Tissue fluid is formed as blood flows through capillaries. Some fluid passes through capillary walls into tissues, carrying nutrients to cells. This fluid, called tissue fluid, bathes cells. Not all tissue fluid returns to blood capillaries. Around 10% enters lymph capillaries, which are part of the lymph system. Lymph vessels drain excess tissue fluid through the lymph system back to the bloodstream, preventing fluid accumulation in tissues.

1. Tissue fluid

2. Exchange across the capillaries Formation of tissue fluid Lymph Tissue fluid

3. As blood flows through the capillaries some fluid passes out of capillaries This is tissue fluid it is very similar to plasma but does not have large plasma protein molecules in it This fluid bathes every cell in the body supplying them with glucose, amino acids, fatty acids, salts and oxygen (Tissue fluid also removes carbon dioxide an other waste material from cells) Tissue fluid - formation

4. As blood enters the narrow capillaries the build up of pressure forces water through the capillary walls into the cells. Other substances move out by diffusion Tissue fluid - formation

5. Tissue fluid – exchange with tissues

6. Quick question – If you take out lots of water from a fluid but leave all the large plasma proteins in it, what happens to the water potential?

7. As fluid leaves the capillaries the blood has lost a lot of its water and so it is far more concentrated (its water potential becomes more negative / lower) So water passes back into the capillary by osmosis Waste products leave the cells and enter the capillaries by diffusion Tissue fluid - drainage

11. Not all the fluid returns to the blood capillary 10% enters a separate system of microscopic tubes called lymph capillaries These are part of the lymph system Lymph vessels have valves that let fluid enter but not leave them The lymph capillaries join to form lymph vessels Tissue fluid - Lymph

12. Flow of liquid through the lymph system is very slow It depends on movement of our muscles, the valves in the vessels. Flow is in one direction only, from tissues back towards the bloodstream. Tissue fluid

15. So… fluid In the Blood is called plasma Surrounding the cells is called tissue fluid In the lymphatic system is called lymph Tissue fluid

16. Tissue fluid

17. Name the system to which the vessel labelled X belongs. (ii) Describe the function of the vessels in this system. (2)

18. lymph / lymphatic; drains excess tissue fluid (because not all re-enters capillaries) / prevents accumulation of fluid ; returns fluid to blood;

19. Explain what causes the decrease in water potential at the arterial end of the capillary. Explain what causes the increase in water potential towards the venous end of the capillary.

20. loss of water ; (due to) hydrostatic pressure/blood pressure; explanation in terms of solute concentration / plasma proteins; 2 max water enters; due to osmosis / more negative water potential (in capillary); 2