The Glomerulus: Anatomy and Function Quiz

The Glomerulus: The Sphere of Filtration

The glomerulus is a specialized structure within the kidney that serves as the primary site for filtration of blood and the formation of urine. This tiny unit, which is the functional unit of each nephron, is a loop of capillaries twisted into a ball shape and surrounded by the Bowman's capsule. The glomerulus is responsible for filtering plasma, which contains dissolved solutes, from blood cells, and its unique structure enables it to handle high volumes of filtration.

Structure of the Glomerulus

The glomerulus is composed of three main structures:

1. Endothelial cells: The glomerular capillary endothelium has many perforations, called fenestrae, which are pores about 70nm in diameter. These pores do not restrict the movement of water and proteins or large molecules but instead prevent filtration of red blood cells. Surrounding the luminal surface of the endothelial cells is a glycocalyx consisting of negatively charged glycosaminoglycans, which functions to hinder the diffusion of negatively charged molecules by repelling them due to like charges.

2. Glomerular basement membrane: The basement membrane surrounds the capillary endothelium and is mostly made up of type IV collagen, heparan sulfate proteoglycans, and laminin. In particular, heparan sulfate proteoglycans help restrict the movement of negatively charged molecules across the basement membrane. The basement membrane consists of three layers: an inner thin layer, a thick layer, and an outer dense layer. These layers help to limit the filtration of intermediate and large sized solutes.

3. Epithelial cells (Podocytes): Podocytes are specialized epithelial cells of Bowman's capsule that form the visceral layer of the capsule. Foot-like processes project from these podocytes and interdigitate to form filtration slits. These filtration slits are bridged by a thin diaphragm (the slit diaphragm), which has very small pores. The pores prevent large molecules, such as proteins, from crossing.

Filtration and Ultrafiltration

The glomerulus is where ultrafiltration, a process of filtration under pressure, occurs. The afferent arteriole dilates at the proximal glomerulus, while the efferent arteriole constricts at the distal glomerulus, creating a pressure gradient throughout the glomerulus. This pressure gradient causes filtration under pressure. The filtration rate of molecules of the same charge across the filtration barrier is inversely related to their size. Small molecules like glucose (180 Da) are freely filtered, whereas albumin (69 kDa) is barely able to cross the barrier. The electrical charges on molecules also play a role in affecting their filtration rate. Negatively charged large molecules filter less easily than positively charged ones of the same size.

Clinical Relevance

The glomerulus plays a crucial role in maintaining renal function, and disorders affecting its structure and function can lead to significant health issues. For example, minimal change disease, which is responsible for 10-25% of cases of nephrotic syndrome, involves pathology of the podocytes that can be detected under an electron microscope. This pathology includes diffuse effacement of the podocytic foot processes, causing widening of filtration slits and visible microvillous changes.

In summary, the glomerulus is a complex structure that allows for continuous filtration of plasma from blood cells. Its unique arrangement of endothelial cells, basement membrane, and epithelial cells (podocytes) enables it to filter small molecules while preventing the passage of larger ones. Understanding the structure and function of the glomerulus is essential for understanding the physiological processes within the kidney and for diagnosing and treating renal disorders.