Human Physiology, 14th edition (2016)

Glossary G-15

recruitment (r ̆ı-kroot 9 ment) In terms of muscle

contraction, the successive stimulation of more and

larger motor units in order to produce increasing

strengths of muscle contraction.

recurrent (re-kur 9 ent) circuits Circuits of neural

activity where neurons activate other neurons in a

sequence that turns back on itself (also known as

reverberating circuits). Such recurrent circuits are

believed to underlie short-term memory.

reduced hemoglobin Hemoglobin with iron in

the reduced ferrous state. It is able to bond with

oxygen but is not combined with oxygen. Also called

deoxyhemoglobin.

reducing agent An electron donor in a coupled

oxidation-reduction reaction.

referred pain Pain originating in deep, visceral

organs that is perceived to be coming from particular

body surface locations. This is believed to be caused

by sensory neurons from both locations synapsing on

the same interneurons in the same spinal cord level.

reflex arc The neural pathway for an involuntary

response to a stimulus, involving sensory information

carried into the CNS in afferent neurons, motor

information carried out of the CNS in efferent

neurons, and in many cases also involving

association neurons within the CNS. No association

neurons are involved in the monosynaptic muscle

stretch reflex arc.

refraction (re-frak 9 shun) The bending of light rays

when light passes from a medium of one density to a

medium of another density. Refraction of light by the

cornea and lens acts to focus the image on the retina

of the eye.

refractory (re-frak 9 t ̆o-re) period The period of

time during which a region of axon or muscle cell

membrane cannot be stimulated to produce an action

potential (absolute refractory period), or when it can

be stimulated only by a very strong stimulus (relative

refractory period).

regulatory T lymphocytes Previously called

suppressor T lymphocytes, these cells inhibit the

activity of B cells and helper T cells to reduce the

intensity of inappropriate immune responses.

relaxin (re-lak 9 sin) A hormone secreted by the

corpus luteum of the ovary that, in many mammals

(but not humans), causes a softening of the pubic

symphysis and relaxation of the uterus to aid

parturition. In humans, however, relaxin promotes

other effects that benefit the developing embryo.

releasing hormones Polypeptide hormones

secreted by neurons in the hypothalamus that travel

in the hypothalamo-hypophyseal portal system to the

anterior pituitary and stimulate the anterior pituitary

to secrete specific hormones.

REM sleep The stage of sleep in which dreaming

occurs. It is associated with rapid eye movements

(REMs). REM sleep occurs three to four times each

night and lasts from a few minutes to over an hour.

renal (re 9 nal) Pertaining to the kidneys.

renal plasma clearance The volume of plasma

from which a particular solute is cleared each minute

by the excretion of that solute in the urine. If there

is no reabsorption or secretion of that solute by the

nephron tubules, the renal plasma clearance is equal

to the glomerular filtration rate.

renal plasma threshold When a molecule in

the blood plasma is filtered in the kidneys by the

glomerular capillaries and reabsorbed across the

nephron tubules by carrier proteins, the renal plasma

threshold is the minimum plasma concentration of

that molecule required to saturate the carriers and

cause the molecule to appear in the urine.

renal pyramid (p ̆ı 9 r ̆a-mid) One of a number of cone-

shaped tissue masses that compose the renal medulla.

specific proteins (enzymes). Such phosphorylation

may activate or inactivate enzymes.

proteinuria (pro-te-noo 9 re-a) The presence of

protein in the urine in amounts greater than normal

levels, as determined by specific standards.

proteome (pro 9 te-om) All of the different proteins

produced by a genome.

proton (pro 9 ton) A unit of positive charge in the

nucleus of atoms.

pseudohermaphrodite (soo 0 d ̆o-her-maf 9 r ̆o-d ̄ıt) An

individual who has the gonads of one sex only, but

some of the body features of the opposite sex. (A true

hermaphrodite has both ovarian and testicular tissue.)

pseudopod (soo 0 d ̆o-pod) A footlike extension

of the cytoplasm that enables some cells (with

amoeboid motion) to move across a substrate.

Pseudopods also are used to surround food particles

in the process of phagocytosis.

puberty (pyoo 9 ber-te) The period in an indi-

vidual’s life span when secondary sexual

characteristics and fertility develop.

pulmonary (pul 9 m ̆o-ner 0 e) circulation The part

of the vascular system that includes the pulmonary

arteries and pulmonary veins. It transports blood

from the right ventricle of the heart through the

lungs, and then back to the left atrium of the heart.

pupil The opening at the center of the iris of the

eye.

Purkinje (pur-kin 9 je) cells Neurons in the

cerebellum that send axons to other regions of the

brain to influence motor coordination and other

functions. Purkinje cells exert inhibitory effects at

their synapses with other neurons.

