorized into successive ranks or grades with each level subordinate to the one above.

High density lipoprotein (HDL)

Lipoproteins are combinations of lipids (fats) and proteins. They are the form in which lipids are transported in the blood. The high density lipoproteins transport cholesterol from the tissues of the body to the liver so it can be gotten rid of in the bile. HDL cholesterol is therefore considered the "good" cholesterol. The higher the HDL cholesterol level, the lower the risk of coronary artery disease.

Highest specific binding

The highest specific binding (HSB) standard is the standard expected to yield the highest bound response.The HSB is the internal response which is used to calculate the normalized response if the appropriate baseline standard was not assayed. The HSB is also used to calculate one of the method control monitor points in the absence of the appropriate baseline standard. In competitive binding assays, the lowest concentration standard is the HSB. In sandwich binding assays, the highest concentration standard is the HSB.

High performance liquid chromatography (HPLC)

A quantitative method for identifying, purifying and analyzing compounds that are dissolved in solution. A form of column chromatography (see Chromatography) used to separate, identify, and quantify compounds.

High temperature/ short-time processing (HTST)

A process by which food is sterilised at very high temperatures but only for a very short period.

High tone hypertonic

Having stiff, rigid, or inflexible muscles.

High-throughput screening (tests)

A technique of rapidly screening for biological effects in very large numbers of compounds; A methodology utilized to quickly screen large numbers of compounds for use as pharmaceuticals or agrochemicals (e.g., herbicides).High-throughput screening tests examine whether a compound binds to a receptor of interest, whether the compound produces a functional response in the receptor, and other tests on a small scale. High-throughput screening is a first-pass test affording a quick initial look at the efficacy of a compound.For example, when screening chemical compounds for potential use as a pharmaceutical, the goal often is to assess differences between diseased and (treated) cells; enabling identification of a pharmaceutical candidate that favorably impacts change in protein level (i.e., gene expression) which characterizes a diseased state, or some other gene expression marker (e.g., apoptosis).When screening compounds for potential use as herbicide active ingredients, the goal is to assess differences between normal and (treated) weed plant cells; enabling identification of a potential herbicide candidate that imparts desired (fatal) change.Although whole living cells or whole microscopic animals such as nematodes could be utilized in HTS, it is more common to use a proxy (e.g., receptors, enzymes, or STATs from applicable cells) whose interaction with candidate compounds can be inferred to cell (and/or organism) effects.

Hill analysis

A method for analysing saturation or competition curves to determine whether the interaction between the ligand and the receptor is competitive. As with the Scatchard plot, the Hill plot has now largely been superseded by computer- fitting of the curves to a one- or multi-site model. The slope of the Hill plot (sometimes called the Hill coefficient or nH) should equal 1 if the ligand binds competitively and reversibly. Slopes significantly greater than 1 may indicate positive cooperativity in the binding; slopes less than 1 may indicate heterogeneity of binding sites or negative cooperativity. However, there are also many artefactual reasons why the slope can differ from the theoretical value.

Fig. F: Hill plot from the saturation curve for a radioligand. The y axis represents the log of the proportion of receptors occupied by radioligand divided by the proportion not occupied (b = the amount of ligand bound, assuming that the NSB has been subtracted; Bmax = the maximal amount of specifically bound ligand). The significance of the slope (nH) is explained in the text.


Excessive growth of body and facial hair, including the chest, stomach, and back

Hip pain

Hip pain is the sensation of discomfort in or around the hip joint, where the upper end (head) of the thigh bone (femur) fits into the socket of the hip bone. Hip pain has a number of causes, most of which are related to degeneration, injury, or inflammation of the muscles, bones, joints, and tendons located in the hip area. Common causes of hip pain include arthritis , bursitis , bone fracture , muscle spasms and strains. Hip pain can also result from disorders causing pain radiating from the spine and back, such as sciatica and herniated discs.


A substance released by injured cells that causes blood vessels to dilate during an inflammatory response.


