Formation of Tissue Fluid; A level OCR Bio?!!

Hi. This is how I learnt it/my notes on it:

Tissues have networks of capillaries called a capillary bed. Some capillaries have small pores in them called fenestrations, which allow molecules to “leak” out of the capillary. Only small molecules pass into tissue fluid, larger ones remain in the capillaries. Blood flows from the arteriole end of the capillary towards the venule end.

At the arteriole end of the capillary there is:
- high hydrostatic pressure
- osmosis occurring drawing water back into blood
- a net flow of fluid out of the capillary

At the venule end there is:
- lower hydrostatic pressure
- osmosis drawing water back into the blood
- a net flow of fluid into the capillary.

The higher hydrostatic pressure at the arteriole end forces the small molecules out of the capillary through the fenestrations e.g. water, dissolved oxygen, dissolved solutes (glucose, amino acids, ions), some proteins such as hormones, few leukocytes (mostly phagocytes) and no plasma proteins.

Tissue fluid bathes the somatic cells within the tissue bed and the majority will return to capillaries after exchanging substances with the body cells. Excess tissue fluid drains into the lymphatic system, where the lymph fluid is eventually returned to the blood in the thorax close to the heart. Lymph fluid contains less useful substances than tissue fluid as they’re used by the body cells but contains more: CO2, more fatty substances from intestinal absorption, and lymphocytes.

That’s pretty much it, hope that helps