Plasma is a palestraw-coloured liquid; contains about 90% water and 10% dissolved substances, e.g. plasma proteins, inorganic salts, organic nutrients, hormones, urea, lactic acid, carbon dioxide
Red blood cells/erythrocytes are small biconcave discs with 7-8µm diameter, continuously made in the bone marrow and live for ~120 days before they're broken down in the liver
Erythrocytes are adapted for carrying oxygen:
Haemoglobin: ~300 million molecules of Hb occur in each RBC and each can bind a maximum of 4 molecules of oxygen (no nucleus to make more room for Hb)
Biconcave shape gives a large SA:V for maximum absorption of oxygen and a flexible plasma membrane enables them to squeeze through narrow capillaries
Platelets (thrombocytes) are small (~3µm), non-nucleated, fragments, of larger cells in the bone marrow, linked to chemical reactions that make blood clot (form a solid barrier over the wound)
Blood Clotting: Platelets surround damaged regions and release an enzyme which converts soluble blood plasma protein fibrinogen into the insolublefibrin threads which traps erythrocytes and leucocytes forming a clot; this eventually dries out and forms a scab; this is important as it prevents excess blood loss and pathogens from entering the wound
Plasma protein have many important functions, e.g. osmotic balance, include: Albumin, fibrinogen, immunoglobulins (antibodies) and lipoproteins (LDLs and HDLs)