Small proteins with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in its chromatin structure; A family of proteins forming organizing complexes to structure chromosomal DNA in eukaryotic cells.They are water- and dilute acid-soluble proteins rich in basic amino acids (e.g., lysine), and are found complexed with DNA in the chromosomes of all eucaryotic cells except sperm. Histones are responsible for the structure of chromatin, acting as spools around which the DNA winds (like the way cotton winds around a wooden spool), compacting the DNA so that it will fit inside the nucleus. They play important roles in gene regulation and, to some degree, the expression of proteins from the DNA, e.g., Acetylation, i. e., addition of acetyl molecular group) of histones results in some genes within the DNA looped around that histone to become (more) accessible to the cell's transcriptional "machinery"; thereby turning-on those genes. Methylation (i.e., addition of methyl molecular group) of a protruding amino acid (e.g., lysine) in a histone; thereby "turning-on"/up-regulating those genes.
Sumoylation (i.e., addition of SUMO protein group) of histones results in repression/"turning off" those genes. Histone modification by acetylation and methylation has been shown to affect large scale changes in chromosome packing affecting the ability of cells to 'read' genes. The more packed, the less access the cell has to genes. This large scale modification is part of epigenetic control of gene expression, i.e., the making of proteins from DNA code.


See Human immunodeficiency virus.

Holoblastic cleavage

A type of cleavage in which there is complete division of the egg, as in eggs having little yolk (sea urchin) or a moderate amount of yolk (frog).


Organisms that remain pelagic throughout their entire life cycle.


Permanent members of the plankton.

Homeobox genes

A 180-nucleotide sequence within a homeotic gene encoding the part of the protein that binds to the DNA of the genes regulated by the protein. A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. They have been found in many organisms from fruit flies to human beings. In the fruit fly, a homeobox appears to determine when particular groups of genes are expressed during development. In many other cases homeobos genes are pattern genes that establish the body plan and position of organs in response to gradients of regulatory molecules.


Evolutionary alteration in the placement of different body parts.


Maintaining a relatively constant internal environment; The steady-state physiological condition of the body. In biology used to describe a condition where an organism maintains a stable structure where in fact a constant flux of molecules occurs. Although many organisms can live for years, all cellular components like proteins, membranes, sugars, and nucleic acids are constantly recycled while never compromising the integrity of the organism as a whole. This turnover processes can be characterized by specific half-life values that for most proteins, membranes, and RNA (but not DNA structures) are measured in hours. In a more narrow sense homeostasis refers to the maintenance of water and salt concentration in cells.


An organism, such as a bird or mammal, capable of maintaining a stable body temperature independent of the environment.

Homeotic genes

Genes that control the overall body plan of animals by controlling the developmental fate of groups of cells.


Humans and closely related primates; includes modern and fossil forms, such as the australopithecines, but not the apes.


Humans and the apes.


Amino acid produced as a byproduct of the metabolism of one-carbon compounds.
Excessive accumulation of this substance in the bloodstream has been associated with an increased risk of coronary heart disease.

Homogeneous magnetic fields

Magnetic field with a constant strength over space.


Process in which the size of the fat globules is reduced to small uniform particles, which are then distributed evenly throughout the liquid. For example, the cream in homogenised milk is distributed throughout the liquid rather than rising to the top to form a layer.

Homogenous Immunoassay

Homogeneous immunoassays are assays which use differences between bound and unbound label signals to measure the bound or the free fractions without the need to separate the two fractions


Similarities in DNA or protein sequences between individuals of the same species or among different species.

Homologous chromosomes

A pair of chromosomes containing the same linear gene sequences, each derived from one parent. Chromosome pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism's father, the other from the mother.

Homologous structures

Structures in different species that are similar because of common ancestry.


Similarity in characteristics resulting from a shared ancestry; A characteristic shared by different groups of organisms that is derived from the same structure or trait in their common ancestor. Two structures are considered homologous when they are inherited from a common ancestor which possessed the structure. This may be difficult to determine when the structure has been modified through descent: Ancestral characteristics may be greatly modified in descendent groups, so that homologous traits may not be similar in appearance or function. Clues of their common ancestry may often be found in structural details, ontogeny, or position relative to other features.


A similiarity between different groups of organisms that is not homologous (due to common ancestry) but rather the result of convergent evolution or character reversal.


