What are the three components of the filtration membrane at the glomerulus and how do they affect filtration? (3) 3. What types of substances pass through the filtration membrane? What types of substances do not pass through the filtration membrane? Provide few examples of each in your answer.
The Correct Answer and Explanation is:
Components of the Filtration Membrane at the Glomerulus
The filtration membrane in the glomerulus consists of three main components:
- Fenestrated Endothelium: This is the innermost layer of the filtration membrane, composed of endothelial cells with pores (fenestrae). These pores allow water, small solutes, and some plasma proteins to pass through while preventing larger blood cells (e.g., red and white blood cells) from entering the filtrate. The fenestrated structure ensures that large molecules do not pass into the Bowman’s capsule.
- Basement Membrane: Located between the endothelial cells and the podocytes, this layer is a gel-like mesh made of proteins such as collagen and glycoproteins. It acts as a physical barrier to larger proteins, preventing their passage into the filtrate. The basement membrane is negatively charged, which also helps repel negatively charged molecules (e.g., albumin) from passing through, providing selective filtration.
- Podocytes: These are specialized epithelial cells that wrap around the capillaries of the glomerulus. They have foot-like extensions known as pedicels, which interlace and form filtration slits. These slits further restrict the passage of large molecules and ensure that only substances small enough to pass through the fenestrae and basement membrane can enter the filtrate.
Substances That Pass and Do Not Pass Through the Filtration Membrane
Substances that pass through:
- Water: Easily passes through due to its small size and no significant charge.
- Ions: Small ions like sodium, potassium, and chloride pass freely through the filtration membrane.
- Small Molecules: Glucose and amino acids, being small and uncharged, can pass through into the filtrate.
Substances that do not pass through:
- Blood Cells: Red and white blood cells are too large to fit through the fenestrae and basement membrane.
- Proteins: Large plasma proteins like albumin are generally too large to pass through, though small amounts can escape if the membrane is damaged.
- Large Molecules: Substances like large polysaccharides or lipids are typically too big to filter through the membrane.
The filtration process at the glomerulus is highly selective and serves to retain cells and large proteins in the bloodstream, while allowing small, essential molecules and waste products to enter the renal filtrate.
