Renal System 2: Glomerular Filtration

What forces govern glomerular filtration?

Glomerular filtration involves passage across a filtration membrane. This filtration occurs through capillary endothelial pores, which are leaky but not overly selective. The solutes then enter a dense layer or Basal Lamina, which is a collagen matrix that separates the cells of the capillaries from those of the Bowman's capsule (tubule). The basal lamina prevents large proteins from crossing, but allows water and solutes to move across. There is also reasonable dependence on charge when it comes to determining what is able to move through this layer. The solutes must then pass through filtration slits created by spaces between podocytes, which line the Bowman's capsule.

Hence,

1. Water and solutes build up in glomerular capillaries
2. Substances move through pores in capillary endothelium into basal lamina
3. Filtration slits between podocytes map route for substances into Bowman's capsule

In order for glomerular filtration to occur, three forces exist. These three forces determine the overall filtration pressure:

1. Glomerular hydrostatic pressure (55mmHg) is another term for blood pressure in the glomerular capillaries. It pushes water and solutes out of the plasma and into the filtrate. In comparison to blood pressure in the systemic circuit, i.e. the vessels exterior to the kidneys, it is much higher. This is because of the capillaries are arranged so tightly at the glomerulus.
2. Capsular hydrostatic pressure (15mmHg) is the pressure in the Bowman's capsule which opposes that in the glomerulus, i.e. the capillary hydrostatic pressure opposes the glomerular hydrostatic pressure. It pushes water and solutes out of the filtrate and into the plasma. Unlike blood pressure which results from smooth muscle contraction and relaxation, the capillary hydrostatic pressure results from resistance to flow along the nephron and conducting system.
3. Blood-colloid osmotic pressure (30mmHg) results from the presence of suspending proteins in capillaries. It draws water out of the filtrate back into the plasma, opposing filtration.

The net hydrostatic pressure can be found using the following equation:

GHP - CsHP = 15-15 = 40mmHg

Thus, the net filtration pressure can be found using the following equation:

NHP - BCOP = 40-30 = 10mmHg

Glomerular filtration generates about 180L of filtrate per day, where 99% is reabsorbed in the renal tubules. The glomerular filtration rate is the amount of filtrate the kidneys produce each minute. It averages at about 125mL/min. The glomerular filtration rate depends on filtration pressure, as just discussed.