Cambridge O Level Biology · Syllabus 5090 · Transport in Humans
Plasma
What is Plasma?
Plasma is the straw-coloured liquid part of the blood, in which the blood cells and platelets are suspended. It is mostly water but is not pure water: dissolved in it are glucose, amino acids, mineral ions, hormones, urea, vitamins, carbon dioxide and plasma proteins such as fibrinogen. Plasma transports all of these around the body and also distributes heat from active organs such as the liver and the muscles.
This definition is part of the Transport in Humans chapter in Cambridge O Level Biology.
Common mistakes with Plasma
- “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.
- “Plasma is just water.” Why wrongIt is mostly water, but the dissolved substances are the entire reason the blood is worth circulating. WritePlasma is the liquid part of blood, carrying glucose, amino acids, ions, hormones, urea, vitamins, carbon dioxide and plasma proteins, as well as the blood cells and heat.
- “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 Plasma
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.