The presence of the same alleles at one or more loci. A homozygote is an individual having two copies of the same allele at a genetic locus.


Identifies the two alleles of a gene in a diploid organisms as being identical to each other. In heterozygous individuals, the two alleles are different versions of the gene.

Horizontal gene transfer

A natural process by which some of the genes of an organism are transferred and incorporated into the genome of an organism belonging to another species.
This contrast with vertical gene transfer, in which the genes of an organism are passed on to its offspring


Messenger substances synthesized in the body and secreted by the endocrine glands; One of many types of circulating chemical agents produced within the body by endocrine glands, instrumental in maintaining homeostasis and regulating reproduction and development. Hormones are secreted by endocrine (ductless) glands into nearby capillaries and are transported throughout the body in the blood. Hormones circulating in the blood diffuse into the interstitial fluids surrounding the cell. Cells with specific receptors for a hormone respond with an action that is appropriate for the cell. Because of the specificity of hormone and target cell, the effects produced by a single hormone may vary among different kinds of target cells.
Hormones can be chemically classified into four groups:
Amino acid-derived hormones are modified amino acids.
Polypeptide and protein hormones are chains of amino acids of less than or more than about 100 amino acids, respectively. Some protein hormones are actually glycoproteins, containing glucose or other carbohydrate groups.
Steroid hormones are lipids that are synthesized from cholesterol. Steroids are characterized by four interlocking carbohydrate rings.
Eicosanoids are lipids that are synthesized from the fatty acid chains of phospholipids found in plasma membrane.
(see Endocrine system, HypothalamusPituitary gland)

Hormone replacement therapy (HRT)

Hormones (oestrogen, progesterone, or both) given to post-menopausal women or to women who have had their ovaries surgically removed.

Horner's syndrome

Sinking in of the eyeball, ptosis of the upper eyelid, constriction of the pupil, anhidrosis and flushing of the affected side of the face caused by paralysis of the cervical sympathetic nerves.


(1) Human, animal, or plant in which another organism lives and nourishes itself.
(2) A recipient of grafted tissue.
An organism in which the genetic material is altered by modification of a part of its own genetic material and/or insertion of foreign genetic material.

Host strain (bacterial)

The bacterium used to harbor a plasmid. Typical host strains include HB101 (general purpose E. coli strain), DH5a (ditto), JM101 and JM109 (suitable for growing M13 phages), XL1-Blue (general-purpose, good for blue/white lacZ screening). Note that the host strain is available in a form with no plasmids (hence you can put one of your own into it), or it may have plasmids present (especially if you put them there). Hundreds, perhaps thousands, of host strains are available.

Host Vector (HV) System

The host is the organism into which a gene from another organism is transplanted. The guest gene is carried by a vector (i.e., a larger DNA molecule, such as a plasmid, or a virus into which that gene is inserted) which then propagates in the host.

Hot flashes

The sudden wave of mild or intense body heat caused by dilation of capillaries in the skin resulting from decreased levels of oestrogen.

Housekeeping genes (also known as constitutive genes)

Genes that are expressed in all of an organism’s cells. These genes are expressed in all of an organism’s cells as they provide the basics necessary for life. They tend to be expressed at a constant or relatively constant level in contrast to Inducible genes.


Hypertext Markup Language. The language used to encode formatting, links and other features on Web pages. Uses standardized "tags" such as and whose meaning and interpretation is set universally by the World Wide Web Consortium.

Human chorionic gonadotropin (hCG)  

A peptide hormone secreted by the chorion that prolongs the life of the corpus luteum and prevents the breakdown of the uterine lining.

Human embryonic stem cell (hESC)

A type of pluripotent stem cell derived from the inner cell mass (ICM) of the blastocyst.

Human gene therapy

Insertion of normal DNA directly into cells to correct a genetic defect.

Human Genome Project

Collective name for several projects begun in 1986 to (1) create an ordered set of DNA segments from known chromosomal locations, (2) develop new computational methods for analyzing genetic map and DNA sequence data, and (3) develop new techniques and instruments for detecting and analyzing DNA.
Initially known as the Human Genome Initiative.

