Glomerular filtration

Glomerular filtration

Glomerular filtrate is the fluid that is filtered through the glomerular membrane into the Bowman’s capsule.

Glomerular capillary membrane

• Glomerular capillaries are relatively impermeable to proteins so that the filtered fluid, glomerular fluid (GF) is essentially protein free and devoid of cellular elements. The glomerular capillary membrane has three layers:
  1. Endothelium of the capillary
  2. Basement membrane
  3. A layer of epithelial cells (podocytes) surrounding the outer surface of the capillary basement membrane

The endothelial cells lining the glomerulus are perforated by thousands of small holes called fenestrae.

Surrounding the endothelium is the basement membrane composed of a meshwork of collagen and proteoglycan fibrillae which can filter large amount of water and small solutes.

The final layer contains epithelial cells called podocytes lining the outer surface of the glomerulus. These cells are not continuous but consists of many finger like projections which form slit pores through which glomerular filtrate filters.

Therefore, GF must pass through three different layers before it enters the Bowman’s capsule.

The permeability of the glomerular membrane is 100 to 1000 times as great as that of the usual capillary and is because of the pores of the endothelium which are approximately 100 nm diameter and also slit pores approximately 25 nm wide.

Despite the tremendous permeability of glomerular membrane, it has a high degree of selectivity for the sizes of the molecules that it allows to pass. Therefore,  plasma proteins are completely impermeable.

Reasons for high degree of selectivity

Size of the molecule: Pores in the membrane allow molecules with a diameter upto 8 nm.

Pores are lined with a strong negative charges and electrostatic repulsion of the protein molecules (proteins are electronegative) prevent their filtration.

Dynamics of Glomerular filtrate

The energy for the filtration is provided by the heart in the form of hydrostatic pressure of the blood inside the glomerular capillaries and the colloidal osmotic pressure of the fluid within the Bowman’s space through the capillary membrane into the Bowman’s capsule. On the other hand, colloid osmotic pressure in the blood and the hydrostatic pressure in the Bowman’s space oppose filtration.

The colloidal osmotic pressure in the Bowman’s capsule is negligible due to very low protein content. The colloidal osmotic pressure in the glomerular capillaries increases,  since 1/5^th of the fluid portion of the plasma in the capillaries filters into the capsule increases the protein concentration about 20% as blood passes from arterial to venous end of the glomerular capillaries.

The colloidal osmotic pressure of the blood entering the capillaries is 28 mm Hg which rises to 36 mm Hg by the time the blood reaches the venous side and so the average colloidal osmotic pressure is 32 mm Hg.

Filtration pressure

Filtration pressure is the net pressure forcing the fluid through the glomerular membrane equals to the glomerular pressure minus sum of glomerular colloidal osmotic pressure and capsular pressure.

For example, If Glomerular Hydrostatic Pressure = 60 mm Hg, Colloidal Osmotic Pressure in glomerulus = 32 mm Hg and capsular pressure (Bowman’s capsule Hydrostatic Pressure) = 18 mm Hg. Then, the Filtration pressure = 60-(18+32) = 10 mm Hg.

Tubular transport

Transport of fluid from the Bowman’s capsule to the renal pelvis is accomplished by a difference in the hydrostatic pressure. Tubular reabsorption involves transport of water and solutes from the tubular fluid to the peritubular capillaries.

Tubular secretion

Tubular secretion is the transport of solute from the peritubular capillaries to the tubular fluid.