General Considerations
All nephrons have one glomerular apparatus each. Each glomerular apparatus is a critical contributor to the renal function. Here are the functions performed by the glomerular apparatus. Notice that the tubular secretion and reabsorption is not performed by the glomerular apparatus.
- Filtration of the substances. This is the first step in the urine formation. (Filtration Barrier.)
- Prevention of filtration for the substances that we don’t want to loose. For example blood cells and proteins. (Filtration Barrier.)
- Blood pressure measurement and maintenance. (Juxtaglomerular cells.)
- Blood osmolality measurement and maintenance. (Macule Densa cells.)
- Renin release to help maintain body fluid volume and blood pressure. (Macula Densa cells.)
- Blood flow regulation to the glomerulus to control the rate of filtration. (Mesangial cells.)
Structures forming Glomerular Apparatus
This apparatus is formed by following structures:
- Bowman’s capsule of a nephron.
- Glomerular capillaries and the basement membranes.
- Podocytes or the visceral layer of the bowman’s capsule.
- Mesangial cells inside the bowman’s capsule and immediately outside of it.
- Juxtaglomerular cells in the walls of the afferent and efferent arterioles.
- Macula Densa cells in the wall of the distal convoluted tubule of the same nephron.
Functional Considerations
Filtration
Glomerular filtration barrier is composed of endothelial cells, basement membrane and the podocytes. Podocytes are the visceral layer cells of the bowman’s capsule. Podocytes and the capillary endothelial cells share one common basement membrane that is sandwiched between them. We will discuss details about this barrier in another lecture. Here it is important to note that this barrier allows the fluids and selective substances to move from the blood compartment to the bowman’s space of the nephron.
Selective Filtration
Filtration barrier mentioned above also prevents filtration of proteins and blood cells. This is accomplished by progressively smaller fenestrations (holes) in the barrier and also be the abundance of negative charges in the barrier. As an example, albumin is about 4nm but still is not able to move out of the blood compartment because of the strong negative charges on the barrier (proteins are usually negatively charged.)
Blood Pressure measurement and Maintenance
Juxtaglomerular cells detect the stretch in the arteriolar wall due to the blood pressure and release renin if necessary. Renin in turn work with Angiotensin to help regulate blood pressure and blood volume.
Blood osmolality measurement and maintenance
When the distal convoluted tube ascends and reaches near its corresponding bowman’s capsule the cells in its wall facing the bowman’s capsule become specialized cells. These cells are called macula densa cells. As these cells are facing the urine compartment (nephron lumen) they are able to measure the osmolality of the urine. Macula densa cells release ATP or Adenosine when the osmolality is not correct. JG cells contract or relax under the influence of this ATP and/or Adenosine. Result is blood flow regulation to the glomerulus which in turn helps regulate body fluid volume and osmolality.
Blood flow regulation to the glomerulus by the mesangial cells
Mesangial cells are derived from the smooth muscle cells. Hence they have contractile activity in addition to the phagocytic activity. In fact these are the only phagocytosing cells that are not derived from the monocytes. It is thought that these cells can also help regulate the blood flow to the glomerulus. This effect seems less important compared to their phagocytic activity.