Why haemoglobin molecules are not found in blood plasma?

I read this information from PRE-U TEXT STPM BILOLOGY, why haemoglobin molecules are not found in blood plasma? It is because if haemoglobin molecules were suspended in the plasma rather than enclosed within erythrocytes, the blood would become too viscous for the heart to pump the blood through the blood vessels. Haemoglobin would also affect the solute potential in the blood. Membrane of Bowman’s capsule in the nephron allows only proteins of molecular weight below 68 000 to pass. The haemoglobin molecules are smaller than the 680 000 threshold molecular weight. If present in the plasma, the haemoglobin can pass through the filtration membrane and be lost in the urine.