619
CHAPTER SIXTEEN
Lymphatic System and Immunity
16.3
TISSUE FLUID AND LYMPH
Lymph is essentially tissue fl
uid that has entered a lymphatic
capillary. Thus, lymph formation depends upon tissue fl
uid
formation.
Tissue Fluid Formation
Capillary blood pressure filters water and small molecules
from the plasma. The resulting fl uid has much the same com-
position as the plasma (including nutrients, gases, and hor-
mones), with the important exception of the plasma proteins,
which are generally too large to pass through the capillary
walls. The osmotic effect of these proteins (called the
plasma
colloid osmotic pressure
) helps to draw fl uid back into the cap-
illaries by osmosis.
RECONNECT
To Chapter 15, Exchanges in the Capillaries,
pages 578–579.
Lymph Formation
Filtration from the plasma normally exceeds reabsorption,
leading to the net formation of tissue fl uid. This increases
blood’s return to the right atrium. Thus, lymph from the
lower body regions, the left upper limb, and the left side of
the head and neck enters the thoracic duct; lymph from the
right side of the head and neck, the right upper limb, and
the right thorax enters the right lymphatic duct
(f g. 16.6)
.
Figure 16.7
summarizes the lymphatic pathway.
The skin has many lymphatic capillaries. Consequently, if the skin is
broken, or if something is injected into it (such as venom from a sting-
ing insect), foreign substances rapidly enter the lymphatic system.
PRACTICE
1
What are the general functions of the lymphatic system?
2
Distinguish between the thoracic duct and the right lymphatic duct.
3
Through which lymphatic structures would lymph pass in
traveling from a lower limb back to the bloodstream?
FIGURE 16.4
Lymphatic vessels merge into larger lymphatic trunks,
which, in turn, drain into collecting ducts.
Jugular trunk
Right lymphatic
duct
Brachiocephalic
vein
Bronchomediastinal
trunk
Intercostal
trunk
Internal
jugular vein
Thoracic
duct
Subclavian
trunk
Thoracic
duct
Intestinal
trunk
Lumbar
trunk
Lymphatic
vessels
FIGURE 16.5
A lymphangiogram (radiograph) of the lymphatic
vessels and lymph nodes of the pelvic region.
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