cycle; characterized by rapid movements of the eyes beneath the
Receptor (re-SEP-tor) A specialized cell or a distal portion of a neu-
ron that responds to a specific sensory modality, such as touch,
pressure, cold, light, or sound, and converts it to an electrical
signal (generator or receptor potential). A specific molecule or
cluster of molecules that recognizes and binds a particular ligand.
Receptor-mediated endocytosis (en'-do-sT-TO-sis) A highly selective
process whereby cells take up specific ligands, which usually are
large molecules or particles, by enveloping them within a sac
of plasma membrane. Ligands are eventually broken down by
enzymes in lysosomes.
Recessive allele (re-SESS-iv) An allele whose presence is masked in
the presence of a dominant allele on the homologous chromo-
Recombinant DNA Synthetic DNA, formed by joining a fragment of
DNA from one source to a portion of DNA from another.
Recovery oxygen consumption Elevated oxygen use after exercise
ends due to metabolic changes that start during exercise and
continue after exercise. Previously called oxygen debt.
Recruitment (re-KROOT-ment) The process of increasing the number
of active motor units. Also called motor unit summation.
Rectum (REK-tum) The last 20 cm (8 in.) of the gastrointestinal tract,
from the sigmoid colon to the anus.
Reduction (re-DUK-shun) The addition of electrons to a molecule
or, less commonly, the removal of oxygen from a molecule that
results in an increase in the energy content of the molecule.
Referred pain Pain that is felt at a site remote from the place of
Reflex Fast response to a change (stimulus) in the internal or exter-
nal environment that attempts to restore homeostasis.
Reflex arc The most basic conduction pathway through the nervous
system, connecting a receptor and an effector and consisting of
a receptor, a sensory neuron, an integrating center in the central
nervous system, a motor neuron, and an effector.
Refraction (re-FRAK-shun) The bending of light as it passes from one
medium to another.
Refractory period (re-FRAK-to-re) A time period during which an
excitable cell (neuron or muscle fiber) cannot respond to a stimu-
lus that is usually adequate to evoke an action potential.
Regional anatomy The division of anatomy dealing with a specific
region of the body, such as the head, neck, chest, or abdomen.
Regurgitation (re-gur'-ji-TA-shun) Return of solids or fluids to the
mouth from the stomach; backward flow of blood through incom-
pletely closed heart valves.
Relaxin (RLX) A female hormone produced by the ovaries and pla-
centa that increases flexibility of the pubic symphysis and helps
dilate the uterine cervix to ease delivery of a baby.
Releasing hormone Hormone secreted by the hypothalamus that
can stimulate secretion of hormones of the anterior pituitary.
Remodeling (re-MOD-el-ing) Replacement of old bone by new bone
Renal (RE-nal) Pertaining to the kidneys.
Renal corpuscle (KOR-pus-l) A glomerular (Bowman's) capsule and its
Renal pelvis A cavity in the center of the kidney formed by the
expanded, proximal portion of the ureter, lying within the kidney,
and into which the major calyces open.
Renal pyramid (PIR-a-mid) A triangular structure in the renal medulla
containing the straight segments of renal tubules and the vasa recta.
Renin (RE-nin) An enzyme released by the kidney into the plasma,
where it converts angiotensinogen into angiotensin I.
Renin-angiotensin-aldosterone (RAA) pathway A mechanism for
the control of blood pressure, initiated by the secretion of renin
by the kidney in response to low blood pressure; renin catalyzes
formation of angiotensin I, which is converted to angiotensin II by
angiotensin-converting enzyme (ACE), and angiotensin II stimu-
lates secretion of aldosterone.
Repolarization (re-po-lar-i-ZA-shun) Restoration of a resting mem-
brane potential after depolarization.
Reproduction (re-pro-DUK-shun) The formation of new cells for
growth, repair, or replacement; the production of a new individual.
Reproductive cell division Type of cell division in which gametes
(sperm and oocytes) are produced; consists of meiosis and
Residual volume (re-ZID-u-al) The volume of air still contained in the
lungs after a maximal exhalation; about 1200 mL in males and
1100 mL in females.
Resistance (re-ZIS-tans) Hindrance (impedance) to blood flow as a
result of higher viscosity, longer total blood vessel length, and
smaller blood vessel radius. Ability to ward off disease. The hin-
drance encountered by electrical charges as they move from one
point to another. The hindrance encountered by air as it moves
through the respiratory passageways.
Respiration (res-pi-RA-shun) Overall exchange of gases between
the atmosphere, blood, and body cells consisting of pulmonary
ventilation, external respiration, and internal respiration.
Respiratory center Neurons in the pons and medulla oblongata of
the brain stem that regulate the rate and depth of pulmonary
Respiratory membrane Structure in the lungs consisting of the alveo-
lar wall and its basement membrane and a capillary endothelium
and its basement membrane through which the diffusion of
respiratory gases occurs.
Resting membrane potential The voltage difference between the
inside and outside of a cell membrane when the cell is not respond-
ing to a stimulus; in many neurons and muscle fibers it is -70 to -90
mV, with the inside of the cell negative relative to the outside.
Retention (re-TEN-shun) A failure to void urine due to obstruction,
nervous contraction of the urethra, or absence of sensation of
desire to urinate.
Reticular activating system (RAS) (re-TIK-u-lar) A portion of the reticular
formation that has many ascending connections with the cerebral
cortex; when this area of the brain stem is active, nerve impulses
pass to the thalamus and widespread areas of the cerebral cortex,
resulting in generalized alertness or arousal from sleep.