Glomerular Apparatus and Filtration - Anatomy & Physiology
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Glomerular Filtration
The role of the renal corpuscle is to selectively filter the blood by allowing small molecules through but preventing plasma proteins from leaving the blood. This filtration occurs extracellulary and is done by what is know as the glomerular filtration barrier. This structure is made up of three layers:
- Fenestrated capillary endothelium
- Glomerular basement membrane
- Podocytes
Fenestrated Capillary Endothelium
- There are small gaps called fenestrae in the cytoplasm of the capillary endothelium.
- They account for approxamately 10% of the surface area of the cells
- They allow water and non-cellular components of the blood to pass through
- They act mainly as a barrier to the cells of the blood
Glomerular Basement Membrane
- Also known as the basal lamina
- This is the main barrier to proteins
- Restricts all but the smallest plasma proteins from passing through
- Made up of a protein mesh in a gelatinous matrix
- Composed of collagen and other matrix proteins
- Prevents filtration of compounds >7,000 Da
- Lower permeability to anions compared to cations. This allows for further selective filtration
Podocytes
The cells of the visceral layer of the Bowmans capsule posses finger like foot processes called pedicels. These wrap around the outer layer of the basal lamina. Filtration occurs through small gaps between pedicels called slit diaphragms. This is the final barrier against proteins
Factors Which Determine Selective Filtration
- Hydrostatic pressure in the capillaries causes filtration but as the filtrate is basically protein free there is no real protein osmotic pressure in the Bowmans capsule. This means the filtration is almost entirely powered by the hydrostatic pressure in the capillaries.
- Molecular size - molecules with a radius of 4nm or more aren't filtered, whilst those with a radius less than 2nm are filtered without restriction. Thus the filtration barrier is selectively permeable.
- Electrical charge
- The barrier has a net negative charge thanks to cells in the membrane having a negatively charged surface and a basal lamina with negatively charged glycoproteins.
- Therefore negatively charged molecules are filtered less than positive or neutral ones
- Proteins are negatively charged this and their size prevents their filtration
- Protein binding
- Drugs, ions or small molecules which are bound to protein are not filtered
- Molecular configuration
- Round molecules fit less easily than ellipsoid ones
- Rigidity
- The higher the rigidity of a molecule the less easily it is filtered
Composition of Filtrate
- Normal filtrate is basically protein free.
- However small amounts of albumin do make it across the membrane. These are completely reabsorbed in the proximal tubules
- Small molecules with a molecular weight <7,000 Da are filtered without restriction these include:
- Water
- Sodium, chloride, creatinine, urea, uric acid and phosphate are filtered in isotonic levels (same concentration as plasma)
- Larger molecules with a molecular weight >7,000 Da such as myoglobin (17,000 Da) are filtered less
- Movement of larger molecules is restricted
- Plasma proteins with molecular weights up to 70,000 Da are heavily restricted from passing through the glomerulus
- Charged molecules over 70,000 Da are not filtered at all
- The threshold for neutral molecules is 100,000 Da
| Glucose | 180 | 1 |
| Myoglobin | 17,000 | 0.75 |
| Haemoglobin | 68,000 | 0.03 |
| Albumin | 69,000 | <0.01 |