An overview of glomerular filtration

Glomerular filtration refers to the process of ultrafiltration of plasma from the glomerular capillaries to the Bowman capsule. Filtration is a process in which blood pressure pushes plasma and dissolved material out of the capillaries. In glomerular filtration, blood pressure pushes plasma, dissolved substances and small proteins out of the glomeruli and into the Bowman’s capsule.

Glomerular filtration occurs from the glomerulus to Bowman’s capsule. The pathway for ultrafiltration of plasma from the glomerulus to the Bowman space consists of the fenestrated capillary endothelium, the capillary basement membrane, and the visceral epithelial cell layer (podocytes) of the Bowman capsule. High blood pressure (60 mmHg) in the glomerulus pushes plasma, dissolved materials, and small proteins out of the blood and into the Bowman capsule. The fluid at this moment is called (filtrate).

Filtration is primarily determined by the molecular size and shape of the filtered solute and, to a lesser degree by its charge, the blood cells and large proteins remain in the blood.

Glomerular filtration rate (GFR)

The glomerular filtration rate (GFR) is defined as the total amount of filtrate produced in all nephrons of both kidneys in the given unit of time. The normal GFR is 125 mL/minute or approximately 180 L/day. Increased blood flow to the kidneys increases GFR and reduced blood flow reduces GFR.

Pressures determining filtration rate

The 3 pressures, which determine the GFR are:

1. Glomerular capillary pressure
2. Colloidal osmotic pressure in the glomeruli
3. Hydrostatic pressure in the Bowman capsule

Glomerular filtration rate (GFR) is determined by either favoring or opposing the filtration by these pressures.

Glomerular Capillary Pressure

Glomerular capillary pressure is the blood pressure in the glomerular capillaries. It is roughly 60 mm Hg and ranges between 45- and 70-mm Hg. Glomerular capillary pressure is the body's highest capillary pressure. This pressure favors glomerular filtration

Colloidal Osmotic Pressure

This is the pressure exerted by the plasma proteins in the glomeruli. The Plasma proteins are not filtered by the glomerular capillaries and remain in the glomerular capillaries. These proteins produce a colloidal osmotic pressure of about 25 mm Hg. This pressure opposes glomerular filtration.

Hydrostatic Pressure in Bowman Capsule

This is the pressure exerted by the filtrate in the Bowman capsule. It is often referred to as capsular pressure. It's around 15 mm Hg. It also opposes glomerular filtration.

Net Filtration Pressure

Net filtration pressure is the balance between the pressure that supports filtration and the pressure that opposes filtration. It is otherwise referred to as effective filtration pressure or essential filtration pressure.

Net filtration pressure is about 20 mm Hg and, it varies between 15- and 20-mm Hg.

Starling Hypothesis & Starling Forces

The determination of net filtration pressure is based on the Starling hypothesis. The Starling hypothesis indicates that net filtration through the capillary membrane is proportional to the difference in hydrostatic pressure across the membrane minus the difference in oncotic pressure. Hydrostatic pressure inside the glomerular capillaries is the glomerular capillary pressure.

Filtration coefficient

The coefficient of filtration is GFR in terms of net filtration pressure. This is the GFR per mm Hg of net filtration pressure. E.g., when GFR is 125 mL / min and net filtration pressure are 20 mm Hg.

Regulation of GFR

The factors which regulate glomerular filtration rate (GFR) include:

1. Renal Blood Flow
2. Tubuloglomerular Feedback
3. Glomerular Capillary Pressure
4. Colloidal Osmotic Pressure
5. Hydrostatic Pressure in Bowman Capsule
6. Constriction of Afferent Arteriole
7. Constriction of Efferent Arteriole
8. Systemic Arterial Pressure
9. Sympathetic Stimulation
10. Surface Area of Capillary Membrane
11. Permeability of Capillary Membrane
12. Contraction of Glomerular Mesangial Cells
13. Hormonal and Other Secretory factors