Why does glucose pass into the kidney but red blood cells and protein do not?

Each kidney contains 1000s of nephrons and this is where a process called ultrafiltration occurs. Blood from the renal artery flows through the glomerulus at high pressure. The glomerulus is a bundle of capillaries at the start of the nephron surrounded by a double membrane called the Bowman's capsule. Small molecules such as glucose, urea, water and ions are able to pass through the capsule membrane and are squeezed out of the blood into the Bowman's capsule space. However, big molecules such as red blood cells and protein are not able to pass through the capsule membrane and so stay in the blood and do not enter the capsule space.