Purkinje (pur-kin 9 je) fibers Specialized

conducting tissue in the ventricles of the heart

that carry impulses from the bundle of His to the

myocardium of the ventricles.

pyramidal (p ̆ı-ram 9 ̆ı-dal) tracts Motor tracts

that descend without synaptic interruption from the

cerebrum to the spinal cord, where they synapse

either directly or indirectly (via spinal interneurons)

with the lower motor neurons of the spinal cord; also

called corticospinal tracts.

pyrogen (pi 9 r ̆o-jen) A fever-producing substance.

Q

QRS complex The principal deflection of an

electrocardiogram, produced by depolarization of the

ventricles.

R

reabsorption (re 0 ab-sorp 9 shun) The transport of a

substance from the lumen of the renal nephron into

the peritubular capillaries.

receptive field An area of the body that, when

stimulated by a sensory stimulus, activates a

particular sensory receptor.

receptor proteins Proteins in target cells for

regulatory molecules that are each specific for

a particular regulatory molecule, and that bind

to it with a high affinity and limited capacity. A

regulatory molecule must bind to receptor proteins in

its target cells in order to regulate those cells.

reciprocal innervation (r ̆ı-sip 9 r ̆o-kal in 0 er-va 9

shun) The process whereby the motor neurons to an

antagonistic muscle are inhibited when the motor

neurons to an agonist muscle are stimulated. In this

way, for example, the extensor muscle of the elbow

joint is inhibited when the flexor muscles of this joint

are stimulated to contract.

portal systems in the body are the hepatic portal

system and the hypothalamo-hypophyseal portal

system.

positive feedback A response mechanism that

results in the amplification of an initial change.

Positive feedback results in avalanche-like effects,

as occur in the formation of a blood clot or in the

production of the LH surge by the stimulatory effect

of estrogen.

posterior (pos-t ̄er 9 e-or) At or toward the back of

an organism, organ, or part; the dorsal surface.

posterior pituitary See neurohypophysis.

postsynaptic (p ̆ost 0 s ̆ı-nap 9 tik) inhibition The

inhibition of a postsynaptic neuron by axon

endings that release a neurotransmitter that

induces hyperpolarization (inhibitory post synaptic

potentials).

potential (p ̆o-ten 9 shal) difference In biology, the

difference in charge between two solutions separated

by a membrane. The potential difference is measured

in voltage.

prehormone (pre-hor 9 m ̄on) An inactive form

of a hormone secreted by an endocrine gland. The

prehormone is converted within its target cells to the

active form of the hormone.

preload The load on a muscle before it contracts.

In the ventricles of the heart, the preload relates to

the tension on the ventricular walls produced by their

filling with the end-diastolic volume of blood.

presynaptic (pre 0 s ̆ı-nap 9 tik) inhibition

Neural inhibition in which axoaxonic synapses

inhibit the release of neurotransmitter chemicals

from the presynaptic axon.

process (pros 9 es, pro 9 ses) cell Any thin

cytoplasmic extension of a cell, such as the dendrites

and axon of a neuron.

progesterone (pro-jes 9 t ̆e-r ̄on) A steroid hormone

secreted by the corpus luteum of the ovaries and by

the placenta. Secretion of progesterone during the

luteal phase of the menstrual cycle promotes the final

maturation of the endometrium.

prohormone (pro-hor 9 m ̄on) The precursor of a

polypeptide hormone that is larger and less active

than the hormone. The prohormone is produced

within the cells of an endocrine gland and is

normally converted into the shorter, active hormone

prior to secretion.

prolactin (pro-lak 9 tin) (PRL) A hormone

secreted by the anterior pituitary that stimulates

lactation (acting together with other hormones)

in the postpartum female. It may also participate

(along with the gonadotropins) in regulating gonadal

function in some mammals.

prophylaxis (pro 0 f ̆ı-lak 9 sis) Prevention or

protection.

proprioceptor (pro 0 pre-o-sep 0 tor) A sensory

receptor that provides information about body

position and movement. Examples are receptors in

muscles, tendons, and joints and in the semicircular

canals of the inner ear.

prostaglandin (pros 9 t ̆a-glan 9 din) Any of a

family of fatty acids that serve numerous autocrine

regulatory functions, including the stimulation of

uterine contractions and of gastric acid secretion and

the promotion of inflammation.

proteasome A protease complex in the cytoplasm

that digests proteins that are tagged with ubiquitin, a

polypeptide that marks molecules for destruction.

protein (pro 9 te-in) The class of organic molecules

composed of large polypeptides in which over a

hundred amino acids are bonded together by peptide

bonds.

protein kinase (ki 9 n ̄as) The enzyme activated by

cyclic AMP that catalyzes the phosphorylation of

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