blood, while carbon dioxide (CO 2 ) diffuses from the
blood into the lungs. Gas is transported in the blood,
and thecirculatory system distributes oxygen through-
out the body while collecting CO 2 for return to the
lungs. Ventilation, or airflow to the lungs, and perfu-
sion, blood flow through lung capillaries, are the main
factors that determine the overall pulmonary function.
The ventilation/ perfusion ratio needs to be fairly con-
stant in all parts of the lung for the most effective
transfer of respiratory gases.
Lungs are saclike structures of varying complexity,
depending on the organism. They are connected to the
outside by a series of tubes and a small opening. In
humans, the lungs can be found in the thoracic cavity
and consist of the internal airways; the alveoli, tiny,
thin-walled, inflatable sacs where oxygen and carbon
dioxide are exchanged; the pulmonary circulatory ves-
sels; and elastic connective tissues. The exchange of
gases in the lungs takes place by breathing, chest move-
ments that inhale air and exhale or force gases such as
carbon dioxide out.
lyase An ENZYMEof EC class 4 that catalyzes the
separation of a bond in a SUBSTRATEmolecule.
See alsoEC NOMENCLATURE FOR ENZYMES.
lymph Aclear, colorless, watery fluid that contains
white blood cells and antibodies. Lymph bathes the tis-
sues, passes through lymph-node filters and returns to
theblood stream. The lymphatic system is a system of
vessels and lymph nodes that returns fluid and protein
to the blood, and it is separate from but parallel to the
circulatory system. Lymph flows through lymph nodes
and is transformed into lymphocytes, a special kind of
white blood cell that helps create the immune system
and comprises up to 50 percent of all white blood cells.
lymphocyte A white blood cell that identifies foreign
objects such as bacteria and viruses in the body and pro-
duces antibodies and cells that target them. There are
different types of lymphocytes, the most important of
which are: B cells that produce specific antibodies to
destroy foreign objects; T cells that attack cells that are
infected by viruses, cancer cells, and other foreign tissue;
NK (natural killer) cells that kill cancer cells and cells
infected by viruses through phagocytosis; and null cells,
also NK-type cells that attack certain targets. These cells
occupy less than 1 percent in circulating blood as they
travel from the blood to the lymph and lymph nodes and
back into the circulating blood. Other types of lympho-
cytes include acidophilic or eosinophilic, agranular, non-
granular or lymphoid, basophilic, granular, heterophilic,
neutrophilic, polymorphonuclear, and activated.lysis Biological term for destruction or killing, as in
bacteriolysis or the rupturing of a cell membrane with
resulting loss of cytoplasm.lysogenic cycle A stage in viral development when a
virus inserts its genome in a host genome and lies dor-
mant until outside factors initiate the new genetic mate-
rial to be transcribed. It then goes into a lytic cycle, a
massive replication of new viruses, and lyses (destroys
the cell membrane) the host cell.lysosome A small organelle found in eukaryotic cells
that is surrounded by a membrane and contains diges-
tive hydrolytic enzymes and chemicals that are strongly
acidic. Lysosomes fuse with the vascular membrane,
grab the food, and digest it, breaking it down into
usable parts. Lysosomes also work with leukocytes by
digesting leukocytes that have engulfed foreign objects
or cleaned up damaged cells.lysozyme A basic enzyme or antibacterial product
that can be found in tears, saliva, perspiration, the
nasal cavity, egg whites, animal fluids, and some plant
tissues. It attacks the cells walls of gram-positive bacte-
ria by cleaving the muramic acid [beta(1-4)-N-acetyl-
glucosamine] linkage in the cell walls of this class of
bacteria.lytic cycle The viral replication that releases new
phages (particles) through the lysis of the host cell.
See alsoLYSOGENIC CYCLE.lytic virus A virus that causes lysis of the host cell.210 lyase