Describe the similarities and differences between ultrafiltration in the kidneys and the formation of tissue fluid.

Both are processes which involve the filtration of small molecules (such as water, glucose, amino acids) out of capillaries, while larger molecules, proteins and cells (such as red blood cells) remain in the vessel. Both processes occur due to high hydrostatic pressure (force outwards) overcoming the oncotic pressure of water (force inwards).There are a few key differences between the processes of ultrafiltration and tissue fluid formation. In the kidneys, the molecules must pass through three layers of cells - the endothelial cells of the capillary, the basement membrane of the glomerulus, and the slit between processes of podocyte cells. Meanwhile, tissue fluid only needs to pass through one layer of cells - the endothelial cells of the capillary. The filtrate in the kidney then enters the Bowman's capsule/PCT for excretion (or possible reabsorption), while tissue fluid bathes cells in the intercellular space. Reabsorption of certain molecules (water, ions, sugars) back into the capillaries is possible in both cases. In the absence of reabsorption, the kidney filtrate is excreted from the body in urine, while the components of tissue fluid may enter the surrounding cells or form lymph.