Cambridge O Level Biology · Syllabus 5090 · Transport in Humans
Tissue Fluid
What is Tissue Fluid?
Tissue fluid is the fluid that surrounds and bathes the body cells. It is formed at the arterial end of a capillary, where the pressure of the blood forces water and small dissolved substances such as glucose, oxygen, amino acids and ions out through the capillary wall. Red blood cells and large plasma proteins are too large to pass through and remain in the capillary, so tissue fluid differs in composition from plasma. Substances are exchanged between tissue fluid and the body cells by diffusion, and much of the fluid returns to the capillary at the venous end as the pressure falls.
This definition is part of the Transport in Humans chapter in Cambridge O Level Biology.
Common mistakes with Tissue Fluid
- “Red blood cells leave the capillary to form tissue fluid.” Why wrongThey are far too large to pass through the wall, which is why tissue fluid is not red. WriteTissue fluid is formed from plasma: water and small dissolved substances are forced out, while red blood cells and large plasma proteins remain in the capillary.
- “Tissue fluid contains the same large proteins as plasma.” Why wrongThe capillary wall holds them back. WriteLarge plasma proteins are too big to pass through the capillary wall, so they remain in the blood and tissue fluid contains very little of them.
Questions students ask about Tissue Fluid
What is tissue fluid and how is it different from blood?
Tissue fluid is the fluid that bathes the body cells. It is formed when pressure forces water and small dissolved substances out of a capillary at its arterial end. Red blood cells and large plasma proteins are too large to pass through the capillary wall, so tissue fluid contains neither, which is why it differs in composition from blood and is not red.