Glomerular Filtration
Objective 4
Describe the structures by which blood is filtered through the glomerular membrane. Compare and contrast the composition of blood and glomerular filtrate. Identify the pressures that contribute to the glomerular net-filtration pressure. Describe the three regulatory mechanisms to control the glomerular filtration rate (GFR).
The first step of filtering blood occurs at the glomerulus. Filtering occurs by size; smaller substances pass into the filtrate and larger substances remain behind. Imagine you made an interesting version of macaroni and cheese. Deciding to be healthier, you add frozen peas and shredded carrots to your macaroni as it’s cooking. A very impressive three-layer strainer helps you strain the water from this concoction. The first layer has holes large enough that the macaroni stays in the strainer, but everything else gets through. The next layer has even smaller holes that keep the peas behind, the water and small carrots pass through. The last strainer has very small holes. These holes keep the carrots behind. The water has successfully been drained while you have kept the part you want to eat. You may not want to try this anytime soon unless you are feeling very experimental.
Just like our three-layered strainer, the glomerulus in the kidneys also has three layers to keep some substances out of the filtrate while allowing smaller substances to pass through. The three layers of tissue that form the filtration membrane are comprised of:
- the endothelium of the glomerular capillaries. These specialized capillaries are fenestrated capillaries, so they are much more permeable than the more common continuous capillaries of the vascular system.
- a layer of connective tissue (basal lamina) is sandwiched between the endothelium and the podocytes.
- the outermost layer of the capsule is formed by the visceral layer of the glomerular capsule. This tissue is formed by unique cells called podocytes. These cells have numerous foot-processes (pedicels) that extend from the podocytes. The pedicels have small spaces between them called filtration slits. The permeability of the membrane can be controlled by the amount of space provided by the foot-processes of podocytes.
The capillary endothelium allows for the passage of fluid and blood solutes (such as ions, glucose, and urea) into the glomerular capsule but does not allow formed elements (blood cells and platelets) to pass through. Very few proteins can pass through this three-layer membrane.
By working together, these three layers of the glomerular membrane effectively restrict the passage of any substance larger than 40,000 molecular weight (40 kDa) into the glomerular filtrate. Capillary blood pressure is the primary driving force that pushes fluids and solutes through the glomerular membrane.
Glomerular filtrate is blood minus the formed elements and the majority of plasma proteins. Normally, 16-20% of the plasma in the afferent arterioles becomes filtra