Blood plasma contains large molecules and proteins that are not present in glomerular filtrate. Glomerular filtration is the first step in making urine, states Springfield Technical Community College.
In glomerular filtration, blood plasma is filtered through the glomerulus, explains Springfield Technical Community College. The glomerulus is a structure in the kidney that filters blood plasma to make urine. The glomerular membrane contains slit pores and small holes that allows smaller sized molecules to pass through, but larger molecules are unable to pass through. Electrolytes such as sodium, potassium, calcium, phosphate and bicarbonate as well as magnesium can pass through and are present in the glomerular filtrate. Nutrients and nitrogenous wastes also can pass through and are present in glomerular filtrate. Small hormones and water can also pass through the membrane. Blood cells and plasma proteins are unable to pass through the glomerular membrane and are not found in glomerular filtrate.
The next step in urine formation is the reabsorption of some of the molecules in the glomerular filtrate back into the plasma, according to Springfield Technical Community College. Water, some electrolytes, nutrients and hormones are reabsorbed, whereas wastes are not reabsorbed. Blood plasma needs to be filtered through the kidney in order to keep the blood clean.