Human immunodeficiency virus (HIV)

The infectious agent that causes AIDS; HIV is an RNA retrovirus. The retrovirus that attacks T-cells in the human immune system, destroying the body's defenses and allowing the development of AIDS.

Human leukocyte antigen

A set of proteins expressed on the surface of cells. These proteins are implicated in the recognition of substances foreign to the body and which allow the body to discriminate between self and non-self.

Human papillomaviruses (HPV)

A group of over 100 types of viruses of which 30 are sexually transmissible.
The major cause of cervical cancer, the second most common cancer in women.


Used to retain moisture in foods by absorbing water from the air to prevent drying out.


the amount of water vapor in the air.

Humoral Immune Response

Refers to the rapid manufacture and secretion by the body of the soluble blood serum components, e.g.,: antibodies (by B cells), complement proteins, lymphokines, cecrophins, etc, in response to an infection. Humoral immunity is the type of immunity that fights bacteria and viruses in body fluids with antibodies that circulate in blood plasma and lymph, fluids formerly called humors.

Huntington’s disease or Huntington’s chorea

A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 years. Caused by an expansion of a trinucleotide repeat and inherited as a dominant trait. It results in degeneration of neurons in the basal ganglia.This degeneration causes uncontrolled movements, loss of intellectual faculties, and emotional disturbance. HD is a familial disease, passed from parent to child through a mutation in the normal gene. Each child of an HD parent has a 50-50 chance of inheriting the HD gene. If a child does not inherit the HD gene, he or she will not develop the disease and cannot pass it to subsequent generations. A person who inherits the HD gene will sooner or later develop the disease. Whether one child inherits the gene has no bearing on whether others will or will not inherit the gene. Some early symptoms of HD are mood swings, depression, irritability or trouble driving, learning new things, remembering a fact, or making a decision. As the disease progresses, concentration on intellectual tasks becomes increasingly difficult and the patient may have difficulty feeding himself or herself and swallowing. The rate of disease progression and the age of onset vary from person to person.
A genetic test, coupled with a complete medical history and neurological and laboratory tests, helps diagnose HD. Pre-symptomic testing is available for individuals who are at risk for carrying the HD gene. In 1 to 3 % of individuals with HD, no family history of HD can be found.

Hurdle Technology

Method of inhibiting or terminating microbial growth and reproduction.
Up to now, about 50 different hurdles have been identified in food preservation. Apart from the most important and commonly used hurdles such as temperature, pH, and water activity, others of potential value include: ultrahigh pressure, mano-thermo-sonication, photodynamic inactivation, modified atmosphere packaging (MAP) of both non-respiring and respiring products, edible coatings, ethanol, maillard reaction products and bacteriocins.

Hyaline membrane disease (also referred to as "respiratory distress syndrome", RDS)

A respiratory disease of newborns, especially premature infants, in which a membrane composed of proteins and dead cells forms and lines the alveoli making gas exchange difficult or impossible.The lungs are less compliant and fatigue occurs as a result of attempting to ventilate "stiff lungs". Due to immaturity of preterm infant's lungs and the absence, alteration, or inactivation of surface-active material (surfactant).
(see Respiratory Distress Syndrome)


(1) Offspring of two parents that differ in one or more inheritable characteristics. (2) Offspring of two different varieties or of two different species.


The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule; the reaction by which the pairing of complementary strands of nucleic acid occurs. In the process the hydrogen bonding of complementary DNA and/or RNA sequences forms a duplex molecule.DNA is usually double-stranded, and when the strands are separated they will re-hybridize under the appropriate conditions. Hybrids can form between DNA-DNA, DNA-RNA or RNA-RNA. They can form between a short strand and a long strand containing a region complementary to the short one. Imperfect hybrids can also form, but the more imperfect they are, the less stable they will be (and the less likely to form). To "anneal" two strands is the same as to "hybridize" them.


A type of hybrid cell produced by fusing a normal cell with a tumor cell. When lymphocytes (antibody-producing cells) are fused to the tumor cells, the resulting hybridomas produce antibodies and maintain rapid, sustained growth, producing large amounts of an antibody. Hybridomas are the source of monoclonal antibodies.

