Page 163 - 84 human physiolofy part-1

The Immune System

protecting an organism from infection by identifying and attacking pathogens. This is a difficult task,
since pathogens range from viruses to parasitic worms and must be detected with absolute specificity as
they are "hidden" amongst normal cells and tissues. Pathogens are also constantly changing themselves
to avoid detection and successfully infect and destroy their hosts.

Lymphatic System

The lymphatic system and the immune system are terms that are used interchangeably to refer to
the body's ability to defend against pathogens. The lymphatic system is comprised of three interrelated
functions: (1) Removal of excess fluids, lymph, from body tissues, (2) Absorption of fatty acids and
subsequent transport of fat, chyle, to the circulatory system and (3) Formation of white blood cells
(WBCs), and initiation of immunity through the formation of antibodies, lending specific resistance to
pathogens.

Lymphatic Pathways

The lymphatic system acts as a secondary circulatory system, except it collaborates with white
blood cells in lymph nodes to protect the body from being infected by cancer cells, fungi, viruses or
bacteria. Unlike the circulatory system, the lymphatic system is not closed and has no central pump; the
lymph moves slowly and under low pressure due to peristalsis, the operation of semilunar valves in the
lymph veins, and the milking action of skeletal muscles. Like veins, lymph vessels have one-way,
semilunar valves and depend mainly on the movement of skeletal muscles to squeeze fluid through
them. Rhythmic contraction of the vessel walls may also help draw fluid into the lymphatic capillaries.
This fluid is then transported to progressively larger lymphatic vessels culminating in the right
lymphatic duct (for lymph from the right upper body) and the thoracic duct (for the rest of the body);
these ducts drain into the circulatory system at the right and left subclavian veins.

Lymph

Lymph originates as blood plasma that leaks from the capillaries of the circulatory system,
becoming interstitial fluid, filling the space between individual cells of tissue. Plasma is forced out of
the capillaries by hydrostatic pressure, and as it mixes with the interstitial fluid, the volume of fluid
accumulates slowly. Most of the fluid is returned to the capillaries by osmosis. The proportion of
interstitial fluid that is returned to the circulatory system by osmosis is about 90% of the former
plasma, with about 10% accumulating as overfill. The excess interstitial fluid is collected by the
lymphatic system by diffusion into lymph capillaries, and is processed by lymph nodes prior to being
returned to the circulatory system. Once within the lymphatic system the fluid is called lymph, and has
almost the same composition as the original interstitial fluid.

Edema

Edema is the swelling that forms when too much tissue fluid forms or not enough taken away. It
can be caused by a variety of conditions such as allergic responses (too much vasodilation), starvation
(lack of albumin in blood lowers osmotic pressure and decreases amount of fluid returning to
capillaries), and lymphatic disorders (e.g. blockage due to parasite in elephantiasis, or removal of

Wikibooks | 163