Production of urine begins with a biological sieving process known as glomerular filtration. Occurring within microscopic functional units of the kidney called nephrons, this process ensures that your blood is cleared of waste while retaining essential cells and proteins.
Anatomy of Filter
Glomerular filtration takes place in the renal corpuscle, which consists of two main structures:
-
Glomerulus: A high-pressure capillary network.
-
Bowman’s Capsule: A cup-like sack that catches filtered fluid (filtrate).
For filtration to occur, fluid must pass through the filtration membrane, a three-layered barrier that acts like a specialized colander:
-
-
Fenestrated Endothelium: Capillary walls have large pores that allow everything except blood cells to pass.
-
Basement Membrane: A negatively charged gel layer that repels large proteins (like albumin).
-
Podocytes: Specialized cells of Bowman’s capsule with foot processes that create narrow slit diaphragms, providing the final fine-tuning of the filter.
-
2. Pressure Gradient
Filtration isn’t active transport; it is a passive process driven by hydraulic pressure. It relies on balance of four main forces, collectively determining the Net Filtration Pressure (NFP):
-
Glomerular Hydrostatic Pressure (Pgc): The push force, Because the afferent arteriole (entry) is wider than the efferent arteriole (exit), blood backs up in the glomerulus, creating high pressure (approx. 55 mmHg).
-
Colloid Osmotic Pressure (pigc): The pull force. Proteins remaining in the blood try to draw water back into the capillaries (approx. 30 mmHg).
-
Capsular Hydrostatic Pressure (Pbs): Resistance from the fluid already inside capsule (approx. 15 mmHg).
The formula for resulting pressure is:
Using standard values: 55 – (15 + 30) = 10 mmHg.
This small & steady pressure is what keeps kidneys filtering.
3. Glomerular Filtration Rate (GFR)
The GFR is total volume of filtrate formed by all nephrons in both kidneys per minute. In a healthy adult, this is roughly 125 mL/min, or about 180 liters per day.
4. What is into Filtrate?
The resulting glomerular filtrate is essentially blood plasma without large proteins. It contains:
-
Water
-
Glucose and Amino acids
-
Electrolytes (Sodium, Potassium, Chloride)
-
Metabolic wastes (Urea, Creatinine)
If the filter is damaged (due to diseases like diabetes or hypertension), red blood cells or proteins may leak into urine—a condition known as hematuria or proteinuria.