Hybrid zone

A region where two related populations that diverged after becoming geographically isolated make secondary contact and interbreed where their geographical ranges overlap.


An organic molecule consisting only of carbon and hydrogen.


A fluid filled sac partially surrounding the testis. Manifests itself as a swelling on the side of the scrotum.


An excess of cerebrospinal fluid (which surrounds the brain or lies in the cavities between the four ventricles) and enlargement of fluid-filled cavities in the brain.
Often results in enlargement of the head with pressure on the brain, which may cause mental retardation, convulsions, and/or visual impairments.


Referring to any event related to the movement of water.

hydroelectric power water use

the use of water in the generation of electricity at plants where the turbine generators are driven by falling water.

hydrologic cycle

the cyclic transfer of water vapor from the Earth's surface via evapotranspiration into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans.


Process of adding hydrogen molecules directly to an unsaturated fatty acid from sources such as vegetable oils to convert it to a semi-solid form such as margarine or shortening. The degree of hydrogenation influences the firmness and spreadability of margarines, flakiness of pie crust and the creaminess of puddings. Hydrogenated oils are sometimes used in place of other fats with higher proportions of saturated fatty acids such as butter or lard.

Hydrogenatied fats (partially)

Partially hydrogenated fats are a the result of chemical addition of hydrogen units to polyunsaturated fatty acids. If hydrogenation is not complete, the process renders trans-fats (as compared to cis-fats found commonly by natural processes). The reason for hydrogenation is to gain optimal texture of fats and oils in processed foods. Trans-fats have been associated to increases in bad cholesterol (LDLs), but fully hydrogenated fats do not contain trans-fats and are not associated to clinical symptoms of heart disease.

Hydrogen bond

A type of weak chemical bond formed when the slightly positive hydrogen atom of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.


The scientific study of water and water masses.


A chemical process that lyses or splits molecules by the addition of water; an essential process in digestion. Chemical reaction of another substance with water, resulting in fragmentation or splitting of the molecules of that substance, for example the splitting of the sucrose molecule into glucose and fructose molecules.


Instrument that measures specific gravity of liquids, used to measure salt, sugar or alcohol concentration.


Water soluble (“water liking”) Having an affinity for water.


Water insoluble (“water hating”); and refers to non-polar properties as opposed to polar properties of water; Having an aversion to water; tending to coalesce and form droplets in water. Water soluble, or hydrophilic, molecules do not easily mix with hydrophobic molecules or solutions. Oil is a hydrophobic substance and mixtures of oily and watery molecules tend to minimize their contact surface. Thus the observed separation of oil from water when left standing. However, oil can be partially mixed with water when shaking a container vigorously indicating that the separation or mixing of molecules is determined by the energy of molecular interaction. The energy to overcome these molecular interactions can be provided by mechanical force (shaking) or high temperature.  The latter is a well known observation; salts, sugars, and fats normally mix better in hot than cold water, and hot water is better in removing stains form dishes or cloths.

Hydrostatic skeleton

A skeletal system composed of fluid held under pressure in a closed body compartment. The main skeleton of most cnidarians, flatworms, nematodes, and annelids.


Relating to hot water or the action of such water. The water is often heated by magma or in association with magma.


Serum in the pleural space

Hydroxyl group

A functional group consisting of a hydrogen atom joined to an oxygen atom by a polar covalent bond. Molecules possessing this group are soluble in water and are called alcohols.


