Anatomy and Physiology
The lymphatic system
The lymphatic system

You will recall that when we looked at blood vessels we
noted that the lymphatic system drained from the capillary
bed. The hydrostatic and colloidal pressure operating in
the capillary bed forces fluid into the venules. Some fluid
remains behind in tissue spaces and this is called the
interstitial fluid. It contains some metabolic waste
products, such as carbon dioxide, and some proteins that
were not pushed through into the veins. We produce
about 3 litres of this fluid each day and clearly, it must be
returned to the general circulatory system. This is done
through the lymphatic system.
Lymphatic capillaries

The smallest lymph vessels are lymphatic capillaries.
Endothelial cells overlap to form a simple one way valve.
Fluid enters through a gap in the valve. Bundles of fine
collagen filaments anchor the endothelial cells to
surrounding structures. Its like a swinging door in which
the door can only open inwards. Fluid builds up on the
outside and eventually the force pushes open the door and
the fluid enters. This allows proteins to enter also. When
the tissues are inflamed the opening becomes larger and
cells and cellular debris can enter. This is both good and
bad. Good because this is the way that we can trap and
filter pathogens and bad because this is a route for cancer
cells to spread.
Lymphatic trunks

Lymph capillaries merge to form lymphatic
vessels. These join to form lymphatic trunks. The
lymphatic trunks merge into two lymphatic ducts.
As lymphatic vessels become larger, the walls
become endothelial plus collagen, elastin and
smooth muscle. At six places in our body we
have a major accumulation of lymph nodes.
These are part of the lymphatic system which is
dedicated to trapping and dealing with invading
organisms. Special white blood cells are located
in the lymph nodes ready to attack any invader.
Lymphatic ducts

The lymphatic trunks converge into two
lymphatic ducts. The right lymphatic duct
drains the upper right quadrant and the
thoracic duct drains the rest. These two
ducts re-enter our vein on both sides of our
body into the subclavian vein. Study the
slide carefully to familiarise yourself with
these points.
Forces that move fluid between
compartments

The forces that move fluid along the
lymphatic system include capillary
hydrostatic pressure (a nick in a hose) and
Interstitial colloid osmotic (because of
proteins in fluid). Note that if proteins
move into interstitial space because of
inflammation then more water is drawn
from capillaries by osmosis.
Flow

Lymph vessels have one way valves and pressure
build up assists lymph flow. Lymph fluid is also
propelled by skeletal muscle movement and
breathing in (inspiration) helps draws fluid
upwards. This is why people who are chronically
confined to their bed often have swollen feet.
The same reason applies if you travel for 24
hours on a jet flight and have little movement.
Lymphostasis

This is a condition caused by stagnation of
lymph. It is caused either by malformation
or obstruction and results in lymphoedema.
Retention of fluid can contribute to
proliferation of fibroblasts and result in
layers of elastic and collagen fibres being
laid down.
Oedema from lymphatic
obstruction

Obstruction can be caused by malignant
tumours, excessive Fibrinous inflammation
surgery or a parasite that blocks the lymph
vessels. Secondary lymphoedema can arise
from treatment of breast cancer where
removal of breast tissue results in
obstruction of lymphatic system.
Activity

Describe the structure and function of the
lymphatic system.