In the kidneys, the glomerulus plays a crucial role in filtering blood to form urine. However, not all substances in the blood are filtered out. Some of the key substances that are typically not filtered through the glomerulus include:
- Large Proteins: Proteins such as albumin are usually retained in the bloodstream. Their large molecular size prevents them from passing through the filtration membrane of the glomerulus, which is selectively permeable.
- Blood Cells: Red blood cells, white blood cells, and platelets are too large to be filtered. The structure of the glomerular capillaries does not allow these cells to pass, ensuring they remain within the circulating blood.
- Some Hormones: Certain hormones and larger signaling molecules are also not filtered due to their size and structure.
The reason these substances are not filtered out lies in the unique characteristics of the glomerular capillaries. The glomerulus is composed of fenestrated capillaries, which have small pores to allow the passage of water, ions, and small molecules. However, these pores are not large enough for larger structures like proteins and blood cells to pass through. Additionally, the basement membrane of the glomerulus has an electrically charged barrier that repels negatively charged molecules, further preventing the filtration of certain substances.