Hyperalgesia (from hyper greateralgesia pain-feeling) is characterised by a lowered pain threshold, increased pain from suprathreshold stimuli and occasionally spontaneous pain. Like all other sensory systems, pain receptors (nociceptors) adapt (reduce activity) to prolonged or repeated application of an adequate stimulus. In contrast to other sensory systems, the pain system is also capable of enhancing its responsiveness as a consequence of adequate stimulation. This sensitisation is characterised by lowered response threshold, induction of spontaneous activity and enhanced responses in the suprathreshold range. These changes cause the phenomenon of hyperalgesia and can even result in allodynia where even gentle, low threshold stimuli can elicit pain. Hyperalgesia follows several types of tissue injury and is a prominent feature of diseases of the nervous system and inflammation generally. There are two types according to the location relative to the injury site. (a) primary hyperalgesia refers to the site of injury and is characterised by enhanced responsiveness to mechanical and heat stimuli, and (b) secondary hyperalgesia refers to changes in surrounding undamaged skin and is characterised by enhanced responsiveness to mechanical stimuli only. The mechanisms for the two phenomena are different. Primary hyperalgesia appears to be due to local sensitization of the C-class of sensory neurons that signal polymodal nociceptive input. These fibres become sensitized by inflammatory mediators such as bradykinin, formed naturally from plasma proteins following tissue damage, to heat and some other chemicals such as histamine.
Secondary hyperalgesia appears most probably to be due to sensitization of the central processes (i.e, the endings in the central nervous system) of the Aδ and C classes of sensory neurons that normally signal pain as well the Aβ class of neurons that receive input from low-threshold mechanoreceptors that normally signal non-painful touch sensation (LTMs).


An excess of bilirubin in the blood.


The set of all points in a plane such that the difference of the distances to two fixed points is a constant.    
The general equation for a hyperbola is

The hyperbolic functions are defined as follows:
hyperbolic cosine:   cosh x = (1/2)(exe-x)
hyperbolic sine:       sinh x = (1/2)(ex - e-x)
hyperbolic tangent:   tanh x =  sinh x/ cosh x


A patch of the primary visual cortex which contains an ocular dominance column for each eye and a complete set of orientation columns, covering 180° of orientation.
(seeOrientation selective columns, or Ocular dominance columns, or Hypercolumns)


Above normal levels of insulin in the blood of a person. The most common cause of low blood sugars seen in newborn babies. Hyperinsulism can also lead to decreased insulin sensitivity which may lead to high blood sugars. Treatment is aimed at keeping blood sugar levels normal and will be based on the cause of the condition.


Constant and excessive movement and motor activity. Hence hyperkinetic = an excess of behavior in inappropriate circumstances and hypokinesis = the absence of a normal amount of bodily movement and motor activity; extreme lack of movement or listlessness.


A voice resonance disorder that occurs when excessive air passes through the nasal cavity, often resulting in an unpleasant twanging sound. Hence hyponasality = A voice disorder involving resonance, where too little air passes through the nasal cavity; denasality.


Overactive parathyroid gland causing too much calcium and low phosphorus levels in the blood. Calcium levels that are too high can cause weakness, dehydration, kidney stones, brittle bones, high blood pressure, seizures, or coma. Treatment is based on the cause of the condition.


An increase in the polarity of a membrane; an electrical state whereby the inside of the cell is made more negative relative to the outside than at the resting membrane potential; A mechanism by which a membrane potential is made more negative inside with respect to the outside of the cell. A neuron membrane is hyperpolarized if a stimulus increases its voltage from the resting potential of –70 mV, reducing the chance that the neuron will transmit a nerve impulse. Hyperpolarization causes neurons and muscle cells to be electrically silent (see action potentials) and stabilize at a resting potential. Hyperpolarization is the result of moving positive charges from in to out of a cell which is usually the result of K+ ions moving out, but can also be achieved by moving Cl- ions into the cell.


An increase of the hormone prolactin secreted from the pituitary gland. An increased prolactin level can cause breast drainage (galactorrhea), irregular menstrual cycles, or enlarged breasts.


Widely spaced eyes.


High arterial blood pressure.


Archaea and Bacteria that grow optimally at temperatures in the range of 95°C to 110°C and can survive to temperatures close to 120°C


An increase in the production of thyroid hormone from the thyroid gland. It causes, trouble sleeping, diarrhea, tiredness, goiter, feeling hot, fast heart rate, weight loss, bulging of eyes, or shakiness. Treatment may include antithyroid medications and management of fast heart rates. In some cases radioactive iodine and/or surgery could be necessary.


Excessive muscle tone; increased resistance to passive stretch

Hypertonic solution

A solution with a greater solute concentration than another, a hypotonic solution.

Hypha (Plural = Hyphae)

Microscopic, tubular filaments that make up the body of a multicellular fungus.


