What is normally present in urine? How does the filtration barrier function to prevent things from entering the filtrate? What does it prevent from entering?

Generally urine comprises about 95 percent water, 2 percent electrolytes, that is, ions of salts, primarily sulphates, chlorides, potassium, bicarbonates of sodium, and others, 2.6 percent urea, 0.3 percent uric acid, and small quantities of ammonia, creatinine, hormones, some of the pigments, hippuric acid, and allantoin.

The filtration barrier comprises of the glomerular capillaries fenestrated endothelium, the filtration slits of the podocytes, and the fused basal lamina of the podocytes and the endothelial cells. The barrier allows the entry of water, small molecules, and ions from the bloodstream into the space of Bowman capsule.

The barrier restricts passing of negatively charged and/or large proteins like albumin. The basal lamina of the filtration barrier comprises three layers. Any small molecules like glucose, water, salt, urea, and amino acids can pass freely into the Bowman's space, however, the cells, large proteins, and platelets do not.