Transport in animals

03.24.2020
09:27:34
Grade9- Biology- Today’s Topic-Transport in the blood, lymph and tissue fluid- course book -pages 121 to 123.
Main points:
The lymphatic system:
Not all the tissue fluid returns to the capillaries. Some of it enters blind-ended, thin-walled vessels called lymphatics.
The lymphatics from all parts of the body join up to make two large vessels, which empty their contents into the blood system.
The fluid in the lymphatic vessels is called lymph.
Some of the larger lymphatics can contract, but most of the lymph flow results from the vessels being compressed from time to time when the body muscles contract in movements such as walking or breathing.
There are valves in the lymphatics, so that when the lymphatics are squashed, the fluid in them are forced in one direction only: towards the heart.
At certain points in the lymphatic vessels there are swellings called lymph nodes.
Lymphocytes are stored in the lymph nodes and released into the lymph to eventually reach the blood system.
There are also phagocytes in the lymph nodes. If bacteria enter a wound and are not ingested by the white cells of the blood or lymph, they will be carried in the lymph to a lymph node and white cells there will ingest them.
The lymph nodes thus form part of the body’s defence system against infection.

The transfer of materials between capillaries and tissue fluid:

The fluid that escapes from capillaries is not blood, nor plasma, but tissue fluid.
Tissue fluid is similar to plasma but contains less protein, because protein molecules are too large to pass through the walls of the capillaries.
This fluid bathes all the living cells of the body and, since it contains dissolved food and oxygen from the blood, it supplies the cells with their needs.
Some of the tissue fluid eventually seeps back into the capillaries, having given up its oxygen and dissolved food to the cells, but it has not received the waste products of the cells, such as carbon dioxide, which are carried away by the bloodstream.
The tissue fluid that doesn’t return to the capillaries joins the lymphatic system.