Unrealistic interpretation of physical signs or sensations as abnormal, leading to preoccupation with the fear or belief of having a disease.


The condition in which the production of sex hormones and germ cells (sperm and eggs) is inadequate.

Hypophysial portal blood vessels

Blood vessels that form a short portal and convey information from the hypothalamus to the pituitary gland.


Pituitary gland. Hence, hyophysial = relatging to the pituitary gland


Defective or incomplete development of any tissue.


Literally "below the fleshy spike". A condition in which the external urinary meatus (opening) opens anywhere below the tip of the penis rather than at the tip.


The longest side of a right triangle.   The side opposite the right angle in a right triangle.


The ventral part of the vertebrate forebrain; functions in maintaining homeostasis, especially in coordinating the endocrine and nervous systems; secretes hormones of the posterior pituitary and releasing factors, which regulate the anterior pituitary.
An area at the base of the brain, just below the thalamus, that contains a number of small nuclei with a variety of functions that control many internal body functions. The aim of this control is to maintain homeostasis, or maintaining the body's status quo. Factors such as blood pressure, body temperature, hunger, glucose concentration in the blood, osmolarity of the blood, reproductive function and other functions are controlled through means that include, as a critical component, the release of hormones.
When deviations from homeostasis occur or when certain developmental changes are required, the hypothalamus stimulates cellular activity in various parts of the body by directing the release of hormones from the anterior and posterior pituitary glands. The hypothalamus communicates directives to these glands by one of the following two pathways:
1. Communication between the hypothalamus and the anterior pituitary occurs through releasing and inhibiting hormones produced by the hypothalamus and delivered to the anterior pituitary through blood vessels. The releasing and inhibiting hormones are produced by specialized neurons of the hypothalamus called neurosecretory cells. The hormones are released into a capillary network (primary plexus) and transported through veins (hypophyseal portal veins) to a second capillary network (secondary plexus) that supplies the anterior pituitary. The hormones then diffuse from the secondary plexus into the anterior pituitary, where they initiate the production of specific hormones by the anterior pituitary. Many of the hormones produced by the anterior pituitary are tropic hormones (tropins), hormones that stimulate other endocrine glands to secrete their hormones.
2. Communication between the hypothalamus and the posterior pituitary occurs through neurosecretory cells that span the short distance between the hypothalamus and the posterior pituitary. Hormones produced by the cell bodies of the neurosecretory cells are packaged in vesicles and transported through the axon and stored in the axon terminals that lie in the posterior pituitary. When the neurosecretory cells are stimulated, the action potential generated triggers the release of the stored hormones from the axon terminals to a capillary network within the posterior pituitary. Two hormones, oxytocin and antidiuretic hormone (ADH), are produced and released in this way.
(see Endocrine systemPituitary gland)


A temporary working explanation or supposition based on accumulated facts and suggesting some general principle or relation of cause and effect; a postulated solution to a scientific problem that must be tested and if not validated, discarded; a proposition that is being investigated, it has yet to be proven; A testable scientific idea that can be proved right or wrong with experiments. A concept or idea that can be falsified (shown to be false) by various scientific methods. A hypothesis is a formulation of a question that lends itself to a prediction. This prediction can be verified or falsified. A question can only be use as scientific hypothesis, if their is an experimental approach or observational study that can be designed to check the outcome of a prediction.


A disorder that prevents the thyroid gland from making enough thyroid hormone for the brain and body to grow and develop normally.Congenital hypothyroidism is a condition found in 1 out of every 4000 newborns. All babies are tested for congenital hypothyroidism within the first week of life. Acquired hypothyroidism is more common in teen girls, but can be found in boys and girls of any age.


A condition of diminished muscle tone. Can result from damage to any part of the brain, usually including the cerebellum and basal ganglia, resulting in decreased stiffness of the extremities and trunk.

Hypotonic solution

A solution with a lesser solute concentration than another, a hypertonic solution.


Reduced oxygen content in tissues; Insufficient oxygen reaching the tissues of the body.


Surgery to remove the uterus and, sometimes, the cervix. When the uterus and the cervix are removed, it is called a total hysterectomy. When only the uterus is removed, it is called a partial hysterectomy.