HUMAN BIOLOGY

glossary  G -13

kilocalorie One thousand calories of heat energy, or the

amount of energy needed to raise the temperature of 1 kilo-

gram of water by 1°C; the unit of measure for the caloric

value of foods.

kinetic energy The energy of motion.

Krebs cycle With a few conversion steps that precede it,

the stage of aerobic respiration in which pyruvate is com-

pletely broken down to carbon dioxide and water and two

ATP form. Coenzymes accept the protons (H+) and electrons

removed from intermediates during the reactions and deliver

them to the next stage.

labor The birth process.

lactation The production of milk by hormone-primed mam-

mary glands.

lacteal Small lymph vessel in villi of the small intestine that

receives absorbed triglycerides. Triglycerides move from the

lymphatic system to the general circulation.

large intestine The colon; a region of the GI tract that

receives unabsorbed food residues from the small intestine

and concentrates and stores feces until they are expelled from

the body.

larynx (lare-inks) A tubular airway that leads to the lungs.

It contains vocal cords, where sound waves used in speech

are produced.

LDL Low-density lipoprotein that transports cholesterol;

excess amounts contribute to atherosclerosis.

lens Of the eye, a saucer-shaped region behind the iris con-

taining multiple layers of transparent proteins. Ligaments can

move the lens, which focuses incoming light onto photorecep-

tors in the retina.

leukocytes White blood cells.

Leydig cell In testes, cells in connective tissue around the

seminiferous tubules that secrete testosterone and other

signaling molecules.

LH Luteinizing hormone, secreted by the anterior lobe of the

pituitary gland. In males it acts on Leydig cells of the testes

and prompts them to secrete testosterone. In females, LH

stimulates follicle development in the ovaries.

life cycle Recurring series of genetically programmed events

from the time individuals are produced until they themselves

reproduce.

ligament A strap of dense, elastic, regular connective tissue

that connects two bones at a joint.

limbic system Brain region that, along with the cerebral

cortex, collectively governs emotions; it includes parts of the

thalamus, hypothalamus, amygdala, and hippocampus.

limiting factor Any essential resource that is in short supply

and so limits population growth.

intermediate Substance that forms between the start and end
of a metabolic pathway.

intermediate filament A ropelike element of the cytoskel-
eton that mechanically strengthens cells.

internal environment The fluid bathing body cells and
tissues; it consists of blood plus interstitial fluid.

internal respiration Movement of oxygen into tissues from
the blood, and of carbon dioxide from tissues into the blood.

interneuron Any of the neurons in the brain and spinal cord
that integrate information arriving from sensory neurons and
that influence other neurons in turn.

interphase Of cell cycles, the time interval between nuclear
divisions in which a cell increases its mass, roughly doubles
the number of its cytoplasmic components, and finally dupli-
cates its chromosomes (replicates its DNA).

interstitial fluid (in-ter-stish-ul) [L. interstitus, to stand in
the middle of something] The extracellular fluid in spaces
between cells and tissues.

intervertebral disk One of a number of disk-shaped struc-
tures containing cartilage that serve as shock absorbers and
flex points between vertebrae.

intron A noncoding portion of a newly formed mRNA
molecule.

in vitro fertilization Conception outside the body (“in glass”
petri dishes or test tubes).

ion (eye-on) An atom (or a compound) that has gained or
lost one or more electrons and hence has acquired an overall
negative or positive charge.

ionic bond An association between ions of opposite charges.

iris Of the eye, a circular pigmented region behind the cornea
with a “hole” in its center (the pupil) through which incom-
ing light enters.

isotonic Having the same solute concentration as a fluid
against which it is being compared.

isotope (eye-so-tope) For a given element, an atom with the
same number of protons as the other atoms but with a differ-
ent number of neutrons.

jejunum Middle section of the small intestine, where most
nutrients are digested and absorbed.

joint An area of contact or near-contact between bones.

juxtaglomerular apparatus In kidney nephrons, a place
where the arterioles of the glomerulus come into contact with
the distal tubule. Cells in this region secrete renin, which
triggers hormonal events that stimulate increased reab-
sorption of sodium.

karyotype (carry-oh-type) A preparation of an individual’s
metaphase chromosomes arranged by length, shape, and the
location of the centromere.

keratinocytes Cells of the epidermis that make keratin.

kidney One of a pair of organs that filter organic wastes,
toxins, and other substances from the blood and help regulate
the volume and solute concentrations of extracellular fluid.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).