Filtration Membrane

The filtration membrane is defined as the interface between the blood capillary and the capsular space of the Bowman’s capsule in a nephron.
Blood enters the filtration apparatus through the afferent arteriole, which leads into a capillary bed.
This capillary bed is a twisted "tuft" of capillaries.
Blood exits the capillary bed via the efferent arteriole, which is notably thinner in diameter than the afferent arteriole.
The Bowman’s capsule consists of two distinct layers:
- Visceral layer: A simple squamous epithelial layer that directly covers the entire area of the capillary blood vessels. It follows the three-dimensional contours, wrapping over all blood vessels.
- Parietal layer: The outer layer of the capsule, formed when the epithelial cells turn backward on themselves.
The space between the visceral and parietal layers is the capsular space, which "swallows" the filtrate like "Pac-Man’s throat."
As the epithelial cells transition from the capsule into the tubular network, they become taller and more specialized for the absorption of nutrients.

Intraglomerular mesangial cells: These are connective tissue cells located within the glomerulus that provide structural support.
Immune cells: Located within the glomerular area to phagocytize ("gobble up") any materials that should not have passed through the barrier.
Extra-glomerular mesangial cells: Located outside the glomerulus; these cells help anchor the structure into place.
Macula densa: A specialized structure that connects the filtration area to the distal convoluted tubule (DCT) or the ascending thick limb when it returns to the area of the renal corpuscle.

In order for a particulate or molecule to move from the bloodstream into the capsular space, it must pass through a specific sequence of layers:
1. Fenestrated Capillary Endothelium: The wall of the capillary endothelial cell contains small pores or breaks called fenestrations. Glycoproteins are present on the basal surface of these cells.
2. Basement Membrane: A layer of extracellular matrix situated between the epithelial cells of the capillary and the podocytes.
3. Podocyte Layer (Visceral Epithelium): These are specialized epithelial cells that cover the capillary.
Podocyte Structure:
- Podocytes have foot processes (also called pedicels) that resemble mini legs or fingers wrapping around the blood vessel.
- The foot processes of adjacent podocytes interlace or interdigitate around the capillary bed.
- Podocytes also feature glycoproteins on their outer cell surfaces.
Slit Diaphragms: These are very thin membranes located in the small spaces (filtration slits) between the interlaced foot processes of adjacent podocytes.

The passage of substances is governed primarily by the size of the pores in the fenestrated capillaries and the slits in the slit diaphragms.
Substances less than $3\,nm$ in diameter:
- These substances pass through the membrane freely and readily.
Substances between $3\,nm$ and $5\,nm$ in diameter:
- These substances can pass through but encounter more resistance and may require a "shove" or force to enter the capsular space.
Substances greater than $5\,nm$ in diameter:
- These substances are generally excluded from the filtrate.

Water ( $H_2O$ ):
- An atom is approximately $0.1\,nm$ in diameter ( $1\,Å$ ).
- A water molecule, accounting for atoms and bonds, is roughly $0.5\,nm$ in diameter (calculated as $0.1\,nm \times 5$ ).
- Because it is far less than $3\,nm$ , it passes easily.
Glucose ( $C_6H_{12}O_{6}$ ):
- Despite its complex carbon backbone and multiple $-OH$ groups, glucose is less than $3\,nm$ and moves freely across the membrane.
Nitrogenous Waste Products:
- Urea and creatinine are small enough to pass through freely.
Amino Acids:
- Basic structures like glycine (with a hydrogen R-group) are less than $3\,nm$ .
- At physiological pH, amino acids exist in a charged state (e.g., $NH_3^+$ and $COO^-$ ), but their physical diameter remains small enough for free passage.
Ions:
- Chloride ( $Cl^-$ ), potassium ( $K^+$ ), sodium ( $Na^+$ ), magnesium ( $Mg^{2+}$ ), and calcium ( $Ca^{2+}$ ) pass through easily.
Vitamins:
- Most vitamins are just under the $5\,nm$ threshold. They pass through the membrane but with more resistance than smaller ions. Riboflavin (Vitamin $B_2$ ) is noted for turning urine neon yellow.
Inulin:
- An insoluble fiber that passes through the filtration membrane.

Red Blood Cells (RBCs):
- These enter the capillary single-file but have a diameter of approximately $7.5\,µm$ .
- Since $1\,µm = 1,000\,nm$ , an RBC is $7,500\,nm$ in diameter, which is vastly larger than the filtration pores.
Albumin:
- This is the most abundant protein in the body. When folded, it has a diameter right at or slightly greater than $5\,nm$ .
- It does not pass through the membrane readily.
Other Large Proteins:
- Beta globulins, alpha globulins, and large hormones are too large to be filtered.
The Role of Negative Charge:
- The basement membrane and the cell surfaces (via glycoproteins) are negatively charged.
- This negative charge creates an electrostatic repulsion that prevents negatively charged substances (like albumin and other large proteins) from crossing the threshold, even if they are close to the size limit.

Filtrate vs. Urine: Most passed substances (water, glucose, ions, amino acids) are later reabsorbed by the tubular network. Waste products and a medium for transport (water) are eventually excreted as urine.
Albuminuria: The presence of albumin in the urine, which is abnormal and indicates a failure of the filtration membrane.
Hemoglobinuria: The presence of hemoglobin in the urine, often resulting from the rupture of dead red blood cells.
Hematuria (Blood in Urine): This should not occur under normal conditions, with an exception noted for menstruating females due to potential cross-contamination.
Normal components in filtrate that should be reabsorbed: Glucose and amino acids.